Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: keithpop on April 01, 2014, 07:28:49 am

Title: Music Man 65 - Power Transformer "HOT"
Post by: keithpop on April 01, 2014, 07:28:49 am

I hope this is the right forum....

I am looking for direction/advice from your collective experience.  Anyone familiar with Music Man amps?

I took a friends 'dead' Music Man 65 (2100-65) on vacation with me to fool around with when I wasn't doing anything else.  I found one of the low voltage power supply electrolytic caps with a dead short.  That explains why the fuse kept blowing.  I decided to do a complete cap job and the amp works for a minute before the Power Transformer gets so hot that the thermal resistor shuts it down.  The main fuse does not blow.  Voltages are difficult to get because the PT gets so hot....fast.  It also made a 'girgly' sound when I flipped the on-off switch for a couple of seconds....and then gets hot.  I think it's probably a shorted winding somewhere.  I'm afraid the power transformer is cooked but wanted y'alls input before I go that route.  I pulled all of the tubes and repeated the test and had the same results.

I could let it cool down to get a voltage reading or two if that would help.

Any ideas out there?

I will attach the schematic (PDF).

Thanks in advance for your help.

keith

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: HotBluePlates on April 01, 2014, 08:13:08 am

The schematic shows 3 secondaries: a high voltage winding, a low voltage winding (2 orange wires?) and a tube heater winding (2 green wires).

I'd start by unsoldering the orange low voltage wires from their rectifier diodes, insulating the ends and turning on the amp. Still get hot and shut off? The try unhooking the high voltage winding. Try isolating windings one at a time to find the circuit that's malfunctioning.

If you unsolder & insulate all 3 windings and still get the thermal shutdown, then the PT (or something on the primary side) is bad.

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: keithpop on April 01, 2014, 10:13:02 am

Thanks for the suggestions.  I disconnected each secondary individually and finally all together and the PT still gets very hot.  Not a good sign....  I also measured the resistance between the primary and chassis ground and it didn't move the needle (no short).  So the primary circuit is OK.

I guess I need to start to find a replacement transformer or getting this one rebuilt.  Any suggestions for a rebuild?   Mojotone sells one but they are out of stock and I'm not sure the amp is worth putting a high priced Mercury PT in it.  I wouldn't mind sending it for a rewind.

Again thank you very much.  Sharing your knowledge is really appreciated.

keith

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: eleventeen on April 01, 2014, 10:32:46 am

I'd be interested in knowing, just for grins, what a rewind would cost. Yes, Merc's are expensive but compared to a rewind...? Can't see there being much difference there but maybe I am mistaken.

I would also look around to see if there is a proper replacement (Mojo or OEM) somewhere, on somebody's shelf.

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: keithpop on April 01, 2014, 11:13:24 am

I heard back from Mojo... They sold Mercury transformers but don't plan on ordering them again.  Mercury's price is around $200....that is pretty steep.

I'll let you know where I land.

thanks,
keith

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: HotBluePlates on April 01, 2014, 02:56:22 pm

Quote from: drgonzonm on April 01, 2014, 01:12:01 pm

Considering, that this a hybrid amp, I wounder if other components are also fried. ...

That's possible, but with the PT out of the circuit and still overheating, it seals the deal that the PT is bad.

Quote from: keithpop on April 01, 2014, 10:13:02 am

...  Any suggestions for a rebuild?  ... I'm not sure the amp is worth putting a high priced Mercury PT in it. ...

I'm pretty sure Mercury started by rewinding bum transformers. I'd call them to ask if they will do the re-wind.

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: keithpop on April 01, 2014, 03:25:46 pm

Thanks again guys for the input.

For the time being I'll put it back on the shelf.  Maybe I'll find one on eBay with a blown output transformer... 

I have a Music Man HD130 and it's a great amp...so I know Music man is good stuff.

thanks again.

keith
 

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: lego4040 on April 01, 2014, 06:32:36 pm

Good luck with that amp, I found one in my building garbage that Was the model Clapton used. I tried my hand at getting it going but had serious water damage once. I plugged it in and it crapped out on me, I wish I would've saved it for parts

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: keithpop on April 01, 2014, 07:00:38 pm

thanks....I wish you had saved it.  That is what I will be looking for... I can't justify buying a $200 PT.  Thanks for the post.

kp

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: Tone Junkie on April 01, 2014, 08:39:21 pm

Keithpop Send me the numbers off the transformer I might have the one you need , i recently canibalized an amp to fix another I had.
Bill

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: lego4040 on April 01, 2014, 09:45:59 pm

 :BangHead:so do I, look into edcor transformers and classictone

Title: Re: Music Man 65 - Power Transformer "HOT"
Post by: keithpop on April 02, 2014, 05:30:50 am

thanks.  My first email yesterday was to Classictone (Magnetic Components).  They don't have a replacement or rewind old transformers.

I will contact Edcor.  Thanks for the lead.  I also set up an automated 'watch' on eBay.

keith