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 Post subject: How do I go about finding a replacement power transformer?
PostPosted: Feb Sun 04, 2018 8:12 am 
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Joined: Nov Wed 01, 2017 4:36 pm
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Location: Lafayette, NJ 07848
How can I get a replacement power transformer for an old Magnavox 9303 stereo tube amp? The schematic says the screen voltage is 300V, so I assume it is a 600 vct transformer. I posted a WTB in the classified section, but I am wondering if there are other power supplies out there that might use a similar transformer that I could swap. How common are these? I really have no idea how to begin looking for an equivalent replacement because I don't know the specs of the current one, and I can't measure it because it no longer works.

How do I get started on this?

Thanks for any help.

Rich

Here is my WTB post with pics and schematic:
viewtopic.php?f=15&t=335390


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 9:57 am 
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Greetings to Rich and the Forum:

The screen voltage will not be the highest voltage in the amp (usually). The plate voltage for the output tubes will give you an idea of the actual voltage requirements for the transformer. The voltage rating of the original power supply filter capacitors will give you a ceiling; the supply DC output voltage will be less than these are rated at.

The filament current requirement can be found by simply adding all the filament currents of all the tubes in the unit together and adding a bit for safety.

The trick will be to find a transformer with the same physical dimensions so that it will fit into the chassis cutout. You can swap end bells with an upright mounting transformer if the bolt pattern is the same, but only if the upright transformer has all of its leads on one side.

Worst case, there are a couple of firms doing transformer re-winding. I don't recall who they are as I have never dealt with them, but if you post a query, someone here will provide the data, I'm sure. Be aware that re-winding is expensive; expect to pay upwards of $200 or so.

Regards,

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 10:44 am 
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Going from AC voltage to rectified DC:
The rectifier in the typical transformer set is a peak detector, so you start with the peak value of the AC waveform. For 300 volts RMS (1/2 of the center-tapped 600-volt winding), the peak voltage is a bit over 420. This is the highest voltage you could expect from an ideal rectifier. The typical tube rectifier is maybe 70-80% of that, so you do get a DC output that is similar to the AC input. The actual voltage available to the tubes is lower due to drops in the filter section. Finally, you need the current rating. Transformers are often rated in terms of rectified DC current, which makes things convenient.

Next, you need the right filament windings---voltages and rated current.

Using voltages and rated currents, you get the "VA" rating. (Same thing as watts, but only when everything is in phase.). One rest of an adequate VA rating is if the transformer has the same core area. Very roughly, two transformers that are the same size and weight are going to have similar VA ratings.

I'll look at the schematic for that set to see if I can narrow things down a bit.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 5:13 pm 
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Location: Mountains of Mourne. Ireland.
300195-1 (Magnavox Power Transformer) specifications.
Image
Service manuals for 93 series amplifiers can be found here, courtesy of Bill J... :)
viewtopic.php?p=2223559#p2223559

Greg.

Edit: At times this Magnavox transformer 300195-1 is shown in Sams with a...
600VCT rating of .130A


Edit: image updated.


Last edited by egg on Feb Sun 04, 2018 7:28 pm, edited 2 times in total.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 5:16 pm 
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Joined: Nov Wed 01, 2017 4:36 pm
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Location: Lafayette, NJ 07848
pixellany wrote:
Going from AC voltage to rectified DC:
The rectifier in the typical transformer set is a peak detector, so you start with the peak value of the AC waveform. For 300 volts RMS (1/2 of the center-tapped 600-volt winding), the peak voltage is a bit over 420. This is the highest voltage you could expect from an ideal rectifier. The typical tube rectifier is maybe 70-80% of that, so you do get a DC output that is similar to the AC input. The actual voltage available to the tubes is lower due to drops in the filter section. Finally, you need the current rating. Transformers are often rated in terms of rectified DC current, which makes things convenient.

Next, you need the right filament windings---voltages and rated current.

Using voltages and rated currents, you get the "VA" rating. (Same thing as watts, but only when everything is in phase.). One rest of an adequate VA rating is if the transformer has the same core area. Very roughly, two transformers that are the same size and weight are going to have similar VA ratings.

