|m1989jmp||6/18/2018 9:08 AM|
|Trace Elliot Brat no reverb|
I got this little practice amp with a loud hum on the speaker, powered down and disconnected the load immediately. No fuses were blown. Measured about 2.5VDC on speaker out. Dropped to 0VDC when I pulled TR1 to TR6 out.
These transistors seem to be obsolete, I think I'll go with BD901/902 for the outputs and straight replacement for driver transistors.
The weird thing is that one of the output transistors was completely covered in thermal compound, both pins and casing. Is this the source of failure?
Any other tests I should do before soldering in new transistors?
Thanks for the answers.
Edit: just to clarify, no reverb refers to the lack of inbuilt effect as opposed to the model with reverb, not the fault.
|Enzo||6/18/2018 9:26 AM|
|No, sloppy heat goo did not kill your transistors.|
HAve you checked all those transistors before just replacing it all? WHat voltage is on collector of TR6? That is roughly what will be on the output.
|J M Fahey||6/18/2018 9:54 AM|
|Maybe that bucketful of goo was an attempt by previous owner to repair it ("hey, this one is getting real hot, letīs add some thermal grease")|
I donīt condone shotgunning but since you already pulled all of the old ones, you might as well put fresh ones in there.
BD901/902 look like perfect replacements and you have to order them anyway, so adding 4 very cheap small transistors wonīt materially add to bill and freight, so ...
Remember to order fresh micas, a little thermal grease and insulating nipples .
Build a lamp bulb limiter, with a 25W lamp, not larger because thatīs a small amp ŋ15W?
Turn amp on, plugged into limiter, no speaker or any load, no signal, and check for DC at the output and proper rails (somewhat less is fine because of the limiter).
Then measure voltage across R3 and R4, the emitter resistors.
By the way, order a couple of those, just in case one or both opened .
Amp in principle "should" work; once DC voltages are confirmed, plug speaker (without turning amp OFF) and play a few chords at low volume, say 1W or so.
If fine, you may try the amp plugged straight into Mains.
Reverb, hiss, hum,etc. are relatively minor problems, if any you check that after power amp works.
|m1989jmp||6/18/2018 11:17 AM|
I first pulled TR1 and TR2, still had 2.5VDC on the output. It dropped to 0VDC after pulling TR3-6, figured I won't find exact replacements, so I've pulled them all without checking one by one.
I'll post voltages on the collector of TR6 as soon as I get new ones in.
|m1989jmp||6/18/2018 11:20 AM|
I'll report back with the results as soon as transistors arrive.
|Enzo||6/18/2018 2:04 PM|
|The point of measuring the collector of TR6 was to diagnose the existing stuff, after all new parts, it won;t matter unless there is still a problem.|
|m1989jmp||7/25/2018 3:24 AM|
|It took a while to find and receive replacement transistors. After replacing TR1 and TR2 I get sound and all voltages are normal.|
However, I get this annoying hiss/white noise whenever the volume is set relatively high. I've scoped the output of IC2-A without signal applied and I see noise right away.
How would one approach this systematically?
|m1989jmp||7/25/2018 1:56 PM|
|Here's a picture of a signal at the point indicated on the schematic. Nothing is inserted into input jack and all controls are on 0.|
The voltage is set at 50mV/DIV.
|Enzo||7/25/2018 2:02 PM|
|Looks like a noisy IC, did you replace the 4558?|
|m1989jmp||7/25/2018 2:23 PM|
|g1||7/25/2018 2:23 PM|
|And just to be sure it's not coming from before the IC, you should plug your guitar in and set all guitar controls to zero.|
|m1989jmp||7/25/2018 2:34 PM|
|m1989jmp||7/27/2018 2:20 PM|