Mate has a pair of standard LS5/8. With a minor fault on one of the amps.
They are BBC modified Quad 405 with a crossover fitted, so one amp drives
the bass unit, the other the tweeter. BBC name AM8/16. I made the big
mistake of saying it would be easy to sort. And having spent two days
getting nowhere, now need the help of experts. ;-)

It is only on the low channel. Switch on cold, not present. After perhaps
a few minutes. a clicking noise starts at a low speed then speeds up to a
buzz. And stops. Then starts again after a variable time. It's well down
level wise, and such that you only really hear it with no signal to the
amp.

Swapped the power amp cards (easy), and the fault stayed with the low
channel, so not a power amp fault.

Disconnect the hot side of the input to the low amp card, noise goes.

The BBC Xover has a 30v regulated supply derived from one half of the Quad
PS. That looks clean on the scope.

The big snag is the noise comes and goes. And of course stops as soon as
you start probing with the scope.

--
*Procrastination is the art of keeping up with yesterday.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
<dave@davenoise.co.uk> wrote:

>Mate has a pair of standard LS5/8. With a minor fault on one of the amps.
>They are BBC modified Quad 405 with a crossover fitted, so one amp drives
>the bass unit, the other the tweeter. BBC name AM8/16. I made the big
>mistake of saying it would be easy to sort. And having spent two days
>getting nowhere, now need the help of experts. ;-)
>
>It is only on the low channel. Switch on cold, not present. After perhaps
>a few minutes. a clicking noise starts at a low speed then speeds up to a
>buzz. And stops. Then starts again after a variable time. It's well down
>level wise, and such that you only really hear it with no signal to the
>amp.
>
>Swapped the power amp cards (easy), and the fault stayed with the low
>channel, so not a power amp fault.
>
>Disconnect the hot side of the input to the low amp card, noise goes.
>
>The BBC Xover has a 30v regulated supply derived from one half of the Quad
>PS. That looks clean on the scope.
>
>The big snag is the noise comes and goes. And of course stops as soon as
>you start probing with the scope.

Don't suppose by any chance you have a schematic?

d

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

In article <60b8ab37.12464171@news.eternal-september.org>,
Don Pearce <spam@spam.com> wrote:
> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
> <dave@davenoise.co.uk> wrote:

> >Mate has a pair of standard LS5/8. With a minor fault on one of the
> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
> >made the big mistake of saying it would be easy to sort. And having
> >spent two days getting nowhere, now need the help of experts. ;-)
> >
> >It is only on the low channel. Switch on cold, not present. After
> >perhaps a few minutes. a clicking noise starts at a low speed then
> >speeds up to a buzz. And stops. Then starts again after a variable
> >time. It's well down level wise, and such that you only really hear it
> >with no signal to the amp.
> >
> >Swapped the power amp cards (easy), and the fault stayed with the low
> >channel, so not a power amp fault.
> >
> >Disconnect the hot side of the input to the low amp card, noise goes.
> >
> >The BBC Xover has a 30v regulated supply derived from one half of the
> >Quad PS. That looks clean on the scope.
> >
> >The big snag is the noise comes and goes. And of course stops as soon
> >as you start probing with the scope.

> Don't suppose by any chance you have a schematic?

I do, yes. But easy to find the BBC original online.

This might be easier for a first glance:-

https://ibb.co/tByTj9c

I'm rather wary of replacing caps other than electrolytics as the BBC seem
to have found some pretty uncommon values.

