I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.

The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.

I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
both directions on an ohm-meter !!!

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

> I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
> is a complex beast and one channel had gone down to almost nothing.
> Luckily the service sheet was available and I traced the fault to the
> board which houses the tone controls and filters.
>
> The gain is in two stages with the tone controls around the second
> stage. On one of the channels, the voltages on the single transistor
> for that stage were all wrong. After lifting the transistor off the
> board, it became apparent that the base bias was far too high and the
> finger of suspicion pointed to the inter-stage coupling capacitor
> feeding in leakage current from the preceding stage.
>
> I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
> both directions on an ohm-meter !!!

You have a new type of device! Save it. It just might be worth
something... someday... maybe...

--
Jim Whitby

For the next hour, WE will control all that you see and hear.
----------------------
Mageia release 9 (Official) for x86_64
6.6.18-server-1.mga9 unknown
----------------------

jim whitby <mr.spock@spockmall.net> wrote:

> > I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
> > is a complex beast and one channel had gone down to almost nothing.
> > Luckily the service sheet was available and I traced the fault to the
> > board which houses the tone controls and filters.
> >
> > The gain is in two stages with the tone controls around the second
> > stage. On one of the channels, the voltages on the single transistor
> > for that stage were all wrong. After lifting the transistor off the
> > board, it became apparent that the base bias was far too high and the
> > finger of suspicion pointed to the inter-stage coupling capacitor
> > feeding in leakage current from the preceding stage.
> >
> > I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
> > both directions on an ohm-meter !!!
>
> You have a new type of device! Save it. It just might be worth
> something... someday... maybe...

It's a 22.09 usecs time constant - at least until it drifts.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On 2024/03/14 1:56 p.m., Liz Tuddenham wrote:
> I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
> is a complex beast and one channel had gone down to almost nothing.
> Luckily the service sheet was available and I traced the fault to the
> board which houses the tone controls and filters.
>
> The gain is in two stages with the tone controls around the second
> stage. On one of the channels, the voltages on the single transistor
> for that stage were all wrong. After lifting the transistor off the
> board, it became apparent that the base bias was far too high and the
> finger of suspicion pointed to the inter-stage coupling capacitor
> feeding in leakage current from the preceding stage.
>
> I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
> both directions on an ohm-meter !!!
>

Can digital ohm-meters be fooled by ESR?

It does sound like a leaky cap mind you - and interesting that the
leakage matches the stamped uf value!

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
#7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

John Robertson <jrr@flippers.com> wrote:

> On 2024/03/14 1:56 p.m., Liz Tuddenham wrote:
> > I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
> > is a complex beast and one channel had gone down to almost nothing.
> > Luckily the service sheet was available and I traced the fault to the
> > board which houses the tone controls and filters.
> >
> > The gain is in two stages with the tone controls around the second
> > stage. On one of the channels, the voltages on the single transistor
> > for that stage were all wrong. After lifting the transistor off the
> > board, it became apparent that the base bias was far too high and the
> > finger of suspicion pointed to the inter-stage coupling capacitor
> > feeding in leakage current from the preceding stage.
> >
> > I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
> > both directions on an ohm-meter !!!
> >
>
> Can digital ohm-meters be fooled by ESR?

Possibly, but not to that extent. The meter was a Philip PM2518, which
I believe uses DC for the ohms measurement.

A new capacitor solved the problem and saved a rather expensive and
special piece of kit from being consigned to the scrap heap.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
>is a complex beast and one channel had gone down to almost nothing.
>Luckily the service sheet was available and I traced the fault to the
>board which houses the tone controls and filters.
>
>The gain is in two stages with the tone controls around the second
>stage. On one of the channels, the voltages on the single transistor
>for that stage were all wrong. After lifting the transistor off the
>board, it became apparent that the base bias was far too high and the
>finger of suspicion pointed to the inter-stage coupling capacitor
>feeding in leakage current from the preceding stage.
>
>I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
>both directions on an ohm-meter !!!

Used to keep a special box for spectacularly wrong parts, but never
found a situation where their re-use was justified, even as a joke.

Everybody takes themselves so seriously these days. At least each
part had a story behind it . . . .

