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Please help! I screwed up SA7500

S

Segasonic91

Active Member
I have recapped most of this amp now and changed all the transistors. There was a ridiculous comedy of erros on my part when testing after getting the relay clicking on, which I will not go into because it makes me mad when I think about it, lol.

The big booboo I made was installing a cap backwards (have NEVER done that before!) and it popped. There was also a spark on the power board, but not sure where. Thankfully I had the cover on! Always pop it on first. Needless to say, no more relay kick. So now I do not know what is wrong with it. Any clue as to where I should start? I am assuming a short somewhere, but no clue WHERE.

I was testing the resistors on the board today and there does appear to be dead and open ones, on the eq board, the power board and the volume board. I am just not sure if these are all correct. I am testing in circuit. Could there really be that many resistors dead or open? Could it also be that they were already dead or open? Or would that have caused no relay kick?

Also, there was a diode I though may be dodgy and I was going to replace it. It is a 500mW diode and I could only find a 1W replacement. Does the wattage have to be exact? Or can the wattage be higher?

Thanks!
 
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What cap went pop?
Power supply board?
It may not be as bad as you think.
 
It was a 220uf 63v on the switch circuit board, AWM-072-C. I put one of the old caps back in as I only had the exact number of caps for each board. I have the replacement now, putting it in today.

As annoying as all the noob mistakes I have made are, I am happy to say that all this learning has really made me more confident. Now if I could just go back and start again, it would be perfect!
 
I’m going to look at the circuit but in the meantime, have you built or are you using a Dim Bulb Tester?
 
Thank you very much! I do not have a bulb tester at the moment unfortunately, it was lost when I moved in 2018. I have only just now started to do this stuff again, so I never got around to building another. I will get on to that though, I will be doing a Sansui AU-9900 next and have some other things lined up, so I certainly need it again.

Thanks again for your help :)
 
There's two. C6 and C11.
Each is part of the regulated supplies.
Which one?
Check all the resistors, diodes and transistors in that circuit. Replace if you can the three other caps (Their probably okay).
One diode, two transistors and three resistors.
Check the resistors in circuit, use the sister resistors on the other regulator side for comparison. Capiche.
Check the transistors in circuit at the legs. Continuity under 1 ohm, between any two of the three legs indicates its shorted.
Use your diode function on your meter. The one diode is a 13v zener. It will test the same but it is only replaced with another 13v zener.
 
I have checked the resistors on all 4 boards, and I seem to be getting quite a few open ones. Is there anything that would cause a false reading? I would have to change 40% or more the way the readings are going.

I have not dealt with resistors before and it seems quite a pain to change them as there are so many types. Unlike caps, I can't just read the rating and voltage on the body. Is there a list of resistors for the 7500? Would make it a little easier than going through the service manual, lol.

There are zero burnt resistors or diodes that I can see, so all the open readings I am getting is strange. If they have been dead for a while, surely the amp would not have worked? I am dying to hear it in its re-capped state! I also have no amp to listen to my records so it is really annoying me. I saw the most ridiculous bargain locally on Saturday. Well by local, I mean Australia. Unfortunately, it was not anywhere near me! They had a serviced SA-7800 and CT-F950 for AU$449!!!! I was told the amp was sold, so I said I would buy the 950, they wanted $140!! The bastard that bought the 7800 bought the 950 as well, just as I thought they would :( They paid AU$550. Unbelievable.
 
zebulon1 said:
Check all the resistors, diodes and transistors in that circuit. Replace if you can the three other caps (Their probably okay).
Click to expand...

Forgot to mention that the caps around C11 are all new. All were installed at the same time. They seem to test okay, but would you recommend changing them? I hope they are fine as is, as I have recently put in a big order for the AU-9900 and will not be doing another order for a while. I am pretty sure I have more of the 220uf/63v because I ordered some as replacements for the popped cap. Ordered 3 I think, thought it was better to have extra if needed.

In the future when recapping, I am going to order 3 or 4 caps over what is required just to be safe. Should have used my brain and done that from the start.
 
