MA230 Power Transformer Replacement?

[mnmmt]

Hi all:

In checking out my new-to-me MA230 (that's been sitting for a number of years) in preparation for restoration, I found that the power transformer has a dead-short HV secondary winding, and a suspiciously low resistance (1.1 ohm) primary winding.

The transformer appears to be quite a simple affair--a single 120v primary winding with a 167v 1.1A high voltage secondary and a 6.3v 4.3A filament secondary. Does anyone know of any aftermarket (Edcor, etc.) power transformer that will substitute in for the original, or must I have the original rewound?

Thanks!

 

[AbSoundLabsMN]

Thanks for posting your question about a defective power transformer for your MA-230

Please accept my apologies for the delay in responding to your inquiry. It would seem that there are never enough hours in a day to accomplish what needs to be done.

I am assuming that you checked the transformer by disconnecting the windings and not in circuit. That being said, I contacted the Factory to see if there were any specifications, in their Bill of Materials concerning it. Regrettably, no information was located. As such, I can't give you the voltages and current ratings for the transformer. We don't have any MA-230's in-house, at the moment to check. I will make note of this request, and if we see any MA-230's I will check and report back.

I am not able to suggest any after market resources for repair/rewinding of your transformer.

I wish I could be of more help.

Mark
McIntosh Service Center

 

[mnmmt]

Thanks for the reply! I did pull it out, and saw some discoloration where the lead wires had been touching the chassis and other components. When totally isolated, it is measuring closer to spec on my dmm. I am going to reinsulate the leads back to the transformer winding and then see how it performs.

I appreciate your reply!

 

[miscrms00]

Hope you get it sorted, the MA-230 is a nice little amp in my limited experience!

Note I'm not an expert on tube amps in any way, and my comments could easily be the result of my lack of understanding

Your specs on the first high voltage secondary sound pretty reasonable to me. If the doubler output follows the AC peak voltage it would be ~460V / 2 = ~230V / 1.414 = ~163V, in which case you are probably right on. 1.1A also sounds pretty reasonable. Looking at TP1-4 each is 0.7V across 15Ohms, or about 50mA bias for each output tube. Maybe double that for max output or 400mA for all four tubes? Looking at the rest of the PS resistor drops, R104 20V/900 Ohm ~ 22mA, R105 70V/3600 Ohm = ~20mA. That just leaves out the -32V unknown, but looks to be smallish given the high value resistors in the bias adjust network. So maybe ~0.5A normal operation, and 2:1 safety margin for cap charging, fault tolerance, etc?

Filament current also sounds pretty reasonable. From what I see 7591 is 0.8A each, or 3.2A. 12AU7 0.3A ea, or 0.6A. 12AX7 0.3A. So total should be roughly 3.2A + 0.6A + 0.3A = 4.1A.

Hopefully yours turns out to be ok and this is all academic!

Rob