Pioneer SA-7500 Replacement transistors: guidance/troubleshooting

[kasbah512]

You can replace as many transistors as you want. It's your call. Replacing Q5 and Q6 will have almost no effect on your output offset voltage. The best solution is to reduce R42 to something like 10k.

I see now. I'm looking at the schematic and that does make sense given that I'm seeing close to -1v at points noted in the initial post. However, I do wonder why this is happening in the first place. That's what I'm failing to understand. I also do have reservations about changing resistors to correct what shouldn't happen in the first place from my understanding (I have a fear of working on it down the line and issues arising from resistor values being inconsistent with the schematic), but if that my best bet, ill give it a try and report back.

 

[ecluser]

What voltage do you have on both sides of your offset trim pots, reference to ground?

 

[spark1]

I think it's a strange ask for them to be verified specifically for a SA-7500. I've worked on a few.

I've been doing this for almost a decade and probably close to 900 units. I'm not making stuff up here, but if you don't believe me - do your research and simply search the forum, you'll confirm my subs.
The KSC2690 would work, but it's compliment the KSA1220 is EOL and we like our pairs to match.

Wow, 2 units a week for 10 years. That's a lot of work. And I saw that you only charge $300 for a full restoration, including parts. Amazingly inexpensive. I'm going to start referring people to you if you don't mind.

 

[ivandezande]

Wow, 2 units a week for 10 years. That's a lot of work. And I saw that you only charge $300 for a full restoration, including parts. Amazingly inexpensive. I'm going to start referring people to you if you don't mind.

Well plenty of units were not restorations. And it was my full time job for a while. Also let’s keep the thread on track we don’t need to post about me.

 

[spark1]

Well plenty of units were not restorations. And it was my full time job for a while.

Oh. I thought it was a hobby/side hustle.

In any case, I can't imagine how you are doing full restorations, including parts, for $300. That seems very (very) cheap.

Heck, it takes me 4-8 hours just to put together a parts list and get it ordered! Then there's disassembly, cleaning all the boards, checking/retouching all the solder work, cleaning all the pots and switches, and post-work testing...all in addition to the actual parts replacements.

Your pricing, given all that's involved, is impressive indeed.

And now back to our regularly scheduled program.

 

[kasbah512]

What voltage do you have on both sides of your offset trim pots, reference to ground?

Im seeing 98.9mV on r41 and 66.0mV on r42 (on the trim pot sides).

Also, how many watts should the new 10k resistor be rated for? Neither r41 or r42 specify in the schematic. I plan on getting at least two so they match in appearance.

 

[ecluser]

The voltage on R42 should be negative. The wattage for R42 is very low. You can use 1/8 or 1/4 Watt, no problem.

 

[kasbah512]

So, I found some 10k resistors that I forgot I even had. What I actually ended up doing is wiring 2 x 10k in series yielding 20k, and wiring that parallel with the 22k r42, to get close to 10k. (I wanted to keep the originals in place because I'm weird) ~10.4k should be the resulting resistance for r42 at this point. However, I was getting around -250mV on each channel for dc offset after this change.

Long story short, I daisy chained 10k resistors until I had enough headroom to make the adjustment. This ended up being 22k ohms for the original resistor wired in parallel with the 7 X 10k ohm in series, giving ~16.7k ohms. I was able to make the adjustment on both sides at this point. Im still working out how I'm going to make something more permanent, but it works for the time being.

 

[ecluser]

You were not able to adjust the offset with the resulting 10.4k at R42? Must I understand that your offset was -250mV or larger (but always negative) when you tried to adjust with the offset trim pot?

 

[kasbah512]

You were not able to adjust the offset with the resulting 10.4k at R42? Must I understand that your offset was -250mV or larger (but always negative) when you tried to adjust with the offset trim pot?

Correct. I cranked the pots all the way over from where they were to try and make it neutral, but I only have like ±25mV adjustments I can make from center. Im also very getting very intermittent snap crackle and pop. The only transistors that are original at this point are 2 (im pretty sure but ide have to double check) on the switch circuit, and the outputs/drivers.

Some bass tones also appear to have a snare like pop at the beginning that I have not noticed before, but I'm saving up for an oscilloscope in the near future.