SA-7500II recap

[loweran]

Well, seems to be working ok now. Maybe the pot just needed to dry out after all the cleaning. I will monitor it.

 

[loweran]

Well it acted up again. Definitely the volume pot. I turned it on cold and the right channel played, left did not. Worked all the pots and switches with no difference in right channel. Moved the volume and left channel kicked in. Going to go over it again. I will reflow the solder and clean it again.

 

[loweran]

All, just wanted to provide an update. The unit has been working correctly on startup for days. I think the issue was the volume pot was wet from all the cleaning and needed to dry out a bit. Not sure if that makes sense, but it's a hypothesis.

Should I be worried about the offset voltages at the speaker terminals? Seems high at 36.5mv and 44.5mv. Also getting bias at ~60mv one channel and ~16mv on the other.

 

[Shadowdog]

Doesn't seem right with channels being different. BTY I have one of those I bought new in 78 & haven't used it in several yrs. Bought a new A-717 in 88 & both are mint.

Hopefully caps are still good on the older one! I don't think you have to replace transistors unless caps or overheating damages or ruins them.

 

[Oldsansui441]

Should I be worried about the offset voltages at the speaker terminals? Seems high at 36.5mv and 44.5mv. Also getting bias at ~60mv one channel and ~16mv on the other

I would be more worried about the high bias in one channel, the dc offset would ideally be closer to zero also.

 

[loweran]

Question is, how do I sort out the high bias. I suppose I could try a different pair of transistors where the double transistor was. Also, not sure what effects the offset. I pulled and tested the emitters resistors and they are within tolerance. I did buy new ones to replace them just in case.

 

[Oldsansui441]

Ahh l see, l assumed the 7500 would have an adjustable bias circuit like the 8500. I had a look at the service manual and it appears to have the same setup (a wire that is to be cut if under 10mV) as some of the smaller Pioneer's like the little SA-5300 l did a while back that l added bias trim pots to.

You could fit a multi turn 1/2 watt 50ohm trim pot in place of resistors R45-R47 and R46-R48. Alternatively you could fit a 20-25ohm trim pot just in place of the R47 and R48 resistors, should work well either way. Here is a link for the 50ohm part https://au.mouser.com/Search/Refine.aspx?Keyword=3296W-1-500LF

Your bias test points are across BOTH 0.5ohm emitter resistors so l would personally go for around 30mV bias which equates to 30mA.

To lower your dc offset you will have to replace the amplifier inputs Q3-Q4 (2sa798) with two preferably matched (although not normally necessary if from the same batch) KSA992's with both emitters fitted through the same centre hole. I had good results recently with the same double diode replacement on a Technics. Here is a link on the subject http://audiokarma.org/forums/index....nsistor-2sa798-is-there-a-replacement.281614/

 

[Oldsansui441]

Sorry, l just looked back through your thread and noticed you have already tried the KSA992's in place of the 2SA798's, maybe try again with closer matched pairs if possible. Alternatively just remove one of the new 992's (of the pair) and fit another from the same batch and recheck.

 

[loweran]

Interesting idea on the trim pot for the bias.

I thought the idea of the double transistors was to reduce fluctuation of the voltage, and not necessarily the voltage itself. Maybe I miss understand the circuit. I well try to test some more of them and swap them out on the high channel.

 

[loweran]

Question about installing a trimmer in place of a resistor. The resistor has two legs where the trimmer has three. How would you mount the trimmer?

 

[Oldsansui441]

I thought the idea of the double transistors was to reduce fluctuation of the voltage, and not necessarily the voltage itself. Maybe I miss understand the circuit. I well try to test some more of them and swap them out on the high channel.

I think because they are in one package they would be ideally matched, ie low offset outcome and also be well thermally coupled to reduce fluctuation.

Question about installing a trimmer in place of a resistor. The resistor has two legs where the trimmer has three. How would you mount the trimmer?

Just tie the centre leg to one of the outer legs, that way if the wiper ever fails the trim pot won't open and cause circuit damage. There are many units that have two of the legs tied together (solder side of board) with their factory trim pots. Just make sure if you do decide to fit trimmers to orient them in such a way that a clockwise adjustment increases bias, in your circuit this will mean more resistance so check your pot with a meter before fitting.

 

[loweran]

Been thinking about this some more. So, lets say I replace R47 (10 ohm resistor) with a 20 ohm trimmer. Would setting it to maximum of 20 ohms reduce the bias as low as it could in the circuit? That would be the initial setting. Then clockwise would lower the resistance and raise the bias, correct?

Did you use a bread board to install it above the board, or just wire it in in your Technics?

 

[Oldsansui441]

I am fairly certain as l mentioned above that when l checked your circuit diagram that more resistance would equal higher bias. Start with trimmers set at 10 ohm (same as old resistor values) to be safe and adjust from there. Orient the trimmers on the board so that a clockwise adjustment increases resistance.

In regard to your second question are you referring to the differential pair that l replaced on the Technics or the trimmers that l fitted to the Pioneer SA-5300? In both cases parts were fitted directly onto original board.

 

[loweran]

I was just referring to what you had said in post #47.

Thanks for the suggestion and advise. I need to finish up an SA-7800 and then plan to pull out the SA-7500II and give it another look. I have been using it and it is quite pleasing to listen to.

 

[Oldsansui441]

Yep, too easy. Actually l have just done this again to an early Denon that had no bias trim pots fitted from the factory. I fitted them with the centre leg soldered to one of the outside legs. You will know which outer leg to tie the centre to as the wrong way will show less resistance when turning clockwise, hope this makes sense. I just used a standard 3296 style like below. Glad to hear you are liking it , l have had a few SA's and liked them all.

 

[loweran]

Got it back on the bench. Replaced one of the 992s to try and address the offset. Also noticed some cold joints so I touched those up. Put it on the MM and reading idles of 33mv on right channel and 18mv on the left. Offests are both in the 30 - 33mv range.

 

[Oldsansui441]

So did you get your bias trimmers in and do they adjust satisfactorily?

 

[loweran]

No, havent done that since I don’t have the parts. I was just going to work on the offset to see if I could get it lower and noticed a resistor with a bad solder joint. I checked the whole board and reflowes anything that looked suspect. Swapped out the 992s with a matched set and tested.

The idle in that channel that was 60 is now 33. That is about right where it should be. The other channel is still a bit low at 18. It’s in spec according to the manual.

Gave it a listen and sounds good.

I may just leave it be for now. I was going to order the trimmers with my next order.

 

[Oldsansui441]

The idle in that channel that was 60 is now 33. That is about right where it should be. The other channel is still a bit low at 18. It’s in spec according to the manual.

Ahh fair enough, yes much better than it was beforehand.

 

[loweran]

We will see I let these things bother me too much sometimes. I may go back in and mod in those trimmers at some point. Thanks for the suggestion!

Really not sure what else I can do about the offsets. I matched two KSA992s and it didn’t make much of a difference.