Forumvibe bulb not lighting

[Spring2019]

Hi guys.
Building a forumvibe with a kit supplied by Musikding.de

I am getting no light at all from the bulb when pedal is engaged.
I have the DPDT switch wired up in the cancel mode.
See doc here: https://www.classicamplification.net/fv/ForumVibe_FV-5rev3.pdf

I am attaching a couple of images.
One shows the build and the other shows the transistor voltages.

Any ideas re where to start troubleshooting this would be much appreciated.

Thanks a lot
Paul

[antonis]

That's quite normal..
(in the mean of there is only 600mV ( 18.4 - 17.8 ) drop across the bulb..)

As far as I can see, Q12 Emitter is biased at about mid-supply(*) but Q11 is out of correct bias..
(its Base stands lower than its Emitter..)

(*)In case of Q11 Base & Emitter voltages was flipped, Q12 should be more happy..

P.S.
I guess you didn't use C-B-E pinout BJTs 'cause all are 180^o flipped..

[Spring2019]

Hi Antonis and thanks for the reply.

For Q11 I need to take a closer look and see why base is lower than the emitter.

Re the transistors.
There are 13 in total.
Q1 is a 2N5210
Q2-Q13 are all 2N5088 so they are indeed all NPN CBE transistors.

When you say they are all 180deg flipped, what do you mean exactly?


Any other thoughts about how I can troubleshoot this?

Many thanks again
Paul

[idy]

It means your transistors are "backwards" from the pictures that come with the musikding docs. The flat and round faces are reversed.
 The docs specify BC549, and they read CBE. 2N5088s read EBC, so you should be OK on that.

[R.G.]

When you say they are all 180deg flipped, what do you mean exactly?

I'm not saying this to be critical. The underlying meaning you should get from this is two things:
1. Never, ever, assume that a non-original type number has the same pinout (that is, function per pin) as the originally specified type.
>and<
2. Get in the habit of looking up the manufacturer's datasheet for any substituted parts.

The world is full of almost-exact replacements. Make certain. It saves a huge amount of debugging, as you are finding out right now.

[Spring2019]

Hi again guys and once again very many thanks for all the info and advice.

Ok so I double checked the data sheet for the transistors and it looks like I have the transistors facing the right way. See image.

So I guess the next step is to find out why the emitter for Q11 is reading higher than the base.
I will check the values of the resistors feeding those two points



Any other ideas/suggestions ?

Thanks again
Paul

[antonis]

So I guess the next step is to find out why the emitter for Q11 is reading higher than the base.
I will check the values of the resistors feeding those two points.
Any other ideas/suggestions ?

Let's have a relevant circuitry brute analysis:
(although I strongly suggest R.G.'s one..!!)

Q13 is considered bulb driver and is biased to be slightly at cut-off region (little current flowing through bulb, set by TR1 & R49)..
Q11 & Q12 form a Darlington voltage follower with its output (Q12 Emitter) swinging according to LFO's (almost everything down-left Q11/Q12) frequency..
For this follower to work properly, Q11 Base needs to stand about 600mV higher than its Emitter/Q12 Base and Q12 Emitter about 600mV lower than its Base..)
Any positive waveform coming out of Q12 Emitter goes onto Q13 Base and makes it work "harder"..
(its Collector voltage lowers resulting into more voltage difference across bulb, hence more current through the bulb -> LIGHT..)

Now, if you're sure about Q11 correct pin-out, check for any short (or discontinuity) in the relevant area..
e.g. no need for R38/R39 proper values/ratio 'cause, despite their junction voltage, Q11 Base should stand about 600mV higher than its Emitter/Q12 Base..)

[Spring2019]

Hi again guys.
So I just pulled Q11.
It tested fine with pinout as expected of EBC and no faults reported.
I resoldered it back to board but still same voltages present.

Will look for shorts next.
Thanks again guys
Paul

[Spring2019]

Hmm don't see any shorts

[R.G.]

Next lower layer. It is often the case that meters load the voltage they are trying to measure if the point they are measuring is high impedance.

The base of Q11 is very high impedance, so your meter could be giving you bad numbers. Instead of the base of Q11, measure the junction of the biasing string for Q11. This would be the junction of the 3.3k, 4.7K and 2.2M resistors. If that point is more than 11.5V or so, you could assume that Q11 is working properly, as the emitter of Q11 and Q12 seem to be roughly correct.

If that tests out, I would next look at whether the lamp is bad by momentarily paralleling Q13 with a 100 ohm resistor. This ought to make the lamp blink on a little at least.
If it does, I would momentarily bridge the base of Q13 to the 18V power supply with a 22K or so resistor. This ought to blink the bulb. Don't hold this on too long, just a blip.
If the bulb blinks, there may be a problem with values or faults in the resistor that connect to the base of Q13 or the pot in the emitter of Q13.

[Spring2019]

Hi RG.
Thanks for the multiple suggestions for plans of attack.

