Have one bad channel. The unit had several burnt resistors and blown transistors.

Replaced the bad components, but am still having problems.

I took the following voltage readings, with the outputs removed:

Q1 ==> Emitter (E) -0.6v shb 0.6v , Collector (C) & Base (B) OK
Q6 ==> E -51.0v shb 1.8v
Q7 ==> E -55.0v shb -1.8v
Q8 ==> B -55.0v shb 1.8v
Q13 ==> E -52.7v shb 0v
Q14 ==> E 25v shb 0v

Thanks for the help.

Update:

Retested, Q4 was bad

Replaced Q4, Q5, Q1

New readings;

Q1 ==> OK
Q6 ==> E 0.7v shb 1.8v
Q7 ==> E -0.7v shb -1.8v
Q13 ==> E 0.2v shb 0v
Q14 ==> E 0.2v shb 0v

When output transistors short, this often blows out the driver transistors PLUS small-value resistors between the drivers and outputs and in the emitter circuits of the outputs.

It's also common for the bias circuit (in the base region of the drivers) to have a problem.

* * * * *

People always hate to hear this: YOU NEED TO BE PARTS INTENSIVE. It's not unusual to have to replace 15 parts. On the Spec 4, many of the resistors are shrouded in an insulating sleeve. These resistors must be checked.

After parts replacement, run the unit up on a variac.

Fred

When I got this particular unit, one of the amps was fried, literally. The large heat sink was discolored from excessive heat.

You're right about the parts intensity. The following items were bad: 3 of the 4 outputs, Q6, Q7, Q8, Q9, Q11, Q13, Q14, R16, R28, R34, R36, R38, R45, and R47.

After replacing the above items and Q10 as the compliment to Q11, Q4 went bad. I then replaced Q4, Q5, Q2, Q3, and Q1. The only original transistor left is Q12 (I don't have a replacement for that one).

After reassembly, the DC balance was erratic at first but now seems to have settled down. The bias is extremely sensitive to the trimpot, but it is dialed in for now.

I'll take some more voltage readings and post them.

The voltage readings seem to be within spec now, but I'll let it run longer over the weekend and see if everything is still OK.

As mentioned in the previous post, I didn't replace Q12, a 2SC869-C. What is a recommended replacement for this transistor, MTF recommended a KSC2310OTA. Those are now discontinued. Would a KSC2310YBU be acceptable? I have some of those, and they're still in production.

Thanks.

The voltage readings seem to be within spec now, but I'll let it run longer over the weekend and see if everything is still OK.

As mentioned in the previous post, I didn't replace Q12, a 2SC869-C. What is a recommended replacement for this transistor, MTF recommended a KSC2310OTA. Those are now discontinued. Would a KSC2310YBU be acceptable? I have some of those, and they're still in production.

Thanks.

Yes, no problem...

it's a r-o-y coded for gain transistor
so OTA is medium (O) gain and on an ammo belt (TA) for automatic insertion
while YBU is higher (Y)gain and is packaged in (BU) bulk

I was able to adjust the DC balance down to 0mv, with drifting between about -10mv and +10mv. While it was in this range, I was able to set the bias to 46mv.

After a while the DC balance went crazy, up to 830mv, at that point the bias automatically adjusted down to 3mv. If I turn the unit off for a while, the balance goes back down, but then after a while will shoot back up???? When the DC balance is in check, the other voltage readings are within spec. Not sure what happens when it shoots up since I turn the unit off as soon as I know the problem exists. Don't want to fry any items.

Mark, thanks for the insight on the transistors. I wasn't sure since the gains were different for the different suffixes.

I did some more testing and I found that if I blow on any of the transistors (excluding Q1, Q2 and Q4) with a straw, the DC balance goes into the 800mv to 2v range, the same thing is true for D4.

This doesn't happen on the good channel.

Any ideas where I should look??

Thanks,

"Replace the diff-pairs", EW says for the 7482'nd time since he's been on AK (and it's not like there isn't a 'DC Offset' sticky with 2500 replies and a quarter of a million views).

Dump the stock offset adjust pot too.

EW, I did replace the diff pairs. With the exception of Q12, all of the transistors have been replaced on this amp board. Most were fried when I got the unit. So were several resistors, that have been replaced.

I will replace the trim pots though. Still wondering why the transistors are so sensitive to slight heat.

Lotsa fried transistors = collateral damage to the resistors. Look for any that are out of spec.

When I start out with a really zorched board (zorched = drivers and outputs kaput, or other indications including ANY fried resistor) I start making a map on a copy of the schematic that I print out, including opens and shorts on individual transistors - to figure out how the faults affected nearby circuits. It is then my "bible" for anticipating problems, and other potentially stressed parts. That currents' gotta go SOMEWHERE....

But I usually don't sweat any resistors above 30k ohms, as it takes a lot of voltage to push damaging current through those. I DO test them for failure tho....

What did you do about Q1, the dual transistor? Q1a&b, Q2&Q3 have to be as thermally coupled as possible.

Vr3 & Vr4 are the power limiter circuits and you may have them biased on enough to be affecting the Q8 to Q9 bias voltage, where they are stealing current they should not be doing, upsetting everything.

