Carver M400 PSU Repair

[Zero Cool]

I picked up a Carver M400 cube the other day. it was blowing fuses. Found a service manual online that is very detailed! following the steps in the manual i found out it had a blown Bridge rectifier for the 25V supply, got that repaired and now i can get it to the point where i raise the voltage on a variac and the triac starts to fire but it is running very rough. looks like the triac is not firing correctly. from what i gather in the SM that at idle I should only see half of each sine wave. that the triac should be cutting off at the peak of each. and what im seeing looks more like a full sine with a glitch around crossover every other sine and the transformer ticks.

changing the triac made no difference so i am assuming that the diac is bad. I cant get it past 60% of line voltage without it drawing lots of current. and adjusting the firing control pot on the PSU board doesn't seem to do anything which i figure is probably normal at this point.

I'm curious why i couldn't use a standard light dimmer adjusted to half sine to fire the transformer just for testing? there wouldn't be any regulation but I could at least test to see if the rest of the amp comes up and runs long enough to verify the problem is in the triac control section. Looking at the schematic that's all this PSU is, is a light dimmer circuit. same exact circuit with the exception of an LDR in place of the pot.


Zc

[Zero Cool]

If you look at both of these pictures you should see that they are pretty much the same circuit! at least in concept anyway.

[birdyman]

Before you pull out the diac , you can check voltage of Q3 collector to see it can be varied by adjusting PR1 and re check sw1 position if your unit has it. If you want to test amp circuit you need to use Vriac ( not the light bulbs) and short the triac or low voltage power supply to feed +-23v rails. Because M400 has PS caps in serial (it stacked mid rail caps on the top of low level rails) for mid rails, its PS circuit is not stable and saw more than few bridges on mid and low level rails failed. You want to seperate PS filter caps and for each rails.

[Zero Cool]

well after much testing tonight i figured out that there are TWO 25V bridge rectifiers. one on the backside of the board and one on the front side. they are in parallel. and while i had replaced the rear rectifier that was shorted. I did not know there was a front rectifier and I failed to check the combination of all leads and in fact the front side rectifier was shorted too! SO....an hour to disassemble and clean the massive amounts of factory applied silicone goop off of everything. I replace the double 2 amp bridge rectifier nonsense with a nice 10 amp heavy duty rectifier. Put it back together and now i can get the amp up to full power and it passes audio.

but. I kept smelling something hot but i never could find it. I figured maybe it was just more silicone goop burning off a hot resistor someplace. and i noticed that sometimes it would not fire up. then i noticed the 50V rail caps were really hot to the touch! So i probably need to replace all the caps. I was finding that there is 40V across the 2nd tier caps (the 50v rails) but they should only have 25 across them as they are stacked on top of the 25V rails...so something is still goofy.

[johnferrier]

I used to repair those! As I remember, the diac and opto would fail more often than the triac.

Rita (I think this is the same one) was probably the best person to troubleshoot amplifier problems. Looks like she is still in the business. You might email/contact her.

http://www.ritasvintageaudio.net/
(This site is down, right now. I found it just the other day and it looks like a nice operation!)

-

It was like a 32 V diac. Nothing special. The thing that burnt up the amplifier boards is if the biasing transistor went bad. Hard on the variacs that we used to power them up for the first time or for troubleshooting.

Good luck!

[Zero Cool]

I have talked to Rita and her office manager Lynda several times. Im quite impressed with her knowledge but lack of parts seems to be the biggest issue for them/these amps.

I got the amp to a point last night where i could power it up and see that both channels would pass audio. both channels seem to clip evenly and I thought everything was ok. So i reassembled it and tried to power it up and this was the first time I had the front display board back on the amp. when i powered it up the display would dimly flicker and the amp wouldn't power up. i thought i had maybe miss aligned the front panel. its quite tricky as there are pins from the display board that need to perfectly line up with a connector on the PSU board. I thought maybe i didn't get a pin in the connector or something even thought i was very careful.

I pulled the front off and powered it up again this time on the variac and she came right up. but i noticed that the AC line current draw was higher then previously. Now about 2 amps at idle. which i thought was odd. I powered down, checked for binding wires or shorts or anything out of the ordinary and didn't find anything. so i powered up again on the variac. and it wouldn't come up. power down. wait power up, no luck, check double check repeat and it comes up.

So this time while its running I check the RP1 voltage setting and check the triac to make sure its firing correctly, check for dc on the speaker outputs, it passes signal and everything appears good but i notice its humming a bit more. look at the AC line current draw and its up around 4 amps now and something is starting to smoke. kill the amp and remove the cover again. that's when i notice that the +50V rail cap is screamin hot! hmmm

Disassemble, pull the +50V cap and measure it and yup within 10% of 2200uf, its a 25V cap so i power it with my regulated DC supply to 20V and little or no current drawn ok, let it sit for 5 minutes and its staying cool...hmm ok, grab a small transformer some alligator clips and a bridge rectifier and run it and everything is good...hmmm doesn't look bulged...dangit i need an ESR meter...but...out of curiosity i put it back in the amp and connect my meter across it. I should have 25V

and BTW...its a 25V rated cap!! zero headroom! But, bring up the amp and i have over 40V across it when i finally get the amp to refire!!!

At that point it was really late and i was tired i shut everything down and went home. Will try again today. I'm wondering if there isn't a shorted cap someplace. if the neg side cap is bad and was shorted or almost shorted to ground, that would shift the positive side up...hmmmm maybe....something is not right. I will dig back into it today and see what i can figure out. In any case the thought of having to pull all those transistors out again and deal with all that goop once again...uggh, what a pain!


Dave

[birdyman]

Have you replaced all caps yet? If it has stock caps, you want to replace with mordern caps. And you want to upgrade and un-stack 50v rail caps to 63v with same capacity and ground them. You want to check for commutators for shortage and also back flow preventing diodes ( ones connect PS to amp circuit). Your amp can still play with defective parts. You might want to check bias and local bias/protection transistors ( ones that shorts drivers and output pairs) before you fire her up. you need to short triac when you connect your amp to Variac and feed her with 1/4 of AC voltage.

Good luck to you

[Zero Cool]

Caps are all stock, the whole amp is/was stock. I would bet the caps are shot and that's what took out the 2amp rectifiers.

Looks like i need to add to my parts order list!


Zc

[Zero Cool]

Ok well it seems the previous tech had the wires on the transformer wrong. I jumpered the triac and brought the variac up to 30Vac and checked voltages on the transformer and they had the 50 and 80 volt lines backwards. swapped them around and now all the voltages are correct and it will sometimes run.

If i power it up cold with a flip of the switch, it runs, but if i shut down was 2-3 seconds and try and power back up it wont come on. seems i have to wait until the psu caps have drained completely before it will power up again. not sure if this is normal???

[birdyman]

Originally Posted by Zero Cool

If i power it up cold with a flip of the switch, it runs, but if i shut down was 2-3 seconds and try and power back up it wont come on. seems i have to wait until the psu caps have drained completely before it will power up again. not sure if this is normal???

Is it with Variac??.
If it is with amp plugged into AC directly, this is normal. You are experiencing Caver protection circuit at work.

Great Work ZC