T-Rex Viper LFO stopped working

Forum dedicated to helping people debug and troubleshoot non-functional pedals or builds. Please use an clear and informative title, indicating circuit and basic problem. Don't forget to mark the issue as fixed if this is the case.
Post Reply
User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Hi guys,

I'm working on a defective T-Rex Viper I'm trying to fix for a friend. Apparently it has worked for years but started to loose modulation depth, I was told. When I got the pedal I checked the bulb and saw that with the internal trimmer it could be set to a steady glow. I've traced the circuit around the LFO and found it is pretty similar to the common Univibe LFO circuit (see attached). Unfortunately all the parts are SMD so it's hard to tell which transistors are actually used (for the simulation I used 2n3904 types).

I used a signal generator and fed a 2V sinewave into the the depth pot wiper. Using this external sinewave source the bulb flashed with the set frequency. Therefore I'm pretty sure the bulb driver is working fine.
Checking with the oscilloscope I couldn't find any oscillation going on around the LFO circuitry. However, when I unplug the power adaptor and plug it back into the pedal (12VDC, internally pumped up to 24VDC) the bulb would sometimes start flashing for a few seconds and then fade to static.
I checked the capacitors and pots with a meter and they all read reasonable values. However, I didn't unsolder them before testing.

What do you think could be the reason for the issue? I suspect the transistors to be defective somehow. Could it be they "lost gain" for some reason? This could explain why the LFO stops even after it starting oscillating first. I've tried to test the transistors with one of those chinese component-testers but didn't want to desolder them, as I'm not yet used to soldering SMD too much. In the circuit they were detected as PNP transistors with a diode across the E and C terminals. But I'm pretty sure they are NPN. I've simulated the circuit in Spice and using PNP transistors, the LFO wouldn't work. However, I don't know if the component tester is bad or if it's just not possible to check the parts while still in the circuit.

Maybe you guys have some more ideas, what to test? I could of course just try to replace the LFO darlington pair with some highish gain NPNs, but as I don't have SMD parts available I would be sure about the actual issue before ordering new parts.

Thanks a lot!

EDIT: I had a few minutes and tried something I read over at DIYstompboxes, suggested by R.G.. I shunt the base of the darlington pair to ground and released it again. This resulted in a short oscillation which declined again after few cycles. So I guess the transistors are just not "strong" enough to keep the oscillation going...?
Attachments
Screenshot 2021-06-23 at 13.27.33.png

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Quick update: I replaced the LFO transistor pair with two 2N5089 types. Unfortunately while desoldering two of the pads fell off. Well, SMD...
However, the 2N5089 each read an hfe of ~850 so I thought they might do the trick. But unfortunately they didn't. I can get the oscillator to start for 2 - 3 cycles but then it declines. Any more suggestions at this point?

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

Are C1, C2 and C3 elektrolytes?

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Thanks for your interest Manfred! Yes, they are electrolytics. Sorry for not using the right symbol here. Essentially I drew up that schematic just to do a test simulation. Do you think the problem could be related to those?

I was wondering about the electrolytics coupling the LFO to the driving stage. They're both 10uF 16V types, but the signal swing of the LFO might be higher than the 16V at some settings, no? It is running from 24V so I guess there's potential to go higher than the 16V. At least the cap will see pretty much it's full rated voltage most of the time. I might desolder lift C4 in the above schematic to see if it helps getting the LFO to run properly. However, I don't want to desolder too much, as these pads come off really quick, so are there any tests I could do to the cap while it's still in circuit?

Edit: According to an LTSpice simulation using models of 2N5089s or 2N3904s for the darlington pair, the output waveform (tapped at the depth-pot wiper) can reach around 14Vp-p at high speeds. This is already pretty close to the rated 16V. Upon power up I can imagine that the LFO could start with a higher swing when it's triggered and by this exceed the maximum rating of the electrolytic, potentially causing damage over time. Without the diodes the steady oscillator signal can even reach about 23V.

