Fender Deluxe 112 bad signal after replacing tip142 & tip147

[Xris]

Hi.
I'm currently repairing a fender deluxe 112. It had shorted
output transistors tip147 and tip142. Since replacing these the amp now has a bad distorted signal, and limited volume.
Tests on the preamp out show the signal to be fine until it hits the power stage. There is no dc on the speaker terminals.
Anybody got any ideas?

[Chuck H]

It's always best to include a schematic when you post about a specific model. Especially when identifying board parts by number.

Have you checked supply voltages for the output devices? It wouldn't be unusual if they took out other components when they failed or if it was the failure of other components that caused the originals to fail.

[Xris]

Schematic, yes. Sorry, new to forums.
The voltages are reading both 38.5v + & -
All voltages seem balanced.
I can confirm the tip transistors degraded due to not being fully mounted to the heatsink.

[Chuck H]

Hmmm...? Unfortunately the schematic only shows the collector voltage. I, myself, am SS impaired. So I can't extrapolate what to look for on the base and emitter. This is what I was wondering about. If there is a failed peripheral circuit fouling proper amplification. I hope one of our repair techs chimes in on this.

Did you check the VAC against the schematic figures at U1 with 5mV at the input to confirm your preamp test?

[lowell]

To be clear on whether it's a preamp or power amp problem, and just to 100% confirm, plug the preamp out into another amp or power amp. Then, plug your instrument or sound source into the PA in. If the PA in sounds bad, continue with with this step accept with a known good speaker. Report results. Completely confirm if its a preamp or power amp problem first. After that we can then suggest some voltage/resistance measurements.

In fact a quick test IF the PA is the problem, there should be ~2.8vdc from BASE of to TIP142 to BASE of TIP147. I THINK I'M RIGHT. Someone please confirm I'm right on that. Those 4 diodes between them on the driver output side should ensure this.

[lowell]

Also confirm the +/-16v and +/-40v rails are good.

[g1]

there should be ~2.8vdc from BASE of to TIP142 to BASE of TIP147. I THINK I'M RIGHT. Someone please confirm I'm right on that. Those 4 diodes between them on the driver output side should ensure this.

That's right.

Do not operate this unit without all heatsinking in place, and all surfaces contacting heatsink correctly.

[Enzo]

Solid state amps are all more or less the same underneath the skin. The TIPs are darlington - two transistors - so the bases ought to be about two drops away from the emitters, roughly a volt. Note the bias is four diodes, CR5-8, That is four drops, which is what a pair of two drops adds up to.

I'd scope it to see what the distortion looks like. Is it clipped, half missing, twisted, what?

The schematic has voltages for the op amp, so is there +/-25v on its power pins?

[Xris]

To test the preamp, I used the preamp out into a mixer. Sound is pure. I then fed a signal into the poweramp jack, that signal is garbled. I dont have a scope but I'm sure I can record the signal and have a look on a wave editor.
Im at work at the mo. So will show you guys the TP voltages and wave form shape when i get back.
Thanks.
Im sure its only round the power amp stage, and theres not many components around there.
Another thought has popped up.
The two darlingtons were purchased from separate places and are made by different companies. How much of an issue does that generally cause?

I got the amp back after lending it to a friend a year late. He said the gain channel had some intermittent fault which i found to be a dirty track on the volume knob.
Unfortunately when testing i hadn't bolted the heatsink fully on, then 5 mins later ka-pow!!! Fuse blows. The old darlingtons showed short in both directons.
The new darlingtons are firmly screwed to the heatsink with mica pads and cpu thermal paste.

The ballast 0r47 resitors are showing as 0.8 on my meter (my meter shows 0.4 when fully closed so they are the correct values)
I also mean to test the signal before and after the mc1436 IC (U1).

I appreciate your help here

[Xris]

Hey hey guys im back and i've got some interesting results.
the test point readings are;

tp16 30.2vdc / 65vac

tp17 -30.2vdc / 0vac

tp18 (variable voltage responding to volume control)

tp19 negligable voltage

tp3 45.5vdc

tp4 45.6vdc

i have some screen shots from the headphone out recorded in a wave editor. Each sample has been gained to -6db. The frequency sampled is denoted in the file name (bottom right of viewed picture).



But lets talk about the 65vac reading at tp16.

It feeds the IC mc1436 + power. The DC voltages are a little high each side of the power rail. But as i understand it, as long both sides are the same then there's no real problem there.
So where is this 65vac bleading from?
could it be from diodes cr5 to cr8?
The vac bleed goes back across r63 and r64 but the exposed sides of the diods reveal nothing.

There's a 0.1vdc difference between the power rails tested at tp3 & 4 does this mean anything?
And does any of this explain whats happening to the waveforms?

What do you reakon?

Many thanks.

[lowell]

My guess is the driver IC is bad. Guessing the 30vdc is cause the IC power is not drawing any current. The 65VAC is most likely feedback from headphone out thru R69. If the IC isnt working then it's possible that a voltage divider is missing. In other words the IC, when functioning, creates the lower portion of a feedback voltage divider. Just a guess.

[Xris]

I have done input/output test on the ic. It is gaining the signal, and the gained signal from the IC is fine.
I'm currently sniffing around the +40v supply as there is no sign of the leaking vac on the negative side of the circuit.
Ill have a look at the data sheet for the IC and see if i can get a better look at it.
Cheers.

[lowell]

I'm trying to figure out your pictures. The lower right doesnt say where the signal was tapped from.

I might check C38 being there's 0vac on tp17. It may be open. Try that first, then see if the 65vac is corrected at tp16. And please clarify the pics if you would.

[52 Bill]

I don't know that I'd trust that meter reading. Can you get 65Vac at that test point every time?

[Xris]

The pics? Erm. If I click on one of them, it pops up, above close there's the file name. *.png

I'll take c38 off now and check.
It may be worth noting, the stray vac disappears behind C37

And the results the same every time.

Cheers