Vintage univox guitar amp/no sound?

[joshdfrazier]

Hi, this is my first time posting!

I happened upon a really cool looking Univox (lafayette badged) guitar amp awhile ago in non-working condition. I've cleaned/tightened the tube sockets, and replaced a few electrolytic oil caps/tubes that I knew were bad. The amp powers up, all of the tubes glow, but there is absolutely no sound whatsoever. not so much as a hiss or a pop from the drivers. (the speakers do work)

There is no schematic for this amp available, but there is one that comes close that i've attached. the other photo is the guts of the amp i am trying to get working.

wondering where to begin?

[Firestorm]

That is a much better laid out Univox than I am used to seeing! You need to do the usual trace: put a signal in the front and see where it stops. Are you sure the OT is intact?

[joshdfrazier]

just a disclaimer, while i'm not completely clueless, i'm much less educated in this area than most people posting on this forum, haha.

so you're saying connect power to the drivers leads and follow the signal until it comes to a stop? would 12v be enough? also, OT?

[HeeBGB]

Wow. I have a U305R and it is nothing like the pic you posted. Mine is on Pc board and has one 15" jensen speaker. Pretty sure the schem you posted is correct for mine except I converted mine to EL84's

[xtian]

Firestorm is suggesting you put a signal in the input, like a sine wave, not a DC voltage. You can use any audio as a source, like an iPod. Search for how to make an audio probe. But first learn where the dangerous voltages are. And learn how to safely discharge your amplifier! Because: death.

[Firestorm]

If you have a signal generator, put it in the input and check where the signal stops. You can use cd player or a tape player; there are posts here on this. But you need to find where the stops passing to the next stage. I'd start at the output: does signal get through the OT - pops, hum, anything?

[joshdfrazier]

I'm considering output or power transformer issues. The output transformer is a thordarson 22S74. There are 3 leads on one side that read P, B, P, and above is labeled "output". These leads are connected to the amps internals, not the driver (which doesn't make sense to me, logically). there are 6 leads on the opposite side which are numbered, two of which are connected to the driver.

i will troubleshoot the preamp in the meantime.

firestorm: there is absolutely no sound at all! no hums, hiss or anything. even with all dials maxed out.

thanks for the replys!

[Firestorm]

There should be three leads on the primary side of the OT. At least two on the secondary. I'd suspect the OT. Do you know how to do basic tests?

[joshdfrazier]

i found this: http://www.geofex.com/ampdbug/outtrans.htm

it doesn't make much sense to me, even the simple tests. i may just do a quick swap with a different OT.

[HeeBGB]

Do you have a volt meter? If so do you know how to use it? A voltage chart might be of great help here.

[Structo]

First thing I would address is that power cable.

A wad of electrical tape isn't a very good technique for safety.

Have you been able to put a date on this amp?

[Firestorm]

First thing I would address is that power cable.

A wad of electrical tape isn't a very good technique for safety.

Have you been able to put a date on this amp?

Looks like a do it yourself strain relief. Still not as good as the real thing. I'll bet you could get a useable date off those silver electros; just can't make it out from the pic.

[Structo]

Could be.

Something else that caught my eye.

[Firestorm]

Hmm. Doesn't it look like one speaker isn't even connected?

[joshdfrazier]

hey thanks for all of your replies.

the electrical tape is not covering any exposed wire. as firestorm said, it's just to keep you from pulling the cable out. definitely not my doing!
also, rest assured, the speakers are definitely plugged in. i may have just had something disconnected in the photo

so i've determined that there is no power at the plate of the output tubes.. there are 6v going to the heater, that's it. I've tried two different rectifier tubes. the power transformer seems to be functioning correctly. when checked with a voltmeter, all leads gave a reading that corresponded with their labeling on the tranny.

the power switch is not the original. it is a 3 way switch. middle is "off" either side is "on". is it possible that when a tech was servicing this that the leads were not connected properly and the amp is "stuck" in standby? is this stupid??

structo, i'm aware of the exposed leads, thanks for pointing it out though! the amp is a bit of a mess, i know (you haven't even seen the OT! )"
goal is to determine that it can function before i clean it up.