Univibe repair advice needed

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Univibe repair advice needed

Postby Isittooloudoutthere » 15 Dec 2018, 11:33

Hi guys,

can you help me fixing my old Univibe? It worked perfectly for many years but when I plugged it in a few weeks ago, there was no sound at all. I still have a Megavibe, but I really want to have the Shin Ei back on my board now, because I dig the non-bypass sound in combination with my old Marshalls for Hendrix and Trower, etc. I have no problems at all doing the technical work and I have all the tools you need for that, but I'm really kind of :slap: when it comes to understanding the logic of the circuit and finding the failing parts... Now what I understand, based on the article of RG ( [smilie=a_goodjobson.gif] ), is that there might be a problem with an LFO transistor or the lamp driver transistor since the lamp is not flashing anymore. But how can I find that out in detail? Where shall I measure with my meter and which are the right measuring values? Which transistor would you recommend as a replacement, since the old ones aren't available anymore?

Thanx [smilie=rlp_smilie_242.gif]
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Re: Univibe repair advice needed

Postby idy » 15 Dec 2018, 20:42

If there is no sound at all I would not begin suspecting the LFO but start signal tracing. It could be something so simple even... you or I could fix. Maybe. Signal tracer... cable with a capacitor to block DC. Hook up a sound source to the vibe and find out where the sound stops. A bad LFO probably would not kill the signal.
Maybe first make sure the power supply is working. What kind of voltage is leaving the transformer? Then start checking transistors. Just write them all down.
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Re: Univibe repair advice needed

Postby Dirk_Hendrik » 15 Dec 2018, 20:54

Fuse still OK?

If the power supply is OK, and the lamp flashes you can discard the whole power supply and LFO as suspects straight away. If not, follow Idy's tips above.
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Re: Univibe repair advice needed

Postby Isittooloudoutthere » 17 Dec 2018, 06:07

Ok, thanks. The fuse is ok, the power indicator lamp is working, the light bulb isn't flashing... When I plug in the guitar, there's no signal coming out. Just a sizzle when I turn the Volume pot. I'll check it and let you know.
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Re: Univibe repair advice needed

Postby Dirk_Hendrik » 17 Dec 2018, 09:51

btw...

I have hard time imagining this thing will not sound way better after all those ancient electrolits are replaced by fresh ones. I know, vintage vs players value...
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Re: Univibe repair advice needed

Postby Isittooloudoutthere » 23 Dec 2018, 16:49

Ok, yes, well, I'm struggling... I have 13.8V leaving the mains transformer. Now to the signal tracing. I never did that and googled to find the most simple explanation for that but it still is not 100% clear to me. So a few questions... Can I hook up the line out of a portable CD-Player as a sound source? Then I try to trace the signal through the circuit just by hooking one cable of a speaker permanently to ground and on the other cable of the speaker I solder a polarized cap with its negative side to block DC and touch certain points in the circuit with the positive side of the cap, especially the bases, emitters and collectors of the transistors. Right? Sorry guys, I really start from zero here... Does the value of this cap matter? A small drawing or further explanation would be great... Is it right to connect the other side of the speaker to ground?

Is there a layout of the original board somewhere out there which can help identifying the transistors on the board?

And shouldn't the small light bulb flash if it is just a problem in the signal path?

Further help is much appreciated...

Holidaze!
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Re: Univibe repair advice needed

Postby Dirk_Hendrik » 24 Dec 2018, 09:13

Yes, that bulb should be flashing.

That 13.8 volts, that's 13.8VAC I suppose? Being exact in the explanation of what you measure is key here. What is the DC voltage on that large black capacitor?
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Re: Univibe repair advice needed

Postby idy » 24 Dec 2018, 17:35

Yes, a CD player or such (looper pedal) is a good signal source. The signal tracer cable doesn't go directly to a speaker but rather to a small, not-to-valuable amp then to the speaker.

The other "simple" way to get started is to measure voltage at all the pins of all the transistors....

No signal and no flashy on the bulb may be two separate problems. Like the man said, old electrolytics could be doing both. Trace the signal (when you do this you will end up knowing the transistors by number) and check the voltages.
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Re: Univibe repair advice needed

Postby Isittooloudoutthere » 25 Dec 2018, 09:07

Thanks guys!

It is 13.8VAC yes. I just checked the DC voltage at the cap, and: nothing... zero. So I checked DC at the diode pin towards the circuit - zero. This could be the explanation that the bulb isn't flashing too. But a diode, failing? Very rarely I think... But it obviously had been replaced or taken out once for whatever reason and soldered back in not quite well, as you can see on the pics. It is an OF173, so early seventies... Well, could that be the solution? And which would be a good and available replacement type?

Thanks in advance!
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Re: Univibe repair advice needed

Postby idy » 25 Dec 2018, 17:02

Diodes burn out if they carry too much current. So there could be a problem "down stream" causing the circuit to demand too much from the power supply. A short circuit or something.
The diode just chops the AC wave in half, what they call a half wave rectifier, very crude way to get choppy DC.
The diode has burned out twice in the past... there is probably something making this happen.
Others will chime in but you probably could use a "4000" series diode, 1n4001, 1n4002 etc. Higher number means more power handling. Maybe test current (amps) as you power up again... This is a "series" measurement where you would leave the new diode half connected and use the meter to complete the connection. This would tell how much power the circuit is drawing.
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Re: Univibe repair advice needed

Postby phase396 » 26 Dec 2018, 01:00

As a sidenote, I wonder if that torroidal transformer is original. Never seen one on a piece of gear that old. I could be wrong of course.
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Re: Univibe repair advice needed

Postby idy » 26 Dec 2018, 03:31

You're new enough here that somebody has to say: be careful with that power transformer and the AC!

