Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: TIMBO on March 09, 2021, 03:49:28 am
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Hi guys, Not my usual gig, but I thought I'd have ago anyway.
Ecaps were replaced by the owner as they were cactus, when fired up blew internal fuses, thought he had the new ecaps wired the wrong way.
So he gave it to me to try for a fix.
Checked his work all good other than the caps were 4700uf, 5000uf were the caps he replaced.
Removed 4A fuses and checked the rectified volts and they were good for the +-47v as per schem also the +-16v.
Online has shown a few problems but nothing stood out (visibly) on this one.
The owner told me when he opened the cab there were some insulation washers, nuts in the bottom of the amp and couldn't find where they came from.
Further inspection of chassis I noticed the 2N3773 power transistors had some pan head screws holding them in place.
A wire with a lug on the end was just hooked on the tread of the screw.
I believe the outer case of these transistors is the emitter and as per the schematic one wire goes to one side of the 4A fuse (Q2) and the other the R29 (Q1), I can see why the fuse blew as the screw was not touching the chassis but the lug was.
So I have kinda answer my own question, do I need to connect the wire via a BOLT that is insulated from the chassis but connecting the outer case of the transistor (emitter)
(https://i.imgur.com/sDRPbri.jpg)
Anything else I need to look for while I'm in there.
Thanks
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Caution!!! Pan head screws are used only when the transistor is mounted in a socket.
The outer case is the COLLECTOR and must be insulated from the chassis using a mica insulator shaped like the transistor case. Apply heat sink grease to the bottom of the transistor case, put the mica insulator on and apply more heat sink grease to the insulator. Now mount the transistor to the chassis (or aluminum heat sink) using two 6-32 machine screws, insulating shoulder washer, flat washer or ring lug, and nut. Tighten. The collector connection will be soldered to the ring lug (which is mechanically/electrically connected to the transistor case. The base and emitter wires will solder to the base and emitter leads of the transistor. Pics should explain better...
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Awesome!! Thanks
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Hope you found all the proper mounting hardware. If not, kits are available on the net pretty cheap.
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Hey sluckey, Remounted the transistors as above.....
Checked required voltages as per schem and all seemed good.
Connected speaker, odd smell no sound, speaker fried.
Checked the voltages B C E and all are showing -46/+46 and speaker jack showing 45v.
Lucky it was a crap speaker.......
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Replaced both 2N3773 transistors and no real change....
Next step the driver transistors TIP29c/TIP30c no luck at the local store, google tells me the next best is TIP31c/TIP32c or TIP41c/TIP42c these have slightly higher current capacity and available at the local store.
Chucked in the 41c/42c and all is good.
A little background hum, checked the grounding points but all good there.