M-4 rebuild begins

Thought I might post a few pics and details of my M-4 resurrection. It's a very nice unit cosmetically. Supposedly the left channel works and it does have all it's original transistors. The right channel, OTOH, is a basket case. It's already been rebuilt at least once as the outputs have been replaced and quite a few of the resistors as well.

So now on this melt down, two of the outputs are blown as well as the emitter resistors, one of the drivers, quite a few smaller resistors, etc. You get the picture. Thanks to some of the AK'ers that know about this amp, I've gotten some advice about replacement parts and the existing M-4 threads were quite helpful.

Up to this point I've rebuilt the blown amp board and replaced caps and double diodes on the pre-driver board as well as replaced a couple of 2SC458's which apparently are known to be noise problems. There are a couple of spots on the amp PCB where flaming resistors have burned into the top layer, but otherwise it cleaned up pretty well. Now waiting for another package from Mouser.

Couple of pics.

Thanks for letting me share.
John

Went through the good channel today: applied new heat sink grease, replaced some caps and the 2SC458's. Also replaced a few caps on the main power supply board. I wonder if I should replace the speaker relays given the years of switching and the fact that one channel has been blown up a couple of times.

I have a question regarding fusistors. If they measure correct values, do I leave them? Or are they a problem waiting to happen so should be replaced now?

Thanks,
John

Yes, replace the speaker relays. Old relays can significantly degrade sound quality.

I agree replace the relays while you're in there. Here one opened up from my M-4, you can see the pitting on the contacts near the outer edge of the relay:

What relay p/n did you use for the replacement?
Thanks

I see you picked this up. I had cold feet when he told me about the smoke :worried:

These are the relays that I have used with good success

http://www.mouser.com/ProductDetail...mWgSZG54Vc8HnVxY%2bugZwXlu4uwXExjwgvdiwN49Q==

Hey, if you need any parts (PCBs etc) I have a carcass M-4. The cosmetics looked good in the pictures so I assume it is worthy.

It's quite good cosmetically. He took an offer which helped although it wasn't a giveaway. I waited a few days since you expressed interest, but then the seller said he hadn't had any offers so I took that as the all clear. The left channel looks completely original and all seems to be ok. I freshened the heatsink grease and touched a few capacitors. The bad channel was quite burned up but I think it will be ok.

These relays don't appear to have quite the same pinout unless I'm misreading the diagrams. How do you install them? I wonder if installing a relay socket would be a good idea. Not sure if there is enough headspace for a socket with the copper bracket over them, but I might look into it.

Best,
John

Avionic might want to step in but if I remember right you have to snap off a couple of pins?

nerdorama said:

It's quite good cosmetically. He took an offer which helped although it wasn't a giveaway. I waited a few days since you expressed interest, but then the seller said he hadn't had any offers so I took that as the all clear. The left channel looks completely original and all seems to be ok. I freshened the heatsink grease and touched a few capacitors. The bad channel was quite burned up but I think it will be ok.

These relays don't appear to have quite the same pinout unless I'm misreading the diagrams. How do you install them? I wonder if installing a relay socket would be a good idea. Not sure if there is enough headspace for a socket with the copper bracket over them, but I might look into it.

Best,
John

Click to expand...

As stoutblock said you will need to snip off a couple unused pins on the new relay.

I attempted to dig into the relays yesterday. Couldn't quite see how to get the copper shield off from over the relays. It's foot seems to be anchored under the main filter caps. I could likely desolder and work them out, but it would tight. How do you get them out?
Thanks,
John

nerdorama said:

I attempted to dig into the relays yesterday. Couldn't quite see how to get the copper shield off from over the relays. It's foot seems to be anchored under the main filter caps. I could likely desolder and work them out, but it would tight. How do you get them out?
Thanks,
John

Click to expand...

Don't even try to remove that copper shield.Desolder the relay pins. Loosen the back panel enough to tilt it back and then remove relays from the PC board.Actually I just desolder and remove the relays.Its not rocket science.

I replace these relays as a matter of course. If they are not bad, they will be soon.

Got another order of parts today from Mouser. I'll try to get it running this weekend. I didn't have the relays in this order, but I'll plan to order then in the near future. Will see how my luck goes this weekend getting it running and replace the relays soon just to be certain.

Thanks to all for the help.
John

Began the startup this afternoon. Left channel, the undamaged one, came up on the variac and adjusted easily. I left the right power plug disconnected.

Then connected the right channel and started to power up while watching the bias test points. The voltage went way up almost right away, so I think I've got something going on that still need to be fixed.

One step at a time. Getting there.

John

nerdorama said:

Began the startup this afternoon. Left channel, the undamaged one, came up on the variac and adjusted easily. I left the right power plug disconnected.

Then connected the right channel and started to power up while watching the bias test points. The voltage went way up almost right away, so I think I've got something going on that still need to be fixed.

One step at a time. Getting there.

John

Click to expand...

Pull the output transistors on the right channel heatsink assembly and check'm with your meter just to verify that they are still serviceable.

It lives. I checked the outputs and then after more searching found that I had installed a pair of drivers crosswise. NPN for PNP. I hadn't got my variac up very far and stopped when the bias current started climbing so I think I caught it before damage. Both sides play nice sine waves. Bias and offset are good although the repaired side seems more stable than the original left channel.

Anyway, will give it a listen soon. Thanks for all the helpful advice about transistor subs and other things. This should be a good amp. I've got a nice pair of ADS 910's that I think will like it.

If it's helpful to anyone, I can post the transistor subs I used.

Best,
John

Well, I listened to music with it for the first time today. Sounds really nice. I'll probably keep it. I already have a C-4 to pair with it. The C-4 is functional and I've replaced a few components, but I'll need to go through it more carefully. Will give them a pairing this weekend and see what happens. I think I'll try it with my tube line stage as well. Might be a good combo.

All the best,
John