"And Now for Something Entirely Different..."

[kharrisma]

Hey Folks,

I find myself in possession of a Univox U-255R 2 x 12 (Jensens) 40-watt combo "lead" amp. It's *rough,* but I think it can be brought back. I've found a schematic for it... if anyone needs one, I'd be happy to p.m. it to you (my permissions don't allow me to post attachments.)

The reason for this post, however, is that I'm LOOKING for an owner's manual for this amplifier, or any kind of original documentation. I know it's a long shot, but I've got to start somewhere.

Thanks for checking it out!

[The Dude]

I was trying to resist, but can't. I have to ask,...... why do you want an owner's manual? I don't see anything about the amp that isn't self explanatory.

[vintagekiki]

I'm curious what you expect to find in the owner's manual, except for the description of the controls, power consumption ...
If you have some technical problem with U-255R describe it, forum will try to find a solution.

http://www.vintageunivox.com/schematics.html

https://www.schematicsunlimited.com/u/univox

https://elektrotanya.com/?q=showresult&what=univox

http://schems.com/bmampscom/univox/

1)
https://www.musicparts.com/products.asp?Company=Univox
Univox U-255R Amplifier Schematic
Contents: Schematics, Controls, Functions & Theory of Operation, Tube Layout Chart ($15)

2)
https://sowseargear.com/wp-content/uploads/IMG_1178.jpg

http://sowseargear.com/1971-univox-u255r-guitar-amp/

[kharrisma]

Thanks for the replies. It's more of a habit thing than anything else; I've always been a "RTFM" guy (read the *bleeping* manual)... I'm by no means a dumb guy, but I still get surprised from time to time by learning something unexpected in a product manual, when I thought the functions were obvious. I was an electric forklift technician for almost thirty years, and on these particular trucks, you pretty much *couldn't* work on 'em without a manual, and the habit just got cemented in place.

These two amps (this Univox and a 1968 Fender Super Reverb (silverface, but with blackface circuitry) along with two others of no real interest (to me, anyway; a "who?" amp and a Fender Frontman... solid state) have been in a storage locker for over ten years, and apparently there was some leakage, as there's a lot of rusting of the metal parts and visible water stains. Some of the other stuff in there showed evidence of rodent activity too, so I'm going to be looking for rodent gnawings in the wiring, too.

The thing I was hoping to find in the manual was a layout chart; schematics are great for tracing signal and power flow, but it can be tedious to try to identify a specific component's location on the board by using schematics. Sure it can be done, but it's a lot easier with a layout chart. I know a layout chart is more a builder's thing, but Fender supplies them; I had little trouble finding the manual, schematic and layout for it.

I'm good with schematics, and can work out what I want to know from it; it's just faster and easier if there is a layout. Reason I'm looking is that it won't power up at all; dead as a brick. I'll start at the plug, check it, the cord, the cord connections at the amp, the fuse (it's good... oh, and it has a FIFTEEN! amp fuse in it; that *can't* be right, and I wanted to know what the correct rated fuse was... again, the Fender has a 2 amp slo-blow, so fifteen seems way off the mark), the wiring to the transformer, the primaries for shorts or open, the secondaries for the same, the rectifier diodes (2; it's a half-wave rectifier), and by then I ought to have an answer. I just have a certain methodology I developed over the years, and I always prefer to have any/all available documentation at hand if at all possible.

Thanks for the links, by the way; they're very much appreciated!

[kharrisma]

Okay... so I've managed to take a a filthy, dead amp and get it to power up, but with awful hum and crackling every time anything is touched. Did a ton of cleaning (straightened/burnished pins on all tubes, burnished pin sockets, jack contacts (barrel bore and tongue), ran DeOxit through the pots... and now have a graveyard-quiet, crystal-clear amp... but at very low volume. At "10" this thing isn't as loud as a cheesy old "Gorilla" practice amp (rated at 10 watts.) I could carry on a conversation without raising my voice with this thing dimed. According to an old ad for this amp I found, it's supposed to be rated at 40 watts; the 6973 power tubes are rated at about 20 watts in AB1 configuration, which this amp is, and with 2 12" Jensens, it ought to be really loud, shouldn't it?

