# Acoustic 450 solid state guitar/bass head repair help



## west portion (Nov 16, 2021)

Hi all! Turning the amplifier on and plugging in greets you with you its signature wall of sound, no issues. But after playing for around 10mins, the volume cuts off drastically to a quarter of the original level and distorts the signal in a guttural choked out way. The most solid bit of evidence I have of a cause to the problem is that it worked fine until I was playing it during a mild storm. There was a power surge that tripped the breaker for the outlet I was using, after that session the problem has persisted.

This heavily screams "overheating" to me, supported by how taking the board out of the enclosure makes normal volume last longer and when adding a fan it can last indefinitely. I've tested the mammoth power supply smoothing capacitor and it reads what it should and have desoldered/tested the other polarized caps. None of them read lower than the rated value, but some of them much higher for some reason. There was a 4.7uF coming in at 7.3uF, would having higher cap values be an issue?

I haven't seen any cracks in the tracers or otherwise burnt/swollen components, but i agree that there might be a spot on the board that has need of more solder. As the components heat up a short might develop? Maybe the input or output jack to the speaker? Has anyone had a similar problem with a solid state amp? I'm looking for advice on what to try next.

Thanks in advance!


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## Paul Running (Apr 12, 2020)

west portion said:


> None of them read lower than the rated value, but some of them much higher for some reason. There was a 4.7uF coming in at 7.3uF, would having higher cap values be an issue?


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.


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## Frenchy (Mar 23, 2011)

Following this. I have a 450 and now a 470. Never heard of this type of problem. 

When I use my 450, its for 5 to 6 hrs straight at min half volume ( very loud drummer... ) and never had a hitch except blown speakers.


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## Paul Running (Apr 12, 2020)

Frenchy said:


> Following this. I have a 450 and now a 470. Never heard of this type of problem.
> 
> When I use my 450, its for 5 to 6 hrs straight at min half volume ( very loud drummer... ) and never had a hitch except blown speakers.


No, you should not have any overheating problems, the 450 has 8 × 150W power devices; at 50% efficiency, it would source 600W with adequate cooling.


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## jb welder (Sep 14, 2010)

It has a series FX loop which is the 'access in/out' jack on the back. All series fx loops are notorious for causing this kind of symptom.
It does sound like a thermal issue but that can also cause the switch in that jack to act up.
Try plugging in and out of that jack a couple times in case it will self clean from some exercise. Or if you have a TRS (stereo) cable, plug one end in there and short the tip to ring on the other end. See if that helps.


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## mhammer (Nov 30, 2007)

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.


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## nonreverb (Sep 19, 2006)

Solder halo cracks due to heat can be a problem with old solid state guitar amps. Look for areas that are discoloured due to the board being heated.
Another thing you might try is get a can of freeze spray. When it fails, hit different components with a quick shot and see if it comes back. If it does, you're in the right area.


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## dtsaudio (Apr 15, 2009)

All you're doing is shooting in the dark. You need to start to follow some form of diagnostic procedure.
When it cuts out, what are the voltages like?
There are two 15V regulators to power the IC's. I guessing one one of them is overheating.


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## Paul Running (Apr 12, 2020)

Yeah, I was taught to check power supply voltages first, monitor the rails with an analog meter, watching the needle pointer for fluctuations, when the issue occurs...a scope works well too if you have one.


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## Granny Gremlin (Jun 3, 2016)

mhammer said:


> 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.
> 
> ...



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.


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## west portion (Nov 16, 2021)

Paul Running said:


> 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.


hi paul thanks for helping me out here. this is the schematic https://musicstudio.bigredroo.com.au/circuits/Acoustic_450_Schematics-Rev_1.5.pdf that i've been using, the one you posted has IC's and the guts of my 450 uses only transistor amplifiers. you mention the thermal sense circuit, but i'm having a hard time identifying it on my schematic. could you lend a hand with that? i do believe that this is the problem, the power package transistors on the back panel are growing hotter than before the problem started. 

i took some bias point readings of the input pre amp and distortion stage and the only one to read as the schematic says is the first transistor base after the input capacitor. the other readings i took were 3V higher than the schematic. is that enough to throw the circuitry out of wack? following the power supply back to the filter cap saw 98VDC on the positive terminal instead of the written 92V. 

theres a panel after the transformer that has 2 switches allowing for worldwide compatibility, switching between 108V/120V/240V (its correctly set for north america). there are 2 capacitors on this panel that are reading 75% lower than the rating on the package or on the schematic. so could the problem here be either a couple cap replacements or a faulty transformer?


