Restoring a Pioneer QA-800A quadra

[flambart]

Hi, I'm trying to restore a vintage quadraphonic amplifier (Pioneer QA-800A).

So far, some few questions about:

. where can I find the central volume knob (a knurled one, with a kind of crown) ?

. there is any preferred equivalence about 2SC793 power transistors ?

Thanks to all in advance, Paolo

[flambart]

Some news about the restoration job: using the headphones (32 ohm), all 4 channels work apparently fine.

Instead, when I connect the loudspeakers, the sound of the left front channel is strongly distorted.

The power circuit is not DC coupled, but there are 4 output capacitors (2200 uF / 35 volt): as the unit is at least 35 years old, I think that one of them could be the guilty.

Am I wrong? Apparently, the output transistors (2SC793) seem OK, but so far I did not perform any accurate test about them

Thanks, Paolo

[jaycee]

In a device this old, first step - replace all the electrolytic caps - especially those big speaker output capacitors. Power supply ones also.

You can probably fit 4700uF 35V capacitors in the same space as the old 2200uF ones, and I would do that if you are keen to save this unit as it is. For replacing transistors, the originals are not too fast, and MJ15024 or MJ21194 would be suitable.

[sakis]

beyond the very good suggestion from jaycee you may not run in to conclusion so fast ...

---one thing that could be wrong is that there is some other failure except electro's may exist you need to proceed with caution

--- finding quadral potentiometers is not an easy task ...though if you have mechanical skils you may convert stereo pots to quadral pots

if you can follow procedure and you have basic electronic skills here is a link regarding restore and upgrade of vintage amplifiers

there you go vintage amplifier repair/upgrade manual

[flambart]

Thanks to you all for your kind suggestions.

I'll go ahead and I will let you know (obviously, with further questions...)

See you soon, Paolo

[flambart]

A continuing story...

I've found the problem, a driver transistor was faulty (a npn 2SC497, B-E junction is open)

I've some trouble to find it, here: there is any equivalence of it ?

Moreover, someone has some schematics about this model (Pioneer QA-800A)?

Many thanks again, Paolo

[serchestrella76]

Hola Paolo:

Me llamo Manuel Estrella, soy de México, vi lo que comentaste acerca de la reparación que realizaste al amplificador QA-800A de Pionner, quiero comentarte que yo tengo uno, lo compró mi papá hace ya varias décadas, tiene un sonido impresionante, con decirte que no me ha gustado ningún otro, apesar de ser más nuevos, yo tengo el manual original, así como los diagramas, si te interesa y aún eres miembro des diyAudio, te los mando, mi correo es serchestrella76@gmail.com.

También me interesa conocer tu experiencia, dado que a mi amplificador no le suenan los canales frontales y me gustaría mandarlo a reparar, pero no me animo, porque no cualquiera sabe de esos aparatos.

Que tengas un buen dia.

[tinitus]

english please

[sakis]

i have the schematic ...email me

as about transistors i think that it will be easy to locate if not you can use anything you like ( figure of speach ) as long as its close to specs, pin compatible, faster but imortand is to make changes symmetric meaning that you can keep the same for vas use the ones you have and make a pair for the drivers that could be as simple as a pair of BD139 140 ....

make changes for both amplifiers so everything is symmetric .... you need also to bias the amplifier properly all over again

in between i ll make a few calls by tommorow about the original transistors and let you know

( local Pioneer service actually keeps a stock of very strange things not because they service this age of equipment any more but since they had the parts from the 80's and thought that they could sell it for 100 USD at that time of course now days they simply get on your knees if you buy from them anything from that days. funny dont you think ?)

[tinitus]

serchestrella, Im sorry but you have to post in english


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