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Moscode 600 Noisy Fan

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I have an old, much loved Moscode 600 that has no modifications. My technical ability pretty much ends at changing out tubes. Lately, the large fan cooling the heat sink has gone into overdrive, drowning out the music from the speakers. I cleaned out all the dust and dirt from the unit, but it doesn't seem to help. The AC fan is connected to a primitive-looking thermostat that looks rather fragile. I haven't changed any caps, but I suspect they ought to be replaced after 25 years. Apart from the noisy fan, the amp still sounds pretty good to me.

At any rate, I was wondering if anyone could provide me some diagnostics to fix the problem.

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I'm assuming this thing has a two speed fan? Normally the fan would be fed by a resistor in parallel with the thermostat on the heat sink, and run slow to minimize noise, when the amplifier is driven hard the heat sinks warm up sufficiently to cause the thermostat to close applying full line voltage to the fan. Once the amp cools down the thermostat opens and the fan returns to low speed operation. Is this the way the cooling system used to work? - If so the thermostat has probably failed in the closed position, easily checked with an ohm meter, with the power **disconnected.**
Thanks, Kevin. I believe you're correct. Here's a picture of the heat sink with the thermostat and the fan in the red boxes. The 2nd pic shows the thermostat (I think).


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I will try to ask George. However, I haven't been able to get a response from him for other questions.

The resisters are 50 ohm 10W.

The issue is: why does the fan (blowing through the tunnel heat sink) switch to a higher speed? There are several possibilities:
1. Amp is overheating for some reason. The fan is supposed to speed up the unit gets overloaded. However, the fan speeds up under any load within minutes of applying the load. It never used to do this. What else causes overheating?
2. The fan is malfunctioning. This is a Howard Industries fan. There are some used fans on ebay, but I can't find the company (out of business?) or a datasheet for the fan (1175-06-4).
3. The heat protection circuit is thinks the unit is overheating when it's not. Perhaps the wire wound resisters have gone bad?
4. The heat sink is filled with dust and debris. Well, it was, but after I cleaned it out, the behavior did not change.
The unit is supposed to shut down completely if a certain temperature (or load?) is reached. This has not happened so far.

If anyone can shed some light, I'd appreciate it. I will direct my question to George in the meantime.
The schematics show something for the fan I can't interpret. At the top left of the image below, you can see the fan and "TS1" "TS2" and "SS1". I don't think there is an actual "thermostat", just two 50 ohm 10 W resistors that regulate the fan operation.


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Are you sure your moscode has not been modified ? The resisters(you labeled as thermostat) that you circled in your picture, looks like an after thought. I found some pictures on the internet of other 600s which doesn't have them. To double check. I've pulled mine from storage and took a picture. I believe the TS-1 TS-2 are thermostat ... one to kick up the fan the second one to shutdown the amp.

Is your heatsink getting very hot ? You might want to bring to a repair tech if you are not comfortable in taking it apart and finding whats wrong. I believe the output stage comes apart pretty easily.


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Thanks for the photo, it's really interesting. When I bought the unit, the original owner took it to George Kay to have it checked out before sending it to me. I dimly remember George telling me that he either replaced or "fixed" the fan. So, I assume George Kay modified it back in the early 90s. The fan running high only started recently.

Can you tell me where the leads to your fan go? Do you know what the unit uses for the thermostats?

I'm not sure if the heatsink is getting very hot. I don't think so, but I'm not really sure how to determine if it's overheating. What is the correct temperature?

I've taken the heatsink assembly out to clean it, and disassembling it doesn't look too hard. I'm just not sure what I'd look for once it's in pieces. Blown mosfets? I haven't tested or replaced those wirewound resisters. I suppose they could go bad and give misleading voltage to the fan?
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