Help with Univox Guitar Amp

God Almight.... VOM's should be banned from public use.....

The first thing to check with this kind of fault, is that the + and - supplies to the front end are present. They are often derived from the main high + and - rails that feed the output stages, by a simple power resistor and power zener circuit. It is quite common in this type of arrangement for the power resistors to go bad jointed, due to the temperature that they typically run at, or the zeners to go short circuit, or the decoupling caps across them to go short circuit. You will normally expect something of the order of + / - 15v for the front end opamps.

Arfa

"Daremo" bravely wrote to "All" (05 Feb 06 21:13:38) --- on the heady topic of "Help with Univox Guitar Amp"

Da> From: "Daremo" Da> Subject: Help with Univox Guitar Amp Da> Organization: Road Runner High Speed Online

Da> I have a Univox Amp I purchased off the back of a guy's truck. It Da> powers on, but all I get is a hum output. I attach a microphone or Da> guitar cable to any of the inputs and *nothing* changes the output Da> sound. The volume control doesn't change the level of the hum output Da> either. I should say, I've been around guitars and amps for about 20 Da> years and the output of this sounds normal, not necessarily a "hum". Da> It soulds like a normal amp with the input (guitar or mic) turned off. Da> I've tested all transistors with a VOM and they seem OK. I've Da> cleaned all pots/boards contacts with contact cleaner. I do not have Da> a o-scope or a signal generator, so I realize troubleshooting will be Da> difficult. I do have a schematic. My question is this: Do any of you Da> have advice for troubleshooting this with just a voltmeter?

Troubleshooting need not be difficult. Nor do you need a signal generator or a scope. Simply inject some audio program into an input and use a small amplifier to trace the audio path in the circuit. Where the signal stops will be the bad part. Easy as pie!

A*s*i*m*o*v

... A stereo system is the altar to the god of music.

Thanks to you all. I just got lucky. I was using the 10% rule, where common components tested within 10% of each other I called them good. I found 2 PNP Germanium Triodes that were identical but tested way different than each other. I removed them from the circuit to test properly, and they tested the same (both returned the same values, I mean). Upon re-installing them, the amp worked fine. Maybe a cold/bad solder joint? (No way, not after 30+years!) I think these were the preamp transistors? Anyway, like I said, I got lucky.

What is a " PNP Germanium Triode " for goodness sake ?

Arfa

Arfa Daily spake thus:

A transistor.

way to go! we all need a bit of luck now and then.

in the early day of transistors technicians were cautioned to heatsink the leads of transistors when soldering so as not to damage them. tiny little aluminum spring loaded clamps were sold to aid in this process. (haven't seen any of those for years now). it's vaguely possible that you fixed the bad transistor by applying heat, however a bad solder connection is the more likely culprit.

It's been about 100 years since I've had electronics training, that's just what the schematic said.

NTE 158

"Arfa Daily" bravely wrote to "All" (07 Feb 06 01:37:49) --- on the heady topic of "Re: Help with Univox Guitar Amp"

AD> From: "Arfa Daily" AD> Xref: core-easynews sci.electronics.repair:357674

AD> What is a " PNP Germanium Triode " for goodness sake ?

And here I thought you knew it all, eh?!

A*s*i*m*o*v

... From small chips to big breasts, silicone is great!

Asimov spake thus:

Silicon != silicone.

-- If the United States government, with all its capacity to collect and interpret information, did not see Hamas doing very well in the Palestinian election in the wake of these other Islamist victories, then it is either willfully blind or totally incompetent?- and neither possibility is a very comforting thought.

- Rami G. Khouri, editor at large of the Beirut-based _Daily Star_

AD> From: "Arfa Daily" AD> Xref: core-easynews sci.electronics.repair:357674

AD> What is a " PNP Germanium Triode " for goodness sake ?

And here I thought you knew it all, eh?!

A*s*i*m*o*v

... From small chips to big breasts, silicone is great!

I know what PNP is and I know what germanium is and I know what a PNP germanium transistor is ( sheesh ! ) but I have never in 40 years seen a semiconductor device referred to as a " Triode " ( and yes, I can see " Tri " for 3 and " ode " for electrode, and yes, I am fully aware that a conventional transistor has three electrodes ). This side of the Atlantic, triodes have always been vacuum tubes, or in English English, valves.

The name " transistor " is a condensation of the words " transfer " and " resistor " and to the best of my knowledge, that's the name it was christened with by Bell Labs in the US, and the name that it has commonly been known by throughout the world ever since.

Arfa

"David Nebenzahl" bravely wrote to "All" (06 Feb 06 21:49:50) --- on the heady topic of "Re: Help with Univox Guitar Amp"

DN> From: David Nebenzahl DN> Xref: core-easynews sci.electronics.repair:357687

DN> Asimov spake thus:

DN> Silicon != silicone.

The Earth's crust is about 25% silicon and 50% oxygen. They should have called it Silicon Oxide instead of Dirt. Go figure...

A*s*i*m*o*v

... Letterman of Borg - "Ok, Top 10 reasons why resistance is futile:"

and the 'vox is only about 35

and i suspect the transistor(s) are much younger

Ack! Those were "the early days of transistors"??? I feel old...I'd better sit down.

the

little

back in the dawn of prehistory when you could actually fix most stuff and replacement parts didn't cost more then the original unit.

way back when the drugstores had a tube checker and sold tubes.

when radios proudly counted the number of transistors in them.

when nuvistors valiantly failed to curb the onslaught of solid state.

when nixie tubes lit up glowing number displays that actually had curves and not straight segments.

when a 10 function calculator cost over $100 and a slide rule course was required.

gasoline had lead in it and was sometimes as low as 15 cents/gal

i feel old too... im gonna LAY down.

I actually have a VOM that I still use (dare I admit this here?) that makes use of exactly these nixie tubes you describe for its readout. Personally, I find them much easier to read than the 7-segmented equivalents.

Alas, I can remember everything on your list...

Best...

and

back when radios had DIALS

and cable lacing with waxed string was an art form.

i bet ya that nixie VOM is really a VTVM... but i think there were a few FET-VOMs that used them.

i built a nixie frew counter for scratch once... blew it up a couple time measuring the +5 with an unshielded probe... some lessons don't 'take' the first time.

IF YOU PURCHASED IT OFF THE BACK OF A GUY'S TRUCK CHANCES ARE IT'S PROBABLY STOLEN.

ENJOY YOUR "HUM"

DOES YOUR BOYFRIEND ALSO "HUM"

Actually, not to drive this side discussion into the ground, but I actually have restrung a few receiver tuning assemblies by hand, years ago. Not for the faint of heart.

You're right, my nixie-tube-readout meter definitely has active circuitry, but it's actually "modern" solid state...no other tubes aside from the nixies.

Thanks for reminding me how old I am!!!

Cheers...