I'll look at the schematic for that set to see if I can narrow things down a bit.


Thanks. I really appreciate your taking a look. When the transformer was working (yesterday) I measured the DC voltage with no tubes installed other than the rectifier (5U4) and I believe it was about 430 VDC (or so). I don't remember the exact value, but it was definitely over 400 and I think less than 450.

The 300 volt figure I mentioned is from the schematic, and it is on the output tube screens. The plates say 295, which is a bit lower due to voltage drop from bias current through output transformers I assume. With no tubes installed though there are no currents of course and I expect these numbers to be much higher as you mentioned, and they were.

I found a post where a guy claimed to have the specs for this same transformer and said it was as follows:
Primary-117v@.92a
Sec.1-600vct@.130a
sec.2-5v@3a
sec.3-6v@4a

However, I have no way of knowing if this is correct.

Here is a link to the post
http://mail.audiokarma.net/forums/index ... st-3882658

I'm thinking now the only way out is to buy another amp just for the transformer.

The failure:
I had just finished integrating a preamp section into my current Magnavox console power amp. Not sure how the transformer failed yet. The wires coming out of the bottom were all very brittle and cracked. It is possible something there shorted. Of course it is also possible some of my new circuitry is shorted, but I don't see any problems so far. I will be taking another look today. I had just installed the 5U4 and was ramping up the variac to measure the DC. I only got to about 60 volts when I noticed there was no HV AC on the red output lines of the trans. Then I noticed the fuse had blown. I replaced it and it blew again. The fuse I had in there initially was 3 amp. I replaced it with 2 amp and it blew again. Then I pulled the 5U4 and put a 5 amp in and decided to ramp up AC voltage very slowly and watch wall current. at 30 VAC I was drawing about 2.6 amps, so I shut it down. Transformer was slightly warm to touch. Clearly something shorted. Disconnected all wires. Measured secondary resistance was 27 ohms from either end to CT, but only 4 ohms from end to end. This seemed a weird way to short.

I took out the transformer and removed the bell. Chiseled off some of the tar to see if I could find a shorted wire. After moving things around a bit I was seeing 60 ohms on the secondary end to end. I was briefly optimistic, then I chiseled a bit more and exposed some very thin winding (presumably the secondary based on diameter). Unfortunately I broke these wires. Now I have 4 exposed wire ends and do not know how they were connected. I can measure all the resistances and have pictures if you think there is any way to put Humpty back together again :)

Two of the 4 wires measure zero ohms to CT output, and the other two are symmetric at 50 ohms and 220 ohms to each respective secondary end, but even with none of these wires connected I am measuring about 170 ohms on secondary end to end. Is there more than one layer to the secondary winding? This made no sense to me as a circuit.




I


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 5:21 pm 
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Joined: Nov Wed 01, 2017 4:36 pm
Posts: 105
Location: Lafayette, NJ 07848
egg wrote:
300195-1 (Magnavox Power Transformer) specifications.
Image
Service manuals for 93 series amplifiers can be found here, courtesy of Bill J... :)
viewtopic.php?p=2223559#p2223559

Greg.


That's amazing! How did you ever find that part spec? I thought the last number on the bell was a 6, but you are right, it is probably a 5.

So now I know for sure what I need. So... how do I find one? Anyone have a transformer for sale?

Thanks!


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 5:38 pm 
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Joined: Nov Wed 01, 2017 4:36 pm
Posts: 105
Location: Lafayette, NJ 07848
Thanks for the link. I went to the Sams pages referenced and I see where you got the transformer info now. Mine is a 9303-00, so it should be this one, but my chassis does not have c2 and c5 (100mmf). I see they were also optional in older models, so maybe Magnavox wasn't sure if they liked them or not.

https://app.box.com/s/z87awqdg3empgrtcnx8e


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 5:50 pm 
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Joined: Nov Wed 01, 2017 4:36 pm
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Location: Lafayette, NJ 07848
I'd like to find out if the Magnavox 8800 series amp also used this power transformer since there are more of these on the used market. It is the 6V6 version rather than the EL84. Does anyone know where I can find a Sams or other specs for Magnavox 8802 perhaps?

