Univox U130L No Power

[Neutronarmy]

Thanks for the hint. I'll be sure to double check them. The short is what I'm most concerned about - is a bad cap likely to fail short? I guess I assumed that failing open/acting errartic is more common. I checked the output transistors way back and found one to be faulty and replaced the both of them. The weirdness of it all is that when one of the output transistors is removed (the right one facing the back of the amp - I gave a guess via schematic at it being Q9 in the attached scem.), the short disappears. The two large electrolytics seem questionable, especially the one with the apparent rattle. I'll also take your advice and reteach myself diode/transistor checking.

[Enzo]

If a filter cap were shorted, it would probably draw current with or without that transistor. If you removed the filter caps completely, the amp would still work. Hum like crazy, but work.

A transistor doesn;t have to be shorted to cause excess current draw. Some other part/parts farther back could be telling it to turn on hard. Not its fauklt, really, but still removing it stops the draw.

SHorted transistors LOOK just like good ones. It is all well and good to look for burnt up parts, but usually, other than resistors, they don;t appear. But a lack of visible failure doesn;t tell us much. Every transistor in it could be shorted and you'd never know by looking.

I check outputs, and if one or more was bad, I ALWAYS check the drivers and predrivers. And every one of those has resistors that connect to them, and when transistors fail, resistors nearby often burn out - whether they appear that way or not.

I don't have a schematic handy, but solid state amps all look similar on paper. You have outputs and drivers and sometimes predrivers on the V+ side and the V- side. Somewhere left of those will be a transistor or a couple diodes tying the driver (or predriver) bases together. That transistor will be where the bias adjust control is if there is one. That transostor or diode will generally be mounted on or in contact with the heat sink. If it or its circuit is open, then the two opposing sides of output transistors will all conduct at the same time. That draws tons of current and blows fuses, even if all those outputs are just fine.

[jrfrond]

Just FYI- If anyone needs Univox or Westbury schematics, I have most of them sitting in a file right here, leftover from my shop in the early 80's when I did Unicord service (or course, I have all of the old Marshall stuff too). Anyway, feel free to ask. I loaned them once to Tim from the Univox.org website to scan, but many are not available and assigned to an "assorted amps" file that is locked.

[sdman]

Schematic for the Univox U130L

HI,

Do you still have schematics for the U130L Univox?

Thanks,

Sam

Just FYI- If anyone needs Univox or Westbury schematics, I have most of them sitting in a file right here, leftover from my shop in the early 80's when I did Unicord service (or course, I have all of the old Marshall stuff too). Anyway, feel free to ask. I loaned them once to Tim from the Univox.org website to scan, but many are not available and assigned to an "assorted amps" file that is locked.

[J M Fahey]

Here´s U200L which seems to have the same power amp with 4 power transistors:

seems to have problems, re attached as .zip

[sousaluis]

Hi, I have one U200L that I'm recovering that suffered a major issue. All power transistors are shorted and major Capacitors blown.
I'm replacing all this elements but it seems the problem could be the Output transformer(AT-80), it's reading less than 2 ohms between each terminal and the ground. When I connect a 4 ohm speaker directly, bypassing the transformer, and put a fresh fuse this one doesn't blow(with new transistors). As soon as I put the transformer in the middle, bye bye fuse in ms :-).

How can I test this output transformer? what should I measure between the terminals and ground?


Thanks...

[J M Fahey]

Busy now to write a complete answer but :
1) "less than 2 ohms" DC resistance is what you should measure there.
In a transformer resistance is way smaller than impedance.
A value between 0.3 and 0.5 ohms should be about right.
2) the output transformer does pass DC current easily , so if you have any significant DC offset atb the output, or have a lossy output capacitor (if applicable) fuses will blow ... but it's not the transformer's fault !!!
In fact it's protecting your speakerts from DC.
Will write tonight

[Jazz P Bass]

Oh yeah, there's an output coupling capacitor.
C9. 4200uf/ 50Volt

I would suggest that you check that for a shorted state.

[g1]

When you bypassed the OT and connected the speaker directly, there was no hum and no DC voltage on the speaker?

[sousaluis]

Very little hum but still present. And yes the Coupling capacitor was dead, It is one of the things that I'm replacing.Still waiting, it is coming from TEDSS in the US to Portugal, could not find this capacitor in europe...

But Ok, good to know that most probably the problem is not on the output transformer :-). I will replace the Caps, the transistors and test the circuitry. Thanks to all for your quick answers, I will give some feedback as soon as I'm able to finish replacing the components in my spare time.

[g1]

U130L and U130B schematics here: http://music-electronics-forum.com/t39401/#post383583

[Neutronarmy]

U130L and U130B schematics here: http://music-electronics-forum.com/t39401/#post383583

g1 - thanks for that link! I stumbled into the U130L schematic a few years ago, but I still haven't taken the time to really tackle it. As it is just a dusty amp laying around the shop it always gets pushed to the bottom of every queue. Last I checked it also had a shorted driver transistor, but that might just be flaky memory.

It appears as though this ancient thread has taken on a life of its own since I last left it. Crazy how little I knew five years ago (and as of a troubleshooting thread a few days ago, how much I still have to learn in some respects). Thanks again to the ever helpful Music Electronics Forum community - I've learned immensely from reading accounts of various repairs. I hope the others with Univox woes solve them with the greatest of ease! Perhaps this summer I'll tackle the ol' U130L again.