Carver PM 1.5 fan mods

[Bjorn Z]

Hi everyone!

We have 8 Carver PM 1.5's and 20 or so PM 350's at work, and I have been servicing most of them over the past week.

The PM 1.5 fan has a high/low setting, and the manual recommends the high setting as 'normal', but this is unfortunately too noisy by modern standards and can be a real problem in a conference type scenario. If possible, I'd like to mod these so that they switch to high mode when required (engineers aren't used to things like fan switches on amps anymore...). I don't have the schematic for the 1.5's, but the PM 350's have a thermal switch that shorts one of the resistors in series with the motor, and I'm hoping that this scheme could be applied to the 1.5's as well.

Does that sound like a reasonable plan, or will I get in trouble?

Thanks in advance for any help on this!

Bjorn

[anatech]

Hi Bjorn,
Sounds like a good plan. First, remove the switch and replace it with a thermal switch near the center of the chassis. That's where the outputs are. Dress the leads close to the chassis.

While you are there, oil each motor with a light, single weight oil with no additives. Both the front and rear bearings. Make sure you do this as normal maintenance.

-Chris

Edit: Bjorn, I forgot to mention that there is + and - 118 VDC (possibly higher) on those high supply rails. Please be very, very careful in there.

[Bjorn Z]

Thanks for that Chris, seems like it should be pretty straightforward. I will be careful, but have had some time with valves, so high voltage is something I'm used to.

Regarding the PM350's, I've had to replace some of the fan motors on these. I've used 6-15V motors from RS, but they are 3-pole rather than 5-pole, but seem to work ok. With the amp pushed hard into a dummy load, I measure about 13V across it, although I imagine the DC is quite bumpy and peaks may be well above that. It does sound like it's working harder than the same motor run from a bench supply with clean 15V...is this something to worry about or is it not that critical? I was also a bit worried a 3-pole motor might not start as easily as a 5-pole, but this doesn't seem to be a problem.

Thanks a lot for your help so far, I've learnt quite a bit from your previous Carver posts as well!

Bjorn

[anatech]

Hi Bjorn,
If they work, use them unless you run into problems. Keep the bearings lubricated. Keep your eyes open for other sources for the motors. I've seen similar ones in hairdryers, so an appliance repair place might be good to wander into.

I don't recall if there were capacitors for the fan supply, you could certainly use them, and beef up the diodes if you do. They will see higher peak currents. This may have varied by model.

Over here we run at 60 Hz, you are at 50 Hz right? The motors may run better at a higher frequency to begin with, not that this helps you. Just thinking what may cause you troubles.

I have seen "muffin" fans used before as well. You could certainly move to those. They might move more air and you may need to play with resistor values. These probably run from the mains. I have never modified one. I would expect them to run more quietly also.

-Chris

[solderhead]

Originally posted by Bjorn Z
The PM 1.5 fan has a high/low setting, and the manual recommends the high setting as 'normal', but this is unfortunately too noisy by modern standards and can be a real problem in a conference type scenario. If possible, I'd like to mod these so that they switch to high mode when required (engineers aren't used to things like fan switches on amps anymore...). I don't have the schematic for the 1.5's, but the PM 350's have a thermal switch that shorts one of the resistors in series with the motor, and I'm hoping that this scheme could be applied to the 1.5's as well.

Bjorn, if you take a closer look at the amp, you should see that the PM-1.5 amps already have such a circuit in place. If you trace the wires from the PCB for the fan, you'll find a pair of wires that lead to a temp sensor that's mounted in the center of the chassis. The temp sensor is a normally open switch that closes if the chassis temperature rises.

The switch is mounted in parallel with the supply voltage to the fan. When the temperature increases and the switch closes , the voltage dropping resistors are bypassed and the fan runs at full speed.