HELP with restoration of 2 Carver PM-1200's

[frickecello]

Hi, some time ago I bought a pair of Carver PM-1200's amplifiers, they werent in very good condition, only one really worked when I bought them and since I've heard lots of things about them I thought that I could restore the fried one.

But the 'good one' stopped working within the next 2-3 months after bought and since then I've been wanting to restore them.

I want to repair the one that worked, because it is complete (it has all transistors) I have checked all transistors and they seem to be ok, I suspect that the amps were fried or damaged by the first owner 'playing' with bridged operation into low Z loads.

There are no fried resistors or blown caps.

These are the 3 rail voltages (around): 126 V, 78 V and 37 V.

I have read a little about how these amps work and I know that there are 3 different voltages, but I dont know if these are ok.


When turned ON the transformer makes a buzzing sound, and when turned OFF the PROTECTION led indicator lights.

Any help from anyone here who owns or knows about this amps will be appreciated.

[anatech]

Hi frickecello,
You might want to rethink the "repair this myself" idea. You may be successful but you have picked one of the more complicated amps to repair.

The triac on the AC line input is not a switch, it is a voltage regulator that works on the mains. The caps will end to stay charged up for a long time, so discharge all of them every time you work on it. You have those three rails you have identified, plus a 15 V pair and a fan supply.

So, if you really feel you can fix this (this should give you pause), be aware that you can extend the damage greatly. Check the two lower rails capacitors you mentioned (four total) and the rectifiers. There will be some hard to get to resistors that may have overheated and some damaged foil on the board. Work slow, work carefully and take many pictures as you go. Damaged solder joints are possible. You had better work very neatly.

-Chris

[frickecello]

Originally posted by anatech
Hi frickecello,
You might want to rethink the "repair this myself" idea. You may be successful but you have picked one of the more complicated amps to repair.

The triac on the AC line input is not a switch, it is a voltage regulator that works on the mains. The caps will end to stay charged up for a long time, so discharge all of them every time you work on it. You have those three rails you have identified, plus a 15 V pair and a fan supply.

So, if you really feel you can fix this (this should give you pause), be aware that you can extend the damage greatly. Check the two lower rails capacitors you mentioned (four total) and the rectifiers. There will be some hard to get to resistors that may have overheated and some damaged foil on the board. Work slow, work carefully and take many pictures as you go. Damaged solder joints are possible. You had better work very neatly.

-Chris

Thanks for your recommendations I will sure follow them.

About the DIY attitude:

I have tried to have the amps repaired but havent found any competent technician who knows something about this amps, I have worked with vacuum tubes mainly and I know that those big 11000 uF powerlytics can be harmful or fatal if not discharged before servicing the amp.

I have examined very carefully PCB tracks and they seem to be ok, I will check all PSU capacitors using an RC circuit to check charge-discharge time with an scope. also I will check for foil damage.

Any other tip that might be helpful?

[anatech]

Hi frickecello,

I have tried to have the amps repaired but havent found any competent technician who knows something about this amps

That is a very sad fact everywhere in the world it seems these days. I can understand your position.

I have worked with vacuum tubes mainly

That's good, because rails as high as these have other common failure modes that most SS amps do not suffer from. The drive circuits run at the full 125 V rails, so think failure modes at 250 VDC. Your tube experience will help here.

I know that those big 11000 uF powerlytics can be harmful or fatal if not discharged before servicing the amp.

That was a great concern. There are no effective bleeders so the charge remains for a while. Keep that in mind and the biggest source for technician induced damaged is removed.

Any other tip that might be helpful?

Two things that come to mind. Check for degraded solder joints due to heat. Check for resistors that have changed value. Mind the voltage ratings of your replacement resistors. Sometimes the input caps on the amp cards suffer from cracked solder joints. Lubricate the fans.

This amplifier can be soft started on a variac, but it's not straight forward. If you can, run the supplies up on other DC supplies. If you short the triac, you can use a variac up until about 40% of full. Things can get hairy after that so I don't recommend you go any higher. This will allow you to troubleshoot without burning anything else.

Do a search for Carver amps to gain other advice I've given over the years. Look carefully at cleaning and lubrication for all models.

Good luck, -Chris