Sunday, October 18, 2009

Damn it

In an attempt to discover what was wrong with my bass amp I further destroyed it. At least before it worked like 60% of the time. Now it's completely fucked.

I set it up to take some voltage readings yesterday. About 5 minutes after I plugged it and turned it on two resistors flamed out. I quickly unplugged it and drained the caps. I poured over the schematic trying to find out what had caused the burnout. Both of the resistors are close in the signal path but not in serial. It seems like if those two components burned out that other resistors should have burned out as well.

The Q13 driver looked suspect in the whole thing so I popped it out to test it. Under one set of testing this transistor indeed looks bad, but under another set of testing (that makes more sense to me) it seemed fine. I decided just to put the driver back in and maybe ship the amp off. Apparently when I de-soldered the driver to test it, the small metal pad that attaches it to the circuit board came loose as well.

This has all gotten quite a bit beyond me. I don't trust myself (especially with the erroneous driver testing) to actually sort this all out. I still can't even figure out why just plugging the amp in and turning it on caused catastrophic failure. And I certainly don't have the money right now to get it fixed. It's a $65 bench fee just to look at it, and on Ebay you can get a new one for $200. It's a shitty amp, but even shitty is better than nothing.

Wednesday, October 14, 2009

Unexpected bullshit. Usually the bull at least eats first.

I went to play my little tweed amp today. That's what I built it for. I played it two days ago, so it wasn't too weird to think I would be able to play it today. No dice. It's just been sitting there, but now it has a crazy hum with no input. Now on top of my bass amp being in pieces I have to take apart the tweed as well. It's troubleshooting week here at the Vernon household.

I know I always promise pictures, but this time I will actually try to post some.

In my tweed the first two filter caps became ungrounded. That's a problem. If I re-ground them that should fix the problem.
*end update*

*another update*
Grounding those caps indeed fixed the problem. The wire coming from both of those caps had come loose from the 'star ground' point back behind the fuse holder and light bulb holder. It is a pain in the ass to get to that grounding point without desoldering 5 other things. Instead I pulled the grounding wire forward and grounded it to another point which I had available. It doesn't look as pretty but it works and is FAR more serviceable.

To diagnose the problem I started testing voltages throughout the amp. Some voltages were a little low compared to the last time I tested the amp out, but all voltages were well within spec of the Fender drawing. From that I assumed no components, tubes, or transformers had gone bad mysteriously. Next I started testing continuity throughout the amp. I am ALWAYS fooled by the positive lead going to the speaker jack being shorted to ground. I do not understand why this is the case. Someone needs to explain it to me. After fooling around with the speaker jack with no progress I started back tracking from that point looking for continuity to ground. Low and behold I found the two caps not grounding out. I gave a little tug to the grounding wire and found it to be loose.
*end update*

Saturday, October 10, 2009

New part. Same old problem.

Well I put in the new relay. The good news is that the relay works. I didn't make the problem worse, and the amp actually seems to be working a little better but still not fixed. I'm going to dive in, perhaps this afternoon, with my multimeter to try and track down the actual problem. I will probably find out it's something expensive to fix, and then not fix it. Good times.

Thursday, October 8, 2009

A new post. A new oppurtunity to electrocute myself

My Ampeg (i.e. Crate) SVT-350H bass amp has been on the fritz for several years. I took it to Musical Instrument Repair on Highland in Memphis twice (that I can remember) each time to be told "There's nothing wrong with it." I hate that place. Never go there. I think it was those experiences that really made me want to learn about the inards of my amps et. al.

Several years ago I had a friend rig the fan so that it runs all the time at high speed instead of the stock "run when a little temperature diode that sucks overheats." The fan wouldn't run and the amp would overheat and shut down. So that's fixed. Fan always runs. It's a loud fan and kind of annoying but at least nothing cookc inside.

I've read online that the fan not running is a problem with this amp build (which is also used for the B2 I think). It's possible that the overheating burned something out, but at times the amp works perfectly. I'm hedging my bets against something being burnt.

There is a relay (the K1 relay) that stops the amp from popping the speakers on power up. I think this relay is causing the problems. I bought a replacement from Mouser for about $17 w/ S&H. I'll wire it in when it shows up tomorrow and see how things go.

I'm a little disappointed in myself with this project. I could really dive in with my multimeter and try to actually hunt down the problem for sure instead of just replaceing the thing I think is broken and hoping for a good outcome. I'm still worried I don't know enough to not electrocute myself I guess. I'd rather run the safe route, but I learn a whole lot less in the process. Eh.