this is a solid state amp, but i've come to trust the knowledge on this board, so..
my girlfriend bought this amp (1967 ampeg bt-15) a few years ago, and it's been reliable(ish). we've taken it on tour for months at a time, and never had any major issues besides a recap, some dry solder joints and a couple of pot replacements. but now this...
at last nights practice, we had it cranked up nearly all the way (not unusual), and it made a loud pop, followed by hum, and the smell of burning components (later found to be a 1/2w resistor). switched it off immediately. i said "that's it. it's dead for good now i think." she cried! she really likes this amp...
after many tests, i found that the the DC power leads from the diodes is closed. when tested with an ohm meter, there is absolutely no resistance between the hot lead and ground. spent hours checking the obvious, bad caps, melted solder (the amp gets REALLY hot).
i removed the heatsink with the transistors, checked the leads again, and now there is no connection. i bought $40 worth of T0-3 transistors, popped them in, and again, same issue. this could be the nature of this circuit and i could be looking at the wrong thing entirely, i have no idea. solid state amps are an uncharted territory for me.
Also, this amp has no fuse. So, when you try to turn the amp on, instead of blowing a fuse, it just started to fry where the PT leads are soldered on to the board. cool design, ampeg. any ideas?
ampeg bt-15 nightmare
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joshdfrazier
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- Joined: Sun Sep 29, 2013 12:23 am
ampeg bt-15 nightmare
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Re: ampeg bt-15 nightmare
The schematic has a fuse on it. I can't imagine it going out of Ampeg without one so it has apparently been removed. I'd replace that to start with.
Next, you say you're not familiar with solid state equipment? I don't know how much you do know but a great deal of care must be taken when using an ohm meter around transistors - reversed leads will blow them in an instant (experience speaking).
I take it you replaced the blown resistor with no joy? Can you identify which one it was on the schematic? That would help isolate the problem.
I've worked on a lot of SS amps but it was a Long time ago (like 30 years), so there's probably someone more qualified and certainly more familiar than I around here to help you out.
Next, you say you're not familiar with solid state equipment? I don't know how much you do know but a great deal of care must be taken when using an ohm meter around transistors - reversed leads will blow them in an instant (experience speaking).
I take it you replaced the blown resistor with no joy? Can you identify which one it was on the schematic? That would help isolate the problem.
I've worked on a lot of SS amps but it was a Long time ago (like 30 years), so there's probably someone more qualified and certainly more familiar than I around here to help you out.
Activax Amplification
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joshdfrazier
- Posts: 109
- Joined: Sun Sep 29, 2013 12:23 am
Re: ampeg bt-15 nightmare
the only fuse in the amp is the one inbetween the 300vac secondary for the electroluminescent panel. none for the 56vac lead which powers the actual amp. the fuse near the primary does not exist in the amp and there doesn't even seem to be a place for it.
yes, changing the resistor did nothing as i expected, considering i haven't located the cause yet. the resistor was unidentifiable, as the only colored band left was the first one (orange). the leads on the board are very hard to follow so i can't tell for certain, but i know that it is directly connected to one of the speaker leads (black). 3.3k? 3ohm? 33ohm? i was convinced for a moment that it was a 300ohm resistor due to its shared connection with the negative lead on the huge 2500uf cap, so i put a 330ohm in place of the burned one. it is more than likely a much higher value.
the replaced resistor is circled. board is backlit so you can see the leads underneath.
yes, changing the resistor did nothing as i expected, considering i haven't located the cause yet. the resistor was unidentifiable, as the only colored band left was the first one (orange). the leads on the board are very hard to follow so i can't tell for certain, but i know that it is directly connected to one of the speaker leads (black). 3.3k? 3ohm? 33ohm? i was convinced for a moment that it was a 300ohm resistor due to its shared connection with the negative lead on the huge 2500uf cap, so i put a 330ohm in place of the burned one. it is more than likely a much higher value.
the replaced resistor is circled. board is backlit so you can see the leads underneath.
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Re: ampeg bt-15 nightmare
I believe it's the 33R circled here.
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Activax Amplification
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joshdfrazier
- Posts: 109
- Joined: Sun Sep 29, 2013 12:23 am
Re: ampeg bt-15 nightmare
so, a diode was (partially) to blame for the short. without the transistors plugged, i get appropriate DC voltages all over the board. with the transistors in (new and old sets) i am STILL getting leakage from + to ground.
Re: ampeg bt-15 nightmare
When a component dies in a DC coupled amp like this it can easily take several other components with it. Finding one bad component and replacing it usually isn't sufficient.
I suggest that before you work any further with that amp you hook it up to some kind of a current limiter that will limit the fault current to safe levels that no longer create further havoc. (e.g. putting the amp in series with a light bulb is usually a good technique).
After that it's systematic measuring and replacing of bad parts.
The BT-15's I've seen had a fuse holder wired to the back like this:
[img:700:525]http://i554.photobucket.com/albums/jj40 ... tshot2.jpg[/img]
I suggest that before you work any further with that amp you hook it up to some kind of a current limiter that will limit the fault current to safe levels that no longer create further havoc. (e.g. putting the amp in series with a light bulb is usually a good technique).
After that it's systematic measuring and replacing of bad parts.
The BT-15's I've seen had a fuse holder wired to the back like this:
[img:700:525]http://i554.photobucket.com/albums/jj40 ... tshot2.jpg[/img]
