Sansui QRX7001 repair

[SPXER]

In capping the tone board F-2425, I see C16 has a symbol that I thought meant bipolar. The cap I removed was not bipolar, plus it was on the board opposite polarity as the board is marked. The parts list does not show a bipolar. Does this symbol mean polarity does not matter even to a polarized cap?

 

[mandrix]

Well two straight lines (with diagonal lines )should be Polar.
They AFAIK should be wired accordingly.

edited for clarity

 

[mandrix]

 

[SPXER]

Well two straight lines should be Polar.
They AFAIK should be wired accordingly.

Thanks for that, mandix.
The cap I removed (C16) was installed with the neg going to the transistor. Opposite what the board shows as + to the transistor. I think it is a misprint on the board. Also there are two TR09s printed on the board and no TR07. Another misprint. I take the upper line to be + and the lower to be neg. as it is marked ground. I would assume (and we know what that means) the positive of the cap goes to the upper line and the negative to the transistor (closer to the ground line). I think I should put it back as I took it out.

Edit: found C12 is put in opposite of the board marking.

 

[mandrix]

I of course have no knowledge of these boards, but where do the schematics come from?
Curious the orientation of the polarized caps are reversed.

 

[DuncanS]

Thanks for that, mandix.
The cap I removed (C16) was installed with the neg going to the transistor. Opposite what the board shows as + to the transistor. I think it is a misprint on the board. Also there are two TR09s printed on the board and no TR07. Another misprint. I take the upper line to be + and the lower to be neg. as it is marked ground. I would assume (and we know what that means) the positive of the cap goes to the upper line and the negative to the transistor (closer to the ground line). I think I should put it back as I took it out.

Edit: found C12 is put in opposite of the board marking.

View attachment 98642

TR04 is NPN so the voltage at the top of R12, R24 should be +Ve with respect to its Emitter. Therefore if C16 is polarised it should have the + pin on the same +Ve supply rail, and the - pin to the Emitter of TR08, otherwise the polarised capacitor will be reverse biased.

 

[SPXER]

TR04 is NPN so the voltage at the top of R12, R24 should be +Ve with respect to its Emitter. Therefore if C16 is polarised it should have the + pin on the same +Ve supply rail, and the - pin to the Emitter of TR08, otherwise the polarised capacitor will be reverse biased.

I agree with that, and the board is marked wrong. As to C12, I take it to be a coupling cap, signal going left to right. Can you analyze that one for me please, DuncanS? This is instructive for me.

 

[SPXER]

I of course have no knowledge of these boards, but where do the schematics come from?
Curious the orientation of the polarized caps are reversed.

The schematic came with the unit and looks to be of the same vintage.

 

[DuncanS]

I agree with that, and the board is marked wrong. As to C12, I take it to be a coupling cap, signal going left to right. Can you analyze that one for me please, DuncanS? This is instructive for me.

So for C12 orientation it would depend on the DC voltage at R08 if it is higher than the voltage at the Base of TR04 then the + pin would be connected to R08 and the - to the Base of TR04. It would be the other way round if the DC voltage at the Base of TR04 is more +Ve than that at R08.

Looking at C48 its orientation of the symbol looks correct for the voltages, i.e. its + goes to the Emitter of TR12. The symbol orientation of C16 looks correct as well. So from the schematic I think C12 should have its + pin connected to R08 and - to TR04 (it looks like the stage de-couplers have their symbols mirrored vertically).

Edit: I've never seen that Japanese Capacitor symbol before!

 

[SPXER]

So for C12 orientation it would depend on the DC voltage at R08 if it is higher than the voltage at the Base of TR04 then the + pin would be connected to R08 and the - to the Base of TR04. It would be the other way round if the DC voltage at the Base of TR04 is more +Ve than that at R08.

View attachment 98648
Looking at C48 its orientation of the symbol looks correct for the voltages, i.e. its + goes to the Emitter of TR12. The symbol orientation of C16 looks correct as well. So from the schematic I think C12 should have its + pin connected to R08 and - to TR04 (it looks like the stage de-couplers have their symbols mirrored vertically).

Edit: I've never seen that Japanese Capacitor symbol before!

Comparing all 4 of the same circuits, the caps C09-C11 are neg to the resistors R05-R07. Positive to their respective transistors. So C12 should be the same orientation and is mismarked on the board. C09-C11 are marked correctly as pulled.

 

[SPXER]

Got it all capped. I will fire it up tomorrow. Too tired now if it goes bad.

 

[par4ken]

If possible coupling capacitors should be replaced with film types, then polarity doesn't matter. Most are of low enough value that those small WIMAs usually will fit.

 

[Owen Smith]

Even 45 years ago doing amateur electronics as a teenager in the UK, all I ever saw on schematics were the capacitor symbols labelled as "modern" in Mandrix's posting, plus trimmer and variable. I've never seen any of the obsolete, USA or Japan ones before even in the late 1970s. We had wiggly line resistors though.

 

[par4ken]

We had wiggly line resistors though.

Aren't resistors still wiggly lines? I think that UK and European schematics used a rectangular box.

 

[Owen Smith]

Aren't resistors still wiggly lines? I think that UK and European schematics used a rectangular box.

Resistors in the UK have been rectangular boxes for at least 30 years. And they seem to be in the US too because schematics from the US head office at work use rectangular box resistors. I thought the wiggly lines went out of use in the 1980s.

 

[SPXER]

If possible coupling capacitors should be replaced with film types, then polarity doesn't matter. Most are of low enough value that those small WIMAs usually will fit.

What advantage are film types?

 

[par4ken]

Resistors in the UK have been rectangular boxes for at least 30 years. And they seem to be in the US too because schematics from the US head office at work use rectangular box resistors. I thought the wiggly lines went out of use in the 1980s.

I don't know about now (I've been retired for several years) but I've only ever seen the boxes in foreign schematics. The boxes were around in the seventies (if not before) in UK and European publications/schematics.

What advantage are film types?

They have lower dielectric absorption, they sound better. Impotent if there are a lot of them in the signal path. They do not age like electrolytics do (so should never need replacement). They are bipolar. They have very low leakage.

 

[DuncanS]

Resistors in the UK have been rectangular boxes for at least 30 years. And they seem to be in the US too because schematics from the US head office at work use rectangular box resistors. I thought the wiggly lines went out of use in the 1980s.

I don't know about now (I've been retired for several years) but I've only ever seen the boxes in foreign schematics. The boxes were around in the seventies (if not before) in UK and European publications/schematics.

My first job straight from Uni in '79 as a junior Electronics Engineer, we had to draw rectangular resistors - yep, draw! on Melinex sheets with a Rotoring ink pen. The PCB layout guy used different coloured tapes to do the layout as well, on vastly oversized Melinex sheets which were then photographed - and even though everything is on a computer, the term tape-out is still used today, though mainly in the semiconductor industry.

 

[Jeff ss]

I got a delivery of another box of caps. I replaced the caps in the protection circuit after all the Thanksgivings festivities were over. So, how does one test the protection circuit safely? Or do I just assume it is good?

You can either short the outputs or raise the dc offset until it trips off. If younraise the DC offset, sound can still be heard through the headphones. It only affects the speaker outputs.

 

[SPXER]

Alrighty then, I fired it up after the tone board re-cap. No smoke, no fire, so that's positive. Hooking up the test speakers, I notice the left channel is noticeably quieter than the right. It wasn't that way before. I don't have any idea how to sort that out. I did recheck the bias. Suggestions are welcome.