Electrolytics have a wide tolerance band...you should be fine. It sounds like the thermal sense circuit is activating...it will reduce current flow which may reduce the volume or cut in and out randomly. If you can read schematics, you should be able to localize the area of concern...is there an area that seems to be hotter than normal? Here's a schematic if you don't have one.
Acoustic 450 solid state guitar/bass head repair help
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My usual instinct is to look for bad caps if there is a problem that gets worse over some short period of time. If something is unstable, chances are reasonably good that something is not providing a needed current or voltage, because the devices that store that current/voltage are faulty.
Another possibility is heat buildup. But why would heat build up over the described interval? I've previously described a repair I did to a member's Diamond Memory Lane pedal. The member had 2 of them, ostensibly identical. One was quiet as a mouse, and the other had an annoying whine.
I had several e-mail exchanges with their tech support person, and it wasn't until I mentioned that the whine took about 5 minutes to start occurring, that it triggered his insight. It seems that in an early batch of the pedals, the regulator chips they ordered (from a reliable manufacturer and distributor) had off-spec heat fins, which they innocently installed, and didn't discover the issue until later. The device looks like this and should have a thick heat fin, so that heat can be dissipated. In some instances, the fin may be bolted to a larger heat sink or to a chassis, to keep the device cool. In this pedal, the regulator was free-standing, hence relying solely on the heat fin to keep the device at a suitable temperature. Of course, enclosed in a box, there wasn't much natural cooling, so it took about 5 minutes of power for it to get too hot, and the whining would start as the voltage produced began to drift. As the tech suggested, I replaced the device with a new one that had the proper thick heat fin, and the problem was solved.
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Why am I telling you this? The Acoustic amp is from 1973, from what I gather. Back when it was made, I'm pretty sure some thermal compound was applied to the power transistors, so that their heat could be easily dissipated through whatever larger surface they were attached to. That stuff generally stays pasty for a long time, but it's been damn near 50 years since it was applied, and I have no idea what sort of storage conditions it was in. It is quite possible that one or more of the power transistors are overheating because their thermal dissipation is insufficient. The transistors themselves are fine and so is the system implemented for keeping them at a suitable temperature. But they just aren't making decent thermal contact anymore, perhaps because the thermal compound dried up.
Just a hunch.
Another possibility is heat buildup. But why would heat build up over the described interval? I've previously described a repair I did to a member's Diamond Memory Lane pedal. The member had 2 of them, ostensibly identical. One was quiet as a mouse, and the other had an annoying whine.
I had several e-mail exchanges with their tech support person, and it wasn't until I mentioned that the whine took about 5 minutes to start occurring, that it triggered his insight. It seems that in an early batch of the pedals, the regulator chips they ordered (from a reliable manufacturer and distributor) had off-spec heat fins, which they innocently installed, and didn't discover the issue until later. The device looks like this and should have a thick heat fin, so that heat can be dissipated. In some instances, the fin may be bolted to a larger heat sink or to a chassis, to keep the device cool. In this pedal, the regulator was free-standing, hence relying solely on the heat fin to keep the device at a suitable temperature. Of course, enclosed in a box, there wasn't much natural cooling, so it took about 5 minutes of power for it to get too hot, and the whining would start as the voltage produced began to drift. As the tech suggested, I replaced the device with a new one that had the proper thick heat fin, and the problem was solved.
Why am I telling you this? The Acoustic amp is from 1973, from what I gather. Back when it was made, I'm pretty sure some thermal compound was applied to the power transistors, so that their heat could be easily dissipated through whatever larger surface they were attached to. That stuff generally stays pasty for a long time, but it's been damn near 50 years since it was applied, and I have no idea what sort of storage conditions it was in. It is quite possible that one or more of the power transistors are overheating because their thermal dissipation is insufficient. The transistors themselves are fine and so is the system implemented for keeping them at a suitable temperature. But they just aren't making decent thermal contact anymore, perhaps because the thermal compound dried up.
Just a hunch.
My usual instinct is to look for bad caps if there is a problem that gets worse over some short period of time. If something is unstable, chances are reasonably good that something is not providing a needed current or voltage, because the devices that store that current/voltage are faulty.
Another possibility is heat buildup. But why would heat build up over the described interval? I've previously described a repair I did to a member's Diamond Memory Lane pedal. The member had 2 of them, ostensibly identical. One was quiet as a mouse, and the other had an annoying whine.
I had several e-mail exchanges with their tech support person, and it wasn't until I mentioned that the whine took about 5 minutes to start occurring, that it triggered his insight. It seems that in an early batch of the pedals, the regulator chips they ordered (from a reliable manufacturer and distributor) had off-spec heat fins, which they innocently installed, and didn't discover the issue until later. The device looks like this and should have a thick heat fin, so that heat can be dissipated. In some instances, the fin may be bolted to a larger heat sink or to a chassis, to keep the device cool. In this pedal, the regulator was free-standing, hence relying solely on the heat fin to keep the device at a suitable temperature. Of course, enclosed in a box, there wasn't much natural cooling, so it took about 5 minutes of power for it to get too hot, and the whining would start as the voltage produced began to drift. As the tech suggested, I replaced the device with a new one that had the proper thick heat fin, and the problem was solved.
