After checked cabling, ground connection, at last the amp turn on peacefully however still read "0" on the vu meter, no idea what's wrong, the DC voltage reading :
(106) (3.5) (3.5) (106) (106) (3.5) (3.5) (106)
(106) (3.5) (3.5) (106) (106) (3.5) (3.5) (106)
(215) (215) (215) (215)
Your supposed to have B+ & B- voltages.
This tell me one of the transistors are bad.
Sorry for the late reply, the damage board area fixed already but the amp still reading "0" on VU meter , totally silence to headphone, still can't find out what's the problem yet
0 on the bias meters is a result of the protection circuit kicking in,
Due to the Problem with PSU giving the output circuit error .
After transistor problem solve, the amp turn on no more crack sound, no more circuit burn, however both VU Meter now light up togather at the same time, I suspect either zener diodes, diodes, bridge retifier, N channel J-Fet and Mosfet having some issue, may be going to change those parts later.
The issue is that the diodes are performing as zener diodes to stable the power going thru the power transistors.
The problem lie that they can burn (with their series resisitor next to them), when there is "no tubes" in the sockets .
This will cause chain reaction to blow your transistors.
Replacement of both the pnp&npn power transistors simultaneously is recomended,
As well as the crappy tiny diodes for greater amperage versions (100v and higher amperage for durability).
Change all four at same time, and you won't have one component blowing out the other..
Swap out the garbage tiny series resistor that is with the (zener) diodes as well to greater wattage.
I always use Mills MR5 wirewound for these applications where the current can get high.
They can give ofd smoke for a few seconds an still perform like new.
This is from my own experience on same PSU.
The Diodes u mention about show 0.245 V under diodes read in DMM, this one seem like unnormal?
I also just found one of the bridge rectifier has an issue so as j fet n channel
Just change out those tiny baby diodes for some heavy duty and not have to worry .
There is something I dont understand, when i took off those diodes from PCB, the DMM reading normal and no error, then I put it back, the DMM show 0.245V, that makes me confuse?
Do you know why ?
The diodes supposed to perform a zener voltage regulation to the base of the power transistors which handle the PSU current.
Without the diodes you will not have regulation and I am not sure but I believe the transistors are configured to fully conduct .
When there is less load on PSU, the diodes will conduct more to absorb the voltage headroom of the PSU , which in return biases the power transistor to put out lower voltage and less demand.
Yet the diode will get super hot and burn their accompanying series resisitors.
This is hottest area of amp.
The heat also travels thru the traces to the last two caps that are suggest upgraded.
So I would replace the zener diodes with larger and with longer leads to help dissipate heat further away from board.
If not done already, I would extend the bottom of the amp for more component space.
Good luck!