I'm not sure how to go about testing the output tubes to see how much power they're putting out; is this one of those things where simple substitution with new tubes is the best way to see if they're any good? Also: wherever this thing has been living for the last decade or so, it was wet; there's a lot of rust on the metal parts, including the lower speaker frame. How likely is it that the voice coil freedom of movement has been compromised, so that even if the tubes are doing their thing, the speakers can't utilize it from being restricted by corrosion? Any good way to test this theory?

Thanks for any ideas or suggestions!

[The Dude]

I'm betting on some bad electrolytics in something of that age. Also, if you haven't, you should clip out the death cap and add a grounded AC cord. On the speakers: Check them with your fingers. Do the cones move in and out freely? Check them with a known good amp to see if they're working properly. This amp should be plenty loud. Check power supplies in the amp. Do they match (at least somewhere close) the schematic? You could also trace signal through the amp and see where you might be losing gain.

[Enzo]

I wouldn't assume the power tubes are the problem. COuld be but both?

[Mick Bailey]

I would be checking the DC voltages as per the schematic, as well as the AC heater voltage. Also check for high resistance on the inputs and outputs. A damp amp can also corrode in the voice-coil gap and reduce the volume to almost nothing, hence The Dude's recommendation.

[kharrisma]

Thanks for these suggestions!

@TheDude: I'm aware that 'lytics degrade over time; what I'm not so sure about is what form problems with degraded caps will take. Is the issue with power caps, coupling caps, biasing caps, or all of the above? Another thing I don't know (and what I don't know vastly outweighs what I DO know ) is this: does the form factor (read: size) of a cap matter, given the same specifications? There's this one ginormous cap, 20mF, 350V, right where the power lead connects to the board (printed circuit, by the way, not a 'tagboard'), and close by is another cap with the exact same values... but it's literally half the size. Is there electrically any difference between the two? And speaking of caps... "Death Cap?" I think I know what you're getting at... it it that 0.002 cap hanging off the neutral AC input leg going to ground? I was thinking the same thing; the power cord looks like a table-lamp cord; not very reassuring! And not in very good shape, either. I've checked only the raw output voltage from the power transformer so far (it's good); just got it to work yesterday evening, and haven't had a chance to walk through it checking voltages yet. Checking the signal path: can I do this with a plain multimeter, or should I be using a 'scope? I HAD one, but sold it a couple of years back when I "thought" I was done messing with electronics. I've seen some PC-based 'scopes' that are functional enough to do this, for pretty short money; should I be looking at these, or is there a trick to using a simple meter? The cones move... haven't checked full-range, or to see if the upper (least rusty) is the same as the lower. I'll take your suggestion to check 'em with a known good amp; I an use that two ways: one, as a speaker for this amp to see if it's loud, and two: to connect the output to these two speakers and see if it's as loud as the good amp; that alone will tell me a lot. Thanks again, it really is very much appreciated. Not new to electronics, but pretty new to tube amps.

@Enzo: Nope, not assuming anything at this point; trying to backfill my relative lack of detailed knowledge of tube amps real quick and in a hurry! I agree; one tube, not unlikely... both? Not nearly as likely. Not knowing how to test them (other than in a dedicated tube tester) makes things a little more problematic.

@Mick Bailey: I've checked the AC heater voltages; initially, they'd plunge as soon as I plugged in a tube. After straightening/burnishing all the pins/sockets, about 70% of the hum and crackle cleared up, and I got steady voltage at 6.3 VAC (using a Fluke 87 meter, for what it's worth.) I'll be taking all suggestions and I'll post any progress (or lack of thereof) soon. Thanks again!!