I'm stopped at the first one as I still read low. It's reading 10.85v at that junction.
I replaced Q11 with another transistor but the reading is the same, still 10.85

So if it is not the Q11 transistor itself, any ideas which components to test next ?

Many thanks again
Paul

[RedHouse]

Spring2019, please follow RG's advice and questions, he knows his way around a uni-vibe circuit. At this point in all our lives I would say you can consider him a Sensei in this area 

Im sorry I've been away from the forums for some years now but I wanted to drop in on this thread and see how you we're doing.

May I ask if those big power supply caps came with your musikding.de kit?

Anyway, from the pictures I can't make out the marking on the transistors. As RG said not all transistors follow the same/correct pinout so that was the first thing I was interested in. I tired to download the pictures and open them in Photoshop to brighten the dark faces of the transistors, but PS can only do so much (does your camera or phone have macro settings?) RG's suspition of orientation is well on the mark.

One thing I might add to the mix is, (all KUDOS to Bob Sweet ((RIP)) if you remove R45 (4K7) and add in a temporary (flying-wire style circuit) of a 4K7 resistor and LED in series (LED - to ground) it can be very informative because you will immediately see if the non-functionality is in the LFO or the Bulb driver circuit. If the LED is flashing at speed but not the lamp/bulb that shimes on the photocells your problem lies in the lamp driver circuit (Q13) if the LED is not flashing, at speed, your problem lies in the LFO area.

Thank you RG for walking him through this, ever patient and helpful to the masses.

To close, it's a known good build (22 year old PCB project) so it just has to be a component malfunction, part insertion issue, or soldering error.

[Spring2019]

Hi RedHouse.
Thanks a lot for the reply.

The capacitor you mention was not part of the kit.
They sent two 470's instead of two 220's so I temporarily subbed the only 220 I had. Waiting for a replacement to arrive.

As previously mentioned. The transistors are all the same brand CEN and the one I checked (Q11) was indeed an EBC as expected. So i was assuming the rest should be the same.

Thanks for the great suggestion re the resistor and LED in series.
I will make that my next check tomorrow.

Many Thanks again.
Paul

[R.G.]

I'm stopped at the first one as I still read low. It's reading 10.85v at that junction.

Let's stay with this for a minute. I was simply ignoring the 2.2M resistor as too big to matter. Maybe that's true, maybe not. To clarify things, unsolder one end of the 2.2M resistor to Q11's base. Now the voltage at the junction of the 3.3K and 4.7K should be 4.7K/(3.3k + 4.7K) times the power supply voltage.

If the 3.3K and 4.7K are exact, this would be a voltage divider of 0.5875 and if the power supply was exactly 18.4 volts, the junction of the two resistors should be 18.0 * 0.5875 = 10.575V. You're seeing 10.81, so allowing for resistor tolerances, I'd say it's fine. Given that the 2.2M resistor was still in the circuit for your measurements, you probably don't have to remove it for another measurement. I'd say the resistors are OK.

What should be happening is that the voltage you can't easily measure on Q11's base should show up on Q11's emitter pin, minus one Vbe. Vbe is itself something between 0.45 and 0.6Vfor a low current transistor setup like this, and Q12's base emitter should do the same thing. So the expected voltage on Q12 emitter ought to be a bit over 9V or so, +/- some of the "ill defined" - and it is. I think Q11 and Q12 have some chance of being fine.

If the emitter of Q12 is not swinging wildly up and down and you do measure a static DC voltage, then the oscillator has not started. This is a common problem in univibe clones, and results from some failure of the resistors and capacitors in the timing circuit to provide enough signal back to Q11 base, or some failure of Q11/Q12 to have enough gain.

Oscillation issues are harder to debug without an oscilloscope - you have one of those?

I would hook up my meter from ground to Q12 emitter and leave it while I watched. If you then short any one of the three 1uF caps in the feedback network, you should see Q12 emitter wobble up and down a time or two even with low feedback. If you don't, check the 1uF caps and resistors/pots as well as the two diodes.

This doesn't deal with the bulb driver yet. The temporary short to kick the bulb driver is independent of the oscillator stuff, and  can be tried at any time.

[RedHouse]

Here is a diagram of how to use an LED for troubleshooting the LFO and Lamp Driver areas...
(I will add this page to the Forum-Vibe rev3 doc when I get some time)





Also, (unrelated to your current problem) if you are using a regulated DC wall wart or power unit, you dont have to use the 470uF/220uF/220uF caps with the 470 Ohm resitors, that was just for the AC powered 1/2 wave rectified original uni-vibes.

If your power unit is not noisy, you dont need those big caps, you can use 100uF/100uF/100uF and 220 Ohm in the power section.

(edit's to correct bad spelling!)

[antonis]

I'd check for R49 proper value AND connections..

It shouldn't make any harm to reflow its pin going to TR1..