Hartke LH500 intitial hum

[Axtman]

I have a Hartke LH500 that I am working on. When I first turn it on it stalls for about 15 seconds before the sound comes on. Then when I get sound it hums for 10 seconds then fades away in 5 seconds. Other than that, the amp appears to be working just fine.

At first I thought it might be a noisy tube. I swapped 12AX7 tube and it made no difference. Once the amp is on for a while, I can turn it off then turn it back on with no hum. But if you let it cool down for a while it will go back to humming on start up.

Here is the schematic.

LH500 v3.5.pdf

[The Dude]

That symptom is usually caused by a filter cap starting to go bad. It doesn't filter cold, but as it warms up, it starts to work. Scope your power supplies and see if one of them has excessive ripple when the unit is cold.

[Axtman]

Thank you Dude. I was suspicious of the filter cap but I just replaced the whole powersupply / preamp board with a new board.

https://www.fullcompass.com/prod/541...-pcb-for-lh500

I will scope it and let you know.

[Axtman]

Well oddly enough the problem turned out to be the 12AX7 tube. It tested good but I swapped it out for another tube and now there is no fading hum at power up.