[Mick Bailey]

Caps have got smaller over the years so they can look a little lost compared to the originals. With an old amp I usually do a full re-cap of the electrolytics - PSU, bias and cathode bypass. Sometimes I go with a higher voltage to get a physically bigger cap with longer leads. So an old 70v cap in a bias circuit may be 100v. Or a 25v bypass cap replaced with 63v or 100v. If an amp is working OK on the bench I prefer to fix any faults before a re-cap, unless the caps are the problem. Just my own approach - other people may have different methods. Film caps in the signal path are often OK, but they can go open, short, or leak DC. I largely use a DMM to trace faults in an amp and with a signal at the input you can use your meter on AC volts to see if there's a signal on a grid, if it appears at the plate, and then check the other side of the coupling cap to see if it's still there. On DC you can check if there's DC leakage on a coupling cap.

The quiescent DC voltages are important because they set the operating conditions for every stage of the amp. You wouldn't troubleshoot an engine without first checking it has fuel.

[The Dude]

Mick already gave you the rundown on electrolytic caps. On the death cap: Yes it's the .002 from AC in to ground. They can short and cause the chassis to become hot. Clip it out and add a grounded AC cable so the chassis is grounded. These are safety issues aside from any performance issues you might be troubleshooting. A scope is best for signal tracing, but you can certainly use a DVM set to read AC volts.

[kharrisma]

Another question (I've got no end of 'em...) If I measure the AC waveform at the input of the first gain stage, and center that waveform on the "zero" graticule (horizontal line at center of screen), and then check for the same (amplified) waveform after the coupling cap to the next gain stage... if that coupling cap is leaking DC voltage, will that show up on the 'scope as the same (amplified) sinusoid waveform but elevated off of the zero graticule by the amount of voltage being leaked? I have used a 'scope in the past, but not very exhaustively; mostly just verifying the presence or absence of a square wave "on-time -vs- off-time" under certain input conditions (or measuring lead and lag of two square waves generated by a steering tiller; A leading B means turn right, and B leading A means turn left, and the amount of lead determines how fast the steering/drive unit slews.) Nodding familiarity, I guess you could call it. This is turning out to be a real crash-course... and I'm really enjoying the ride! Thanks again for your help!

[Helmholtz]

If I measure the AC waveform at the input of the first gain stage, and center that waveform on the "zero" graticule (horizontal line at center of screen), and then check for the same (amplified) waveform after the coupling cap to the next gain stage... if that coupling cap is leaking DC voltage, will that show up on the 'scope as the same (amplified) sinusoid waveform but elevated off of the zero graticule by the amount of voltage being leaked?

If you use DC coupling, yes. But this method isn't very reliable.

To test for leakage, the most sensitive and reliable method is to disconnect the end of the coupling cap which leads to the next stage but leave the other end connected to the plate of the preceding stage. Now connect a DVM having an input resistance of 10M between the disconnected lead of the cap and ground and measure DCV with the amp turned on. Will take some time for the reading to settle. A new good quality cap will give a reading of a few mV. In most applications DC values of up to 1V are ok. But leakage of old caps often increases with humidity and temperature.

[Enzo]

If your scope is set for AC coupling, then it blocks any DC. So 5v peak to peak of signal around zero DC will look the same as 5v p-p sitting atop 200v DC.

If you set the scope for DC, then a signal around zero will center on teh screen, or wherever you set zero. And if you scope a signal sitting on DC, then that will appear offset by the amount of voltage. problem with that is that 5v of signal on top of 200vDC means the signal will appear tiny at the top of the screen.

But all this ignores that this is what your volt meter is for. Of your cap is leaking, it will leak even with no signal, but either way the DC offset will show on a meter.

[kharrisma]

Ahhhh... light dawns over Marblehead! (old New England north-shore area saying!) Though I've never messed with either of those settings, I understand exactly what you're saying. Huh... and I thought I'd figured out a nifty way to kill two birds with one stone! Live and learn.

As I understand my (pretty basic) theory, a cap passes AC but not DC. So, if I correctly understand the last part of your reply, if I'm reading any DC voltage after a coupling cap, that's not good and the cap most likely needs replaced... and that's even with no incoming AC signal. That seems pretty simple; I was going to make a quick-and-dirty sine-wave generator around 330 Hz (high E, close enough) and THEN start digging. Your way sounds a lot easier. Unless of course I'm getting THIS wrong too....?