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find replacement for power transformer

sherrilb

May 28, 2022
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I bought a Magnavox Stereo 300 tuner amp recently. I bought it to get back a few of the electronics repair skills I had 50+ years ago. (Anybody remember vacuum tubes in cheap radios?) This tuner amp is transistorized.

I have traced the issues to a failed power transformer. The Sams manual I purchased says it is a 117vac primary with two secondaries: 44 vac and 6 vac.

The transformer coils are not shorted together or to the core (measured with my multimeter.)

The Sams manual says the 44 vac secondary should have 5.2 ohms (reads 1.1 ohm) and the primary 30 ohms (reads 17.9 ohms.)

I persuaded myself the problem was in the power transformer by applying -28 vdc from the output of the two rectifiers to ground.

That being said, I would like to purchase a power transformer and I do not know how to search for one with the appropriate criteria.

Please help.
 

kellys_eye

Jun 25, 2010
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Stereo amp and no centre-tapped secondary (or two separate secondaries)? I find that strange. Most push-pull output circuits require a +/- rail.

Surely you mean a 22-0-22 secondary? These should be fairly common (or voltages thereabouts). The 6V secondary might deliver a low DC to a pre-amp? Maybe that could be derived from the main winding using modern SMPS circuitry?
 

sherrilb

May 28, 2022
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The schematic shows a center tap on the 44 vac secondary but I can't find it. I tried reading from the two leads on that secondary to ground and got infinity. There are two secondaries; the 6 vac is for the dial lights and goes to the collector of a pnp transistor marked as "indicator control."

The schematic shows 7 leads from the transformer but there are only 6 physically.

When I applied the -28 vdc to the anodes of the rectifiers, the box worked well on the am and fm bands.

(The schematic shows -28.6 vdc for the output of the rectifiers but the parts list shows the secondary is 44.8 vac.)
 

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Bluejets

Oct 5, 2014
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Power trans is indeed centre tapped as above suggested.....it clearly shows that in the diagram......centre tap is referenced to ground( maybe not frame of the amp) so trace it back, should be obvious.
I'd be checking whatever controls the primary and testing primary before saying it is kaput .
 

kellys_eye

Jun 25, 2010
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You could use a transformer with an 18V AC secondary (no centre tap) and a bridge rectifier/smoothing cap to deliver around 25-26V to the circuitry. No special reason for the centre-tapped version.
 

sherrilb

May 28, 2022
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kellys_eye, thank you very much for your help. As I said, I'm just getting back into this after a 50 year layoff.

Your suggestion was very helpful. I put a bridge rectifier with a cap and a discharge resistor and played with it today.

The follow-up question I have is where do I find a resource for selecting the filter cap?

I have a variac and I used a 4700 uf cap at first without a resistor and it looked like it would take forever to discharge. I then went to a 10 uf cap with an 8k ohm resistor in parallel and that was much more reasonable.

I have also purchased an o-scope which I am trying to figure out so I can see just how effective the filtering is.

Another question: how much of a transformer do I need to provide the current necessary? I found 16 and 24 vac bell transformers that are reasonably priced with 30 va output; my calculation ability is a little rusty but that would seem to provide pretty close to the 1 amp the schematic says I need.

I'm having a wonderful time.
 

kellys_eye

Jun 25, 2010
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Rule of thumb for smoothing caps is 1,000uF per amp. Anything larger (in capacity) is ok too. Rule of thumb capacitor voltage should be 1.5x the DC output of the bridge rectifier (in your case a 50V or higher capacitor will suffice).

The bridge rectifier will drop 1.2V (0.6V across each pair of diodes) so the delivered voltage will be 1.41 x the secondary AC volts MINUS 1.2V dropped in the diodes so for a 24V AC transformer you are looking at 24 x 1.414 = 33.9V MINUS 1.2V = 32.7VDC.

If your current requirement is 1A (DC) then the transformer AC current should be around 1.7A (meaning a 40VA transformer) although in your application doubt using a 30VA will make much difference unless you intend to run the unit a full volume for extended periods.

Discharge resistors are not often used - the actual load (amplifier) will cause the voltage to discharge at switch-off anyway. There is a time constant (CxR) that defines the actual discharge rate and resistors in the kohm range will make the discharge time 'excessive' - something in the 100's (or less) ohms would be better although you must take into account the wattage rating as you are discharging 'quite a lot' into it.
 

Bluejets

Oct 5, 2014
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Did you ever determine the transformer was actually faulty.
There are several components in the transformer primary that would be suspect as well as a resistance check of the transformer windings.
Seems to me as a rather useless exercise chasing up alternative supplies etc. unless this has been tested and proven defective.
 

sherrilb

May 28, 2022
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Bluejets, yes, you are right and I probably jumped to that conclusion although I did test (as thoroughly as I know how) the other components in the primary and secondary, so I "might" still have been right.

But, I followed kellys_eye's suggestion regarding the full bridge rectifier and I've ordered a bell transformer ($12.00 US).

Since, the primary objective of the exercise was to refresh my electronics skills, this is working out well.

Somewhere I've read that the way to learn things is by blowing things up and burning them to the ground. You should be happy to know that I am well on my way in that regard. I hooked the rectifier I built backwards (positive where it should have been negative) and smoked (lots of smoke and some noise as well) a 1000 uf 30 vac can capacitor. The cap should have been replaced in any case since the dc voltage was 28+vdc which would have made the peak to peak 42 volts (I removed my filter cap thinking the one I smoked would suffice.

Thank you all for your help. You are a great bunch of folks.
 

Bluejets

Oct 5, 2014
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Somewhere I've read that the way to learn things is by blowing things up and burning them to the ground

No, I've never heard that quote nor would I ever suggest anything along those lines.
In my opinion a dangerous exercise in any mains related device and just plain ridiculous statement.
 
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