Hartke HA5000 output module repair

[TransLucid]

Hi -

There are two pots on the power amp modules. I assume they are meany to be adjusted when the output devices are changed. Does anyone know the procedure?

Tnx

[Enzo]

One is the output DC offset, trim for zero volts DC on the output. DO that without a speaker load.

The other is bias, which you would adjust as for any solid state power amp.

[TransLucid]

tnx Enzo. Which is which, and what is a normal bias current (which is I assume is adjusted by measuring the voltage across the .47 ohm resistors)?

[Enzo]

I don't have the 5000 schematic handy, so I won't assume the part numbers are the same as on other models which MIGHT use the same board. No load, look at the output with a voltmeter on DC and carefully twiddle a control, If it moves the DC voltage, then it is the offset, if not, it is probably bias. Offset control is usually closer to the input stage of the power amp.

I have no idea what "normal" idle current is through those ballast resistors. I don't like that method. I use a scope and set the bias for minimal crossover distortion without current rise from the supplies, than I back off just a hair towards cold. The point at which crossover notch just disappears is also the point at which the opposing sides of the push pull start to both conduct, so mains draw or power supply draw starts to increase. I like to find that point, then back away from it towards the crossover notch a little for safety.

[Jazz P Bass]

The complete schematic can be found here: Hartke HA5000 schematic
I have attached the relavant area of the schematic.
The Bias control (on the schematic) is SVR 302. The test points are TP 301 & 302.
As Enzo said, you want a minimal voltage across the test points.
5-10 Mv's is enough.
The DC offset control is SVR 301.

[TransLucid]

tnx to you both - I'll give it a shot and see what happens.