Dunlop Stereo Pan Tremolo - No power

[beedoola]

I cannot find the schematic for this. It looks like it may have been posted on Music Electronics Forum but it has been down for awhile.

I track the 9v DC power up to Q1 which is a MPSA18. The Collector has 9v on it nothing on the Emitter. I tried a 2N5088 in its place but same voltages. Anyone have any thoughts or the schematic?

[PRR]

> 9v DC power up to Q1 which is a MPSA18. The Collector has 9v on it nothing on the Emitter.

3 legs, right? What is on the 3rd leg? Trace away from 3rd leg to battery and common. Broken joints, loose wires, burnt parts, new solder blobs?

[antonis]

I track the 9v DC power up to Q1 which is a MPSA18. The Collector has 9v on it

Very good if Q1 is Emtter follower..  

[beedoola]

I track the 9v DC power up to Q1 which is a MPSA18. The Collector has 9v on it

Very good if Q1 is Emtter follower..  

What do you mean?


No burned pads.

[antonis]

What do you mean?

Nothing without a schematic..

e.g. in case of Q1 being an Emitter follower (buffer), 9V on Collector is fine..
As Paul said above, we need voltage reading on Base and definition of "nothing" (short or open circuit..??)

[beedoola]

Was able to track down the schematic. I'll do some more power testing tonight.

[antonis]

To make some schematic corrections..

Q1 is n-p-n so its Emitter arrow should be reversed..
D1 polarity should also be reversed - it's there to prevent Q1 reverse bias..
(as it is in your schematic, it bypasses both voltage regulation (R26/C18) and overvoltage protection (D2/D3)..)
Battery negative pole should be connected on DC jack GND..



Base & Emitter voltage values are somehow dependent both on supply and load..

[beedoola]

Thank you. Show id D1 is blown, that could account for the no voltage on the emitter?

Thanks for those voltages reference.

[antonis]

Show id D1 is blown, that could account for the no voltage on the emitter?

No but with D1 blown there is strong possibility for Q1 been back-biased (no bueno)..

What is the voltage on Q1 Base..??

[beedoola]

Q-1 voltages:

Collector: 9v

Base voltage: 1.5v

Emitter: .9v

D1 doesn't appear to be blown.

[antonis]

Q-1 voltages:
Collector: 9v
Base voltage: 1.5v
Emitter: .9v
D1 doesn't appear to be blown.

This calls either for reversed(of defective) Zener (D2) or wrong(very high) R26 value..

[beedoola]

I pulled D2 and testing it on my DMM:

DMM negative probe to anode and postive probe to cathode: .204dc

DMM postive probe to anode and negative probe to cathode: .186dc

277 ohms measuring across the diode.

If its bad, what are some substitutes I might be able to use?

[antonis]

Before D2 replacement, put a 27k to 33k resistor in place of D2 to verify no other damage..

P.S.
A brief circuit analysis:

R26/C18/Q1 form a capacitance multilied Low Pass filter of 0.159 / R26*C18*h_FE -3dB corner frequecy..
Q1 output stands at Collector voltage minus Q1 V_BE minus I_B*R26 (roughly -700mV)

D2 is a Zener voltage stabilizer but it contributes only for Collector voltage higher than V_ZENER + V_D3..
(in the mean of overvoltage protection - it does nothing for Collector voltage lower than about 9.8V to 9.7V..)
D3 off-sets Q1's Base voltage by a V_BE loss..

You could, of course, check your circuit without D2 in place but you should be sure about your power supply voltage level..

[beedoola]

Thank you! It works. Tried the 33k in D1 - the schematic as the 1N5239 as D2 but on the PCB it's D1.

I also replaced Q1 with a 2N5088 as the stock MSP18 was bad.

Used a 1N4739A for D1