Hafler DH101 preamp repair and mods?

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#141
Hi Aaron,
Sometimes the electrolyte can be conductive enough to upset circuitry just by being there. Try and clean the board in that area with alcohol, or even better a "no residue" electronic cleaning product. Try to keep this fluid away from any capacitors on the board. Of course you could simply clean the PCB and then replace the capacitors.

If a transistor was installed incorrectly (I think they have a reversed pinout), you would normally get no sound or extremely weak sound accompanied with lot's of distortion. Transistors will not generally change the gain in these circuits. The stage gain is set by fixed value resistors.

From your description, depending on how far different the gain is, it is possible you have a transistor in backwards that would swap the emitter and collector. That would drastically reduce the gain. Capacitors would be my normal assumption had you seen this before the transistor change.

It's difficult to diagnose a problem when the unit isn't on the bench (mine in this case). So, recheck your work and look at the pinouts and compare between the two transistors first. Check that your solder connections are good and of course whether you have electrolyte contamination on the PCB or parts.

-Chris
 
#144
While I'm not an expert I really like this little pre. I liked it so much that I took the AA mods, removed the tone controls, updated the RIAA and had boards made. I built a few pre amps. One is at a buddy's house.

For me the case is always the hardest part of any build. Here is a board. Not sure if I ever used it. I also did a PS board and phono board. The phono sounded great.
 

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#145
Hello Chris,

After carefully inspection and test, finally I checked out the problem is the cold solder joint of R23 due to my mis-soldering it when I replaced the transistors. Now this pre amp is playing flawlessly. I like it do much.

Thank you and other members again for your professional advice.

Best Regards

Aaron
 
#147
Values of P3 and P4

Not meaning to hijack the thread, but this seemed an appropriate place to ask the question does anyone know the values (in ohms) of the tone controls (P3 and P4)? I am trying to sim the line stage in the DH-101 and don't really want to remove the tone controls from mine in order to get an accurate measurement.

Thanks.
 
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