Beta bass transistor question

[rdobirdman]

I have a beta bass that is blowing 6 amp fuses, I tested the power transistors and found a couple of bad ones, they are labelled SJ954 and SJ6888 I cannot find any reference for the SJ6888 ones, the schematic looks like it says 22-3155 in Q8 Q9 and Q10 and the parts list doesn't have it on there. I replaced the SJ953 with NTE219, can I replace the other one with a NTE130, any ideas?

[JoeArthur]

I haven't replaced beta outputs myself, but I did see where the SJ954 crossreferenced to the NTE219 (npn output) on the NTE site.

So if the SJ6888 is in position Q17, Q18 and Q19 then these ARE complements, which would be the NTE130 (pnp output).

[rdobirdman]

Here's the latest, with no transistors in Q9 and Q10 the fuse doesn't blow and I can get sound out of the amp, with a new transistor in either of those positions the 6amp fuse immediately blows. I also noticed that the filter cap on that  +side of the circuit retains it's voltage after the amp is off whereas the other - side discharges, I still measure +44V on that side, does this tell me anything?

[JoeArthur]

Insufficient information.

Where is this 6 amp fuse that is blowing?  The mains should be a 2.5amp slow blow and the secondary power supply fusing is 10 amps. You wouldn't have replaced the mains fuse with a 6 amp... right?

44 volts seems a bit high, but that could be possible without the load of the output transistors - and without the transistors in the circuit there would be more resistance for the cap to discharge through which will take longer - especially if the secondary fuse is the one that is blowing.

Have you verified that Q8 isn't shorted? In order for Q9 and 10 to conduct enough to blow a fuse they have to be pulling current through their base.  If Q8 is good, the next likely suspect would be Q7 causing Q8 to conduct... causing Q9 & 10 to conduct - these are all cascaded together, and a fault in one will affect all others after.

And just to make sure I never overlook the obvious - you did use new mica and grease between the transistors and heat sink?

[rdobirdman]

It is one of the secondary power supply fuses, it has "use 6 amp fuse" written right on the circuit board. The new transistors didn't come with new mica so I left the old one there along with the grease that was already there. I replaced Q8 Q9 and Q10

[rdobirdman]

OK, duh, this was my first time working on a solid state amp so I didn't realize the importance of the mica and grease. I put new micas on the power transistors  and voila, it works. Thanks for the help.

[JoeArthur]

New mica and grease is very important.  Mica because it electrically insulates the collector (case) from the heat sink, and grease because it can become contaminated and conductive (due to dust and water vapor in the air) and... without enough clean grease the outputs will not last over time due to an inability to push the heat they generate to the heat sink.

It is possible to reuse mica... but never reuse grease.  And if in the future you do decide to reuse mica - please perform a resistance check between the transistor case and the heat sink - and accept no less than infinity.

Glad to hear your amp is now working!!  Cool!!

[rdobirdman]

Is the grease supposed to go between the transistor and the mica or the mica and the heat sink? or both

[JoeArthur]

Both.

It increases the thermal conductivity... getting the heat of the transistor to the heat sink.