Dunlop TS-1 Tremolo question

[Helmholtz]

I have a thermal cam and the only thing throwing any heat whatsoever is Q1 and that's all of 89 degrees. My battery voltage under no load is 8.75v, when plugged in it reads 8.5V, and on V+ of Q1 it now reads 7.65v. Using my bench supply I get a hard 9v into Q1, but emitter V+ is now 5V.

Q1 shouldn't get warm at all when powered from a battery or a 9V supply.
Is that 89°C?

Check all regulator components.

[stevenrb718]

Q1 shouldn't get warm at all when powered from a battery or a 9V supply.
Is that 89°C?

Check all regulator components.

Sorry, nope, °F lol...so basically nothing. I think I'm going to inject around 8V into V+ and see of the pedal functions properly after checking through the components.

[stevenrb718]

Can't seem to find anything obvious. Injected 8v into the V+ rail and the pedal didn't function any differently. I just don't understand how I can pull the LDR legs and the effect still functions. Does the optocoupler ONLY control the intensity, and it's just a bad opto?

[Mick Bailey]

Maybe remove the vactrol and test it out of circuit. It would also be interesting to see how the pedal operates with it completely removed. You can also test them in circuit, but the results can be skewed with parallel resistance or other semiconductors. After checking the LED, check the dark resistance on the LDR side, and then energise the LED with a 9v supply and 1.5k series resistor and see if the LDR resistance drops.

Maybe it's also worth contacting Dunlop for an official schematic.

Edit; Vactrols usually fail high resistance. With most pedals this would mean loss of modulation, but I'm unclear about the pedal's operation from the schematic. Their placement with regard to the analog switches (and the operation of those switches from their control voltages) doesn't make sense to me, unless I'm missing something.

[stevenrb718]

Of course when pulling the opto out completely, the unit functions exactly as it did, cuz duh (wtf). Having an LDR for the intensity only but other settings being IC-controlled just boggles my mind. So when checking the resistor of the cell when the LED is not powered, I get open lines no matter the configuration. When the LED is powered up i do get resistance readings. I guess I'll drop the coupler back in and test the resistances or use a different resistor on the LED to see if I get different readings.

[stevenrb718]

Ok. So I connected the LED of the coupler back up and measured resistances of the cell. When the intensity knob is turned down to off I do get various resistances, usually around 1M or upper 2M depending which legs. As I turn the knob up the resistances get larger and larger quickly, until going OL on all of the legs at MAYBE a quarter of the pot sweep. Does this mean anything at all?

[Mick Bailey]

You say that when the LED is powered you get resistance readings, but don't say what these are. The dark/light resistance readings should correspond to the spec sheet for the device - taking a look at the link you provided there are two device types listed. The dark resistance Should be 1M ohm or higher (depending on type) and the light resistance lower than 55k ohms (or lower - depending on type). Is there a part number on the device so you can check exactly what the readings should be?

[stevenrb718]

The resistances are never really steady enough for solid readings, but like I mentioned, with intensity set to 0 the resistances are already pushing 1M or more. By 1/5th of a turn they're upwards of 30M and by about 1/4 they're all reading open lines. I'm going to try to cut open the coupler and see if the LED is intact, then I'll more than likely look to replace the entire 3/2 coupler.

[Mick Bailey]

Checking the operation of the Vactrol under operating conditions is difficult because the resistance changes faster than you DMM can register, also parallel resistance or other circuit components can skew things. You need to check it under static conditions. It is possible to energize the LED in-circuit and measure the resistance change from dark to light, but you can't be guaranteed that the results will match with the spec sheet even if the Vactrol is good. I've never had to cut one open to see if the LED is working - a normal forward/reverse bias diode test is all that's needed and then check the LDR side. Forward voltage should read around 2v.

I have a set of clip leads I often use with a pair of DMMs in MESA amp switching circuits. One DMM is set to diode mode on the LED side, and the other set to resistance on the LDR side. The meter voltage under diode test illuminates the LED. This setup works in-circuit with MESA amps, but I don't know if the same method of testing will work in your pedal. Outside of that I just wire a 1.5k resistor in series with a 9v battery for out of circuit testing.

[stevenrb718]

With the coupler out of circuit and directly connected to resistor/9v, I get readings of 1-2: 1.3K ohm, 2-3: 1.6k ohm, and 1-3: 2.5k ohm. All 3 are OLs with the LED dark.

[Mick Bailey]

Those readings are OK. 1-2 and 2-3 should add up to 1-3, but that's a minor point. I think the problem is elsewhere. I'm away for a week after today but will give it some further thought. One thing that's bugging me is as well as being a trem, the unit pans between stereo outputs. I wonder if the maximum depth you're hearing is the unit panning to the 'other' channel even though its in mono mode. Does the switch have any effect? Maybe try running the thing up in stereo to check operation.

[stevenrb718]

I tried that already. The sound is affected with the mono/stereo switching, but I can't tell specifically what it does as I'm still outputting to one amp. It either slows down or has a wider sweep. Either way, the intensity doesn't adjust in any mode.

[Mick Bailey]

I've stared at this circuit and can't make sense of it from the schematic. V+2 would have to be modulated in order to vary the Vactrol, yet on the other hand this voltage would need to be static as it's used as a pullup voltage elsewhere. If modulated, then it's likely that U4a and U4c would switch logic levels accordingly and in turn repeatedly switch the U5 bilateral switches on/off. I would expect this to generate a lot of switching noise and can't see this being a viable function. The only thing I can suggest at this stage is to trace the pedal against the schematic and correct it to give good basis to work from. I've searched extensively for a factory schematic and there doesn't appear to be anything available.

[stevenrb718]

I've contacted Dunlop/MXR and they didn't provide a better schematic. I noticed with a demo video that the coinciding LED on the mono/stereo selector does not cycle like the LED does on mine. The demo model LED seemed to just flip back and forth between solid green and solid red. Mine *seems* to keep green on while it pulses red, and the pulse is at a rate of triplets but it misses the 3rd triplet. I got a wild hair and swapped the 4011, and that didn't change anything. I guess my next shot in the dark is to swap the mc14016. It certainly does feel like this is a power issue somewhere...I guess

[Mick Bailey]

Did you get anything at all off Dunlop?