Univox U130L No Power

[Neutronarmy]

Helpful Folks,

I've been given a Univox 130L solid state guitar head in non-working condition. I should note in advance that though I can use a soldering iron, my repair skills are more of the guess-and-check than the grounded-in-theory variety.

I was told that it was working and then a resistor caught fire in the power amp section. Assuming the resistor was a fuse, I replaced the resistor with one of a very similar value. It worked for a short while but seemed a little off. The power amp section was also mounted really shoddily so perhaps a short occurred. I replaced the power cord and turned it off for a while. Upon returning to it today, the power light blinked for a second and then it died. No power, no output, nothing.

There is a fuse that I checked as my first guess and it appears to be in fine shape. The only visible issue is that the trim pots in the power amp section (or what I assume to be power amp section) have seen better days.

I have a multimeter, a basic understanding and some time, but that is where my functioning knowledge ends. If perhaps you point me in some direction I would greatly appreciate it. Schematics seem to be missing from the internet as well - so the advice of others is really the only direction I have right now. Thanks in advance for any help.

-Chris

[olddawg]

If a resistor caught fire in the output section, you probably have a short drawing excesive current. Most likely an outout device. If it is an old Univox it most likely has matched pairs of pnp and npn transistor in a push pull configuration for an output stage. Find a schematic. Check all of the outputs and driver transistors for a direct short, probably emitter to collector. Meter out all of the associated rsistors. You really should have a schematic and a variac or at least a lightbulb current limiter so you just don't keep burning up parts. It also helps to have a basic idea of how active components and bias works.

[Neutronarmy]

olddawg,

Thanks for the help and direction. I tested the output transistors - RCA 2N4348s - and they both appear to have shorts. Some reading about suggested NTE284s as suitable replacements (as 2N4348s seem hard to come by). I'll spend some time checking resistors as well and likely replace the two trim pots that seem flaky.

I wish I had a schematic to go from, but the internet yields no results in searching for one. Luckily the bottom of the PCB has the values listed in tiny writing. Should get me close.

Do you have any suggested reading on active components and bias? I've learned as I went over the last few years, but my repair skills have always been pretty weak. Perhaps it is time to learn some theory to put into practice.

Thanks again for the help. If you, or anyone else, can think of where I might also be forgetting, it is greatly appreciated.

-Chris

[olddawg]

The trim pots you are talking about may be bias pots. If you can find an old TCG or ECG (Sylvania) replacement reference manual there was always a great section in the front on how active components work and how to test them. Sounds like you need to rebuild the output stage. It isn't a big deal, but difficult to achieve cluelessly and without the proper tools. If it isn't right you will just keep blowing outputs everytime you turn it on. Look closely at the spec on any subs you use.

[Speedy454]

New to thread, lots of experience on the U130L head... Still need help?

[J M Fahey]

If you could scan and post the schematic and any other printed info about it, that would be great.
Thanks in advance.
PS: I'm specially interested in the transformer coupled output and the "ESP" protection circuit.

[Speedy454]

I have a schematic for a U200L. Paid big bucks for it, but here it is for all to use. Also one for the similar U-50.
The U130L preamp is remarkably similar in design to the preamp on the U-50 amp. Same FET input, same BC169 2nd stage. Resistor values are slightly different due to higher power supply voltage. Interstage caps are same or similar. Power amp on U130 is virtually same as U200, but lower power supply voltage and only 2 output drivers instead of 4. I have seen several amps come in with the ESP circuit removed. (SCR cut off board.) The way it is supposed to work is if Q3 on the power amp is driven too hard, the SCR fires and shuts the power amp down. Nice thought. Seems the scr shorts out and the amp stays disabled. On the U130L, the output cap C9 is rated at 50wvdc, and has 45vdc on it. It eventually fails. I like to put in something like 100wvdc. The 2n4348 can be hard to find, but the ECG280 works well. If you are popping outputs as soon as you turn it on, (OUCH!) look close at Q2, Q3, Q4, Q5. They all set up the bias for the outputs. If things aren't just right, the outputs will fail dead short as soon as the switch is thrown. Resistance values need to be verified. Trimpot R10 sets the DC bias of the power amp. Adjust it for exactly half of the power supply at C9. Trimpot R18 sets the bias current through the drivers and outputs. I like about 20 to 50 mA. Some amps creep above 30 and some do not. I have sen one set at 480mA, and it worked. Outputs got hot even at low listening levels. Correcting the idle current corrected the overheating problem.

[J M Fahey]

Thanks for posting.
I have been curious about that Univox line for ages.

[Speedy454]

preamp section U130L vs. U200L

The U130L has a decent FET input section. These overdrive nicely with external pedals. Due to the way the channel volume and master volume are configured, you cannot self-overdrive it like most amps. A few have a Pull PB (power boost) on the master volume control that changes the gain of the FET, causing it to saturate very easily. This is an interesting sound, but not all that useful compared to modern stomp boxes. The U200 has an entirely different preamp. It is all BJT, 4 stages. The only thing really unique about it is the mid range control. It is a resonant R-L-C circuit, very early in the preamp. It provides a huge mid range boost or cut. This can provide some very interesting tones. Some U200L amps also have the Pull PB on the master volume. It changes the gain in the early preamp stage and provides a little overdrive. The volume and master volume configuration on this amp are a little more conventional, and do provide a way of over driving the final preamp stages at low overall listening levels. In summary, I like the way the U130L sounds over the U200L due to the super high impedance FET input. It sounds similar to a Fender Twin clean. It responds well to any pedal I have played through it. I have personally has one since 1976, and have repaired several.