--
*A fool and his money can throw one hell of a party.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

On Thu, 03 Jun 2021 14:23:41 +0100, "Dave Plowman (News)"
<dave@davenoise.co.uk> wrote:

>In article <60b8ab37.12464171@news.eternal-september.org>,
> Don Pearce <spam@spam.com> wrote:
>> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
>> <dave@davenoise.co.uk> wrote:
>
>> >Mate has a pair of standard LS5/8. With a minor fault on one of the
>> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
>> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
>> >made the big mistake of saying it would be easy to sort. And having
>> >spent two days getting nowhere, now need the help of experts. ;-)
>> >
>> >It is only on the low channel. Switch on cold, not present. After
>> >perhaps a few minutes. a clicking noise starts at a low speed then
>> >speeds up to a buzz. And stops. Then starts again after a variable
>> >time. It's well down level wise, and such that you only really hear it
>> >with no signal to the amp.
>> >
>> >Swapped the power amp cards (easy), and the fault stayed with the low
>> >channel, so not a power amp fault.
>> >
>> >Disconnect the hot side of the input to the low amp card, noise goes.
>> >
>> >The BBC Xover has a 30v regulated supply derived from one half of the
>> >Quad PS. That looks clean on the scope.
>> >
>> >The big snag is the noise comes and goes. And of course stops as soon
>> >as you start probing with the scope.
>
>> Don't suppose by any chance you have a schematic?
>
>I do, yes. But easy to find the BBC original online.
>
>This might be easier for a first glance:-
>
>https://ibb.co/tByTj9c
>
>I'm rather wary of replacing caps other than electrolytics as the BBC seem
>to have found some pretty uncommon values.

So this will be the driver stage that the power amp connects to?

d

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

On Thu, 03 Jun 2021 14:23:41 +0100, "Dave Plowman (News)"
<dave@davenoise.co.uk> wrote:

>In article <60b8ab37.12464171@news.eternal-september.org>,
> Don Pearce <spam@spam.com> wrote:
>> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
>> <dave@davenoise.co.uk> wrote:
>
>> >Mate has a pair of standard LS5/8. With a minor fault on one of the
>> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
>> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
>> >made the big mistake of saying it would be easy to sort. And having
>> >spent two days getting nowhere, now need the help of experts. ;-)
>> >
>> >It is only on the low channel. Switch on cold, not present. After
>> >perhaps a few minutes. a clicking noise starts at a low speed then
>> >speeds up to a buzz. And stops. Then starts again after a variable
>> >time. It's well down level wise, and such that you only really hear it
>> >with no signal to the amp.
>> >
>> >Swapped the power amp cards (easy), and the fault stayed with the low
>> >channel, so not a power amp fault.
>> >
>> >Disconnect the hot side of the input to the low amp card, noise goes.
>> >
>> >The BBC Xover has a 30v regulated supply derived from one half of the
>> >Quad PS. That looks clean on the scope.
>> >
>> >The big snag is the noise comes and goes. And of course stops as soon
>> >as you start probing with the scope.
>
>> Don't suppose by any chance you have a schematic?
>
>I do, yes. But easy to find the BBC original online.
>
>This might be easier for a first glance:-
>
>https://ibb.co/tByTj9c
>
>I'm rather wary of replacing caps other than electrolytics as the BBC seem
>to have found some pretty uncommon values.

And the problem lies somewhere between TR5 and TR10. I would suggest
first checking all the transistor voltages for those. Compare with the
other speaker

d

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

In article <60b9dcb4.25132656@news.eternal-september.org>,
Don Pearce <spam@spam.com> wrote:
> On Thu, 03 Jun 2021 14:23:41 +0100, "Dave Plowman (News)"
> <dave@davenoise.co.uk> wrote:

