MC240 - Back to Life

[dshoaf]

Been running the amp now for many hours and its sounding pretty good and not getting too hot either at the tubes or the transformers. Left all the original resistors in but measure them all while replacing the caps. The bass slam is definitely happening which is really cool. Really enjoying it. Will run it for a few weeks and then check voltages and resistances again.

Now you're onto it, Chris. This is the way to run a 'shakedown cruise' with a restored Mac amp. Your '240 is pretty characteristic of what to expect for a restoration. Never make assumptions!

Oh, and the advice on a set of _known good_ tubes is essential. Once you have a set - or at least one of each kind for your amps - keep them safe and never mix them with the other stash of tubes. They're your gold standard.

Cheers,

David

 

[chrisinsc]

I have a Vibro Champ that I'm thinking about putting on the bench. She is an early 70's model I think. I had her gone over in the early 90's and now she sports some truly budget tubes. I didn't know any better and the amp tech did what he did. It went in because it was spotty on volume. It would be solid and loud for 10 minutes and then cut out to 2/3 volume with no life. That bit was fixed. And she sounds good. But I want to know what is in there.

The schematics are out there for your champ too of course and lots of forum info...go for it.

 

[chrisinsc]

Now you're onto it, Chris. This is the way to run a 'shakedown cruise' with a restored Mac amp. Your '240 is pretty characteristic of what to expect for a restoration. Never make assumptions!

Oh, and the advice on a set of _known good_ tubes is essential. Once you have a set - or at least one of each kind for your amps - keep them safe and never mix them with the other stash of tubes. They're your gold standard.

Cheers,

David

I just ordered the rest of the tubes to go with the new 6l6s. With new tubes and all new caps any issues with resistances and voltages should be with the resistors right? After I get the new tubes I am going to send all the tubes I got with the mc240 and m110z to be tested. There are some telefunkens, ge and rca tubes which would be nice to know the status of. I only have access to one kind of tester and someone with multiple testers and a zillion test cycles under their belt will yield a better analysis.

 

[chrisinsc]

So, got the rest of the new tubes today to complement the new power tubes and holy cow, the 240 is in the house! I swear as I listened all evening it got better and then I put the mx110 in along with the empire 598 witha shure v15 iv with sas stylus and put on my original but tired chuck mangione feels so good album on and it was awesome. I cant imagine what will happen when I update the mx110. What fun this is. The family is out tomorrow for holloween and I have half the new box of beatles in mono vinyl left to sample. Gonna be along day at work tomorrow....

 

[damacman]

Have fun! That sounds like a fun day. I miss my MC240s!

 

[chrisinsc]

Some interesting things I have noticed since now having all new tubes. First, its sounds much better and second now when it has been running for hours the power transformer now only gets to 107F and stays there. Before it would top out at 120F which I only did once after I was done recapping and it only had new power tubes. Runs much cooler now with the only change being new little tubes.

 

[sxty8goats]

Some interesting things I have noticed since now having all new tubes. First, its sounds much better and second now when it has been running for hours the power transformer now only gets to 107F and stays there. Before it would top out at 120F which I only did once after I was done recapping and it only had new power tubes. Runs much cooler now with the only change being new little tubes.

What tubes did you go with? I'm still running the original tubes as far as I can tell. Mac branded 6L6GC's and the pre amp section matches each other as well. I'd like to have a new / known set but I get lost when looking at available tubes from current manufactures.

My 110 is done, I got the email the other day. I'm just waiting on Terry to send me an invoice.

 

[chrisinsc]

I asked for a good set of new tubes which were tested and matched. I picked the tung sol reissue big bottle 6l6gc cause this amp has no cage. Then I was sent a sovtek 12ax7lps for v1, the rest were tung sol except the 12bh7's which are eh. So now eveything is new except the resistors. Mike samra insists I now replace or rechrome the existing chassis and refinish the transformers to halt the rust. I will probably have my original chassis redone though I may just paint it to retain the factory writing on the underside. Just enjoying it now and running it 12 hours a day. I cant say enough how mike, jim, and you folks here have made this project fun and satisfying. Its not a trivial endeavor by any means and now the whole thing has to come apart!

 

[ron-c]

You may want to go with USA 12bh7a tubes, GE, RCA, Sylvania etc. as these were used in TVs so they can still be found.