[J M Fahey]

Nice to know, thanks.
I imagined something like that.
I remember an around 1976 Univox (or Westbury) publicity about a 100W, 2x12" combo, where a guy who seemed to be a respected professional musician claimed: "the only difference I find between this amp and a Fender Twin is the price tag". Of course he referred to the clean sound.
All reviews commented something similar.
If possible, please post that 130 preamp schematic, even a hand drawn one will be useful to many.
T I A.

[Neutronarmy]

Speedy454/J M Fahey,

Thank you so much for the schematic and explanations. Sorry it took me so long to notice - I thought the thread was dead and I haven't had the time to work on the amplifier.

I have a few questions about the trimmers and transistors. How exactly do you set the trim pots? You mentioned two specific calibration points (R10 at one half of the voltage of C9; R18 to 20-50 mA). My question is, how exactly do you obtain these readings? Do you merely measure across the trim pots with a multimeter for voltage and current? Are these values still accurate if the amplifier is run through a current limiter (like the light bulb limiter referenced earlier)?

Another question regarding the transistors: are there any usable substitutes for the 40360 and 2N3567? I couldn't find any available to purchase currently and I was planning to grab some replacements with my next electronics order in the case one of the them is bad.

And one last question, which portion of the circuit is the ESP circuit? I have very little previous experience with amplifiers and looking through the U200 schematic I couldn't find it. If that circuit is a source of fault I didn't want to neglect it from my troubleshooting.

Thank you all for the help! I'm very excited to start working on the amplifier, but I hope to do so with enough caution to not ruin excessive components.

[J M Fahey]

Hi neutronarmy.
Let's go step by step:
1) Replace 2N4348 with MJ15022 , not NTE. These are much stronger, and cheaper to boot (worst case they will cost the same).
2) Midpoint set trimmer (R10 on the U200 schematic): set it so the positive side of the output capacitor (C9) measures 1/2 the B+ voltage.
If your PSU provides , say, 80V, set the trimmer so you have 40V.
You *can* adjust with the series lamp connected, later you can make the minor adjustment needed with full voltage.
3) Set the current trimmer (R18?) so you measure 20 to 50mV across *any* of R25/26/27/28.
As before, set them now and readjust later without lamp limiter.
4) Don't worry now about 2N4360/2N3567 . As a general rule, don't fix what's not broken.
5) ESP (Electronics Short Proof) is based around the Q10 SCR, which triggers when too much current passes through Q6/Q8 and kills power to the preamp (Q1) muting the amplifier.
I find it slightly moronic to pull the SCR and not replace it, it's akin to pulling the oil pressure or low fuel light from your car instrument panel "because it is annoying you all time".
Oh well.
Good luck.

[Neutronarmy]

J M Fahey,

Thank you so very much for the suggestions. I appreciate the help and encouragement. Though I've done a myriad of minor repairs and DIY projects of the year, I've always avoided amplifiers (and power supplies) as much as I could - paranoia if nothing else.

I asked about the transistor substitutions merely because local electronics stores are nonexistent outside of Radioshack. Thus if the parts are inexpensive I try to keep a few on hand so I don't have to wait a week for Mouser orders. Regardless, I will take these suggestions and run with them once school finally quiets down. I do greatly appreciate it!

[Neutronarmy]

Every six months I seem to get a kick to get this amplifier running again. So I replaced a resistor on the board that had caught fire long ago (and was earlier replaced with an incorrect value), replaced the 2N4348s, and midpoint set the trimmers to begin bias adjustment. However, once hooked up to the lightbulb current limiter, the amplifier still shows a short. I noticed that when removing one of the two power transistors, the short disappears. I tried swapping transistors to ensure that it wasn't the transistor itself that is shorting, but regardless of which is socketed (in the right socket if looking at the back of the amp - Q9 if I'm reading the U200 schematic correctly; I know it is not 100% the same power amp, but the shorted transistor connects directly to the positive terminal of the giant 3000uF [C11] capacitor on the A+ line) the short still appears.

I do not see any visible shorts in the wiring itself and as I said, if I remove the output transistor the short disappears. I am befuddled. I noticed that the C11 3000uF capacitor to which the transistor connects is original and after discharging it and removing it from the amp I noticed it rattled pretty significantly. I'd assume the rattle means dried and thus not properly functioning capacitor. However, as I admittedly don't complete understand power amplifier topology I'm not certain if said capacitor could be the root of concern or if the rattling is a totally normal thing.

Anyway, just thought I'd throw another confused update into the Music-Electronics population. Thank you folks for all of your help up until this point and perhaps one day this amplify will live to amplify again.

Thanks in advance!
Chris

[olddawg]

The power supply filter caps are probably bad. Suspect any electrolytic caps in an amp this age. Have you metered out the output transistors to see if any of them are shorted? If you are going to work on SS equipment, the first thing you need to knows how to measure the junctions in bipolar transistors, diodes and such.