Dunlop TS-1 Tremolo question

[Mick Bailey]

That's interesting - pulling R141 means that Q2 cannot conduct, so the effect should stop working altogether as Q2 is in series with the LED. With R141 lifted there should be no voltage across TP1.

[stevenrb718]

That's why I say, this thing has had me baffled. There is no voltage across tp1 with the resistor pulled, yet there's still the trem effect. If you wanna hear a real doozy, I disconnected the sensor side of the LFO fully expecting the effect to drop out... it continues. Is the LFO literally only in there to control the intensity, while the speed and depth are actually IC-driven?? Why have the photocell at all?

[g1]

Can you verify the number on the opto and find a datasheet for it? Might provide some insight.

[stevenrb718]

Pdf of what I *think* is the coupler. I'm tempted to just cut into it and see if the LED is even still working. ecb1ed91959ae54d97cf67f5103d9b2b.pdf

[Mick Bailey]

I've just been pondering the schematic and there's something I missed. I'd assumed that the voltage on R157 was modulated (and therefore the LED side of the Vactrol as you'd usually expect), but according to the schematic it's connected to V+2, which is a steady DC source. I overlooked that. Could you confirm that in post #5 the LED you had in parallel did actually pulse?

[stevenrb718]

It sure does. I just tried it again for a triple-check and it functions as you'd expect. I think my next step is to try and rig up a makeshift coupler to determine if it's actually that or something down the line. I only have 2 pin photocells instead of 3, so I'm not sure how I'm going to do that one yet.

[Mick Bailey]

Going by the schematic there's no connection between the LED/intensity section and the oscillator. The only voltage is V+2, so it would be worth checking the PSU section on the second sheet of the schematic to see if the DC voltages are steady, or are themselves being modulated (it's worth checking this anyhow). Check if R157 connects to V+2 to establish whether the schematic is correct.

[stevenrb718]

It's there...I wouldn't necessarily call it steady as it oscillates quickly in the 3.5V-3.8V ranges.

[Mick Bailey]

Does R157 connect to C23 (V+2)?

V+2 seems to me to be too low, unless the modulation is beating your DMM display change. Given that D7 clamps the voltage on RP1 to 5.1v, it would mean that V+2 would need to be higher than your reading. I'm thinking that V+ would be around 8v and V+2 a little less (dropped across R106). What's V+ reading?

Some of the resistor values don't make sense - the total series resistance for the LED (VR1) is 138.3 kohm with TP1 at mid position. This is far too high to power an LED.

[Helmholtz]

V+ should be close to 9V. A 9V power supply won't work here. More likely needs something like 12VDC.

Where's that schematic coming from?
When I first saw it, I thought how could this ever work.

E.g., Q1 has wrong polarity.
And I would expect that the LFO signal is applied to the base of Q2.

3.5V to 3.8V roughly is the sum of the voltage drops of LED 6, VR1 and D11, so supply voltage should be higher than that.
Especially the 100k resistor in the collector path of Q2 doesn't make sense

I don't trust the schematic at all.

[stevenrb718]

I'm getting about 5.2V at V+. The resistor does go to C23 (labeled R157 in the schematic but not that on the PCB). I don't trust the schematics either which is why I mentioned not knowing how "official" they are when posted. I *believe* the MXR reissues ran off of 18V, but the old Dunlop versions run on 9V or battery.

[Mick Bailey]

The instruction manual says it's powered of a 9v battery or 9v regulated supply. The pedal's internal regulator circuit seems to be redundant.

[stevenrb718]

I actually get closer to 5.8V at V+ when on battery, which is a bit weird.

[Helmholtz]

I'm getting about 5.2V at V+.

Powered from a 9V battery V+ should be 8.3V as D1 bypasses the regulator.

Looks like something draws a lot of current. Might be getting hot.

Check D1. Should drop around 0.7V

Also measure loaded battery voltage.

[stevenrb718]

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.