Univibe repair advice

[soggybag]

After some fiddling and swapping of parts I'm getting the following voltages:

Darlington
E 11.5v
B 12v
C 22.4v

Lamp driver
E 4.5v
B 5.2v
C 22v

I tried a new lamp from RS, Which is rated 12v 60ma. Looks like I still have the same problem as before. If I were to swap both transistors in the darlington pair, would it be good to replace them both with 2n5088 types?

[soggybag]

Can anyone point me to some typical voltages for the transistors in the univibe?

[Eb7+9]

SOG,

if you were getting a pulse or two at power-on then almost certainly your Darligton pair and bulb driver had nothing wrong with them - rather what this describes is a critically overdamped loop (lack of phase shift and/or device current gain) ...

the emitter voltage on the second Darligton device is the one to measure if you want to know if the LFO is at least biased right - that voltage alone will indirectly give you bias check on the whole LFO stage ... Ve = 11.5v sounds fine to me - this says biasing resistors and device current gain are all likely ok ... my guess then would be that one of the three identical caps in LFO went bad or one of the limiter diodes across the middle cap is shorted ...

I use 1.5v/25mA bulbs in my vibes but the original ones probably used 6v/25mA, 12v/25mA or similar - that single bulb driver transistor can't dissipate too much heat on its own so try finding a bulb with a light-up current that's not too high ...

good luck ...

~JC

[R.G.]

rather what this describes is a critically overdamped loop (lack of phase shift and/or device current gain)

I believe that's what I said earlier - the darlington's gain is not high enough if it only runs a cycle or two on power up.

The condition is not critically overdamped - it's under-gained. There is not enough gain in the forward path to make up the feedback loop losses, so the oscillation resulting from the power-up transient dies out.

[soggybag]

Thanks for all the help, I really appreciate it.

I replaced all three of the 1u caps with new 1u 25v caps. I also replaced the two diodes in the LFO section with new 1n4148 types. I took out the two LFO transistors and ran some jumpers to my bredboard. I put two 2n5088 type transistors int eh breadboard for testing.

I still get about the same voltages for the darlington pair.

E 11.7v -- at the 10u cap
B 10.8v
C 22.8v

I notice that reading the EB connection between the two I read 7 to 20v that cycles for a few minutes before settling in to 20v.

No matter what I always read about .3v across the lamp (22.3v on one side and 22.6v on the other).

Using the replacement lamp, 12v 60ma, it never lights. But the original lamp will give a dim orange glow.

[Eb7+9]

No matter what I always read about .3v across the lamp (22.3v on one side and 22.6v on the other).

Using the replacement lamp, 12v 60ma, it never lights. But the original lamp will give a dim orange glow.

dim orange glow at idle is what you want - 60mA bulb in my experience is way too much ... 

I guess the next thing to look at is the bridging between the LFO output (at second emitter of Darlington pair) and the input of the bulb driver circuit ... have you tried bridging those two circuits directly with one new cap instead of going though the intensity control and the two caps tied around it ?? Intensity pot or electro caps feeding it could be screwed - make sure these haven't been replaced with overly large values, they can drag the LFO down too ... some people try that to get more output at low-LFO speeds but you quickly hit diminishing returns ...

In my circuit I Darligton the bulb driver device - I have found that if the bulb driver loads the LFO too much it will stop working, that's why there's a 47k in series with the Base of the bulb driver device ... see with a scope if your LFO works with that feed disconnected

btw - does it look like any resistors have been replaced anywhere ?? ... if not I would try measuring them ... the 2.2Meg resistor in series with the Base of the Darlington pair in the LFO is crucial here ... also, the bias resistor on the Darlington emitter should be 4.7k and not 47k like it appears in the hacked schematic ...

some ideas ...

[R.G.]

Using the replacement lamp, 12v 60ma, it never lights. But the original lamp will give a dim orange glow.

