MA-230 output xformer rebuild or replace

toddbailey

Active Member
Hi All,

Got myself into a bit of a pickle, I made the mistake of listening to this amp with out replacing all the psu caps.
And as fate would have it, something failed which may have caused a output transformer to fail as well.

Needless to say, I need to figure out what to do about it.


Does anyone,

know how to rewind a transformer such as this?
Perhaps know of an affordable shop that can do the job?
Maybe have one or two in a box of parts?
know the technical specs of this part , If so I might find a external source such as mouser electronics or digikey ?

One guy I emailed wanted over 5 bills, for a 60 dollar part.

If mac labs doesn't have them anymore I may be forced to part out or something even more drastic, retro fit a pair of eico st-70's transformers.

Before I take such a leap, is there any reason why I couldn't do the job myself.

after all it's just turns of wire on a bobbin....

All in all I am a bit disappointed, these amps are touted as being bullet proof. so much for touts'...
 
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It would be worthwhile to talk to Heyboer Transformer - they do rebuilds and reproductions for lots of audio and guitar output transformers, and it's likely they can help you for less than 5 bills.

This is a very good reason never to use vintage amps with original caps. But one thing - I'm confused that you say a power supply cap failed - was it the bias supply? Are you 100% sure that an output transformer is bad?
 
Are you 100% sure that an output transformer is bad?

Same thing I was thinking! Usually, an output transformer can take a little abuse. It takes something like hard driving and red plating tubes along with bad coupling caps, for a long period of time before an output transformer really gives up!
 
Pull the output tubes and measure across the plates of the output transformer. Each one should have roughly the same DCR. I would be surprised if you took it out.
 
This is a very good reason never to use vintage amps with original caps. But one thing - I'm confused that you say a power supply cap failed - was it the bias supply? Are you 100% sure that an output transformer is bad?

Yes Me bad, but hindsight is always 20/20...

I checked all the output voltages and they all looked ok on the scope so then I decided to plug in tubes, and I got about 5-6 hours to listen before failure

Wasn't driving very hard, just a normal to moderately loud. Small room and efficient Speakers (JBL L36)

I tried to rebias and get closer to .7 per spec, best I could get is .350 but decided to do another listen check and 25 minutes later left channel distortion for a few seconds followed by loud ac hum or right channel, and no output on left. I quickly killed the power. I partially checked dc resistances comparing left and right output transformers. both cathode windings including center to ground. all ok, then the plate winding this is when I discovered the failure infinite on one low resistance on the other. and that is as far as I checked, I still have the fb winding and speker taps to check.

I'm not 100 % positive if the ps failed, but it's the best starting point.
All electrolytics and bypass caps are being replaced

So, right now I have a new set of caps in the mail, a new set of tubes on the shelf (they still test ok) and a dead output transformer. I see if this new lead to rewind transformer is economically viable.

thanks all
 
I, too, am doubtful of a transformer failure - unless there are absolute signs like insulation burned off the wires leading to/from the transformer (I've seen that before!). My conversations with Audio Classics seems to confirm that xformer failures are the exception rather than the rule.

As to your hand-rewinding, be aware that Mac output transformers will be using bi-filar or tri-filar wiring, depending on the transformer. The handfull of good rewinders in the US today all use machine winders in order to lay the wires properly. I doubt it can be done by hand with the same quality and reliability.

The one rewinder you've contacted I have used in the past when my MX-110 power transformer developed AC on the chassis. I did a lot of diagnosing in order to narrow it down to the failed component. He has the 'recipes' for most of the Mac transformers, too. He also does not repair them, he completly rebuilds them with new wire. Lastly, for output transformers, he puts them under test at full power then tests for bandwidth under full stress after rewinding.

I paid a lot for my MX-110 power xformer but feel it is worth it. BTW, Mac still uses the same winding jigs today to build transformers for their new tube gear that were used in the 60s. The quality of the wire has gone up but most everything else is the same. Mac does not, however, rewind old transformers.

Cheers,

David
 
Thanks David,

I don't discount that he does quality work, but you do have to take into consideration the values of the amp and the repair cost involved.
I am way over budget on this project already.

