1979 Fender Champ - no output (FIXED)

[bigdaddydiesel]

Hi all, new member here with an amp that has me a bit stumped. 1979 Champ let loose smoke and blew the mains fuse while on loan. When I got the amp back I took a look inside and didn't see any blown components on the board, so I assumed the power tube had simply gone bad. Replaced the power tube and the 1A slow blow fuse, and the amp was up and running again. Flash forward a month and now there's no output or idle hum when the amp is on, but the mains fuse remains intact and all tubes show signs of life on the heaters and get hot. Primaries from the PT read 380 volts AC while the secondaries are up to 500(!) volts DC. All three of the B+ nodes on the left hand side of the board from the multi-cap capacitor sit around roughly 400-450v. Multi-cap can and all other electrolytic capacitors were replaced recently. Looking at the printed voltages on a schematic for an AA764 Champ shows that is quite a discrepancy.

In addition, the blue wire from the output transformer should have 350V on the plate of the power tube at pin 3, but I'm getting nothing. Not sure if I have a power transformer with a shorted wind, a bad output transformer, or both?

Would appreciate some guidance. :)

Edit: 1K and 10K resistors on the left hand side of the board ohm out almost bang on printed values. Voltages on pins 1 and 6 of the 12ax7 also read above spec; in the neighborhood of 250VDC. Across the 5 watt 1K cathode resistor I get 19VDC, which is about the only voltage reading I've taken so far that is to the same spec as the schematic.

Edit 2: Looks like the output transformer may have gone bad which would explain the issue of zero output. Removed the leads from the circuit. Across red & blue primary wires it reads open and across the black and yellow secondaries it reads 1.3 ohms. I would expect the fuse to blow in this case but maybe I'm testing it wrong?

Also, thinking about it now the voltages in the amp vs. those on the schematic are probably explained by modern AC mains voltages pushing everything up. I might have figured this one out on my own but would appreciate any feedback as I am a hobbyist.

[nandrewjackson]

Red and blue OT primaries reading open (infinite resistance) sure seems like bad OT.

[sluckey]

Zero volts on output tube pin 3 is consistent with OT primary winding being open. An open OT primary would also explain all the high dc voltages. However, you also say you have 19V across the cathode resistor and that can't be if the OT primary is open. Recheck the voltage across that cathode resistor.

Nothing explains the PT primary voltage being 380VAC! That's got to be a meter error or operator error.

[bigdaddydiesel]

Zero volts on output tube pin 3 is consistent with OT primary winding being open. An open OT primary would also explain all the high dc voltages. However, you also say you have 19V across the cathode resistor and that can't be if the OT primary is open. Recheck the voltage across that cathode resistor.

Nothing explains the PT primary voltage being 380VAC! That's got to be a meter error or operator error.

Correct, the 380 AC volts was operator error. Made my initial post early in the morning.  It’s 380 *DC* volts.

Here’s my readings from inside the amp. From the plate of the power tube to ground I have nothing. With the output transformer out of the circuit it is open between the primary wires. Across the cathode resistor reads 19 vdc.

[shooter]

19V across the cathode resistor and that can't be

without an OT getting VDC to plate, tube can't conduct, tube can't conduct, resistor can't have voltage.
You sure that's 19VDC instead of 19mVDC?
I'd pull the tube then ohm cathode pin to ground

[sluckey]

Correct, the 380 AC volts was operator error. Made my initial post early in the morning.  It’s 380 *DC* volts.

I guess the afternoon is no better.    It's impossible to have 380V *DC* on the primary of the PT.

With an open circuit between the primary leads of the OT it is impossible to have 19V on the cathode of the output tube.

[bigdaddydiesel]

Correct, the 380 AC volts was operator error. Made my initial post early in the morning.  It’s 380 *DC* volts.

I guess the afternoon is no better.    It's impossible to have 380V *DC* on the primary of the PT.

With an open circuit between the primary leads of the OT it is impossible to have 19V on the cathode of the output tube.

