Pioneer SA-8500 relay not engaging after transistor replacment

[crustypaul]

I have a really baffling issue with my Pioneer SA-8500.
The right hand channel developed crackling and popping due to dying transistors on the power amp board, so i replaced the 2SA726's at Q2 and Q4 on the right with KSA992's and the 2SC1451 at Q6 with a KSC3503. This went ok and the amp seemed to be working perfectly again, but i thought i should change the transistors on the left as well. I swapped them over in the same way and switched it on, and the speaker protection relay wouldn't engage.
I took out one of the new KSA992's on the left and replaced it with one of the original 2SA726's and still nothing, so i replaced the other new KSA922 with another original 2SA726 and the amp came back to life....

After much messing around i found the only way to get it working was if Q1 and Q3 on the left were both the original 2SA726's. If either of them are KSA922 then it won't switch the relay when you turn it on.
I was wondering if somehow the pinout diagrams on the board were wrong, but that makes no sense as surely its exactly the same as on the right. On the right the 2SA726's are BCE and the KSA922's are ECB so i just installed the KSA922's flipped around.

I feel like i'm going mad here. It doesn't seem like this should be possible. Am i missing something embarrassingly obvious?

 

[mrk229]

I have a really baffling issue with my Pioneer SA-8500.
The right hand channel developed crackling and popping due to dying transistors on the power amp board, so i replaced the 2SA726's at Q2 and Q4 on the right with KSA992's and the 2SC1451 at Q6 with a KSC3503. This went ok and the amp seemed to be working perfectly again, but i thought i should change the transistors on the left as well. I swapped them over in the same way and switched it on, and the speaker protection relay wouldn't engage.
I took out one of the new KSA992's on the left and replaced it with one of the original 2SA726's and still nothing, so i replaced the other new KSA922 with another original 2SA726 and the amp came back to life....

After much messing around i found the only way to get it working was if Q1 and Q3 on the left were both the original 2SA726's. If either of them are KSA922 then it won't switch the relay when you turn it on.
I was wondering if somehow the pinout diagrams on the board were wrong, but that makes no sense as surely its exactly the same as on the right. On the right the 2SA726's are BCE and the KSA922's are ECB so i just installed the KSA922's flipped around.

I feel like i'm going mad here. It doesn't seem like this should be possible. Am i missing something embarrassingly obvious?

If in doubt use the schematic to determine what component say the transistor base connects to, then find that component on the PC board and follow the traces to confirm what transistor lead it connects to. There are many documented cases of incorrect silk screens.

Since it works with 726's and you know that the 992's need to be flipped then that is probably not the issue. Are you trying to match the 992's at all? Ideally differential pairs should be as closely matched as possible for best performance.

The 992's may be different enough that the offset voltage is out of spec and triggering the protection. Put the 992's back in and find the offset adjustment section of the manual and measure the offset voltage and if needed see if you can adjust it to as near 0mV DC as possible. You should also check the other channel offset as well.

Diagram below is for a regular 8500 not the MKII, if you have the MKII use that adjustment procedure. Ignore the manuals (b) step to turn the pots all the way down, leave them where they are and adjust as and if needed.

You also do not need to use the RCA resistors plugs inserted into the amp in sockets. We just want to see what is going on. If you want to circle back later and make those plugs and re-check then that will be fine.

Volume control all the way down and no speakers connected.

 

[crustypaul]

Thanks so much for the reply.
This is a 8500 mkI.

I haven't made any attempt to match the 992's, but it didn't occur to me that a mismatch that must be vanishingly small could trip the protection relay. Is that normal?
I just tried a different pair of 992's on the left hand channel in place of the working 726's and the amp powered up and engaged the relay fine. I just bodged together some 5.1K load phono plugs and made the adjustments to 0v as accurately as is possibly with those slightly dubious trimmers. The other 30mv adjustment is fine as well.
Do you think its worth attempting to match the 992's for each side or do the trimmers take care of any mismatch? I didn't see any mention of matching them in the dozens of posts on here about replacing the transistors on these things, but i'd happily do it if its of any benefit.

I'm now getting sound from both sides, but mysteriously it does seem to randomly trip the protection relay for a second or so every now and then. Annoyingly it seems to leave it for the exact time where you think its stopped doing it, and then randomly pings off for a second. I'm thinking this might possibly be a problem somewhere else though...

 

[mrk229]

It is very important to match them if the amp does not have an offset adjustment, not as critical if you can adjust. For best performance of a differential amplifier pair the transistors should be closely matched. This type of offset adjustment does not really compensate for a mismatch in the diff amp as much as it "corrects" the mismatch to have 0mV DC at the outputs. that said it should not wobble around much once set.