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## west portion (Nov 16, 2021)

jb welder said:


> It has a series FX loop which is the 'access in/out' jack on the back. All series fx loops are notorious for causing this kind of symptom.
> It does sound like a thermal issue but that can also cause the switch in that jack to act up.
> Try plugging in and out of that jack a couple times in case it will self clean from some exercise. Or if you have a TRS (stereo) cable, plug one end in there and short the tip to ring on the other end. See if that helps.


jb thanks for the reply. i did what you said and hit the jack with some contact cleaner. hoping that this makes a dent in the problem, in fact i know a couple people that are going to be incredibly grateful for this info.


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## west portion (Nov 16, 2021)

mhammer said:


> 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.
> 
> ...


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.


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## west portion (Nov 16, 2021)

Granny Gremlin said:


> 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.


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.


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## mhammer (Nov 30, 2007)

Make sure there are no mica spacers in there that you're passing off as "plastic". Mica spacers are frequently used with TO-3 style power transistors, when you don't want the unit to ground out against whatever surface you are attaching it to. I take it the mica is thermally conductive but not electronically conductive.


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## Granny Gremlin (Jun 3, 2016)

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).



















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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## Paul Running (Apr 12, 2020)

west portion said:


> i took some bias point readings of the input pre amp and distortion stage and the only one to read as the schematic says is the first transistor base after the input capacitor. the other readings i took were 3V higher than the schematic. is that enough to throw the circuitry out of wack? following the power supply back to the filter cap saw 98VDC on the positive terminal instead of the written 92V.


Those readings are 3V higher because the main supply is higher. If you verify the 34VDC reference, it will be high too because it is fed off the main supply...the supplies are loosely regulated. When the amp is loaded, at what voltage does that 98VDC stabilize to?


west portion said:


> there are 2 capacitors on this panel that are reading 75% lower than the rating on the package or on the schematic


What's the identifier of those caps on the schematic? rating as in capacitance or working voltage?


west portion said:


> you mention the thermal sense circuit, but i'm having a hard time identifying it on my schematic.


Usually diodes mounted near the output transistors. They sense the temperature of the heat sink which controls the drive to the output devices...adjusts by tracking the temperature of the driven devices.


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## Granny Gremlin (Jun 3, 2016)

Not sure the vintage 450 had the thermal sense circuit. Here's a schem (with handy V reference points!) - don't see any diodes around the output transistors but I only took a cursory glance and it's a dense diagram.



https://musicstudio.bigredroo.com.au/circuits/Acoustic_450_Schematics-Rev_1.5.pdf


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## jb welder (Sep 14, 2010)

It is critical that all the output transistors have mica insulators between their casings and the chassis.
The output transistor sockets are on the inside of the chassis, the output transistors mount on the outside of the chassis. The chassis itself is used as a heatsink.
The transistor casings are the collectors, all of which have voltage on them. The chassis is grounded. The mica insulators prevent the collectors from being shorted to ground (which would let the magic smoke out).


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## Paul Running (Apr 12, 2020)

Does your amp have the case insulators installed on the power transistors?


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## Granny Gremlin (Jun 3, 2016)

If all (or any of) those transistors weren't properly insulated wouldn't something a bit more drastic be happening than volume drop and distortion? I mean been there, done that, blew the power supply (in this case most likely one or both of the fuses as the power rail just pours into ground?).


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## Paul Running (Apr 12, 2020)




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## jb welder (Sep 14, 2010)

Paul Running said:


> Does your amp have the case insulators installed on the power transistors?
> View attachment 388630


Those are there to protect the user from the voltage on the transistor casings. Mica insulators are still required between the transistors and the chassis.



Granny Gremlin said:


> If all (or any of) those transistors weren't properly insulated wouldn't something a bit more drastic be happening than volume drop and distortion? I mean been there, done that, blew the power supply (in this case most likely one or both of the fuses as the power rail just pours into ground?).


Usually this is correct. But you can still have broken or missing mica insulators and the heatsink compound (and in this case chassis paint) 'mostly' insulates. Then the paint or heatsink grease can act like a resistor and you get some leakage through to chassis and they might just run hot. Not necessarily the case here but it's easy to check that they are in place and not damaged/cracked.


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