RL

> On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
>
>>I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
>>is a complex beast and one channel had gone down to almost nothing.
>>Luckily the service sheet was available and I traced the fault to the
>>board which houses the tone controls and filters.
>>
>>The gain is in two stages with the tone controls around the second
>>stage. On one of the channels, the voltages on the single transistor
>>for that stage were all wrong. After lifting the transistor off the
>>board, it became apparent that the base bias was far too high and the
>>finger of suspicion pointed to the inter-stage coupling capacitor
>>feeding in leakage current from the preceding stage.
>>
>>I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
>>both directions on an ohm-meter !!!
>
>
> Used to keep a special box for spectacularly wrong parts, but never
> found a situation where their re-use was justified, even as a joke.
>
> Everybody takes themselves so seriously these days. At least each part
> had a story behind it . . . .
>
> RL

Way back when...
I used to keep some blown fueses around, just for people who wanted "a
fuse exactly like this one".

--
Jim Whitby

Let us remember that ours is a nation of lawyers and order.
----------------------
Mageia release 9 (Official) for x86_64
6.6.18-server-1.mga9 unknown
----------------------

On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
>is a complex beast and one channel had gone down to almost nothing.
>Luckily the service sheet was available and I traced the fault to the
>board which houses the tone controls and filters.
>
>The gain is in two stages with the tone controls around the second
>stage. On one of the channels, the voltages on the single transistor
>for that stage were all wrong. After lifting the transistor off the
>board, it became apparent that the base bias was far too high and the
>finger of suspicion pointed to the inter-stage coupling capacitor
>feeding in leakage current from the preceding stage.
>
>I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
>both directions on an ohm-meter !!!

Was it a light blue Sanyo?

Chuck <chuck23@dejanews.net> wrote:

> On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
>
> >I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
> >is a complex beast and one channel had gone down to almost nothing.
> >Luckily the service sheet was available and I traced the fault to the
> >board which houses the tone controls and filters.
> >
> >The gain is in two stages with the tone controls around the second
> >stage. On one of the channels, the voltages on the single transistor
> >for that stage were all wrong. After lifting the transistor off the
> >board, it became apparent that the base bias was far too high and the
> >finger of suspicion pointed to the inter-stage coupling capacitor
> >feeding in leakage current from the preceding stage.
> >
> >I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
> >both directions on an ohm-meter !!!
>
> Was it a light blue Sanyo?

It was indeed!

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On Sat, 16 Mar 2024 20:44:58 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>Chuck <chuck23@dejanews.net> wrote:
>
>> On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
>> (Liz Tuddenham) wrote:
>>
>> >I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
>> >is a complex beast and one channel had gone down to almost nothing.
>> >Luckily the service sheet was available and I traced the fault to the
>> >board which houses the tone controls and filters.
>> >
>> >The gain is in two stages with the tone controls around the second
>> >stage. On one of the channels, the voltages on the single transistor
>> >for that stage were all wrong. After lifting the transistor off the
>> >board, it became apparent that the base bias was far too high and the
>> >finger of suspicion pointed to the inter-stage coupling capacitor
>> >feeding in leakage current from the preceding stage.
>> >
>> >I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
>> >both directions on an ohm-meter !!!
>>
>> Was it a light blue Sanyo?
>
>It was indeed!

Sounds like a known issue, then. Unless Chuck is psychic, of course.

On Sun, 17 Mar 2024 17:52:39 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>On Sat, 16 Mar 2024 20:44:58 +0000, liz@poppyrecords.invalid.invalid
>(Liz Tuddenham) wrote:
>
>>Chuck <chuck23@dejanews.net> wrote:
>>
>>> On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
>>> (Liz Tuddenham) wrote:
>>>
>>> >I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
>>> >is a complex beast and one channel had gone down to almost nothing.
>>> >Luckily the service sheet was available and I traced the fault to the
>>> >board which houses the tone controls and filters.
>>> >
>>> >The gain is in two stages with the tone controls around the second
>>> >stage. On one of the channels, the voltages on the single transistor
>>> >for that stage were all wrong. After lifting the transistor off the
>>> >board, it became apparent that the base bias was far too high and the
>>> >finger of suspicion pointed to the inter-stage coupling capacitor
>>> >feeding in leakage current from the preceding stage.
>>> >
>>> >I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
>>> >both directions on an ohm-meter !!!
>>>
>>> Was it a light blue Sanyo?
>>
>>It was indeed!
>
>Sounds like a known issue, then. Unless Chuck is psychic, of course.
Those Sanyo caps caused many weird problems in Pioneer receivers. Back
in the day, I worked in the main repair department of which is now the
largest electronics retailer in North America. Since we worked on
only commission, the receiver repairer sometimes got stuck on an odd
problem and didn't want to take a financial hit. I used to love
challenges so I would repair these receivers. I would say, other than
power amp issues, these caps were the main culprits.