![]()
Why am I telling you this? The Acoustic amp is from 1973, from what I gather. Back when it was made, I'm pretty sure some thermal compound was applied to the power transistors, so that their heat could be easily dissipated through whatever larger surface they were attached to. That stuff generally stays pasty for a long time, but it's been damn near 50 years since it was applied, and I have no idea what sort of storage conditions it was in. It is quite possible that one or more of the power transistors are overheating because their thermal dissipation is insufficient. The transistors themselves are fine and so is the system implemented for keeping them at a suitable temperature. But they just aren't making decent thermal contact anymore, perhaps because the thermal compound dried up.
Just a hunch.
Yeah I've had exactly this happen with a Traynor TS a year ago. It looked like it had survived a house fire tho. Also didn't the Acoustic heads use TO-3 format power transistors mounted directly on the rear with heat sinks sticking out the back (just like Traynor Monoblocks, Peavey Mark IVs etc). Those things should be fine - they dissipate heat much better than the TO-220 package like you pictured (and as used in the TS series) - not even sure if they ever used compound on those - you see them socketted half the time.
The issue starting after a storm surge seems to be more than coincidence. The fx loop jack thing @jb welder mentioned is worth checking because quick/easy and fits the criiterea for such a coincidence, but after that I agree with @dtsaudio - gotta start methodical troubleshooting (and that would eventually cover most of the other things suggested). At the very least, popping the top to get in there with a meter may expose something obviously burnt.
damn hammer spoken like an experienced vet. i checked the bias points for the input pre-amp and found values off by about 3V too high, with the positive side of the power supply filter cap being 6V higher than the written value from the schematic (https://musicstudio.bigredroo.com.au/circuits/Acoustic_450_Schematics-Rev_1.5.pdf). the thermal paste on the power transistors is almost surely compromised so if the issue cant be solved by taming the input voltage than i can try 6 new 2n3773 with some fresh thermal paste.My usual instinct is to look for bad caps if there is a problem that gets worse over some short period of time. If something is unstable, chances are reasonably good that something is not providing a needed current or voltage, because the devices that store that current/voltage are faulty.
Another possibility is heat buildup. But why would heat build up over the described interval? I've previously described a repair I did to a member's Diamond Memory Lane pedal. The member had 2 of them, ostensibly identical. One was quiet as a mouse, and the other had an annoying whine.
I had several e-mail exchanges with their tech support person, and it wasn't until I mentioned that the whine took about 5 minutes to start occurring, that it triggered his insight. It seems that in an early batch of the pedals, the regulator chips they ordered (from a reliable manufacturer and distributor) had off-spec heat fins, which they innocently installed, and didn't discover the issue until later. The device looks like this and should have a thick heat fin, so that heat can be dissipated. In some instances, the fin may be bolted to a larger heat sink or to a chassis, to keep the device cool. In this pedal, the regulator was free-standing, hence relying solely on the heat fin to keep the device at a suitable temperature. Of course, enclosed in a box, there wasn't much natural cooling, so it took about 5 minutes of power for it to get too hot, and the whining would start as the voltage produced began to drift. As the tech suggested, I replaced the device with a new one that had the proper thick heat fin, and the problem was solved.
![]()
Why am I telling you this? The Acoustic amp is from 1973, from what I gather. Back when it was made, I'm pretty sure some thermal compound was applied to the power transistors, so that their heat could be easily dissipated through whatever larger surface they were attached to. That stuff generally stays pasty for a long time, but it's been damn near 50 years since it was applied, and I have no idea what sort of storage conditions it was in. It is quite possible that one or more of the power transistors are overheating because their thermal dissipation is insufficient. The transistors themselves are fine and so is the system implemented for keeping them at a suitable temperature. But they just aren't making decent thermal contact anymore, perhaps because the thermal compound dried up.
Just a hunch.
gg palin' thanks for the input. the power transistors on the back dont have the heat sinks that im used to. theyre connected flat to the chassis on the back but on the inner side before the circuit board they rest on flat squares seemingly cut from plastic sheets. these could be only for use as spacers... they dont seem to be good for much else but i of course am probably out of my experience league. i'll include a picture for clarity.
Those are sockets, like I suggested might be there. I'm not certain, so someone more experienced please jump in, but I think you don't need mica spacers. There's actually only 2 leads on a TO-3, with the 3rd (collector) being the metal can (and screw mounts). If it does need to be insulated, it would be a matter of grommets on the mount screws which is much easier and doesn't break down over time, slide out of place or off and get lost etc (i.e. they should already be there).
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This socket pictured is ovular vs square but you can see the same central metal tab across the screws and middle pin as in the amp pic. So the good news is if you wanna try a quick swap to see if the transistors are burnt, it's super easy (no soldering - pop in and pop out).
Also note that if your Acoustic has these TO-3s, then the schematic posted above is NOT the correct one (it's for the modern "reissue" B450 which uses all ICs everywhere).
Anyway, even without heatsinks, T0-3 transistors dissipate heat very well (better than other packages anyway) because of the huge metal surface area and being mounted on the outside of the chassis rather than the inside.
This socket pictured is ovular vs square but you can see the same central metal tab across the screws and middle pin as in the amp pic. So the good news is if you wanna try a quick swap to see if the transistors are burnt, it's super easy (no soldering - pop in and pop out).
Also note that if your Acoustic has these TO-3s, then the schematic posted above is NOT the correct one (it's for the modern "reissue" B450 which uses all ICs everywhere).
Anyway, even without heatsinks, T0-3 transistors dissipate heat very well (better than other packages anyway) because of the huge metal surface area and being mounted on the outside of the chassis rather than the inside.
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