> >In article <60b8ab37.12464171@news.eternal-september.org>,
> > Don Pearce <spam@spam.com> wrote:
> >> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
> >> <dave@davenoise.co.uk> wrote:
> >
> >> >Mate has a pair of standard LS5/8. With a minor fault on one of the
> >> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
> >> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
> >> >made the big mistake of saying it would be easy to sort. And having
> >> >spent two days getting nowhere, now need the help of experts. ;-)
> >> >
> >> >It is only on the low channel. Switch on cold, not present. After
> >> >perhaps a few minutes. a clicking noise starts at a low speed then
> >> >speeds up to a buzz. And stops. Then starts again after a variable
> >> >time. It's well down level wise, and such that you only really hear it
> >> >with no signal to the amp.
> >> >
> >> >Swapped the power amp cards (easy), and the fault stayed with the low
> >> >channel, so not a power amp fault.
> >> >
> >> >Disconnect the hot side of the input to the low amp card, noise goes.
> >> >
> >> >The BBC Xover has a 30v regulated supply derived from one half of the
> >> >Quad PS. That looks clean on the scope.
> >> >
> >> >The big snag is the noise comes and goes. And of course stops as soon
> >> >as you start probing with the scope.
> >
> >> Don't suppose by any chance you have a schematic?
> >
> >I do, yes. But easy to find the BBC original online.
> >
> >This might be easier for a first glance:-
> >
> >https://ibb.co/tByTj9c
> >
> >I'm rather wary of replacing caps other than electrolytics as the BBC seem
> >to have found some pretty uncommon values.

> And the problem lies somewhere between TR5 and TR10. I would suggest
> first checking all the transistor voltages for those. Compare with the
> other speaker

Sadly, only got the faulty amp here.

--
*Never miss a good chance to shut up *

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

On Thu, 03 Jun 2021 15:14:59 +0100, "Dave Plowman (News)"
<dave@davenoise.co.uk> wrote:

>In article <60b9dcb4.25132656@news.eternal-september.org>,
> Don Pearce <spam@spam.com> wrote:
>> On Thu, 03 Jun 2021 14:23:41 +0100, "Dave Plowman (News)"
>> <dave@davenoise.co.uk> wrote:
>
>> >In article <60b8ab37.12464171@news.eternal-september.org>,
>> > Don Pearce <spam@spam.com> wrote:
>> >> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
>> >> <dave@davenoise.co.uk> wrote:
>> >
>> >> >Mate has a pair of standard LS5/8. With a minor fault on one of the
>> >> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
>> >> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
>> >> >made the big mistake of saying it would be easy to sort. And having
>> >> >spent two days getting nowhere, now need the help of experts. ;-)
>> >> >
>> >> >It is only on the low channel. Switch on cold, not present. After
>> >> >perhaps a few minutes. a clicking noise starts at a low speed then
>> >> >speeds up to a buzz. And stops. Then starts again after a variable
>> >> >time. It's well down level wise, and such that you only really hear it
>> >> >with no signal to the amp.
>> >> >
>> >> >Swapped the power amp cards (easy), and the fault stayed with the low
>> >> >channel, so not a power amp fault.
>> >> >
>> >> >Disconnect the hot side of the input to the low amp card, noise goes.
>> >> >
>> >> >The BBC Xover has a 30v regulated supply derived from one half of the
>> >> >Quad PS. That looks clean on the scope.
>> >> >
>> >> >The big snag is the noise comes and goes. And of course stops as soon
>> >> >as you start probing with the scope.
>> >
>> >> Don't suppose by any chance you have a schematic?
>> >
>> >I do, yes. But easy to find the BBC original online.
>> >
>> >This might be easier for a first glance:-
>> >
>> >https://ibb.co/tByTj9c
>> >
>> >I'm rather wary of replacing caps other than electrolytics as the BBC seem
>> >to have found some pretty uncommon values.
>
>> And the problem lies somewhere between TR5 and TR10. I would suggest
>> first checking all the transistor voltages for those. Compare with the
>> other speaker
>
>Sadly, only got the faulty amp here.

Measure the volts anyway - even without any values on those resistors
I should be able to say whether it makes sense. But did you say it
only does it sometimes? If so, look for hairline cracks in pcb traces
- or poor solder joints. Prod around with a piece of wood and see if
you can provoke the fault by pushing on a component. And if you have
any freezer spray, try that on one transistor at a time.

d

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

First thought, is the output cap on the amp board or is it on the
chassis? Since the fault is clearly not on the amp board that is the
first place I would look in case the cap has gone leaky - the date on
the label should give some ideas. Assuming it is a 405 and not a 405II
that cap could be pushing 45+ years old given it was in production
between 1973 and 1977!