What do you think, does this sound more or less reasonable?

btw, if somebody is interested: Using FFT plot with LTSpice shows that the range of speeds goes from 0.9 Hz (@ ~14Vp-p) to about 5.8 Hz (@ 2.5Vp-p). Once I get the oscillator going again, I'll confirm this with the oscilloscope.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

For a phase shift oscillator to oscillate there are two conditions, the gain of the stage must be high enough and the phase shift of the RC network must be 360°.
I asked about the electrolytes because they can lose their capacitance value due to aging,
then it may be that the phase shift is no longer maintained and the oscillation stops.

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Hi again, thanks for your help.

I tried to check the capacitors in circuit with my component tester but this wasn't really successful. Then I unsoldered the coupling capacitor to the depth pot to isolate the LFO (those freakin' pads came off again...) and checked it with the oscilloscope. I could get the LFO to oscillate from about 9 o'clock to 2 o'clock on the Speed knob. At lower or higher settings the oscillation would fade out. I tried to add a parallel 0.47uF capacitor to each of the Cs in the LFO network, one at a time, which not really seemed to help. It bumped up the oscillation when the C was touching the pads (and slowed it down of course) but the oscillation would decline again afterwards.

With the described method I was able to get the LFO running from about 1.5 Hz to 3.5 Hz but anything higher or lower and the oscillation would decline. Those values are well within the range of simulated LFO so I guess, the parts values should be fine, no? If the capacitors would have lost their capacitance adding some capacitance should be helping I guess...? Otherwise, what about their ESR? Could this values have changed causing these problems? In a build with THT components I would simply change all the Cs in the LFO network but that's not that easy in this case here.

The Speed pot reads about 86k maximum on both systems when its fully CW.

Any more ideas/suggestions? I'm happy to carry out all tests necessary.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

If all values of the components for the phase shift are in order, the only remaining assumption is that the stage gain decreases and thus the oscillation stops.
A last crazy idea.
The 10uF coupling capacitor is charged to a certain DC potential after switching on.
The higher the DC voltage at the electrolytic capacitor, the higher the leakage current, which could have already been increased by aging and additionally flows through the transistor.
This could reduce the amount of positive feedback so that it is no longer large enough to maintain the oscillation.
I would be curious if the oscillator works without the coupling capacitor.

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Hi Manfred, thanks for your input!
Well, I have already disconnected the 10uF coupling capacitor, sorry if I wasn't clear in my description above. This didn't really help. All the description I wrote in the last message here apply to the case of a disconnected coupling capacitor.

Tonight I build the LFO circuit on a breadboard using new parts and powered it from the 24V coming from the actual pedal. I put an LED in series with the emitter-resistor of the Darlington pair to see if the oscillator would work. And yes, it does work also at low and high settings of the speed pot (which I replicated with fixed resistors here using values up to 100k). The oscillation at low frequencies isn't very strong but it never declines.
At least this tells me that the 2N5089 transistors (also tried 2N3904 and those worked as well) work well in that application and the issue must be somewhere else. I guess one of the electrolytics is bad but I really don't want to desolder them all and risk losing the pads again. It would be best to test them first while in the circuit, but I still wasn't successful doing this. Especially for C2 its difficult as my component tester will only detect the two diodes in parallel with it.

Any further suggestions are highly appreciated. Thanks!

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

I guess one of the electrolytics is bad but I really don't want to desolder them all and risk losing the pads again. It would be best to test them first while in the circuit, but I still wasn't successful doing this. Especially for C2 its difficult as my component tester will only detect the two diodes in parallel with it.
I agree with you, this also speaks for the fact that the oscillator was working before and has now failed.
The SMD components to unsolder is a difficult thing without the appropriate desoldering tool to have.
I had considered buying such a tool but it is much too expensive.

