Need help with Fender Ultimate Chorus - one amp side dead

[lassoharp]

My Fender Ultimate Chorus has one dead channel (meaning one amp side) with no output. I checked the voltages on the output transistors and get the following:


Q9: Base... 26.5, Collector.... 40.6, Emitter..... 0.635 V

Q8: Base..... 22.7, Collector..... 40.8, Emitter.... 0.679 V

Comparing to the working amp,

Q11: Base..... 1.14, Collector.... 40.5, Emitter..... 21.9 mV

Q10: Base ...... 1.12, Collector..... 40.7, Emitter ..... 27.1 mV


I'm not well versed with SS circuits and was wondering if someone could suggest where to look next to find the problem.

Thanks

[The Dude]

You're missing some minus signs there. Please retest and be sure to note whether the voltages are negative or positive. Without knowing this, we don't know if you have open transistors or the transistor bias is the wrong polarity.

[lassoharp]

Sorry about that. Here are the correct polarities


Q9: Base... - 26.5, Collector.... - 40.6, Emitter..... - 0.635 V

Q8: Base..... - 22.7, Collector..... + 40.8, Emitter.... - 0.679 V

Comparing to the working amp,

Q11: Base..... - 1.14, Collector.... - 40.5, Emitter..... - 21.9 mV

Q10: Base ...... + 1.12, Collector..... + 40.7, Emitter ..... + 27.1 mV



Note - Q9 Emitter starts out at about - 20.6 V then drops to the lower value after a short time

[The Dude]

According to those voltages, the base/emitter junction of Q9 is open. It would be a good idea to check or change both outputs. Also check the ballast resistors (R112 & R113)and all of the diodes just left of the output transistors on the schematic. You can check them initially in circuit for shorts. Unhook the speaker from that channel until you get everything sorted out and verify that there is no DC voltage on the output. You may want to also build a light bulb limiter if you don't have one before firing up the amp so that you don't damage any new parts you replace.

[lowell]

Q9 b/e may or may not be the issue. If any of the biasing diodes are open this will cause problems. Please check:

CR32/33 and 36/37. My guess is there is a bad diode OR 2 in BOTH of those strings. Please list voltage drops across each diode. (not voltage to ground). And disconnect the speaker for now as the Dude said.

[The Dude]

Q9 b/e may or may not be the issue....

Respectfully: a transistor with the correct polarity bias cannot have 26V base to emitter unless the junction is open. Q9 is bad.

[lowell]

Thank you for correcting me. I realize that the max Vbe is 5v so I guess this means if the Vbe is reading MORE than -5v (pnp) then that junction must be open. However those diodes still stand to be tested and replaced.

[The Dude]

Again respectfully and not to "pick nits", but max Vbe doesn't have as much to do with it as does the expected voltage drop on a standard transistor junction. Think of it as a diode junction for simplicity. On a standard transistor, it's going to be about .5 to .7 or so volts max (biased full on). In this case, we are talking about darlington transistors, so I would expect roughly double that, again biased full on. It may be less, but really can't be more than that unless there is a problem with the device. Even at 5V, I would be questioning the device.

A darlington transistor diagram:

[lowell]

Righto thanks.

[g1]

That max Vbe is not for the forward biased direction. The forward drop will never exceed the approx. .6V (or approx. 1.2 for darlingtons like this one). The max vbe is for when reverse biased if memory serves.
So, like Dude stated, if you see a transistor with correct polarity voltages at E and B, but exceeding normal drop, it's bad.
Just clarifying about the 5V vbe issue. Your point about the diodes is valid.

edit: Um, simulpost. Nevermind.

[lassoharp]

I followed up with an in circuit diode check and, as The Dude observed, it looks like Base to Emitter is reading Open in Q9 (Q8,11, and 10 all check identical - good)


Checking the diodes, CR33 and 32 check good and give approximately the same readings as CR 45 and 44 in the good amp

Same for CR 37 and 36


I took voltage readings as lowell suggested and here are the results (drop across each and not to ground)

**Note - just for reference I measured all with negative lead to cathode - black stripe)

CR32...... + 0.735

CR33...... + 0.762

CR36...... - 6.88

CR37...... - 0.492 *** (see below)


Comparing to the good amp,

CR 44 ....... + 0.586

CR 45 ........ + 0.585

CR 48 ........ - 0.940

CR 49 ........ + 0.229


*** For the measurement on CR 37, I only got the -0.492 value right at power on, as I went back to double check it later the meter was bouncing all over the place and would not give a steady reading, if that's a clue to something.


So, if I'm interpreting things correctly, we have Q9 B-E open, CR 36 and 37 seem to check good with in circuit diode checks but we get odd voltage readings presumably due to the open B-E junction in Q9? Or possibly something else?

Also, C61 and 62 (emitter caps) had already been replaced before any of the data I posted, so they should be in good shape


As far as replacing Q9 and Q 10, any opinions on these?

http://www.mouser.com/ProductDetail/...fbv%252bN2E%3d

TIP142 STMicroelectronics | Mouser

[g1]

Those should work fine.
As long as the diodes are ok with your meter diode check, once the transistor is working the diode voltages should come around.

[nickb]

CR36 seems wrong to me. It's a 3.9V zener so the voltage is limited to approx +3.9V to -0.7V yet it's got 6.88V? I think it's open.

BTW, the diodes CR34, C35, CR36 and CR37 are there to crudely limit the max output current in normal operation but under fault conditions can be destroyed quite readily by excessive base current from a failed output transistor.

[lassoharp]

I need to also add that there is currently -21 volts DC on the positive speaker lead in the bad channel

[lowell]

Being that CR36 seems open or not correct... That will skew the quiescent speaker out voltage towards the negative rail because Q9 is turned on hard. (gross) 90% confident in my guess.

C32/33 are definitely suspect too as there should be no more than 1.2v-1.5v on Q9 base. Actually anything in the string of R108-R111 including CR30-33 are suspect.