That's not unexpected. The 60ma rated bulb needs more current than the circuit provides. The 25ma bulbs can work on the low currents available in the circuit.

I notice that reading the EB connection between the two I read 7 to 20v that cycles for a few minutes before settling in to 20v.

When you first turn on power, everything is given a sudden jolt as power comes up. The LFO is designed to oscillate all the time, so the jolt sets it moving. But something is keeping it from providing enough feedback into itself to keep that oscillation going. That's why it settles out.

Intensity pot or electro caps feeding it could be screwed -

That's a good point. To test that, pull OUT the capacitor which feeds the hot lead of the depth control, and then see if the second darlington emitter keeps wiggling and doesn't die out. If so, it's being too loaded down by the cap/depth pot/cap/driver stuff and the problem may be in the feed to through the depth pot. I've never seen that happen in quite a number of univibe repairs, but it is possible.

the bias resistor on the Darlington emitter should be 4.7k and not 47k like it appears in the hacked schematic ...

That's one reason I had him look at the DC voltages first. If the bottom resistor was 47K, then he would not be getting about mid-power supply like he is. The darlington would be pulled up to near the power supply at both bases and both emitters.

The schematic in my "neovibe" project probably *is* easier to interpret than the hand-written values in the copies of the original schematics on the net. It's been tested - a lot.

[soggybag]

Thanks for the ideas. I'll take a look and see what I can find.

It seems the 60ma bulb may be a problem. The Neo Vibe and JC's site talk about a 12v 25ma bulb from RS. I looked but couldn't find this one. I'll have to look for it again at a different store.

[Eb7+9]

the bias resistor on the Darlington emitter should be 4.7k and not 47k like it appears in the hacked schematic ...

That's one reason I had him look at the DC voltages first. If the bottom resistor was 47K, then he would not be getting about mid-power supply like he is. The darlington would be pulled up to near the power supply at both bases and both emitters.

if you do the analysis of that stage you'll find the DC voltage at the emitter of the second Darlington is set by the Base voltage (set by the 4k7/3k3 divider) minus the two Darlington diode drops below it (Vbe only varies with the Log of current) ... I specified checking the emitter resistor precisely because the mid-rail bias does NOT change significantly in that case, like I said it just confirms the string of voltage sums holds ... with 47k in the emitter circuit you get roughly ten times less current going through the second device as with 4k7 but with less than 0.1v Vbe difference total in that device (in the first device it's even less) and therefore no more change in the emitter voltage ... more important the output drive of the LFO is significantly lower with the 47k emitter bias resistor - this is mistake some people make and it should be pointed out just in case ...

the bulbs that I use in my circuit are 1.5v/25mA to take advantage of the faster cells I use, coupled with a lower power bulb having a quicker cooling down period gives me a slightly extended fast range ... I would say you want something around 6v/25mA - I think 12v/25mA for a stock Vibe might be a little too strong for that bulb driver (theoretically that's 1/3 watt at full brightness) ... don't forget to check the bulb driver device doesn't get too hot when running full on after insering new bulb and doing bias adjustment ...

g'luck

[R.G.]

if you do the analysis of that stage you'll find the DC voltage at the emitter of the second Darlington is set by the Base voltage (set by the 4k7/3k3 divider) minus the two Darlington diode drops below it (Vbe only varies with the Log of current) ...

Down, boy, down.

As you well know, I did the analysis of that circuit years ago. I missed the word "emitter" because my copy of the original was bad at the base divider side. I'm well aware that the base voltage sets things up for the emitter voltage - I teach people that all the time. I had mentally eliminated that someone would have a 47K in either the base divider or the emitter because he's working on a factory built univibe, and it would never have made it out of the factory that way. Yes, a repair person might have put in the wrong one, but agai, it would never have worked after that, so the story should have been different.

I believe that what you're trying to say about the bulbs is that
(a) the heat up/cool down time sets the fastest speed that the bulb can respond in turning on/off
(b) the mass of the bulb filament controls the heat up/cool down time for the bulb
(c) the mass of the bulb filament is set by the total power rating (voltage times currrent) of the bulb

If so, yes, you're right.