Hard to believe that many resistors are bad. When they go open from a sudden unregulated flow of current they get burn and toasted.
Remove the transistors from the blown circuit and retry checking the resistors.
If they still check wonky, De-solder one leg and lift the resistor leg from the trace and remeasure.
For me, checking the transistors is a measure of how bad the damage was.
In this case, I should of recommended replacing the transistors. If they survived the first blast they won't survive on the first power up.
 
Thank you, I will try that today.

One other thing I will mention is that the power transistors on the power board do appear to have black legs, but not on the part of the legs that were soldered. I have the replacements for the 537s, but couod not get the screws on the heat sinks to budge at all. I will get some new heat sinks and install them. I have a big part of the Sansui order coming today, so hoping the replacements for the other two are in there as well.

Is there any particular heat sink recommended for the Pioneers? I saw some that looked similar, but no legs at the front to go into the board. They are TO-220 from memory. There was one that had legs, but the screw hole was too big.

I may see if I can get the replacement diode and resistors at the same time. This shop chain is not very extensive, not by a long shot! So no idea if they will have them. I do know they have the diode because I looked that up a few days back.
 
I was reading some other threads about burnt and fried resistors and have decided that I may as well replace all the resistors on each board. They are dirt cheap, although with this manny it will add up I guess. I have spent the time and money to replace the caps with high quality Nichicon caps and new transistors for the power, eq and volume boards so this will be the last step to restoring this awesome little amp to its former 1975 glory.

In the other threads I read, there were mouser parts lists, are there lists for the 7500s resistors/diodes?

Thanks again.

EDIT: I will post pictures of the switch board later today (currently 4:45am) to show the state of the area around Q11. As I said before, there are no black, burnt areas at all thankfully. I just hope the output transistors are okay! I don't think I ordered any for the Sansui.
 
Replacing things that could be left alone will bite you in the end.
Why make more work for your self?
This older equipment is fragile and can be traumatized easily.
Keep studying the board and schematic until you get familiar with the layout.

YouTube is a great source. “Xraytony” has several long videos on different Pioneer equipment and touches on the techniques used when working on this equipment. Other videos explain how to test the individual components using a meter.

Tip:
The images of the boards in the service manual are of the bottom, trace side. So - If you were looking from the top side of the board, it would be a mirrored image.
Kinda messed up but that’s the way it is.
 
I guess you are right. I just thought that a full refresh may help the unit in the long run.

I know that the scematic is the solder side, or "foil side" :) I have checked the resistors on the legs and also on the solder points on the foil side and they are giving the same result. I wish I had a print out of the particular schematic for the switch board so I can mark each bad transistor as I test them. I have tested them multiple times to make sure the reading is the same, but need to do it once more and write down each number. I will then list the resistors that are bad, and hopefully that will be able to produce a list of needed items.
 
You mean like this?
Found at "HIFi Engine.com"
Transistor equivalents:
2SA945, 512-KSC2383YTA
2SD325, 512-KSC2073TU
Zener diode:
WZ-130, 78-TZX13A

c11 sa 7500.PNG
 
Last edited:
zebulon1 said:
You mean like this?
Found at "HIFi Engine.com"
Transistor equivalents:
2SA945, 512-KSC2383YTA
2SD325, 512-KSC2073TU
Zener diode:
WZ-130, 78-TZX13A

View attachment 1912563
Click to expand...

Pretty much, yes. I have the PDF, it would be nice to have a printout so each thing can be highlighted as bad. Did you find the equivalents at hi-fi engine? Or do you just know the equivalents to those old 2SA transistors?
 
Looking for the resistors to order with the diodes and transistors. I am ordering carbon film, 250mv for the carbon listings in the manual. The Mouser numbers listed for modern replacements I have founx on AK do not come up for me on the site I am using (Element14/Farnell) so I am searching manually.

I have only looked for the ones in that cutout you posted from the service manual as they are the ones I need most. I could not find a 16k carbon film at all, and no 250mv 10k either. Is it okay to order 500mv 10k? Or for that matter, any 250mv in general. As for the case of the 16k, is it okay to use a 16k/250mv/200v? Or can they not have voltages at all? I will replace the metal film 2w resistors around the relay as well.
 
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