Fender Ultimate Chorus power amp repair

[dwmorrin]

I've got a Fender Ultimate Chorus here, arrived with a blown mains fuse (FX1), and two shorted power transistors (Q8, Q9, shorted collector to emitter on both).

Combed through the power amp components. Can't find anything else bad in there.
Replaced Q8 and Q9 with same type transistors (TIP142/TIP147).
Power up with dim bulb, and no load. Seems OK.
Take off dim bulb, power voltages match the attached schematic just fine.
Monitor idle current by measuring voltage across emitter resistors R112, R113, and it's shooting up fast - turn off amp. (Still no load, no signal.)

R131, R132, emitter resistors for the OK other power amp stay put at around 56mV. R112, R113 shoot past 1V quickly.
The power transistors bases are all at about 1.2V to ground, positive for the NPNs, and negative for the PNPs. Collectors are at +/-41V, and emitters start at about 100mV and steadily rise. Transistors quickly get hot, so I don't keep the amp on for even 1/2 a minute.

Checking U8/U9, the MC1436 op amp drivers, the good amp (U9) output pin 6 locks in at about 23mV, and the bad amp (U8) output pin 6 steadily creeps up about a mV a second.
Still can't find a bad part, so I swap U8 with a new MC1436, no improvement. (U8/U9 power supply pins are both at +/-26V.)
Both U8 and 89 have similar input pin voltages. Here's all the voltages:
pin 1 - nc
pin 2 - ~2-3mV
pin 3 - ~100µV
pin 4 - -26V
pin 5 - nc
pin 6 (good amp) = 23mV
pin 6 (bad amp) = starts around 23mV, steadily rises about 1mV/sec in sync with the increasing current in the power transistors.

I've tried checking in-circuit resistances just about everywhere and I can't find any clues, and every measurement squares with the working amp. So I've now taken to removing components for out-of-circuit testing, but still can't find a bad component.

The biasing diodes all check OK (CR30-CR33) and there is a total drop of 1.48V when I measure Q8 base to Q9 base, forward bias, and open circuit with reverse bias (this is with the diode checker on the meter, amp powered off). This matches the other side. Voltage at the top of R109 is 13V, and the bottom of R110 is -13V, and matches with the other amp.

Something is turning these transistors (Q8/Q9) on and I can't figure it out. I really want to blame the op amp circuit, maybe something wrong with the dc feedback paths. R102, R101, R100, and C58 have been checked, as well as R98 and C56, and the resistances between pins 2&6 and those pins to ground match perfectly between the two power amps. And, as mentioned earlier, I've already replaced the op amp.

Any ideas on where to look next?

[52 Bill]

It seems that you have done all of the right things and have checked what should be checked. As the bias starts to rise what is happening to the voltage at the bases of the outputs?

And I assume that the new transistors are the correct type and have been correctly mounted to the heatsink.

[dwmorrin]

The base of Q9 starts at about -1.2V and slowly moves more negative.
The base of Q8 starts at about 1.18V and drops, a bit faster than Q9, gets to 1.160V in about 5 seconds, while Q9 in 5 seconds moves from -1.20V to -1.21V.

Seems like a clue. What do you think?

The new transistors are TIP142/147 and they are bolted to the heatsink with new insulators and the heatsink is bolted to the chassis when I power up.

[g1]

You mentioned bias diodes CR30-33, but not CR34-37. I would be more suspicious of them, especially CR35 from what you said about the drop at Q8 base.

[Enzo]

I don't see the op amp as the problem, how would it selectively turn both positive and negative sides on at once? The IC could drive the whole output over to one rail, but not both.


CR34-37 are limiters, unsolder and lift CR34, CR37, not those four diodes are out of circuit, now how does the channel work?

A 1.48v bias string sounds great for unpowered, but what is the voltage base to base under powered condition. Stable or climbing?

The resistors R108-111, you mentioned some voltages, so what are the voltages ACROSS each, and when using Ohm's Law, do they all calculate to have the same current through each?

Just to see, you could tack a shorting wire across CR30 and CR33. I mean one across each, not a wire end to end of the string. Does that cool it off.

[dwmorrin]

Lifting CR34/37 had no effect.
The voltage taken between the base of Q8 and the base of Q9 is 2.34V, and seems relatively stable (dropping ever so slowing in the µV region) compared to the rising voltage across the emitter resistors.

Here are the requested voltages across each of R108-111
R108 - 28V
R109 - 13V
R110 - 13V
R111 - 28V

Tacking a wire across CR30, and a separate wire across CR33 does indeed "cool it off." The emitter resistors don't even break 1mV.

After that (while short was still in place, amp is ON), I started comparing voltages at the other power amp, and was surprised to see that those emitter resistors (R131/132) were at 125mV, and rose up to 145mV before I shut it off. I removed the shorts across the other amp, and measured R131/132 again... back down to 60mV while the other amp's emitter resistors were at 250mV, so shut it down again. Not sure if I was able to leave it on longer it the R131/132 would go up further or not.
(EDIT: This seemed to be due to neglecting to put back the heat sink screws on that side of the heat sink bar. See next post.)