The 8500 has some other known problematic transistors in it that might be making noise and causing a "spike" that triggers the protection? Was it doing anything like this before?

It has 2SA725's and 2SC1312's in the control amp and 2SC1313's on the switch board. The phono EQ board has both the 2SA725's and the 2SC1313's. And last but not least you have the infamous 2SC1451's in the power amp.

 

[crustypaul]

What are we matching for here? Hfe or Vbe?
i'm assuming Hfe.

I've already replaced the 2SC1451's in the power amp.

Its never tripped the relay protection before. Maybe i disturbed something elsewhere replacing the transistors in the power amp. I was hoping to not have to go through it and replace the lot, but i guess it can't hurt.

 

[mrk229]

I use my DCA75 to match by Hfe, then have a look at the Vbe when I find a good Hfe match.

It does seem very coincidental that it started popping up when you made some changes.

 

[crustypaul]

Well, i just tested about 40 KSA992's and they are astonishingly well matched already, mostly with a Hfe of dead on 400 and VERY close Vbe, to the point where i was somewhat suspicious of my testing as i've never seen a load of transistors this close before. A bag of 2N3904's i got from aliexpress are all over the place as expected though so presumably it must be correct.

Having tested the pair of 992's i originally had in the left hand channel it seems i have somehow randomly picked the only two outliers with Hfe's of 320 and 440 or so. I guess that combination was somehow enough to trip the protection relay.

I've installed a couple more pairs with the tightest matching specs and it hasn't tripped the protection again so far. Fingers crossed thats sorted it out, although i can't help thinking there must be something somewhere else thats a little borderline if something like this was causing the relay to disengage.

Many thanks for the advice. It seems transistor matching was a lot more important than i had expected, at least it is in this specific amp!

 

[mrk229]

You are welcome, hope it remains stable.

 

[soccerfan]

Well, i just tested about 40 KSA992's and they are astonishingly well matched already, mostly with a Hfe of dead on 400 and VERY close Vbe, to the point where i was somewhat suspicious of my testing as i've never seen a load of transistors this close before. A bag of 2N3904's i got from aliexpress are all over the place as expected though so presumably it must be correct.

Having tested the pair of 992's i originally had in the left hand channel it seems i have somehow randomly picked the only two outliers with Hfe's of 320 and 440 or so. I guess that combination was somehow enough to trip the protection relay.

I've installed a couple more pairs with the tightest matching specs and it hasn't tripped the protection again so far. Fingers crossed thats sorted it out, although i can't help thinking there must be something somewhere else thats a little borderline if something like this was causing the relay to disengage.

Many thanks for the advice. It seems transistor matching was a lot more important than i had expected, at least it is in this specific amp!

Dont trust any transistors that you buy from amazon/ebay/aliexpress etc. Most are fake!!
Buy from mouser/digikey/newark or any other authorized seller

 

[crustypaul]

Transistors are all from mouser.

Having spent a few days with this amp i can say that it now sounds awful. Replacing the 2SA726's and 2SC1451's in the power amp has made the whole thing sound horribly grating and far too in your face or messy in the midrange. Most of the bottom end has just vanished. Theres no depth to it at all now.
It sounds so messed up i had to check that i didn't have the speakers out of phase.

To be honest i'm really wishing i'd never done it. A little crackling on one side was actually preferable to how it sounds now.

 

[mrk229]

Transistors are all from mouser.

Having spent a few days with this amp i can say that it now sounds awful. Replacing the 2SA726's and 2SC1451's in the power amp has made the whole thing sound horribly grating and far too in your face or messy in the midrange. Most of the bottom end has just vanished. Theres no depth to it at all now.
It sounds so messed up i had to check that i didn't have the speakers out of phase.

To be honest i'm really wishing i'd never done it. A little crackling on one side was actually preferable to how it sounds now.

Wow that has never been my experience in the many many Pioneer amp boards I have refurbed. I am assuming you checked the bias after the replacements?

You could try putting back just the original 1451's or just the original 726's and see if it changes anything.

 

[ivandezande]

Transistors are all from mouser.

Having spent a few days with this amp i can say that it now sounds awful. Replacing the 2SA726's and 2SC1451's in the power amp has made the whole thing sound horribly grating and far too in your face or messy in the midrange. Most of the bottom end has just vanished. Theres no depth to it at all now.
It sounds so messed up i had to check that i didn't have the speakers out of phase.

To be honest i'm really wishing i'd never done it. A little crackling on one side was actually preferable to how it sounds now.

Something’s wrong…..

I’ve never even once restored a unit for a client and have them complain it was more forward or mid heavy.