Author Topic: Magnavox console I picked up this weekend... Blown transformer  (Read 628 times)

thermofin

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Hello everyone, this is my first post.
I picked up a Magnavox Console this weekend for $75 and was aware that it was not working. The original punch card came with it and lists the model as a 3ST661A. After removing the back panel it is clear the transform exploded. I am looking for some help to determine where I can find a replacement.
I am also looking for schematics but not sure how to identify them. The Amp has several numbers on it and I'm not sure which numbers identify the chassis.
I am also looking for some advice as to what may have caused the transformer to do this and what other problems may have been caused by this failure.
I have checked the tubes and the 5U4GB is still good but all 4 EL84's are gone. One of the two 6EU7's are also shot.
I am not terribly experienced with the electronics but will be learning as I go.
I have refurbished a 62 Motorola and 59 Zenith that are in good working order.

Thanks for any help.

electra225

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #1 on: July 31, 2018, 08:37:36 AM »
That model is what we refer to as a "little" Imperial.  Dandy performers.  One of Magnavox's best.

I have never seen a transformer that looks like that one.  I appears to have been pretty warm at some point.  You need to do a thorough check of the B+ circuit to determine what is drawing so much current. 
I don't need Google.  My wife says she knows everything.

thermofin

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #2 on: July 31, 2018, 08:58:20 AM »
Thank you for the reply.
This transformer must have literately exploded. The 5U4 was covered with the black substance on one side. I have to believe the transformer is shot. Would you suggest plugging it in to test the circuit? I don't have the equipment to vary the current.
I'm hoping I didn't get in over my head on this one....I can't seem to find a replacement transformer online.

SeniorSteve

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #3 on: July 31, 2018, 09:16:39 AM »
The problem with many of the manufacturers at that time is they never put fuses on them.  Looking at the damage I don't think there's a chance it is still working.  I would bet the primary is open at this time.  It must have been left running unattended - that surely must have stunk up the house.  Anyway, besides the transformer you probably should replace the electrolytic and paper capacitors at a minimum on the amplifier chassis.  One of the electrolytic capacitors may have shorted causing this mess.  When you do the repairs (I'm hoping you do), be sure to add a fuse to the primary to protect the next transformer.  The cabinet on your stereo is in very good shape.
Good luck with your repairs.

Steve

thermofin

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #4 on: July 31, 2018, 11:28:53 AM »
Any idea where to get a replacement transformer? I have no problem replacing the caps.

SeniorSteve

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #5 on: July 31, 2018, 12:50:52 PM »
Here are the specs for the transformer, I got it from Antique Radio Forums (ARF).  It's another site for good information. 
There is a website called oldradioparts.com you might take a look for a used transformer.  Otherwise you can get a new transformer, but it's going to cost as much as this console you just purchased.  You save the labor costs, but it still can be an expensive hobby. 
BTW That was one HOT transformer with pressure to push the tar out of it like that.

http://antiqueradios.com/forums/viewtopic.php?f=15&t=335390

electra225

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #6 on: July 31, 2018, 02:42:59 PM »
By the time you buy a PT, a quad of output tubes and a 6EU7 or two, you are going to have a pile of money into the project.  You might be better off trying to find another amp on ebay and going that way.  Do a good tune-up after you get it and you should be good to go for quite awhile.  There will be less risk buying an amp that works on ebay than there would be trying to fix yours.  That thing has had a hard life.

Senior Steve, I had not read about shorted hum controls burning up PT's.  I appreciate knowing that. 
I don't need Google.  My wife says she knows everything.

thermofin

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #7 on: July 31, 2018, 04:16:33 PM »
I think that's some good advice.
I don't mind putting some money into this console, I'm planning to replace the Motorola that I restored with this one.
I also read where the hum control can short out the transformer. Is it worth having and can it be bypassed?
I am planning to add a fuse, I'm surprised they didn't have one. If you would have seen the old wood house where this came from, it could have gone up in flames. This transformer grounds out on several leads.

electra225

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #8 on: July 31, 2018, 04:34:32 PM »
The hum control provides part of the bias for the output tubes, so, no it can't be bypassed.  I intend to investigate the hum control on the amp of the Stereo Theater I'm working on.  It never hurts to investigate, just in case.  I have not had an issue up to this point.  I put a fuse in Magnavox amplifiers I have restored.  I'm not convinced that will save a transformer, but it might save the house. 

I would like more information on the potential problem with the hum control.  I have never had a problem with one.  I clean them to make sure they work properly, then set the bias as specified on the schematic.  IIRC, the 9300 series amp wants 11 volts on the cathodes of the output tubes. 
I don't need Google.  My wife says she knows everything.

SeniorSteve

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #9 on: July 31, 2018, 04:41:48 PM »
I would probably side with Electra225's comments about the costs.  Unfortunately I see these working amps on eBay for incredible amounts.  I did look at Hammond transformers and they have two models that should fit the bill, but the prices are $88.00 to $118.00 from Mouser.  The more expensive is the style that burned and the less expensive is an "upright" version of the transformer.  I'm surprised that you haven't mentioned the smell from the burned transformer.  I'm just wondering if it happened a long time ago and they just left it sit.  That amount of transformer carnage would have left a stink.
Electra225, I wasn't the one that mentioned the shorted hum control, but looking at the circuit I can see it doing a slow burn on the PT.  Why didn't these manufacturers just use a fuse?  There is an additional 6 volt winding in the transformer I don't know where it goes (no schematic)- and the replacement transformers don't have.

Edit:  I looked it up and see it goes to the tuner filaments

TC Chris

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #10 on: July 31, 2018, 10:11:12 PM »
Antique radio c lun=b and ham radio swap meets and such often have people selling parts like transformers.  You need the right high voltage and current.  You can always use separate filament transformers if you need to (6 V for most tubes, 5 V for the rectifier).

Chris Campbell

thermofin

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #11 on: August 01, 2018, 07:08:01 AM »
There was no noticeable stink with the amp, just the typical "grandmas old furniture" smell. I suspect this console has been sitting idle for some time. It was only being sold because the house was.
I think I'll hold out for an original amp or transformer and start working on the tuner and changer. I don't intend to give up on this console.
Thanks for the help and if anyone runs across an amp chassis or transformer, please let me know.

thermofin

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #12 on: August 01, 2018, 08:56:12 AM »
I'm curious about the model number for this console. It begins with a 3 which I guess is the run #. In reading through the posts, I see a lot of models that begin with 1 and some with 2 but have yet to see one that is a run 3. What does the run mean? Also what is the "A" at the end stand for?

electra225

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #13 on: August 01, 2018, 10:58:26 AM »
The Run number refers to some manufacturing procedure.  The A suffix refers to whether it had an MPX adapter, most likely in this model.  The 661 would make it part of the "little" Imperial series, basically a Symphony with horns instead of speakers. 

The hum control problem is a curious one to me.  I'm wondering if the hum control is not being slandered by a problem with the cathode bias resistor and/or the cathode bypass capacitor.  Those two components are in series with the hum control, and are far more likely to cause a problem.  I'm not sure it would do a slow roast of the power transformer, though.  One would think it would fry one of those components and open the circuit before it took out the robust Magnavox PT. 
I don't need Google.  My wife says she knows everything.

Motorola Minion

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Re: Magnavox console I picked up this weekend... Blown transformer
« Reply #14 on: August 14, 2018, 01:11:45 PM »
I have two spare Magnavox transformers. One from an AMP-142BC.
Tubes - Magical - Tubes

Dave