Quite frankly at this point I would remove D6, Q6, D7 & Q7 to de-energize those circuits, get the amp set up and running RIGHT, THEN add them back in and fiddle-fart with their settings.

I have a SPEC-4 in for repair right now that was at another shop. Not only did the "tech" there use counterfeit 2SD424/2SB554 output transistors, he removed the 470 ohm bias pot from the circuit altogether, and then evidently turned the amplifier on!!!!!!!!!! As you might guess, most everything downsteam of the class-A drivers went up in smoke.

Jeeezzz . . . what a moron!

Fred

EW, I did replace the diff pairs. With the exception of Q12, all of the transistors have been replaced on this amp board. Most were fried when I got the unit. So were several resistors, that have been replaced.

What did you do about Q1, the dual transistor? Q1a&b, Q2&Q3 have to be as thermally coupled as possible.Oops...I forgot that the Spec 4 has a dual transistor.

I find it odd that Pioneer call out a 2SA798 for Q1, when the schematic shows a C-E voltage of about 67V (A798 is rated for 50V). Would seem to call for a 2SA979.

Hmmm...

Based on past guidance from MTF and EW, I did print out the schematic for the amp. I then tested all of the resistors,diodes, and transistors and replaced all of the electrolytic caps. I marked all of the bad components with a red highlighter and all of the good components with a green highlighter on the schematic. This gives a pretty good overview of the path of destruction.
As Fren Longworth said, the SPEC 4 is parts intensive.

In this particular case there was a lot of destruction. As Fren Longworth said, the SPEC 4 is parts intensive.

I did replace all of the transistors with the exception of Q12 (most were toast) and any bad or questionable resistors (there were several). EW pointed out that every resistor must be checked, which I did. Several were obvious from the discoloration and burn marks, but others physically looked OK but turned out to be bad when measured. In some cases the resistors have to be removed from the circuit to be tested.

For Q1, there was a factory cover to insulate it, which I placed on the replacement transistor. From past experience, Q1 is very sensitive to temp changes and causes large changes and drifting in DC Balance if not shielded. Q2 & Q3 are unshielded.

I don't have an oscilloscope or signal generator, so I haven't touched VR3 & VR4, since I couldn't perform the adjustment procedure without that equipment. This could be a problem given the extensive replacements that have been made.

Any suggestions as to good, small oscilloscopes? I may look for a used one.

<snip>
Vr3 & Vr4 are the power limiter circuits and you may have them biased on enough to be affecting the Q8 to Q9 bias voltage, where they are stealing current they should not be doing, upsetting everything.

Quite frankly at this point I would remove D6, Q6, D7 & Q7 to de-energize those circuits, get the amp set up and running RIGHT, THEN add them back in and fiddle-fart with their settings.

<snip>

I don't have an oscilloscope or signal generator, so I haven't touched VR3 & VR4, since I couldn't perform the adjustment procedure without that equipment. This could be a problem given the extensive replacements that have been made.

Any suggestions as to good, small oscilloscopes? I may look for a used one.

It sure did...

Getting those circuits out of the way will postpone the need for a scope to do troubleshooting.

You may want to check all voltages to see if they are within spec. Some schematics give you point to point voltages at normal condition. If it is not on the schematic, then someone needs to figure out what the readings should be. Compare that with abnormal voltages may show some part is miss behaving.
If above does not get to the answer, the output drift looks like the feedback at DC is not enough. In other word not enough feeback to control the thermal effect. Simplest thing to check is if feed back resistor is OK, the high pass gain cap at the feed back may be semi to fully shorted.

With Q6, D6, Q7, D7 removed, things stabilized at lot. The maximum change in the DC balance was about 15mv depending on the transistor being warmed by blowing on it with a straw. I was able to set it to zero and the Bias to the recommended 50mv.

What next??

With NO SPEAKER LOAD, and the idle current at a reasonable level, the power limiter circuit should not be activating. I would reinstall the 2 diodes and 2 transistors and then turn it on while watching the idle current- it may drop, find which way (CW or CCW) adjusting the limiter pots (VR3 & VR4) brings the circuit back to (or close to) the idle current that was set.

Experiment slightly with the pot rotation, there will be a sweet spot where it starts to kick in, with no effect on idle current before that.

It may be that with the idle current correct and no speaker load it doesn't kick in ANYWHERE in the pot's rotation.

After I reinstalled Q6&7 and reconnected D6&7, had the same issues. I decided to take the ohm readings of VR3 & VR4 before I started adjusting them for a frame of reference. I found that VR3 was toast. Replaced it with a nice Bournes and problem solved. No more issues with DC balance or Bias settings after several hours of use.

I will test more extensively over the weekend and make sure there are no more issues.

As mentioned before, I checked all of the resistors, diodes, transistors. Unfortunately, I didn't check the trim pots. It sure would have saved a lot of time and effort.

As a side note, one thing I did different was that I used an MJE340 for Q8 and an MJE350 for Q9. Those were the On-semi recommended replacements. Seems to work fine.

I've worked on 4 SPEC 4's recently, and all of the amplifiers were different. Some had additional diodes, some had fewer caps, but none of the 4 units were the same.

Thanks for all the help, they're all working now.