You will get some people who have done this exact repair and will tell you what to be careful about when powering this up again. I've built clones but never messed with an original unit with an issue with the AC...

Something happened maybe twice to cook a diode...there are various strategies for either powering up slowly with variable voltage, or putting in a series resistor or... maybe testing for shorts before you replace the diode and power up. Just saying...
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Re: Univibe repair advice needed

Postby Dirk_Hendrik » 26 Dec 2018, 13:20

Isittooloudoutthere wrote:And which would be a good and available replacement type?

Any rectifier diode. A 1N4001 is quite common but any of it's higher numbered brothers will do just as well. 1N4002, 1N4003 etc
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Re: Univibe repair advice needed

Postby ppluis0 » 26 Dec 2018, 19:10

Isittooloudoutthere wrote:It is 13.8VAC yes. I just checked the DC voltage at the cap, and: nothing... zero.


Disconnect the supply cord and check (with your multimeter set to ohms) if the main electrolytic condenser becoming short circuited.

If you read a low resistance value, lift one terminal of the mentioned condenser and confirm if the short is in that component or the low value still can be measured at the board.

Cheers,
Jose
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Re: Univibe repair advice needed

Postby Isittooloudoutthere » 26 Dec 2018, 20:16

Don't worry, although I don't understand much theory, I know how to deal with 230VAC and more. I worked a lot in my '65 JTM45 and also in other old PTP Marshalls, Fenders and a Hiwatt.

Idy, I understand. It's unlikely that the diode is the only problem and there might be a short on the board. This might have happened before, although I have it on my board now for years and never had a problem with it...

Jose, I checked the resistance of all 3 electrolytic caps onboard and it keeps increasing, I don't get a stable value. So I guess the short is not here... Is that right? Or shall I carry on with what you said?

How shall I carry on now? I don't have a variac. And I don't know how to look out for further shorts... How do you do that? :slap:
What about putting in a new diode in series with a resistor - how many ohms? - and a meter and measure the current?

Thank you all for your support, I really appreciate that.
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Re: Univibe repair advice needed

Postby idy » 27 Dec 2018, 04:00

The resistor in series is a cautious thing. Maybe 100ohms? Just something in case. It will drop some voltage, your circuit won't get all the juice it should. The power supply voltage seems a bit low for a univibe, and yes that transformer looks modern... But you say it was working, so it's big enough.

When you check a capacitor with an ohm meter you first see a low resistance because you are charging it up, it is sucking up current. Then it is charged and should show "overlimit," a "1" or whatever your meter shows.

At some point you have to "cautiously take the plunge" by powering it up again to see what happens. Replacing the electrolytics first is something some folks would do automatically. Others call that "shotgunning." Replace everything, you might fix the problem. And antique elctros really do explode.

Power disconnected, what kind of resistance do you see across the power supply? What about the ohms on the other side of the diode, what kind of "load" is the circuit providing?
Something to check before powering up.
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Re: Univibe repair advice needed

Postby Dirk_Hendrik » 27 Dec 2018, 09:47

Isittooloudoutthere wrote:
Idy, I understand. It's unlikely that the diode is the only problem and there might be a short on the board. This might have happened before, although I have it on my board now for years and never had a problem with it...


Desolder the old diode and replace with a fresh one.

Scenario 1:
All works again and you're done, instead of rambling on for days here about the possibility of another defect that caused this one. Yes' defects cause other defects but just as yes, components can malfunction. Ge-Diodes as rectifiers do fall into that category.

Scenario2:
It still fails. Let's move on from there if needed.

Now, solder in that replacement diode.
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Re: Univibe repair advice needed

Postby ppluis0 » 28 Dec 2018, 02:51

Dirk_Hendrik wrote:Desolder the old diode and replace with a fresh one.


Hi Dirk,

After re-read the entire post I realize that the main rectifier diode was replaced by someone with a tiny germanium not able to deal with the current required by the entire board.

I'm waiting to read what happen after the substitution with a 1N400X as you suggested.

Cheers,
Jose
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Re: Univibe repair advice needed

Postby Isittooloudoutthere » 28 Dec 2018, 08:34

Ok, I'll get some new caps today and diodes and put them in on the weekend. That can't be wrong, although they have no bulbs... And an old friend of mine who works for TI will visit me tomorrow and teach me some basics about signal tracing and looking out for shorts. I'll do some measurements with him. I'll let you know what happens. Thanks very much guys!
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Re: Univibe repair advice needed

Postby Isittooloudoutthere » 28 Dec 2018, 16:46

Yes! It's alive again. :D :D :D
With a new diode and caps. I put in 100uf caps instead of the 220uf that had been inside.
The RG article said 100uf, so I guess this is ok... The 220uf weren't available today.
I have 17,5VDC after the diode.
Thank you again. You've been a great help. [smilie=rlp_smilie_242.gif]

Now some further thoughts.

It definitely sounds like it should again. But I noticed that the bulb is flashing really just very lightly. And as you see in the pics, it leans pretty much towards only two of the photocells. I put it in the middle and back again and didn't notice any significant difference with a low volume amp here at home. What are your experiences with different positions and light intensity of the flashing bulb? I'm just curious...

And did anybody check out the increase of Unity Gain mod in RG's article to avoid the slight signal drop?
Maybe somebody can chime in tell his opinion about that. Makes sense to me...

Have a great weekend y'all and a happy new year!
8)
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