I put a set of pads for a TO-220 device in my Neovibe layout just so someone could put in a bigger power transistor for the lamp driver, as the TO-92 package is marginal in this application. The stretched TO-92 uniwatt packages that Zetex and some Japanese devices come in is better than a stock TO-92, but a TO-220 is even better. But most people get away with the stock package.

[soggybag]

I found a 12v 25ma bulb at RS, of course they only hhad 1 at the shop I went to. I'll have to try and get a spare at another shop. Later today when I have some time I'll try out some of the suggestions.

[soggybag]

Thanks for all of the help. My friend decided to take his Univibe to a professional. I think, with the help of the forum (of course), I may have fixed it. My friend is heading down to LA and knows a guy down there that will fix it for $150 (I was surprised at the price).

[parser]

Hello,
my univibe is working perfectly with LFO speed and depth (intensity of the lamp), unfortunately it doesn't vibe. Any advice where to start debugging ?

[duck_arse]

https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

go there, follow instructions, start a new thread. also post photos and the circuit you are using, or no-one will look.

[Eb7+9]

my univibe is working perfectly with LFO speed and depth (intensity of the lamp), unfortunately it doesn't vibe. Any advice where to start debugging ?

... no signal from wet side signal path ?
 
first - check the vib/chorus switch for continuity

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quick DC test on the wet side ...

each photo-cell has one side DC connected to Darlington emitters thru a 4k7 resistor
grab your meter and see if you can see this DC on all four photo-cells (just cuz those are easy to find)
this will tell you if any of the stages aren't biased (ie., a dead stage)

---

otherwise, you could have a signal break elsewhere along the wet side path ...
check all soldering with good eye piece


btw, have you tried playing the unit while randomly pressing down on parts to see if
you can make the effect kick in?

[parser]

https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

go there, follow instructions, start a new thread. also post photos and the circuit you are using, or no-one will look.

Thank-you for redirecting me to the link above, I'll go through the check-list.

[parser]

Thank-you Eb7+9, I'll follow your advices. As Duck Arse wrote, I'll go through the check-list (I read a good advice about checking the voltage +/- on electrolithics capacitors) and eventually start a new specific post.

p.s.: By the way, my project is NeoVibe from Geofex (http://www.geofex.com/Article_Folders/univibe/vibeupdate.pdf), it worked perfectly the first time as was built, then stopped to vibe (signal passes through the circuit (volume pot works, for instance) but it doesen't vibe).   

[parser]

I'm rebuilding from scratch and I was wondering if using in Neovibe:
- some 1/2 W resistors instead of the reccomended 1/4 W may create issues
- a smaller value of capacitance (i.e measured 8,4 uF instead of 10 uF) may affect biasing of the unit
- a lamp bulb of 12 V 40mA (or higher amperage) instead of the specified 25mA affects the performance of the unit 
Thankyou

[idy]

Going for higher wattage R is always OK, if the physically larger resistor will fit your board.

Electrolytic Cs are high tolerance, we expect them to measure out of spec. And designers know that. 16% out is not unusual. I think the cap you are interested in feeds the LFO signal to the lamp driver. A cap that is too small might cause the signal to fade. You could use a laser one if you want. Nothing to do with bias.

Bulbs do affect these things. I would be be prepared to either use a known good value or experiment. Higher amperage would make the LFO driver work harder. What is that transistor rated for? Data sheet...

[Eb7+9]

- a lamp bulb of 12 V 40mA (or higher amperage) instead of the specified 25mA affects the performance of the unit 

that's way too high power for the vibe bulb driver
the whole filament cool-down thing is complete bs as the vibe won't run "deeply" at high speeds

I use 1.5v/25mA bulbs in my vibes


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here's a vibe rebuild/de-laming and zero-hum Twin restoration job I did not too long ago