Second, a single rail amp will give a switch-on thump as the output cap
charges. Have Quad put some sort of isolation between the cap and the
speaker with a resistor that allows the cap to charge before, say, a
relay closes so that the thump is not heard? If so is the relay getting
enough drive to shove the contacts hard together and/or have the
contacts got burnt or corroded.

Finally, and I have to ask this question: are you sure it is a 405 and
not a 303? The only cct diags I can find for a 405 show it with +/-
supply rails, whereas the 303 is single rail. When I got my used ex-BBC
vanilla BC1's from M&B Radio in Leeds nearly 30 years ago Brian also had
a stack of BC1's with built-in amps - which were 303's. Just a thought.
Another thought is for your friend to contact the manufacturer (if they
still exist, of failing that Spendor who probably built them as well)
and see if they have any remnant data.

On Thu, 03 Jun 2021 14:24:57 GMT, spam@spam.com (Don Pearce) wrote:

>On Thu, 03 Jun 2021 15:14:59 +0100, "Dave Plowman (News)"
><dave@davenoise.co.uk> wrote:
>
>>In article <60b9dcb4.25132656@news.eternal-september.org>,
>> Don Pearce <spam@spam.com> wrote:
>>> On Thu, 03 Jun 2021 14:23:41 +0100, "Dave Plowman (News)"
>>> <dave@davenoise.co.uk> wrote:
>>
>>> >In article <60b8ab37.12464171@news.eternal-september.org>,
>>> > Don Pearce <spam@spam.com> wrote:
>>> >> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
>>> >> <dave@davenoise.co.uk> wrote:
>>> >
>>> >> >Mate has a pair of standard LS5/8. With a minor fault on one of the
>>> >> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
>>> >> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
>>> >> >made the big mistake of saying it would be easy to sort. And having
>>> >> >spent two days getting nowhere, now need the help of experts. ;-)
>>> >> >
>>> >> >It is only on the low channel. Switch on cold, not present. After
>>> >> >perhaps a few minutes. a clicking noise starts at a low speed then
>>> >> >speeds up to a buzz. And stops. Then starts again after a variable
>>> >> >time. It's well down level wise, and such that you only really hear it
>>> >> >with no signal to the amp.
>>> >> >
>>> >> >Swapped the power amp cards (easy), and the fault stayed with the low
>>> >> >channel, so not a power amp fault.
>>> >> >
>>> >> >Disconnect the hot side of the input to the low amp card, noise goes.
>>> >> >
>>> >> >The BBC Xover has a 30v regulated supply derived from one half of the
>>> >> >Quad PS. That looks clean on the scope.
>>> >> >
>>> >> >The big snag is the noise comes and goes. And of course stops as soon
>>> >> >as you start probing with the scope.
>>> >
>>> >> Don't suppose by any chance you have a schematic?
>>> >
>>> >I do, yes. But easy to find the BBC original online.
>>> >
>>> >This might be easier for a first glance:-
>>> >
>>> >https://ibb.co/tByTj9c
>>> >
>>> >I'm rather wary of replacing caps other than electrolytics as the BBC seem
>>> >to have found some pretty uncommon values.
>>
>>> And the problem lies somewhere between TR5 and TR10. I would suggest
>>> first checking all the transistor voltages for those. Compare with the
>>> other speaker
>>
>>Sadly, only got the faulty amp here.
>
>Measure the volts anyway - even without any values on those resistors
>I should be able to say whether it makes sense. But did you say it
>only does it sometimes? If so, look for hairline cracks in pcb traces
>- or poor solder joints. Prod around with a piece of wood and see if
>you can provoke the fault by pushing on a component. And if you have
>any freezer spray, try that on one transistor at a time.
>
>d

I found a problem with the schematic. TR12 emitter has no DC
connection so TR12/13 can't be biased properly. Presumably this is
just a drafting error as the HF path is actually working.