I am waiting for a reply from Mac labs, hopefully they can provide a replacement.
 
If you've already done continuity tests on the O/T and have confirmed and open or dead lead, I would take it apart a little and see if the issue is as simple as a broken tap wire to output lead. Those tap wires (windings) are damn near microscopic.

Ive had one break right at the tape up connection just inside the cover wrap before and was able to fix it.

You're right though. Cost and the PIA make "rebuilding" any but the most rare transformer a losing proposition. You could easily replace both OPT's for relatively cheap. If it comes to this, just post a thread here asking which OPT's to get and you'll get several good leads from guys here that are familiar with suppliers.


..
 
Well, that is an idea, with a slight chance to save the project.
worth taking a look. Microscopic you mean like 40 gauge wire?
That makes the idea of rewinding virtually impossible. Any thing much smaller than 30 and the strength of the wire becomes a issue.
 
any but the most rare transformer a losing proposition.

Fortunately the MA-230 output transformer is not very rare. As recently as a decade ago you could find them for sale new and not mounted to the amp. Mc must have been sitting on tonnage of them and released them into the general public. IIRC, Glass Audio even had an article on building an amplifier around them. I'd bet there are quite a few sitting unused on someone's project shelf.
 
Well, this does sound hopeful, so far the cost to reverse engineer and have a pro shop rebuild them is a losing proposition.

But I have yet to find one. IIRC and Glass Audio, ok, more places to look.
 
So that you don't waste time looking for a shop called "IIRC", please be advised that IIRC is an acronym standing for "If I Recall Correctly".
 
No worries, I thought it might be a typo and you really meant to say check out irc avenues aka chat rooms for audio gear, parts etc.

I did a brief google and found several possibilities, but couldn't research it in detail...
 
If you've already done continuity tests on the O/T and have confirmed and open or dead lead, I would take it apart a little and see if the issue is as simple as a broken tap wire to output lead. Those tap wires (windings) are damn near microscopic.

Ive had one break right at the tape up connection just inside the cover wrap before and was able to fix it.

You're right though. Cost and the PIA make "rebuilding" any but the most rare transformer a losing proposition. You could easily replace both OPT's for relatively cheap. If it comes to this, just post a thread here asking which OPT's to get and you'll get several good leads from guys here that are familiar with suppliers.


..

I doubt seriously if you can replace these transformers for any thing near "cheap".......these are highly specialized outputs and while they arent as complicated as the outputs on MC225 240, or 275, they do have windings that figure into the cathode circuit. Bite the bullit and have it rewound. I bought some MA 230 reproductions in the 90s that were around 200 each. It was a place called Sterling audio, a gentleman by the name of Eric Gagne. I do know he had a rewinder reproduce them for him. Rewinding these isnt a DIY task. The Mcintosh outputs are real precision units, with windings that are used in circuit for the particular unit it was designed for, and are not plug and play. Classic audio may have spare units, at least they used to. This amp is worth fixing, look at the price nice units go for on Terry's site. (AK sponser) Otherwise sell it to someone willing to restore.

Russellc
 
Fortunately the MA-230 output transformer is not very rare. As recently as a decade ago you could find them for sale new and not mounted to the amp. Mc must have been sitting on tonnage of them and released them into the general public. IIRC, Glass Audio even had an article on building an amplifier around them. I'd bet there are quite a few sitting unused on someone's project shelf.

Yes, I built and still have that amp, and its a winner.

Russellc
 
I bought two of those transformers for a project some day. I may copy the MA230 circuit and use 7591's.

Find the glass audio article. Basically its the MA 230 circuit, different power supply, ( about 600 volts) and use 6550/KT88 outputs. It also uses a 6L6 gc triode wired, 1/2 a 12ax7a and a diode sting to regulate the front end and the screens on the output tubes. The same feedback is retained as well.
It will produce between 65 and 70 watts per channel and it a very impressive sounding P-P amp.

Russellc
 
BRB ? BTW ? who or what are they?

Destructor, Where did you buy these? I'm batting zero so far.

Mac doesn't recognize their own part numbers, not good
 
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