Marshall 2098 lead 100 repair

[Mick Bailey]

I'm very curious how to measure 10mA at 0.33 Ω.
Ohm's law says it is 3.3 mV. With analog Vmeter can hard to measure, but DVM has walking on display +/- 1 digit.

Even a cheap DMM has a resolution of 0.1mV. That's accurate enough for me for setting bias. The other way is to set it off mains current draw - turn the bias right down and then carefully increase it to the point where the current just begins to rise. At that point you know the transistors are conducting.

[Dave H]

Even a cheap DMM has a resolution of 0.1mV. That's accurate enough for me for setting bias.

And you could always measure across the two 0.33R resistors in series to double the accuracy.

[Luisflores]

Well i couldn't send the images due to the size, another cuestion to the thread XD. But well i bypassed the OT and a strange pop noise disappeared. Now I'm measuring the voltage on the. 33 ohms resistor and like with the current, the voltage dropped from 1.6 milivolts to .35 in something like 10 minutes. It doesn't get hot or something like that but than voltage seems to low for me. Thanks

[Luisflores]

I checked the choke when arrived. The isolation was something like burned out but the choke seems good, just checked the soldering and put on some thermofit . All the transistors, diodes, electrolytic capacitors and a lot of resistors were changed because the amp was far to damaged. Also a lot of rails have been replaced with wire because were burned out. This amps seems like s... But i don't want it to just go directly to the garbage

[Mick Bailey]

A failing component doesn't need to get obviously hot to give thermal problems. Bias will drift slightly, and I always re-check after 20 minutes or so to be sure an amp is OK when warmed up, but what you're seeing points to a faulty component. This is where a can of freeze spray can narrow down the problem. If you don't have any an airline off a compressor with a blow gun can work. Your bias circuit is certainly not working correctly. I would re-check the bias transistor and associated components.

[vintagekiki]

Now I'm measuring the voltage on the. 33 ohms resistor and like with the current, the voltage dropped from 1.6 milivolts to .35 in something like 10 minutes.

In order not to make a breakdown without need, between power supply and PCB pwr, in series with power supply voltage +40V and -40V set current limiter in form of resistors 100 Ohm / 50W.
So you can safely perform measurements and interventions to repair amp without fear that it may burn.

[vintagekiki]

Even a cheap DMM has a resolution of 0.1mV. That's accurate enough for me for setting bias. The other way is to set it off mains current draw - turn the bias right down and then carefully increase it to the point where the current just begins to rise. At that point you know the transistors are conducting.

Resolution is not a problem, accuracy is a problem.
Adjusting to point where idle current just begins to rise, measured with 200mV DVM and read on penultimate digit, is only possible with helipot potentiometer.

Darlington transistors are known as "fast fuses"
While idle current adjust, and DVM measure 3.3mV, darlington with smoke has moved to recycle bin.

[Luisflores]

Sorry for being offline to long. The Friday i tried to check the amp with the new transistors, new output resistors and changed the md8003 with two 2n2222a. Everything went ok but in a point current started growing and the output transistors with the output resistors went shorted. Really doesn't know what the hell is going on here. Tried with the resistors and in the beginning everything were ok but in something like 20 minutes the two darlingtons were just garbage

[vintagekiki]

In order not to make a breakdown without need, between power supply and PCB pwr, in series with power supply voltage +40V and -40V set current limiter in form of resistors 100 Ohm / 50W.
So you can safely perform measurements and interventions to repair amp without fear that it may burn.