Marshall DSL100 - bogus bias voltage measurement

[Tone Meister]

Later model DSL100 with bias test points inside the chassis (CON5). Owner brings amp by because it's making intermittent noises even with all volume controls down. Ran the amp for a few minutes and the right pair (viewed from rear) were redplating, so I installed 4 fresh EL34s, which eliminated the noise. Checked the bias at the test points and measured 13.5 VOLTS DC, not millivolts, but VOLTS at the CON5 test point. Yes, the Fluke 87 I'm using meter has a fresh 9V battery.

Checked bias voltage at pin #5 grid of each tube socket and get -45.8V on one pair and 46.6V on the other pair. Located right beside the bias trimmers TR1 and TR2, the bias measurements on R93 and R98 are consistent with the pin #5 readings at each tube socket. Plate voltage is 460V at each power tube.

I expect to see readings at the test point between say 45 mV and 120 mV, roughly. Any idea why my readings would be in volts rather than millivolts?

EDIT: Incorrect schematic removed

[Tone Meister]

Even more weird. I now measure 415V DC on one side of the test point and 0.015V DC on the other side. <<<< That is NOT a typo, and I triple checked my measurements. Even used another meter to confirm.

Measurements on the tube sockets are still the same as above.

[Enzo]

I don't understand what the two sides of a test point are.


Why would your current reading be so high? Perhaps the resistor that is used has gone up from damage? If there is a 1 ohm resistor, across which we expect to read 60-80mv, and the resistor is toasted and now reads 100 ohms, the voltage across it will be much larger.

[Tone Meister]

The test point CON5 has three pins. The center pin is common. One outer pin is the measurement point to bias one pair of output tubes, the other pin is for the other pair. I am measuring voltage, which SHOULD translate to current, but the measurements at the power tube grids are within a safe range.



I have attached the wrong schematic, I believe.

[Tone Meister]

I don't understand what the two sides of a test point are.


Why would your current reading be so high? Perhaps the resistor that is used has gone up from damage? If there is a 1 ohm resistor, across which we expect to read 60-80mv, and the resistor is toasted and now reads 100 ohms, the voltage across it will be much larger.

It was a 1 ohm resistor Enzo, specifically R68. Not having the exact schematic (DSL 100-60-00) had me chasing my tail there for a few minutes.

[Enzo]

OK, your wording got me, I call that three test points. But I get it .

SO did you find that 1 ohm resistor bad then?

[Tone Meister]

Sorry about the poor explanation. Yes, one of the 1 ohm resistors read high mega-ohms, so I swapped them both out. Found no other issues after burning the amp an hour or so at half throttle.