Univox U-1011 question

[The Dude]

My thoughts in no particular order:

1) It's unlikely you have that many bad caps and used caps are probably ok.
2) If there was a problem caused by the grid or leaky coupling caps, the plate voltage should have changed substantially (more than 7 volts) when you grounded the grid. I'm starting to wonder if the schematic voltages are wrong.
3) After "re-skim-reading" the thread, we are chasing voltages, I'm not sure what the actual symptom is. In the first post, one channel worked and the other didn't. Both channels share what we have been calling V2, so I'm not sure how we got on that tube.
4) Have you tried the amp since you changed the tube in post #8. If so what works and what doesn't
5) Other than some voltages that don't match the schematic, what are we fixing?

[57nomad57]

yes I have a signal generator and i'm putting signal through the amp as I go along. I'm getting more signal at the speakers now but it's still weak.

I'm not completely sure the schematic is totally accurate . I noticed on Channel #1 right at the grid at the 470k resistor on the board ( both boards , I'm doing a board swap, so both boards have this ) there is also a resistor (56k) connected in parallel going to ground. This 56k resistor is not on the schematic. I didn't notice this until Last night because the trace is so small at that point. There is no similar resistor on channel 2. As far as voltages go I'm just trying to get in the general ball park of what is on the schematic.

When I said one channel worked( not well) and the other one didn't , that was with the first board. It had been substantially reworked and had some damage so that is why I secured a second board to swap out( supposedly from a working amp, but the pictures of the amp looked pretty bad).On these boards The traces are on the top and the components are on the bottom. It makes working or modifying the amp hard. I relocated the componets on the top , over the traces, so the work would be easier.

Also on both boards the tone stacks had been messed with. I rebuilt the tone stack according to the schematic ( again is it correct?) and it seems to kinda work as some signal is getting through but it's distorted.

My approach to fixing the amp is systematic. Get the first and second stages of gain fixed properly, then look at the tone stack, presence, then phase inverter, then reverb then tremolo.Then look at the power stage if it needs tweaking. At this point the trem and reverb aren't even connected. I wanted to get the basic amp working properly then look at trem and reverb. A little at a time. Does this sound logical? or should I have everything hooked up?

So the symtom at this point is there is little gain. the tone stack is half way working but not really, presence doesn't seem to do anything, but since there is not much gain I can't really tell. Testing with my home made signal tester I'm getting gain after the 2nd stage of gain( but only at 73 vdc at the plate instead of 145 ) to the tone stack, but after that it's distorted.

So my next step is to get the coupling caps working so they don't leak DC. Then work on the tone stack.

Thanks for your comments and help.

[57nomad57]

Also the original literature talks of a boost pull switch on the first volume pot, but there is nothing like this on the schematic.

[57nomad57]

So if the coupling caps aren't the problem -- it seems the only way voltage could get to the grid is maybe through the traces ? the b+ supply runs along the plate return to the volume pot an perhaps there is something causing DC conductivity. I'll inspect that today.

Other work done: I changed out the output transformer to an Univox U-1011 orginal part - that I was able to get. What was in there was a Shumacher-Woodward OT (part number: 582200 EIA-606-005) It had 2,4 and 8 ohm terminations. I can't find any information about this or it's original application. Other work was to correct the filter cap wiring. The amp was holding voltage ( 700DC!!!) after shut off. THe 100UF filter caps weren't connected properly and the power supply voltages weren't right. tis has been corrected.

Dude - thanks for your welcome - I feel less like an interloper.

[57nomad57]

Hey The dude --you were right - it wasn;t the caps -- it was the 470K resistor out of place and the 500Pf cap out of place after the first stage of gain. -- after it was corrected the plate votage at the 2nd stage of gain shot up to 125vdc. The amp is much much louder now. On to the tone stack.


I want to get it working pretty well. then tweak it.

[g1]

It sounds like the schematic may be different from what you have there. I would suggest you make a few copies, and draw in the old bits as you make your changes, just in case you need to look back at how it was. Some of it may be mods, but it also could be a variant of the circuit.

[The Dude]

Hey The dude --you were right - it wasn;t the caps -- it was the 470K resistor out of place and the 500Pf cap out of place after the first stage of gain. -- after it was corrected the plate votage at the 2nd stage of gain shot up to 125vdc. The amp is much much louder now. On to the tone stack.


I want to get it working pretty well. then tweak it.

That's good news. Glad you got it figured out. Thanks for your post #17. That clarifies a lot of things. Also, ...... no need to feel like an interloper. I sincerely hope you enjoy the place and stick around after you get your amp repaired.

[57nomad57]

I've gotten past the tone stack ( I simplified it ALA fender ) and am on the the phase inverter -- something not quite right there... but I'm almost finished with the basic amp. It's getting more powerful now.