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big_tabasco

HD130 head repair

Hi I was wondering if anyone had any tips, I have an HD130 head I bought in non working condition,(my 3rd HD130 in all!) it has no preamp tubes, and it came with a 4x KT77 tubes which I understand are supposed to be interchangeable with EL34

Repairs I've done so far:
* replaced power transistors w/ a matched 2N6488 pair
* replaced electrolytics on the power board
* replaced the LM1458 chip (these first 3 steps solved my other HD130 combo so I just went ahead and this to start with!!)
* replaced the 220R resistors on the tubes
* replaced the 1.5kohm 10W resistors.
* the largest electrolytic caps actually seemed fresh I think they are marked 2016 by whoever installed them

Symptoms:
* hi power mode burns out the 3rd 220R resistor on the 3rd tube. eventually causes more issues if you keep it running, the 2nd pair of tubes starts redplating as eventually the 1.5kohm 10W resistor burns out as well, and then eventually itll burn out one of the power/bias transistors (I know all this unfortunately I was not watching the internals when I attempted to repair it so far!)

* Oddly enough it seems to work and sound perfect in low power mode.

thanks for any help!!!

mgriffin155

Check Power Supplies.

Since the tubes are red plating, remove the tubes to safely do some static mode troubleshooting. Since there has been work done, go back and check the Lo voltage power supply zener diodes (D5,6 ) for correct voltages. Also check D17 zener diode for correct tube control grid bias voltage. Next check the Hi voltage supply outputs in both Lo (580V) and Hi (700V) Mode. Fix any out of spec voltages. Other problems will be easier to diagnose once all the power supplies are in spec. After that, reinstall the tubes for active mode troubleshooting. Good Luck. -mgriffin

big_tabasco

Thanks, so much for the

Thanks, so much for the response, I'll give it all a shot and report back!

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