As luck would have it, I have another Ultimate Chorus here that has two working power amps. After giving it 30 seconds to warm up, R112/113 measure 40mV, and R131/132 measure 44mV. (This is a completely different Ultimate Chorus.)

So the first question I have is: what seems like the a correct range of mV to be looking for with no load, no signal across the emitter resistors? At this point, I'm only assuming that the 40-44mV amp is OK; for all I know it is biased cold. What's the maximum idle current you'd expect, just looking at the schematic?
Secondly would be, since shorting CR30/33 "cooled it off", are we looking at perhaps altering the bias string in this case? I did have an Ampeg that Enzo helped me with several months ago that required such a fix, so perhaps this is a similar situation.

Perhaps the big change in R131/132 between shorting and un-shorting the other amp's diodes is a clue? I had been assuming that amp was OK, but not so sure now. Guess I'll have to investigate over there now.
Any more suggestions/comments are appreciated.

[dwmorrin]

Update to that last post.
Scratching my head as to what I did to make that other side jump, I realized I had neglected to screw in the heatsink on that side of the power amp (there are 5 screws, and I only had 2 in there).
After screwing the heat sink down, I got about 56mV across R131/132 with the other side blazing, and then it jumps up to about 84mV when I short the bias diodes in the other amp.
I suppose that could be normal due to the unregulated power supply, but someone smarter than me would have to verify that.

[g1]

At this point I think I would try swapping the output devices between the two sides of the amp. Sounds like your base voltages are stable, you are properly heatsunk (thermal grease?), so the outputs seem to be going into thermal runaway.
Maybe you received some bad/counterfeit output devices?

[Enzo]

yeah, that occurred to me too, the output transistors.

Where did you buy them? And are they the same package as the old ones?


You reported driver base to base of 1.48v unpowered on your meter, but 2.43v measured in a live circuit. I would observe that the thing is thus biased hot.

You want to compare to another working amp or channel? COmpare THAT. What is the vase to base live reading on them? Just me, but I;d expect more like 2v than 2.43. That is why I suggested shorting those two diodes. And did we try the amp with those shorted diodes to see how it sounds? The result of that cold bias will be greater crossover distortion, but it may not be objectionable.


I don't use output emitter current as a benchmark, but even 40ma sounds high to me. 40-44mv across a 0.47 ohm resistor is about 90ma through it. That sounds like a lot of idle current.

[dwmorrin]

The output transistors are from Mouser. Mouser part numbers are 511-TIP142 and 511-TIP147.
They are in TO-247 packages. I am straying from the originals there as they are TO-218. I recall perhaps more than one thread in which the virtues of TO-247 have been touted, but perhaps I'm wrong? Should I get the TO-218 package instead?
I haven't been able to get back the unit yet, but I will compare the base to base readings with that other unit next time, and swap the transistor pairs and let you know.
And, yes, I cleaned and then applied new thermal grease to the heatsink.

[Enzo]

TO247 is fine. They also come in TO220, and I was afraid you might have gotten those, but the big ones are fine.

Those bias diodes are in thermal contact with the heat sink, mounted through little holes. Is there a daub of heat grease on each?

[WinChuckster]

, I'm aware this is an old post but I'm struggling with the same amp.

Was a solution ever figured out?

I just replaced the 4 output transistors, zener diodes, and .47 ohm resistors. Powered it up, played thru it for 5 min maybe, was on clean channel, volume maybe 3, and poof, q8 let the magic out! Same channel that let go originally. I didn't get a chance to get any readings from it while powered up cuz it sounded good ... For a minute ....

Power on now gives me flames from q8!!

Note - i also switched to TO-247 with sil or mica pad (not sure the difference but i just used the originals

Anything that might help me bring it back to life would be appreciated. Thanx in advance ....
Chuck......

I should have mentioned i also replaced R30, the "gain" pot as it had gotten sheared off at the knob ....

[Enzo]

I suggest you start a new thread for your amp, so we don't get confused. Someone reading over the thread might think the voltage readings above are from your amp

[WinChuckster]

Copy that! I just don't have any numbers to put up like this thread does.

It would be nice to know if the problem on this thread was resolved. Could gain some insight.

Enzo.... Your last post on this thread mentions hear sink compound on the bias diodes thru the heatsink, is that an issue, as there wasn't any originally ....

[rybeka]

New member. I am hoping someone has resolved the issue mentioned here as I have the same problem. A search of the forum provided little help. Q10 in the right channel has a B to E, B to C and a E to C short. Q11 has a E to C short. CR46 and CR 47 are also shorted. After replacing these components, the resistance readings for the left channel (which has no issues)and the right were the same. The amp will work for about three minutes and promptly repeat the same failure. I have replaced all of the caps, diodes and U9 in the right channel. I could really use some help with this one.