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Help Needed With Pioneer SA-7800 Repair

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BargainBrad

Member
Hi all,

I'm fairly new to this, but I'm working on repairing a Pioneer SA-7800 that I picked up a few months ago. There was a problem with the power amp board (some resistors had badly burned up) that the previous owner had tried to repair.

To make a long story short, I rebuilt the power amp board (checking all resistors and diodes and replacing all transistors (except the 5-leg ones) and capacitors) using advice and recommendations from this website. I also replaced the outputs. The board looks great and I'm fairly confident that I assembled it correctly.

I installed it last night and immediately blew the 6amp fuse. I then attached a dim bulb tester (which I should have done the first time), installed a new fuse, and confirmed that there is definitely a short of some type, as the bulb is staying bright.

From the threads I've reviewed, a short in the outputs is the most likely cause of this, but I checked the outputs again and they are fine. I'm wondering if the problem is on the power supply board.

Can anyone recommend the best measurements to take to try to identify the problem? I have a good multimeter but not an oscilloscope. With the DBT on, I checked the DC bias on the power amp board and it read 0.

Any guidance would be greatly appreciated.
 
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using your ohmmeter, unit un-plugged, start to check resistances on all the supplies to ground, including the big ecaps.
resistances should start at lower value and then you should see the R increase as caps charge up. if something is steady short or low R to ground, investigate why.
If they look okay, then start to compare the other, hopefully good channel, against the bad one. you can compare same nodes to ground and also resistances across components. They should all match within 10% or you have differences that point to the areas affected.
It is difficult to isolate when they have everything hard wired.
 
I'm not sure that I follow when you say "all supplies to ground." Which "supplies" are you talking about? I can post the layout if it would be helpful.
 
6A fuse is on transformer primary. There are 0.8A fuses on
secondary for regulated supply but no fuse on unregulated supply
so probably a power amp issue.

Power supply test points are,
pins
15 or 16 or 17 +48Vdc (regulated)
19 or 20 or 21 -48Vdc (regulated)
29 or 30 +46Vdc
31 or 32 -46Vdc

Results from transistor testing can be somewhat flaky, sometimes a few volts is need before
the transistor goes faulty. Suggest power up on DBT and measure Vdc starting at the outputs
measure e/c/b voltages to ground/chassis, you may have a c/e short under load.
 
Did you replace the BIG capacitors.
And were you especially observant of their polarity.
A reversal here could have (and HAS) DONE THAT.

The sa-7800 is a NSA amp - these tend to be VERY picky about repairs.
BUT
IF you remove the output transistors the amplifiers WILL DC balance. (this is important)
There are several aspects of powering up an NSA amp that can be tricky with regards to trimpot replacement and initial settings.
SOME of them go away when there are no outputs connected.
Still check the 0.22 ohm emitter resistors with one leg lifted - sneak currents to fool you DO exist, and they can even LOOK like the correct resistance.



Unfortunately, the information about which original resistors burned has probably been lost.

I will only be able to pop in for a few minutes every couple of days for the next weeks, so I am trying to point directions to travel.

the main two places that can carry that much current is: 1) reversed main caps and 2) through the output transistors.
 
There are plenty of experts helping you out here - but I wondered as a very basic test - did you test the amp out on the DBT with the powersupply disconnected from the other boards? If you check it with only the trafo, rectifier and then powersupply connected, that way you could isolate the powersupplyas being the problem.
 
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In response to mbz's post, I tested the Vdc from the following points to the chasis and they are WAY off:

15/16/17 - ~1.7 Vdc
19/20/21 - ~1.5 Vdc
29/30 - ~1.5Vdc
31/32 - 36 mV

Four the four outputs:
For the two NPN (MJL3281A): E = 1.03V C = 9mV B = 1.5V
For the two PNP (MJL1302A : E = 1V C = 1.5V B = 1.5V

Mark, I have not touched the big capacitors. I did not touch the power supply board at all. Although I do plan to recap it once I get the amp stabilized and running. The emitter resistors are brand new and I tested them at .22 ohm before installing them. I can test again if you believe it necessary. Should i disconnect the outputs and start the amp and redo the measurements above?

Slimecity, I will do that if Mark and others recommend it. Good idea.

Thanks for all the help.
 
BargainBrad said:
Slimecity, I will do that if Mark and others recommend it. Good idea.
Click to expand...
Yeah, I also like the idea of testing with no load on the power supply and/or with the output removed
however, despite having done a SA-7800 refurb, I don't know the amp well enough. The refurb helped me
learn a little.

EDIT: Do not unwrap wirewrap connections.
Low readings on both regulated and unregulated pins suggests multiple issues.
 