Thanks,
Ron-C

 

[chrisinsc]

You may want to go with USA 12bh7a tubes, GE, RCA, Sylvania etc. as these were used in TVs so they can still be found.

Thanks,
Ron-C

I have some of these tubes but they need to be tested. I just needed a good set for using now and before the next set of tests after I run it. But I probably should be collecting them for when its ready.

 

[cc43fan]

This schem is a bit different. I think I got it from Jim when he sent the parts list files for the rebuild kits. It has a few minor differences and a couple unexplained notes with corresponding dotted lines. I'm not sure if it is any value to this discussion but here it is.

I realize this thread is a bit old, however I'm attempting to put together my plan of attack before heating up the solder iron. It appears that the amp I purchased is the "newer" version (it doesn't have the voltage selector switch). My concern with the schematic is the mention of "notes". I'd like to find out what the notes refer to? I'm really hoping someone has that complete service manual which contains the schematic in question. I've attached the files for reference.

 

[Pio1980]

If the amp has never been serviced, use it as the guide with the assumption that it passed QC and provided many years of satisfactory service as-is. Recapping the elyticaps and striped tubular caps goes without saying, the bias supply should get silicon rectifier and a 1/4 Watt flame proof series dropping resistor of 220-470 Ohm, whatever gives correct bias voltage with heater bus at 6.3 VAC.

 

[c_dk]

Sometimes you get tired old saying the same thing..........

But the resources of the akdatabase should be a go to for all these questions. A far superior service manual is available there......might have to enter through the Marantz portal to get to the " M " section.

 

[cc43fan]

Sometimes you get tired old saying the same thing..........

But the resources of the akdatabase should be a go to for all these questions. A far superior service manual is available there......might have to enter through the Marantz portal to get to the " M " section.

I'm sorry if you feel compelled to make yourself tired "saying the same thing"....I did in fact check the AK DB, and found the only service manual which contains only one schematic.
http://akdatabase.com/AKview/thumbnails.php?album=5
Clearly the "notes" referenced in the schematic not listed in the AK DB are not important, got it, thanks.

 

[dcgillespie]

cc43 -- The notes provided by Mac are as follows:

Note 1: In units with serial numbers below 32G66, C29 and C30 do not appear: R9 & C2 and R32 & C12 are connected together as shown.

Note 2: In units with serial numbers below 32G66, R17 & R40 were 27k; R18 & R41 were 30K; R10 and R33 were 10K; and R57 was 10K 2W.

Note 3. In units with serial numbers below 32G66, R58 (100K) is used. When this resistor is used, the connection is broken at point indicated by "X".

Note 4. In units with serial numbers below 45G45, power transformer T3 is connected as shown in detail "A".

I hope this helps!

Dave

 

[Pio1980]

Rust -- The notes provided by Mac are as follows:

Note 1: In units with serial numbers below 32G66, C29 and C30 do not appear: R9 & C2 and R32 & C12 are connected together as shown.

Note 2: In units with serial numbers below 32G66, R17 & R40 were 27k; R18 & R41 were 30K; R10 and R33 were 10K; and R57 was 10K 2W.

Note 3. In units with serial numbers below 32G66, R58 (100K) is used. When this resistor is used, the connection is broken at point indicated by "X".

Note 4. In units with serial numbers below 45G45, power transformer T3 is connected as shown in detail "A".

I hope this helps!

Dave

Did Mac recommend any changes/upgrade be performed based on these service bulletins?

 

[dcgillespie]

Not to my knowledge. It appears by all accounts that the driving goal for the basis of the changes was to get a little more drive capability out of the inverter stage. This was done by increasing the value of the plate resistors -- which then required changes to the amplifier B+ distribution network. Changes were also made to (likely) account for the change in supersonic response this produced.......

Dave

 

[cc43fan]

cc43 -- The notes provided by Mac are as follows:

Note 1: In units with serial numbers below 32G66, C29 and C30 do not appear: R9 & C2 and R32 & C12 are connected together as shown.

Note 2: In units with serial numbers below 32G66, R17 & R40 were 27k; R18 & R41 were 30K; R10 and R33 were 10K; and R57 was 10K 2W.

Note 3. In units with serial numbers below 32G66, R58 (100K) is used. When this resistor is used, the connection is broken at point indicated by "X".

Note 4. In units with serial numbers below 45G45, power transformer T3 is connected as shown in detail "A".

I hope this helps!

Dave

Dave,
Thanks very much for your help and knowledge!