Carver pm-1200 fan woes

[pjaneiro]

Got my hands on another used pm-1200, it seems to work nicely but the fans are not stable, by not stable i mean that they operate slowly on normal setting and are irratic on high speed, (spining slow to FAST), this amp has been fitted with the new service bulletins, new pcb with 4 caps, so i was wondering where should i start to look to solve my problem, Thanks...

BTW : this amp was on all the time, and was in a heavy smoke environment, so the interior is all coated with this greasy residue....so i don't know if this can affect something

[mrshow4u]

...I'd start with thermal sensing stuff. Thermistors on the heatsink, etc. Do you have a road map (Schematic)?

[anatech]

Hi mrshow4u,
The problem is simpler than that. Carver used thermal cutouts, not linear sensors and the fans run off their own unregulated supply (HINT!)

Hi pjaneiro,
I've replaced a ton of fans over the years. Right now, use a touch of Methyl Hydrate on the shaft of the fans as they clear the bearing. I'm trying to wash out the smoke residue. After you've cleaned out the residue, use a light weight (single weight, NOT 3 'n1 or anything like that) machine oil and lubricate the bearings. Turn the shafts only by hand, do not run the amp yet! Once they are turning freely with a touch of excess oil around the bearing, you can turn the amp on. You should be able to clearly feel the poles as you rotote the shafts.

The fans were normally wired in series. One stops and the other turns faster.

If the supplies in your amplifier are unstable, the fans will also do as you describe. Fix that problem first.

-Chris

[mrshow4u]

Hi Chris,

The problem is simpler than that. Carver used thermal cutouts, not linear sensors and the fans run off their own unregulated supply (HINT!)

I see, ala Hafler 500's and not ala Crown. That is good to know. I've worked on a lot of Carver Consumer stuff, but not their Pro line. Little sintered bronze bearings, eh?

[anatech]

Hi mrshow4u,
The consumer amps didn't use the fan normally, the cut out was the same. They can be very similar otherwise.

Little sintered bronze bearings, eh?

Yep, smoke residue gums them right up. If you just add oil, you end up with a sticky mess that will go further into the bearing. I hold the motor shaft down so the goo runs away from the bearing. You will remove older lubricant also, so this isn't a bad thing. New lube will then extend the motor life.

Sometimes the brushes will be shot already, or the bearing seized. Just do what you can.

-Chris

[pjaneiro]

Thanks guys, i just cleaned both fans, and when i power up them up with an external power source they run nicely, so my problem is probably the voltage supply, i have the schematics, but where should i start, these are beasts to work on, , also i just noticed that my right channel runs a bit higher than my left !, the clipping light even lights up a bit when i play it hard, would the bias need adjusting ? or then again the problem is elsewhere ?

Thanks again for your help...

[mrshow4u]

Hi pj, I think Chris was saying the fans are in series. That would indicate a mechanical friction difference in the two fans, if they're turning at different speeds. If one amp channel is warmer than the other after just idling for a while, that would indicate a bias/idle current difference between the two channels.

Clipping light might be coming on early? Do you have a way to check at what power level? A ot of clipping circuits turn on when the input to output (minus gain) difference becomes large. This could happen from open emitter resistors, or the commutator transistors not switching the higher B+/B- rails onto the output devices.

[anatech]

Hi pjaneiro,
Troubleshooting and repairing Carver amps requires more skill and understanding than I've seen the average repair shop exhibit.

Bias adjustments will not affect this. Do not adjust any controls!

Please take your amplifier to a really good service tech who is used to servicing Carver. Most every tech will tell you they are, look for an older tech that was once Carver warranty. This is the least expensive way to get your amplifier repaired. He probably looks tired and is busy.

The greatest amount of damage I've seen comes from small shops or techs who refuse to admit someting may be beyond them. A "friend" that knows electronics is even scarier second only to an electronics teacher that decides the amp was designed incorrectly. An electronics engineer (now professor) stands alone as the single most destructive force in the universe. Just observations from servicing equipment for a long, long time. Please don't cause harm to your amplifier.

-Chris

[pjaneiro]

I admit that I am more used to reparing std amplifier designs like akai's, technics, NAD and such, even some of the older models like the bgw's 750's and 8000's, or the good old Dynaco's...Problem is that there is no Carver service center here in montreal, or if there is or was, well i am unaware of where they are so if anybody knows where i could find a guy here in the montreal region, that would be nice...

Thanks

[anatech]

Hi pjaneiro,

Problem is that there is no Carver service center here in montreal

Yes, I know. Try to find a good technician if you can, otherwise you may have to look outside your area.

The amount of destruction coming from Quebec was out of proportion to the rest of the country. Some dealers had their service status removed due to dishonesty in billing. Others for incredibly bad workmanship.

Everyone, just a word about warranty claims if you are a warranty shop. Each product has it's own "normal" percentages of service items. When claims are falsely billed, you stand out as being dishonest, or really very stupid. You can tell the good shops from the bad ones very easily. A store's history will stand out as well. There is no beating "the system" in the long run. Distributors talk to each other too, so you kill your reputation over many brands instead of one only.

-Chris