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So I started it up without the outputs connected. Unfortunately, not good news. First, the bulb dimmed more and the voltage jumped to about 26V, which was much higher than where it was before. So that was good to some degree. But after about 20 seconds, R60 (at least I believe it is R60) started to smoke and I shut it down. I did a few more tests after that and it looks like something is definitely wrong on the power amp board. For example, B C & E on Q24 and Q26 are also showing the full voltage around 26V.

Unless anyone has any specific suggestions, I am planning to remove the power amp board and recheck everything. I'm worried that I damages something the first time I started it up with the outputs connected and no DBT.

I'm also worried about 2SA1930 + 2SC5171, which I used to replace the original transisotrs in Q24 and Q26. I bought them from KP Components, which seemed reputable, but now I'm worried. Can anyone suggest a replacement pair for the original 2SC1913 and 2SA913 that I can get from Digikey or Mouser? The other options I saw have all been discontinued.
 
Q24, Q26 and R60 should be replaced. Symptoms/measurements are a bit thin so concerned that these parts
may not be root cause (Q26 c-e short?). Check R52, R54, if these appear un-stressed then go with above
replacements on DBT, outputs removed. Look elsewhere if R52, R54 are stressed
 
Q24 and Q26 tested fine, but I replaced them on both channels to be safe. R60 also measured fine, but I am going to replace it, along with R52 and 54 to be safe. I will then do another run through of testing the other components on the board, but I'm a little at a loss as to what could be causing the problem on both channels. This is definitely not a single component that failed unless it is a component common to both L and R, since the high voltage on the legs of Q24 was found on both sides of the PCB.
 
BargainBrad said:
B C & E on Q24 and Q26 are also showing the full voltage around 26V.
Click to expand...
Check D17,18 otherwise Q5,6

BargainBrad said:
Can anyone suggest a replacement pair for the original 2SC1913 and 2SA913
Click to expand...
Not strong on transistor matching, some threads show people
using MJE15032 & MJE15033

Because of amps history I would consider replacing the
trimmers VR5-VR8 with multi turn types and set the
initial value of each to minimum (fully counter-clockwise)
as per adjustment section of sm.
 
MJE15032G and MJE15033G are exactly what I used. I replaced the trimmers when I rebuilt the board and just re-tested them. They seem fine.

I am going to test some more components tonight, including D17, D18 and Q5 and 6 and will report back.
 
I spent a lot of time tonight testing those components and many more, and everything tested fine. I'm wondering if the problem could be on the power supply board or one of the other components.
Does anyone know if I could power up without the power amp board installed and check each of the connections to the board to see if they have the right readings? For example, +48 and -48V for the rail connections. Thoughts or other suggestions for further trouble shooting before reinstalling the board and trying again? I am loathe to do that because I haven't actually identified any problem. So the odds are the same bad things happen.
 
There is an inconsistency between the voltage measurements in post #7 (results might indicate amp was powered off???)
and post #9 where you are seeing 26V, Suggest redo power supply measurements on post #7.

Certainly would be nice to test stand alone power supply, need to wait for someone to ok.

I would be careful believing that devices that test ok are ok. Seen too many devices pass diode test but fail
under load. I still like D17,18, Q5,6, need to measure b/c/e/anode/cathode voltages to GND.
Measure voltage at L or R channel output transistor collectors to double check unregulated supply.
 
With outputs removed, what are the power supply voltages as per post #7.

EDIT: Forget this. With outputs removed R60 smoked and unexpected 26V
 
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BargainBrad said:
For example, B C & E on Q24 and Q26 are also showing the full voltage around 26V.
Click to expand...
Q26collector should be a negative voltage.
If all other voltages are positive then candidates are Q24,C16,D18, Q6

EDIT: Suggest try a lower wattage bulb in the DBT 30W? so you can power up without
further damage. Measure Vdc at Q6 b/c/e
 
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I ordered the caps for the power supply board, so I think I am going to remove it and test its components before I do anything with the Power amp board. Let me know if you think this is a bad idea.
I wish I knew if I could power up the amp and test the power supply voltages without the power amp board installed. Anyone have insight on that?
 
I removed the power supply board today and removed all the eletrolytic caps. They all tested decently after removing them, but I will install the replacements tomorrow. I'll then check and/or replace the transistors, diodes, and resistors tomorrow.
I'm still hoping someone can tell me whether I can test the power supply board without hooking it up to the power amp board and the other components. I'd love to make sure it works on its own before reinstalling that piece.

I've also been thinking about the trimmers and the difference in resistance between what I installed and the original. The more I think about it the more I think that the resistance being too high in the trimmers could have been what caused the problems when I installed the power amp board. I am going to pull them and put the resistance as low as possible before reinstalling it.
 
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