Magnavox made many different console amps, and I'd like to compile a list of all products that would have a transformer I could swap if possible.

Thanks.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 6:56 pm 
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rjp wrote:
That's amazing! How did you ever find that part spec? I thought the last number on the bell was a 6, but you are right, it is probably a 5.

The 6.3v secondary labels (S3, S4) penciled on the schematic are reversed.

S3, the 0.3-amp secondary, is the top winding (BRN) on the schematic transformer (Y1, Y2).

S4, the 4-amp secondary, is the second winding (GRN) from the top with the tube filaments.

- Leigh

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 7:29 pm 
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Thank you Leigh, I've corrected the error.

Good to have you ridin' shotgun... :)
Greg.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 7:46 pm 
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The only thing I see odd about the xfmr is the additional low-current 6.3v winding.

My inclination would be to find a main transformer that meets all the other specs.
Then get a little 6.3v transformer to handle that other low-current winding.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 8:01 pm 
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Leigh wrote:
The only thing I see odd about the xfmr is the additional low-current 6.3v winding.

My inclination would be to find a main transformer that meets all the other specs.
Then get a little 6.3v transformer to handle that other low-current winding.

- Leigh

Good option.....

Also, you always have the option of having it rewound.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 8:22 pm 
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Joined: Nov Wed 01, 2017 4:36 pm
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Location: Lafayette, NJ 07848
Leigh wrote:
The only thing I see odd about the xfmr is the additional low-current 6.3v winding.

My inclination would be to find a main transformer that meets all the other specs.
Then get a little 6.3v transformer to handle that other low-current winding.

- Leigh


Thanks for noticing this. As it turns out, I do not need the low current 6.3v output at all. That was for a second filament run in the tuner chassis which I am not using.

So I suppose this makes it easier to find a replacement, but I still don't know where to look.

NOTE: Some people are using the second filament output in a bucking circuit. By wiring it in series with the primary it reduced the overall EM field and consequently reduces all the AC output levels. Supposedly this helps compensate for the increase in line levels from 117 to 120+ since the 1960's.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 8:26 pm 
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rjp wrote:
NOTE: Some people are using the second filament output in a bucking circuit. By wiring it in series with the primary it reduced the overall EM field and consequently reduces all the AC output levels. Supposedly this helps compensate for the increase in line levels from 117 to 120+ since the 1960's.

On edit:

I changed my mind.
That winding may not have adequate insulation for connection to the primary,..

- Leigh

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Last edited by Leigh on Feb Mon 05, 2018 3:06 pm, edited 1 time in total.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Sun 04, 2018 10:42 pm 
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Check Playthings of the Past. You might find something similar.

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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 1:10 am 
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Location: Lafayette, NJ 07848
Update:
I went over the circuit carefully this afternoon and everything looked good, so I put in a smaller 500vct transformer that I had laying around as a test and the amp plays well (other than I can't turn it up loud without distortion), so I believe the circuit is good. The failure must have been due to the transformer itself or its brittle wires.

I added a 1 tube preamp with volume, bass, and treble controls and I am pleased with the response so far. I extracted the parts from the matching tuner. The single 6EU7 tube fits nicely in the empty socket hole already in the chassis. I do have some 60Hz hum. I will have to track down the source. Maybe my preamp is too close to the filament lines? The original wires are not twisted. Probably I should wait until I replace the transformer before investigating further. Found one on classifieds today.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 2:39 pm 
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Edcor XPWR008 would work just fine as a fall back option. Specs are almost spot on.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 2:46 pm 
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Yes, that's a great transformer. Better than OE, but website says 8 weeks lead time to build.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 3:02 pm 
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In re the comment about using the low-current winding for bucking.....I'm not sure the rating of 0.3 amps is adequate for that.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 5:27 pm 
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Location: Lafayette, NJ 07848
pixellany wrote:
In re the comment about using the low-current winding for bucking.....I'm not sure the rating of 0.3 amps is adequate for that.


That's an excellent point. No, the 0.3 amp rating of the second filament does not appear high enough given that the primary is rated at 0.92 amp.

The little filament winding should should burn up with 0.92 amps running through it, right? So why are so many people using it successfully I wonder?


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