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Acoustic 450 solid state guitar/bass head repair help

2020 Views 22 Replies 8 Participants Last post by  jb welder
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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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 should be fine. It sounds like the thermal sense circuit is 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 there an area that seems to be hotter than normal? Here's a schematic if you don't have one.


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