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Pioneer Amp keeps going Pop

Pottyf430

Pottyf430

New Member
Hi everyone.
I have a Pioneer A757 Mk2 Reference Amp.
I was using it one day and It just went bang and smoke came from the vents.
I was not pushing the amp hard, just normal volume.
I took apart the amp.
I replaced the amplifier transistors and various other components like fuse-able resistors, and all 4 centre tap resistors and a few other components.
It took me a long time to get to this point and a lot of time was spent.
I powered up the amp and it worked. I used it for a week, an hour here and there. The top and bottom covers were still off. I then was using the amp and it had been on for a couple of hours. It seemed fine. Then I started to turn up the volume, not to loud. The volume knob was just over a quarter and then the amp went pop. More smoke.
1 channel’s amplifier transistors had blown. Diode has gone. Other Transistors had gone short circuit. More fuse able resistors had gone open circuit.
I have not fixed the amp yet. I still have voltage issues which I need to fix but was wondering why the amp went pop again.
Any ideas would be greatly appreciated.
 
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I would say that you originally fixed the symptoms, but not the root cause of the fault.

I am no expert on fixing these. I'm sure there will be someone coming by soon who will have some real suggestions on what to do.
 
SmileyFaceEQ said:
Voltage regulator?

Did you check the board for random solder blobs or broken or cold solder joints?
Click to expand...

Hi there, yes I have checked for dry joints and cold joints both on power supply board and power amp board.
I’m still repairing the damage from the last blow up as there is a power problem but I am concerned that once I have repaired that fault and the amp is working again, how long will it last? Will it go pop again?
 
BilboBaggins said:
I would say that you originally fixed the symptoms, but not the root cause of the fault.

I am no expert on fixing these. I'm sure there will be someone coming by soon who will have some real suggestions on what to do.
Click to expand...
Yes you are probably right. I have repaired the damage but not the cause hence the amp going pop again.
 
Problem is I have sold a lot of SA-717 to 757 MKII (technically they were all mostly identical) but I have never seen a blown one! These things were bulletproof! However there was one problem common to nearly all Pioneer receivers and amplifiers.
Stabilising transistors in the power supply were nearly in each case not outfitted with cooling so they did unsolder themselves over time. Sometimes hard to see!
 
Thorsten R. said:
Problem is I have sold a lot of SA-717 to 757 MKII (technically they were all mostly identical) but I have never seen a blown one! These things were bulletproof! However there was one problem common to nearly all Pioneer receivers and amplifiers.
Stabilising transistors in the power supply were nearly in each case not outfitted with cooling so they did unsolder themselves over time. Sometimes hard to see!
Click to expand...

Hi there, Thanks for that.
I will check the transistors tomorrow to see if the joints have become unsoldered. Probably be best to reflow them anyway.
I still have a power issue. I don't have +3v and -3v on Ic1 of the power amp board.
I need to do some more investigation to find out why.
Thanks again
Paul
 
Pottyf430 said:
Hi there, Thanks for that.
I will check the transistors tomorrow to see if the joints have become unsoldered. Probably be best to reflow them anyway.
I still have a power issue. I don't have +3v and -3v on Ic1 of the power amp board.
I need to do some more investigation to find out why.
Thanks again
Paul
Click to expand...

Update.
I checked all the transistors on the power supply board. They all looked ok even with magnifying glasses on.
I decided to reflow the solder joints anyway.
I switched the Amp on and hey presto the relays click. I connected some speakers and gave the Amp a source and i have Audio again.
I'm really nervouse at this stage not wanting the Amp to go Bang again. I dont understand Why i still dont have 3v on IC1 even though i have audio.
Any ideas anyone??
 
Please tell us what "IC 1" is. The ICs in the A 757 MKII have 3 numbers in the designation (IC 101, 301, 501, and so on).You can get the manual from Hifiengine.com.
 
Ah I see! You mean the voltages at pins 8 and 14 of the non-switching circuit! That is not trivial. I see no other way than carefully taking measurements at the resistors and diodes around the ICs. When I do this I normally protect the power transistors with 10 W (appr. 100 Ohm) resistors in their collector paths (the direct connection to + or - supply). So that only a weak current can go through those transistors if something bad happens with the test probe ... Only when all voltages are ok I remove those resistors.

I found a good discussion about the PA0016 here: Pioneer amplifiers - with PA0016 chip - elektroda.com In this translated discussion read "tension" as "voltage".
 
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Thorsten R. said:
Ah I see! You mean the voltages at pins 8 and 14 of the non-switching circuit! That is not trivial. I see no other way than carefully taking measurements at the resistors and diodes around the ICs. When I do this I normally protect the power transistors with 10 W (appr. 100 Ohm) resistors in their collector paths (the direct connection to + or - supply). So that only a weak current can go through those transistors if something bad happens with the test probe ... Only when all voltages are ok I remove those resistors.

I found a good discussion about the PA0016 here: Pioneer amplifiers - with PA0016 chip - elektroda.com In this translated discussion read "tension" as "voltage".
Click to expand...

Hi again, thanks for the response. I have had a look around IC101 and I suspected an IC failure.
I removed both IC101 and IC102 carefully and replaced them with IC sockets so I could swap the ICs over.
When I swapped the ICs over the fault move with the Chip, so now the other channel (we’re IC 102 was) hasn’t got +and - 3V and the original location is now ok.
So i now need to get a genuine PA0016 from somewhere.
I have also replaced Q1 to Q8 with genuine upgraded Toshiba transistors. I was wondering if the previous one’s I installed were not genuine and maybe why the amp blew.
Like I say the Amp is now in a working condition. But I have not had it on for long due to the fact of the issue with IC101 and the fact of why it went pop in the first place.
The trouble is when it does blow it does so much damage and I have spent so much time getting to where it is now. I really don’t want to go backwards.
Thanks for all you input so far.
 
Very good action! I did not realize at first that this happened at one channel only. PA0016 are still available, make a search in the net.

But still: Take a closer look at the resistors! Especially those that are marked with a warning sign in the service manual. And the diodes also.
 
Hi All,
Thanks for the responses.
I have ordered 2 PA0016 ICs from Fix Part Uk.
They claim to be genuine Pioneer so I can only hope.
I will do some research about placing resistors on the collector side of the power transistors before I go any further.
I have replaced all damaged resistors with fuseable ones as specified by the schematic drawing. These have all come from reputable suppliers. So I am taking every precaution I can.
The only other component I wasn’t sure about which I replaced was Q121 which had a short across it.
The replacement came from china. It’s a 2SC1845.
When the amp blew the second time, this transistor failed again. I’m not sure if this was the cause or the damage to the transistor was cause by something else failing causing the amp to blow.
 
When the differential amp Q109 / 111 is asymmetrical (transistor blown, diode or resistor defect) or Q117 is defunct it could open Q121 completely so that it (and the parts behind it) get destroyed.
 
Do not use questionable parts from China such as old nec 2sc1845 when you can get onsemi ksc1845 from mouser
You said you used toshiba outputs from where did you get them I hope not china too, mouser sells 2sa1943n and 2sc5200n they are 150w parts onsemi makes 200w parts not to say that these will fix it if it’s the front end having problems as you discribe. Damn pioneer for using there own silicon, unreliable circuits with no support with service parts.
 
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