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Sorry for the delayed reply. I've unsoldered the two diodes to test C2 and it gives the same reading as C1, both read an ESR of about 0.14k at 1uF, does this sound about right? I couldn't get any reading on C3 so I went ahead and replaced it with a 470n foil cap but this didn't help at all. Oscillation is still only starting over a relatively narrow range of the speed pot.
I could replace C1 and C2 as well but I don't know if this would improve something. I'm starting to get really confused, haha. There's not much left that could be defective apart from a bunch of resistors and the two capacitors. By the way, the diodes test fine as well .

Any more suggestions?

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

First of all I need to understand how this circuit really works, I have not found a description anywhere.
Who knows more about this circuit?
It looks like a current-current positive feedback.
I am working on it.
Which transistor types are currently installed in the circuit the 2N3904 or the 2N5089?

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Hi Manfred, thanks for your continuous interest!

Myself I don't completely understand why this arrangement causes an oscillation, however, it seems it's the common design for the Univibe LFO so in theory it should work as it is. At the moment I'm using 2n5089 transistors for the darlington pair. They measured around 800 hFe each.

On the geofex sites you can find some more information about the technology of the Univibe: http://www.geofex.com/article_folders/u ... ivtech.htm

The circuit of the viper just differs by the two 220k resistors parallel to the Speed pot. But that's the factory circuit and it works on all the other units (and worked on this one before), so these shouldn't be causing the issues here, I guess.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

Thanks for the link, great!

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

The article on the linked page does not describe how the oscillator works.
I ran a simulation and looked at the circuit structure.
There I noticed a circuit that I have seen many years ago in my book precision electronics of Philips from 1972. R6+R8, R5+R7, C2 and C3 form this filter circuit structure with the difference that C2 and C3 have the same value.
This circuit has the special property that there is a small increase in the frequency response and in this range a slightly higher voltage than the input voltage.
This could be the explanation for how the oscillator starts to oscillate.
As you can see in the oscillogram the amplitude increases with each oscillation period, only when the voltage at C2 is limited by the diodes D1 and D2 the value of the oscillation amplitude remains stable.
At each amplitude after the start, the output voltage increases a little bit more due to the above mentioned increase in the amplitude response.
The oscillator frequency is the one at which the value of the phase response is zero degrees.
That the oscillation stops should be due to noticeable deviations from the component values,
so you need to look at the signal voltage at the junction of C1 and R1 depending on the setting of the potentiometer.
SpecialFilterCircuits.jpg
SpecialFilterCircuits.jpg (8.7 KiB) Viewed 1932 times
Frequency response using C = 100n and R = 100k:
SpecialFilterFrequencyResponses.jpg
T-Rex-OszillatorFilterCircuit.JPG
T-Rex-OszillatorFilterCircuit.JPG (13.37 KiB) Viewed 1932 times
Amplitude responses and phase responses for the circuit above.
Potentiometer set to 0%:
Responses@PotSetting0%.png
Potentiometer set to 50%:
Responses@PotSetting50%.png
Potentiometer set to 100%:
Responses@PotSetting100%.png
Start oszillation:
AC-Voltages T-Rex Oscillator.jpg

User avatar
iefes
Information
Posts: 18
Joined: 24 Nov 2013, 12:35
Has thanked: 1 time
Been thanked: 19 times

Post by iefes »

Hi Manfred,

wow, you've really put some effort into this problem. Thanks a lot!

I guess you're right and there should be a component whose value significantly deviates from the original value. I have changed the transistors and C3 already, tested C1 and C2 (in circuit though) and they tested fine. I also tested the diodes which were fine. The Speed-Pot reads right values as well (in circuit). I might go around and measure the voltages around the transistors and scope the signal on the base of the upper transistor. Maybe the electrolytics are bad even though they read fine in the circuit, might be one of them doesn't behave as it should when there's some voltage applied. I have no better explanation and don't know what else to check. I mean, one of the resistors could have gone bad, but this should show up when checking the voltages.

I'll try and report back, thanks!

Post Reply