Another peavey 400 bh problem

[robgrif]

repaired one of these 400 bh previously thanks to
this forum many thanks for the great help
this problem is another peavey mark 111 bass amp I have that is not working
400 bh board produces no output
I changed a shorted out power transistor q9 with new mje15003 ,tl074,two 4148 diodes
and a p600 diode i
used light bulb limiter
amp powers up ok
checked all cement resistors none are open
removed u2 no effect
tested preamp it is ok
vr's seem to be working 15 volts out
no hum when touching connector at corner of board
getting voltage at output transistors 51 volts rails
low voltage .02 at q6,q5 and q4 but q3 reads correct
any one have any ideas as to possible culprit for no output and the low voltage readings
thanks in advance
regards
robgrif

[Enzo]

OK, power seems OK but won;t pass signal.

SO you don;t blow fuses, and you get both +/-50v rails, and no DC on output? Sounds like the right half is working.



There is that big crude looking output inductor L1, with the 22 ohm resistor in parallel. Measire resistance across that. If it measures a dead short, that is normal. if it says 22 ohms, then the coil is broken at one end. If the thing reads open, then the cooil is broke and the resistor burnt out and that would cause no output right there.

Pull U2, apply a strong signal to the input. Is the signal at pin 14 of the TL074? At the collector of Q9? At the bases of the drivers?

Power off. Is there continuity between the bases of Q4-5-6 and the emitter of Q3?

There are those two short three-wire cables conecting the main board to the transistor board. Are they connected - and properly? Are the female pins snug?

[robgrif]

400bh

thanks for taking time to respond to my questions Enzo
checked inductor L1 ok reads zero
checked continuity on q 4 5 and 6 to emitter q3 ok reads zero
removed U2 applied signal through preamp to board no sound
Is there a better place to apply signal on the board directly?
If so would you describe a method or procedure for doing so and a method to detect the signal at locations you suggested
do I require a signal probe of some type to detect the signal or inject it?
hope these question do not seem too naive as you can tell I am new to
electronic repair although I do have a basic understanding of the function
of various components in circuits and the use of my dimm

thanks for your time
regards
robgrif

STOP THE PRESSES!!
noobi mistake mine of course
two things combined to make this amp inoperable after initial repair attempt
number one the speaker must have taken a beating on initial failure of amp
and subsequently failed later on and I did not recheck it ,it was working !!
secondly I replaced tl074 when there were active diode problems with the amp
and the replacement part failed
anyway I remedied both problems and now the amp is functioning
thank you for all your help and tips
on this forum lots of information on many problems and many answers
regards
robgrif
embarrassed noobi

[Bigroo76]

I am having problems with my Peavey 400BH aswell... Its constantly hums while stand by or playing through it. If I turn the volume down on bass and amp. You can still hear the hum... What is wrong? I pulled the fuse is says its a F8A, is that the right fuse? Please help. I do not have the funds to retube my Ampeg SVT and the Peavey is a back up. The only way I can quieten the hum is with my sans amp DI pedal. What am I doing wrong?