valvastate 8008 blowing secondary fuses

[Poffa]

Need help
valvestate 8008 blowing both secondary fuses (6.3a)
checked cables, tranny,rectifier and pilot-led.
thinking problem is further in.
any ideas ?

[Enzo]

Well, yes. When fuses blow, the MOST likely failure is a shorted power transistor. Check the output transistors on the heatsinks for shorted condition. probably only one of the channels is blown. You will probbly find the transistors are BDV64 and BDV65. if one of the pair is shorted, replace the other one too.

You can replace the BDV types with TIP142 and TIP147. Watch your polarity. and those TIP types come in several shapes, so get the right ones. (TO3P if I recall.)

[Poffa]

You were right.

desoldered both bdv64 transistors, and there was a difference, when measuring between the mid and right leg. the one (of witch i presume is faulty) had a connection. the other transistor didn't.
now i'm gonna order and change all four transistors.

BUt... there is also a small transistor on the same rail..
(tip 29 c)
could this one be damaged as a result of a damaged bdv 64 ??

thank you very much

[Enzo]

Don't know, you would have to test it. In my experience, it will be OK, but every case is different.

Check any of the rectangular cement power resistors you see. They can burn open without looking different. Values are printed on them.

[Poffa]

Hello

Thank you very much for the help.
changed all four transistors today.
and measured for any short circuits, before plugging in.
the amp worked like a charm.

[Cygnus X1]

Digging up this old thread to say thanks, and point of clarification for
others who might be searching.
My 8008 shorted out on both TR4 and TR18.
Checked shorted on both with 1 ohm between middle and right side pins.

BDV65C, replaced with NPN TIP142
BDV64C...replaced with PNP TIP147.
Working fine and running cool.

Thanks again!