Pioneer SA-7500 - zero bias on one channel - need help

[mubs13]

Hi everyone,

This is my first post here, so I apologize for jumping directly to asking question without introducing myself first. But I have my beautiful and beloved Pioneer SA-7500 that has got a strange issue on one channel that I have not been able to fix. I thank you in advance for any advice will be much appreciated.

I did post about this issue in another popular forum here in Australia but unfortunately the problem was left unidentified. I am really counting on the experts here to point me to the right direction.

The main issue is that the bias current is 0mv on right channel. Left channel works and sounds fine. The R channel of course outputs a weak distorted sound. Turning the bias trim pot doesn't seem to have any effect.

In my attempt to fix and narrow down the issue, I have tried the following:

- Replaced all of the following transistors on both channels

2SA726 -------> 512-KSA992FBU
2SC1451 -------> KSC3503DSTU

- Checked all the transistors (including driver and power) on the faulty channel using the junction check method. Both NPN and PNP types measure the voltage drop of roughly 0.6v on base-emitter junction and nothing on collector-emitter. Though the output transistors only showed around 0.5v drop but it also measured consistently on both pairs.

- Swapped output power transistor pair between the channels, problem remains same.

- Swapped bias diodes D2-STV4H between channels, problem still remains same. I can measure -2.1v across on each channel diodes (positive lead connected to cathode).

- Have checked the trim pot in R channel using multimeter, it seems to be working as i can measure varying resistance as I turn it.

- Have checked most of the resistors and they all seem to measure OK.

- Visually inspected each component on power amp board, nothing looks odd or damaged.

- I have also recapped the amp with Nichicon PW caps for power supply and Elna Silmic for audio boards.

- DC offset measures within spec on both speaker terminals.

I tried to take some voltage readings on the R channel and I found the base voltage on one of the driver transistor 2SB527 is off. As per service manual it should be -1.2v but it measures always somewhere between -200mv to -500mv based on the bias pot setting. Collector voltage looks ok at -40v. The other driver transistor 2SD357 measures OK with around 1.2v on base and 40v on collector.

After all the above, bias voltage still measures 0mv on R channel and it sounds distorted. L channel sounds fine.

Please note that I am only a diy enthusiast and still learning as and when I can. So please excuse me if I take a while to follow on an advice or if I ask silly questions Any help will be much appreciated!

Thank you.

 

[chazix]

Welcome to AK! I expect that someone who knows the SA-7500 better than I will chime in soon. But meanwhile, from your description of the things you've tried and a look at the schematic, I think the likeliest explanation is that at least one of the driver transistors has failed. If you know how to do an ohmmeter junction check, I think you would probably find the base-collector junction on at least one of the drivers no longer acts like a diode.

If that's correct, you'll want to replace both drivers in the bad channel as a minimum. If you search for the type numbers in this forum, there's a good chance one of the real Pioneer experts has recommended currently-available replacements. (With a little more luck, they'll be available in Oz. I understand that can be a problem.)

Note that even if you confirm that the drivers are bad, it doesn't necessarily mean that the outputs are good. I'd be a bit suspicious of the outputs that came from the bad channel.

And also suspicious of the bias trimpots - there's a school of thought that says that those should be replaced as a prophylactic measure on vintage amps.

Cheers,

chazix

 

[mubs13]

Welcome to AK! I expect that someone who knows the SA-7500 better than I will chime in soon. But meanwhile, from your description of the things you've tried and a look at the schematic, I think the likeliest explanation is that at least one of the driver transistors has failed. If you know how to do an ohmmeter junction check, I think you would probably find the base-collector junction on at least one of the drivers no longer acts like a diode.

If that's correct, you'll want to replace both drivers in the bad channel as a minimum. If you search for the type numbers in this forum, there's a good chance one of the real Pioneer experts has recommended currently-available replacements. (With a little more luck, they'll be available in Oz. I understand that can be a problem.)

Note that even if you confirm that the drivers are bad, it doesn't necessarily mean that the outputs are good. I'd be a bit suspicious of the outputs that came from the bad channel.

And also suspicious of the bias trimpots - there's a school of thought that says that those should be replaced as a prophylactic measure on vintage amps.

Cheers,

chazix

Hi chazix,

Thanks for the advice mate. I forgot to mention this in my original post, but I have already checked all the transistors (including driver and power) on the faulty channel using the junction check method. Both NPN and PNP types measure the voltage drop of roughly 0.6v on base-emitter junction and nothing on collector-emitter. Though the output transistors only showed around 0.5v drop but it also measured consistently on both pairs.

I do not remember correctly if I tried swapping the driver transistor pair or not, but I have already swapped the output transistor pair between the channels, so the working L channel has the power transistor pair from the faulty channel and it works fine. And the R channel with the pair from L still has issues so it's possibly something else...?

I will try swapping the driver pair between the channels tonight and report the outcome.

Cheers.

 

[chazix]

Both NPN and PNP types measure the voltage drop of roughly 0.6v on base-emitter junction and nothing on collector-emitter.

The junction that I suspect in the driver transistors is the base-collector. For a "bipolar" transistor (an NPN or PNP type), the B-C junction should mirror the B-E in an ohmmeter check. That is, "ohm-ing" between the base and either of the other terminals should produce pretty close to the same readings; conductive with one orientation of the ohmmeter leads, and non-conductive with the other orientation.

No question that doing channel swaps is an effective test technique. I just think Murphy's Law looms larger on that path, but I'm probably a little late with that particular gem of unsolicited advice .

 

[mubs13]

The junction that I suspect in the driver transistors is the base-collector. For a "bipolar" transistor (an NPN or PNP type), the B-C junction should mirror the B-E in an ohmmeter check. That is, "ohm-ing" between the base and either of the other terminals should produce pretty close to the same readings; conductive with one orientation of the ohmmeter leads, and non-conductive with the other orientation.

No question that doing channel swaps is an effective test technique. I just think Murphy's Law looms larger on that path, but I'm probably a little late with that particular gem of unsolicited advice .

Finally, some good news! Yes, it was one of the driver transistor. I initially only tested them on base-emitter junction and it tested OK, and I did not know that the B-C junction should mirror the B-E. Thank you very much for the hint and helping me out here man, truly appreciated

I actually also swapped the pair between channels just to be sure and voila, the bias issue moved to L channel from R. So that confirms it as well.

Now i just need to find good substitutes for the drivers. I suppose I should change both pairs on both channels?

 

[mubs13]

Based on forum search result, MJ15030 MJ15031 have been suggested as good substitutes for 2SD357 and 2SB527 drivers. Is that a good substitute to go with for SA-7500?

 

[chazix]

MJ15030 MJ15031

Those will be fine. I believe the part numbers are missing an "E", though - should be MJE15030 and MJE15031.

In my opinion it isn't important to do the replacement in both channels. I don't believe replacing the drivers will make any perceptible difference to the amplifier's performance.

Cheers,

chazix

 

[chazix]

don't believe replacing the drivers will make any perceptible difference to the amplifier's performance.

Whoops - I don't like how that reads literally. I do think replacing the drivers is (at least part of) an effective repair. What I meant to convey was that I think the amplifier channel repaired with the ON Semi parts will perform indistinguishably from the working channel that still has the original drivers.