As for the rest, you just have three unity gain buffers with high in
and low out impedance. The first one, TR5 and 6 is literally just a
buffer. Then you have a high pass filter leading into TR8/14.
Presumably this is just a rumble filter. The crossover lowpass filter
itself is built around TR9/10. Finally a bandpass filter with a low
shelf completes the circuit to feed into the PA

d

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

In article <5936bad380dave@davenoise.co.uk>, Dave Plowman (News)
<dave@davenoise.co.uk> scribeth thus
>Mate has a pair of standard LS5/8. With a minor fault on one of the amps.
>They are BBC modified Quad 405 with a crossover fitted, so one amp drives
>the bass unit, the other the tweeter. BBC name AM8/16. I made the big
>mistake of saying it would be easy to sort. And having spent two days
>getting nowhere, now need the help of experts. ;-)
>
>It is only on the low channel. Switch on cold, not present. After perhaps
>a few minutes. a clicking noise starts at a low speed then speeds up to a
>buzz. And stops. Then starts again after a variable time. It's well down
>level wise, and such that you only really hear it with no signal to the
>amp.
>
>Swapped the power amp cards (easy), and the fault stayed with the low
>channel, so not a power amp fault.
>
>Disconnect the hot side of the input to the low amp card, noise goes.
>
>The BBC Xover has a 30v regulated supply derived from one half of the Quad
>PS. That looks clean on the scope.

Check or just replace the Electro caps on that they may have lost
value..
>
>The big snag is the noise comes and goes. And of course stops as soon as
>you start probing with the scope.
>

Dave..

Has this amp been altered or modified in anyway do you know?.

It might be taking off at RF frequencies thats not unknown, and not that
obvious..

QUAD's didn't really use any odd caps as such..

Have you got the full service manual if not let me know and I'll mail it
over..

If you get really stuck s contact Keith Snook that guy knows more about
the 405 than what Peter Walker did i reckon!..

http://www.keith-snook.info/QUAD-405-Modification.html

I do know he has a few AM8/16's around!..

--
Tony Sayer

Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.

On 3/06/2021 7:57 pm, Dave Plowman (News) wrote:
> Mate has a pair of standard LS5/8. With a minor fault on one of the amps.
> They are BBC modified Quad 405 with a crossover fitted, so one amp drives
> the bass unit, the other the tweeter. BBC name AM8/16. I made the big
> mistake of saying it would be easy to sort. And having spent two days
> getting nowhere, now need the help of experts. ;-)
>
> It is only on the low channel. Switch on cold, not present. After perhaps
> a few minutes. a clicking noise starts at a low speed then speeds up to a
> buzz. And stops. Then starts again after a variable time. It's well down
> level wise, and such that you only really hear it with no signal to the
> amp.
>
> Swapped the power amp cards (easy), and the fault stayed with the low
> channel, so not a power amp fault.
>
> Disconnect the hot side of the input to the low amp card, noise goes.
>
> The BBC Xover has a 30v regulated supply derived from one half of the Quad
> PS. That looks clean on the scope.
>
> The big snag is the noise comes and goes. And of course stops as soon as
> you start probing with the scope.
>

**Check all the earth connections. Betcha one is high resistance. It's
possible that one of the decoupling electros is high ESR/low capacitance.

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

In article <s9aulu$hd7$1@dont-email.me>,
Woody <harrogate3@ntlworld.com> wrote:
> First thought, is the output cap on the amp board or is it on the
> chassis? Since the fault is clearly not on the amp board that is the
> first place I would look in case the cap has gone leaky - the date on
> the label should give some ideas. Assuming it is a 405 and not a 405II
> that cap could be pushing 45+ years old given it was in production
> between 1973 and 1977!

Output cap? 405 doesn't have one.

> Second, a single rail amp will give a switch-on thump as the output cap
> charges. Have Quad put some sort of isolation between the cap and the
> speaker with a resistor that allows the cap to charge before, say, a
> relay closes so that the thump is not heard? If so is the relay getting
> enough drive to shove the contacts hard together and/or have the
> contacts got burnt or corroded.

