Marshall TSL100 Problems

[tcampos]

Hey guys,

Really nice forum you have here!

I have a Marshall TSL100 having problems only in the crunch/lead channels. Low volume and low distortion in both. Clean channel is noticeably much louder and distorts almost the same as the other channels all at full gain/volume. This is clearly a preamp problem but I cannot figure it out.

All plate voltages are ok, I've measured everything. First I thought V1 was the problem since the plate at pin 6 is only at 170V but since the plate resistor is 220k in my amp I guess this voltage is pretty expected.

I followed the rest of the amp and noticed that the resistor R33 (100k) was measuring 40 approximately ohm. I cutted one of its pins and measured it and I figured it was actually ok, measuring 100k. Transistors TR1 and TR2 near V2 measure 40 and 70 ohm from drain to source. So, do you think this might be the source of the problem?? I'm almost losing my faith...

You can see the schematic in http://www.drtube.com/schematics/mar...03)%20iss7.pdf

Thank you.

[Enzo]

TR1,2,3 are JFETs. JFETs are ON until you turn them OFF with voltage on the gate pin. With power off, it is normal for them to measuer low resistance. IN parallel with that 100k resistor, it would make it appear very low.

Measure that resistance across those two transistors when power is on and see how they measure.


REsistors almost never short. A resistor that is supposed to be 100k and measures 40 ohms is almost certainly not 40 ohms itself, but rather it has a parallel resistance - such as that JFET.

[tcampos]

TR1,2,3 are JFETs. JFETs are ON until you turn them OFF with voltage on the gate pin. With power off, it is normal for them to measuer low resistance. IN parallel with that 100k resistor, it would make it appear very low.

Measure that resistance across those two transistors when power is on and see how they measure.


REsistors almost never short. A resistor that is supposed to be 100k and measures 40 ohms is almost certainly not 40 ohms itself, but rather it has a parallel resistance - such as that JFET.

With amplifier ON TR1 measures 40 ohm from drain to source but now if i measure the terminals of the 100k resistor I have 100k. Strange... How is that possible if they are in parallel?

I just measured all anode and cathode voltages with no power amp tubes inserted:

V1B:
Pin 1 (Anode): 261VDC
Pin 3 (Cathode): 2.2VDC

V1A:
Pin 6 (Anode): 194VDC
Pin 8 (Cathode): 1.65VDC

--
V2B:
Pin 1 (Anode): 281VDC
Pin 3 (Cathode): 2.7VDC

V2A:
Pin 6 (Anode): 315VDC
Pin 8 (Cathode): 3.7VDC

--
V3B:
Pin 1 (Anode): 427VDC
Pin 3 (Cathode): 245VDC

V3A:
Pin 6 (Anode): 244VDC
Pin 8 (Cathode): 1.8VDC

--
V4B:
Pin 1 (Anode): 275VDC
Pin 3 (Cathode): 49.6VDC

V4A:
Pin 6 (Anode): 291VDC
Pin 8 (Cathode): 47.4VDC

Everything seems ok considering the following voltage chart from dsl100 Marshall DSL Voltage Chart

[Enzo]

Look at the schematic. The 100k is parallel to TR2, not TR1.

Since they are channel switchers, you should be able to change channels and have those transistors turning on and off.

You should find the voltage at the gate turns on and off as channels are switched. If no gate voltage gets there, the JFET won;t turn off.

I don;t know that those JFETs are necessarily the problem, but they are easy to test as we have discussed. If you find one that never turns off , then look at its gate. If voltage comes and goes at the gate, then the thing is probably bad. If no voltage change occurs, then the transistor is not being controlled, so we work back into the control circuit.

[Alex R]

v1 and v2 heaters are fed with DC by a rectifier (BR1) that quite often goes wrong, you might want to check they are getting their DC, just to eliminate that from your enquiries.