sick m400

I have a early power supply m400 that has had new caps and triac installed. It has been working fine, and it got bridged for the first time and is now upset. It runs bridged or unbridged fine at low power. If it gets pushed to 25% or so it does the commutator buzz in the output. quit powerfully I might add. It flickers the power lights and buzzes until it settles down. lower the output level and it is fine. Any suggestions? Do I maybe have a rail that has a bad rectifier? It has been bullet proof for a couple of years now. It is easier to fix if it is plain broke. I can see the pattern on my lab scope when it goes nuts. Thanks for any help!
John

There are a few possibilities doctorpipe. When you say new caps, I'm assuming that you replaced ALL of the electrolytics, not just the ones in the power supply section. I also assume that you have checked both the 50V and 75V rails for proper operation and correct bias as per the service manual instructions. Otherwise you are probably looking for a bad diode or transistor in the commutation section. I think that BillD would have more info adn ideas for you to check out as he has serviced his M-400s before.

Keep us posted.

What load were you running it into bridged?

Man, when you were in there, you should have replaced those rectifiers. IIRC, one of them you have to shift it forward to not hit one of the output devices. Here's what I replaced in mine.

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Part Type    Parameters  Vendor    Part #        Cost Qty Total    M-400      Assy   Manufacturer
Electrolytic 4700µF 100V Newark   95F4473      $11.05  2  $22.1   C30-C31      P/S   United Chemi-Con
Electrolytic  700µF  35V Digi-Key 493-1881-ND   $1.51  6  $9.06   C20-C25      P/S   Nichicon
Electrolytic   10µF  35V Digi-Key 493-1850-ND   $0.23  3  $0.69  C1,C5,C16     P/S   Nichicon
Electrolytic  1µF    50V Digi-Key 493-1886-ND   $0.23  4  $0.92 C6,C8,C10,C3   P/S   Nichicon
Electrolytic  22µF   16V Digi-Key 493-1769-ND   $0.15 10  $1.51   C7,C9,C14    P/S   Nichicon
Electrolytic  22µF   16V Digi-Key 493-1769-ND   $0.15            C10-C11 L/R L/R Amp Nichicon
Electrolytic 4.7µF   25V Digi-Key 493-1808-ND   $0.23  2  $0.46    C6 L/R    L/R Amp Nichicon
Electrolytic 470µF  6.3V Digi-Key 493-3014-ND   $1.15  2  $2.30    C3 L/R    L/R Amp Nichicon
Rectifiers      8A  400V Digi-Key GBU8G-BPMS-ND $1.60  3  $4.80  BR2,BR3,BR4   P/S   Micro Commercial
Fuse Holder              Digi-Key 283-2347-ND   $3.45  1  $3.45              Chassis Cooper/Bussmann
Vactrol                  Allied   980-0710      $2.94  1  $2.94  OC1-VTL5C3    P/S   PerkinElmer
                                               $48.23

BR2, BR3 and BR4 are the rectifiers. Because I used caps that were a little larger, it required me to replace the fuse holder with a more slender one. The Vactrol is the type of optocoupler that the original M-400 used (Light Dependent Resistor).

Here is what happened to make the little cube sick. I have a system designed for three amps. So getting the tfm-35 caused a problem, (now there are four amps). You can't have a perfectly good amp and not use it! So I decided to bridge two cubes and let them run the subs. Sounds like a good idea. The subs are 8 ohms and there are two of them, left and right. It did some weird thumps and pops when I was checking the operation. Then the l.e.d.'s started freaking out, like I was melting the voice coils or something. It was not even pushing five watts at the time of this fault. I moved the cable and it hummed really loud! And tripped out again. I hooked it up to the pair of subs (one left and one right) as before and it worked but made commutator noise,but no pops or thumps, if you pushed it even a little. I found out the cable (even though it is a gold one) had a bad ground. When it was hooked to the left and right channels, the other cable took care of the ground. So by bridging the unit, it lost the ground and the input voltage went sky high. I found it killed the triac, so I put another in and replaced the opto-coupler, because I keep them on hand. With a minor adjustment, it is alive and well again. Both of the bridged amps have a separate ground wires now, for safety's sake. So if you decide to bridge an amp, consider a ground wire to save on future repairs. I am amazed that the amp is fine. Tough little bugger! So that is the scoop. Thanks for the parts list. I will be stocking up.

Congratulations on resurrecting your amp, doc!

Great information! Will keep this in mind as I acquire another M400 for similar applicaiont.

Congrats on the fix.

Stupid question doctorpipe; what kind of pipe does your handle refer to?

The kind of pipes are exhaust pipes. I have lots of fun with that question. It is interesting what some people guess. It is because of a coanda cone (dynamic exhaust check valve) I built for a SCCA ITB car years ago. The name stuck. I am a Engine builder/developer, and sales and tech support for Audie Technology (Cam profiling equipment, Flow bench data aquisition, and engine simulation). So when I apply all of this, the engine gets developed on the computer, and optimized from there. That also means figuring the correct exhaust sizes and lengths. I also repair and modify German cars, and Jaguar v-12's. Nobody local understands them, so I do that too. So my hobby is the stereo and electronics. I repair a few amps/receivers a year for other people as a hobby. You can see my mug online at www.audietech.com. Sorry for being so long.

You are right about the weird questions the word pipe brings up doc! Reason I was asking is because I am a pipe smoker and a fellow pipe smoker who uses the nickname pipedoc is a Urologist!!! Pipes are also used in plumbing so there are several possibilities!

Most people think it refers to smoking a Cheech and Chong pipe. My favorite, a woman wondered if I was in the porn industry and that was my stage name! Definitely not either of those!

Hehe, I hear ya'. Glad to see that you are back up and running and welcome to the forum.

TREX