These BBC modified amps are known for a switch on squawk. A function of
their crossover rather than the power amps. No muting relays.

> Finally, and I have to ask this question: are you sure it is a 405 and
> not a 303? The only cct diags I can find for a 405 show it with +/-
> supply rails, whereas the 303 is single rail. When I got my used ex-BBC
> vanilla BC1's from M&B Radio in Leeds nearly 30 years ago Brian also had
> a stack of BC1's with built-in amps - which were 303's. Just a thought.
> Another thought is for your friend to contact the manufacturer (if they
> still exist, of failing that Spendor who probably built them as well)
> and see if they have any remnant data.

The amps built into BBC BC1 were badged Spendor. Not having seen a circuit
for them, it possible they were based on half a 303.

The main supplier of the LS 5/8 is Rogers. It's definitely a 405 with a
+/- 55 volt supply. The crossover volts are picked up from one side of
this. The BBC also made use of the current limiting on the 405 (for the
ESL) to limit the output of the amp driving the tweeter.

The full manual for the speaker is quite easy to find online. Written by
Rogers.

--
*The last thing I want to do is hurt you. But it's still on my list.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

In article <60b9081e.36246718@news.eternal-september.org>,
Don Pearce <spam@spam.com> wrote:
> I found a problem with the schematic. TR12 emitter has no DC
> connection so TR12/13 can't be biased properly. Presumably this is
> just a drafting error as the HF path is actually working.

My error. My original is a poor quality photo copy, so I re-drew it ages
ago when I was using these amps and speakers. There is a dot missing
between R43 and C26 so it connects to rather than passes R45 to TR13

> As for the rest, you just have three unity gain buffers with high in
> and low out impedance. The first one, TR5 and 6 is literally just a
> buffer. Then you have a high pass filter leading into TR8/14.
> Presumably this is just a rumble filter. The crossover lowpass filter
> itself is built around TR9/10. Finally a bandpass filter with a low
> shelf completes the circuit to feed into the PA

The original also has descriptions of each part of the circuit beside that
part. I never completed by drawing. ;-)

--
*Why do overlook and oversee mean opposite things?

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

In article <CTXD6+mfSUugFwLg@bancom.co.uk>,
tony sayer <tony@bancom.co.uk> wrote:
> In article <5936bad380dave@davenoise.co.uk>, Dave Plowman (News)
> <dave@davenoise.co.uk> scribeth thus
> >Mate has a pair of standard LS5/8. With a minor fault on one of the amps.
> >They are BBC modified Quad 405 with a crossover fitted, so one amp drives
> >the bass unit, the other the tweeter. BBC name AM8/16. I made the big
> >mistake of saying it would be easy to sort. And having spent two days
> >getting nowhere, now need the help of experts. ;-)
> >
> >It is only on the low channel. Switch on cold, not present. After perhaps
> >a few minutes. a clicking noise starts at a low speed then speeds up to a
> >buzz. And stops. Then starts again after a variable time. It's well down
> >level wise, and such that you only really hear it with no signal to the
> >amp.
> >
> >Swapped the power amp cards (easy), and the fault stayed with the low
> >channel, so not a power amp fault.
> >
> >Disconnect the hot side of the input to the low amp card, noise goes.
> >
> >The BBC Xover has a 30v regulated supply derived from one half of the Quad
> >PS. That looks clean on the scope.

> Check or just replace the Electro caps on that they may have lost
> value..
> >
> >The big snag is the noise comes and goes. And of course stops as soon as
> >you start probing with the scope.
> >

> Dave..

> Has this amp been altered or modified in anyway do you know?.

> It might be taking off at RF frequencies thats not unknown, and not that
> obvious..

> QUAD's didn't really use any odd caps as such..

> Have you got the full service manual if not let me know and I'll mail it
> over..

> If you get really stuck s contact Keith Snook that guy knows more about
> the 405 than what Peter Walker did i reckon!..