You'll have to excuse me. Checked the schematic again and unless I'm reading it wrong the power transformer primaries are measured in AC volts. Correct?

Schem here: http://www.prowessamplifiers.com/schematics/fender/champ_aa764_schem.pdf

I am still learning and am doing this work before and after night shifts so I reflexively corrected myself, assuming it was fatigue.

I went through the amp again and this is what I have. I added a 470ohm screen grid resistor to the amp previously so the grid wire is coming in at the normally unused pin 6 of the output tube socket. The 470ohm resistor reads good.

With all tubes in the amp -

B+ on board from the half moon leg of the cap can: 460vdc
B+ from square leg: 435vdc
B+ from triangle leg: 405vdc
OT to plate: 0vdc
Across 1k cathode resistor on the board: -18.5mvdc
Across 470 screen grid resistor (pin 4 and 6): 9vdc
PT primary to pin 4 of rectifier: 373vAC
PT primary to pin 6 of rectifier: 373vAC
PT Secondary to pin 2 of rectifier: 454vdc
PT Secondary to pin 8 of rectifier: 455vdc
From pilot light: 6.5vdc

With the output tube removed:

B+ on board from the half moon leg of the cap can: 513vdc
B+ from square leg: 513vdc
B+ from triangle leg: 507vdc
OT to plate: 0vdc
Across 1k cathode resistor on the board: 0vdc
Across 470 ohm screen grid resistor (pin 4 and 6): 0vdc
PT primary to pin 4 of rectifier: 381vAC
PT primary to pin 6 of rectifier: 381vAC
PT Secondary to pin 2 of rectifier: 519vdc
PT Secondary to pin 8 of rectifier: 519vdc
From pilot light: 6.8vdc

The voltages I'm getting from OT secondary's blue wire to pin 3 and the screen of the output tube are not at all consistent with the schematic.

19V across the cathode resistor and that can't be

without an OT getting VDC to plate, tube can't conduct, tube can't conduct, resistor can't have voltage.
You sure that's 19VDC instead of 19mVDC?
I'd pull the tube then ohm cathode pin to ground

Ah, correct. I checked with a different meter and am getting -18.5mVDC across the cathode resistor. Pulled the output tube and checked ohms reading from pin 3 to ground and found no reading. I also removed the OT from circuit and ohmed between leads. 1.3 ohms between primary wires and open between secondaries. Would this be congruent with a blown output transformer?

[PRR]

> power transformer primaries are measured in AC volts. Correct?

Transformers only work on AC.

Power transformer primaries are universally 120VAC, except when they are 240VAC. You may be thinking of some other winding.

[uki]

Welcome to the forum !

PT primary to pin 4 of rectifier: 373vAC
PT primary to pin 6 of rectifier: 373vAC

Wires coming from PT to rectifier are the secondary wires. Check out picture.

[st]

Also, the schematic you should be looking at is the AB764 vibro champ/bronco schematic, not AA764. The AB schematic contains errors in the voltages because of poor updating from the AA schematic, but at least the voltages at the b+ nodes are correct...

[bigdaddydiesel]

Thanks for the replies everybody. I am still learning as I go and I appreciate the patience and hand-holding from all those involved.

I felt confident the output transformer was the issue and ordered a ClassicTone 40-18030 earlier this week. $38 to my door. It arrived yesterday. Quickly got to work removing the old Schumacher and wiring the ClassicTone ot in. I turned the amp on to find...no hum from the speaker... 

...until I rocked the preamp tube in its socket and heard crackling coming through the speaker. Plugged a signal generator into the input of the amp and confirmed it works again. Even better, in fact. It is *much* quieter than it was before the incident. The original output transformer was probably subject to a lot of mileage over the past 40 years. Sorted the crackle from the preamp tube, checked bias and plate voltages, and buttoned it all up.

Again, thank you all.  I will commence lurking and hoover up knowledge.