> http://www.keith-snook.info/QUAD-405-Modification.html

> I do know he has a few AM8/16's around!..

I'm fairly certain it's not a power amp fault as I tried swapping the
cards, as it's easy to do. And the fault stayed with the LF output.

As you likely know, the connections to the power amp are all 1/8th spade
connectors. And removing the 'hot' wire only (leaving the ground) to the
amp input kills the buzz.

The odd value caps are all in the BBC X-over. They seem to have been
sourced from a variety of makers.

--
*Shin: a device for finding furniture in the dark *

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

It has to be a dodgy joint or faulty capacitor or diode I'd say, poke the
rectifiers a bit and the electrolytic as well. I had a vcr that had this and
that was a bridge rectifier module, and in another case a rustling or
burping in a mixer turned out to be an electrolytic that was somehow varying
and because it was in a sensitive amp it was amplified.

I'm sure others will think of more things, but often its only by replacing
things that you actually find it. I really miss being able to do this
myself, as it keeps the old grey cells going.
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Dave Plowman (News)" <dave@davenoise.co.uk> wrote in message
news:5936bad380dave@davenoise.co.uk...
> Mate has a pair of standard LS5/8. With a minor fault on one of the amps.
> They are BBC modified Quad 405 with a crossover fitted, so one amp drives
> the bass unit, the other the tweeter. BBC name AM8/16. I made the big
> mistake of saying it would be easy to sort. And having spent two days
> getting nowhere, now need the help of experts. ;-)
>
> It is only on the low channel. Switch on cold, not present. After perhaps
> a few minutes. a clicking noise starts at a low speed then speeds up to a
> buzz. And stops. Then starts again after a variable time. It's well down
> level wise, and such that you only really hear it with no signal to the
> amp.
>
> Swapped the power amp cards (easy), and the fault stayed with the low
> channel, so not a power amp fault.
>
> Disconnect the hot side of the input to the low amp card, noise goes.
>
> The BBC Xover has a 30v regulated supply derived from one half of the Quad
> PS. That looks clean on the scope.
>
> The big snag is the noise comes and goes. And of course stops as soon as
> you start probing with the scope.
>
> --
> *Procrastination is the art of keeping up with yesterday.
>
> Dave Plowman dave@davenoise.co.uk London SW
> To e-mail, change noise into sound.

I well remember Pioneer having problems with this issue in a lot of their
amps, that was a faulty batch of 2sc something or other transistors used in
the pick up input, Rather than return the whole thing I bought a couple of
equev devices from Tandy, that dates it, and put those in and never any more
issues again. Nothing ever looks wrong with the transistors just occasional
increases in noise, Very peculiar.
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Don Pearce" <spam@spam.com> wrote in message
news:60b8e587.27391453@news.eternal-september.org...
> On Thu, 03 Jun 2021 15:14:59 +0100, "Dave Plowman (News)"
> <dave@davenoise.co.uk> wrote:
>
>>In article <60b9dcb4.25132656@news.eternal-september.org>,
>> Don Pearce <spam@spam.com> wrote:
>>> On Thu, 03 Jun 2021 14:23:41 +0100, "Dave Plowman (News)"
>>> <dave@davenoise.co.uk> wrote:
>>
>>> >In article <60b8ab37.12464171@news.eternal-september.org>,
>>> > Don Pearce <spam@spam.com> wrote:
>>> >> On Thu, 03 Jun 2021 10:57:08 +0100, "Dave Plowman (News)"
>>> >> <dave@davenoise.co.uk> wrote:
>>> >
>>> >> >Mate has a pair of standard LS5/8. With a minor fault on one of the
>>> >> >amps. They are BBC modified Quad 405 with a crossover fitted, so one
>>> >> >amp drives the bass unit, the other the tweeter. BBC name AM8/16. I
>>> >> >made the big mistake of saying it would be easy to sort. And having
>>> >> >spent two days getting nowhere, now need the help of experts. ;-)
>>> >> >
>>> >> >It is only on the low channel. Switch on cold, not present. After
>>> >> >perhaps a few minutes. a clicking noise starts at a low speed then
>>> >> >speeds up to a buzz. And stops. Then starts again after a variable
>>> >> >time. It's well down level wise, and such that you only really hear
>>> >> >it
>>> >> >with no signal to the amp.
>>> >> >
>>> >> >Swapped the power amp cards (easy), and the fault stayed with the
>>> >> >low
>>> >> >channel, so not a power amp fault.
>>> >> >
>>> >> >Disconnect the hot side of the input to the low amp card, noise
>>> >> >goes.
>>> >> >
>>> >> >The BBC Xover has a 30v regulated supply derived from one half of
>>> >> >the
>>> >> >Quad PS. That looks clean on the scope.
>>> >> >
>>> >> >The big snag is the noise comes and goes. And of course stops as
>>> >> >soon
>>> >> >as you start probing with the scope.
>>> >
>>> >> Don't suppose by any chance you have a schematic?
>>> >
>>> >I do, yes. But easy to find the BBC original online.
>>> >
>>> >This might be easier for a first glance:-
>>> >
>>> >https://ibb.co/tByTj9c
>>> >
>>> >I'm rather wary of replacing caps other than electrolytics as the BBC
>>> >seem
>>> >to have found some pretty uncommon values.
>>
>>> And the problem lies somewhere between TR5 and TR10. I would suggest
>>> first checking all the transistor voltages for those. Compare with the
>>> other speaker
>>
>>Sadly, only got the faulty amp here.
>
> Measure the volts anyway - even without any values on those resistors
> I should be able to say whether it makes sense. But did you say it
> only does it sometimes? If so, look for hairline cracks in pcb traces
> - or poor solder joints. Prod around with a piece of wood and see if
> you can provoke the fault by pushing on a component. And if you have
> any freezer spray, try that on one transistor at a time.
>
> d
>
> --
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus
>

In article <s9cjrp$isj$1@dont-email.me>,
Brian Gaff \(Sofa\) <briang1@blueyonder.co.uk> wrote:
> I well remember Pioneer having problems with this issue in a lot of
> their amps, that was a faulty batch of 2sc something or other
> transistors used in the pick up input, Rather than return the whole
> thing I bought a couple of equev devices from Tandy, that dates it, and
> put those in and never any more issues again. Nothing ever looks wrong
> with the transistors just occasional increases in noise, Very peculiar.
> Brian

The noise (to me) sounds very like from a cap, as the frequency of the
buzz starts low and increases. And then stops. Oddly, left it on soak all
day yesterday, and it was clean at switch on this morning.

--
*People want trepanners like they want a hole in the head*

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

On Fri, 04 Jun 2021 10:49:33 +0100, "Dave Plowman (News)"
<dave@davenoise.co.uk> wrote:

>In article <s9cjrp$isj$1@dont-email.me>,
> Brian Gaff \(Sofa\) <briang1@blueyonder.co.uk> wrote:
>> I well remember Pioneer having problems with this issue in a lot of
>> their amps, that was a faulty batch of 2sc something or other
>> transistors used in the pick up input, Rather than return the whole
>> thing I bought a couple of equev devices from Tandy, that dates it, and
>> put those in and never any more issues again. Nothing ever looks wrong
>> with the transistors just occasional increases in noise, Very peculiar.
>> Brian
>
>The noise (to me) sounds very like from a cap, as the frequency of the
>buzz starts low and increases. And then stops. Oddly, left it on soak all
>day yesterday, and it was clean at switch on this morning.

Time to start swapping electrolytics. Has to be favourite.

d

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

Found some time to look at it again.

Re-soldered the entire ground side on the BBC PCB - it's all discrete
components so easy enough. And seems to have sorted it.

Thanks for all the pointers.

--
*Why is the time of day with the slowest traffic called rush hour?

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.