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Palomar 300a Transformer hum

Know this is a old thread...but on my 300A yesterday for no reason at all, it decided to pop the fuse...Before it popped the fuse I herd a loud "BUZZZ" then no power.

Had the box out on the table today and replaced the fuse, but soon as I flipped the power switch on, that loud BUZZ I herd yesterday I found out came from the transformer...then it blows the fuse...

So I figure the transformer must be bad...question is, can these things be fixed, or am I just S.O.L. and going to have to try to find another transformer?
 
What you're seeing is one of two things.

A severe overload caused by decades-old filter caps, and the rectifier diodes that get damaged when the filters short.

-- or --

A bad transformer. Good chance if it's bad that a prior breakdown in the rectifers and filters shortened its life. Even if it happened 30 years ago.

Transformers almost never just fail spontaneously. Much more common to see damage from overloading with a fuse that's just too big in line.

If unhooking one of the two wires from the transformer's HV winding to the rectifier board stops the overload, your transformer might still be okay. If it still blows the fuse, the transformer may have actually failed. Just don't forget to check the HV rectifiers and filters before installing a replacement. Wouldn't want to fry it like this one.

The way it's built, a failed tube almost never does this until you key it.

Almost never. Tubes fail more frequently than any other part in the amplifier. Would be wise to be sure none of them has a short in it.

73
 
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Although blown, the fuse's are both factory spec. (one on the radio and one on the transformer) and most likely the ones that came from the factory for as that go's...Cap's I'm sure aren't quiet up to snuff due to age, just never had anything let-go like this one did, just sitting there not even being keyed.
 
jessejamesdallas said:
Although blown, the fuse's are both factory spec. (one on the radio and one on the transformer) and most likely the ones that came from the factory for as that go's...Cap's I'm sure aren't quiet up to snuff due to age, just never had anything let-go like this one did, just sitting there not even being keyed.
Click to expand...
The voltage is present even if not keyed.
 
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It is on all but the oldest versions. If you see a relay perched on the rear corner of the rectifier-filter board, you have the old version. That relay turns on the HV only when it's keyed. All later versions of the 300A have 800 Volts DC coming out of that board on standby.

If the HV filter caps are original, the only question is how much collateral damage did they do when they finally shorted. That transformer is pretty tough. It usually survives this sort of episode.

73
 
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nomadradio said:
It is on all but the oldest versions. If you see a relay perched on the rear corner of the rectifier-filter board, you have the old version. That relay turns on the HV only when it's keyed. All later versions of the 300A have 800 Volts DC coming out of that board on standby.

If the HV filter caps are original, the only question is how much collateral damage did they do when they finally shorted. That transformer is pretty tough. It usually survives this sort of episode.

73
Click to expand...
The high voltage is still present but not connected to the tubes.
 
Well...come Saturday, off to the shop it go's!

My first thought was the transformer was toast, and I was probably going to have to turn it into a paper weight, because finding another transformer would probably be next to impossible to fine without buying another amp too...If it's just caps and rectifier's that shouldn't be that big a deal to fix...Even if some of the tubes are toast, I can deal with that...
 
If you ever have to replace the original tubes with the taller ones, install flat washers on the 6 screws holding the top cover on. Don't thread the screws through the holes on the bottom cover but rest them on the top edge of the bottom cover and tighten the screws. That will give the needed cover height without arcing the plate caps to the cover. The added flat washers provide enough grip and surface area that you can still pick it up by the cover without it coming off. Also, check the screws holding the PC boards in place. They provide RF grounding to the tube boards and are almost always loose after all these years. Tube socket pins often need re-soldering due to cold joints.
 
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If you score a transformer in the case with the cable for one of these, it's 50-50 as to whether it's the right one to match your RF unit.

One version has a HV secondary winding of 560 Volts. The HV rectifier/filter board in the RF deck that matches this version has a bridge rectifier, to get just over 800 Volts DC.

The other version of the RF deck will have four 3-Amp rectifier diodes on the HV board. The correct transformer voltage for this RF deck is half this much, around 280 Volts AC. The full-wave doubler still gets you the same 800-plus Volts DC.

There are absolutely no markings on the transformer's cabinet to distinguish one from the other. They look the same. Even on the inside, there is no marking to identify which transformer you have.

A mismatch of the low-voltage transformer to the 'bridge'-version RF deck gets you an amplifier that is perpetually on low side, no matter which way the High/Low switch is set.

A mismatch of the 560-Volt transformer to a RF deck with the doubler circuit gets you 1600 Volts.

Briefly.

Very briefly.

Just long enough to make the filter caps go POW!, or something like that.

With any luck, breaking the connection between the transformer's HV winding and the HV rectifiers will allow it to power up on standby, run the fan and the tube heaters. If so, your transformer is probably still good.

73
 
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nomadradio said:
A bad rectifier can cause a transformer to "grunt", and make noises under load.

As a rule, if this is the cause, it will get louder when you're keyed and drawing current from the transformer's HV winding.

And if it's just loose laminations, it will hum all the time.

73
Click to expand...
If it hums all the time, you can hum along with it and harmonize. Or, you might be able to teach it the words to the music....:whistle:
 
nomadradio said:
If you score a transformer in the case with the cable for one of these, it's 50-50 as to whether it's the right one to match your RF unit.

One version has a HV secondary winding of 560 Volts. The HV rectifier/filter board in the RF deck that matches this version has a bridge rectifier, to get just over 800 Volts DC.

The other version of the RF deck will have four 3-Amp rectifier diodes on the HV board. The correct transformer voltage for this RF deck is half this much, around 280 Volts AC. The full-wave doubler still gets you the same 800-plus Volts DC.

There are absolutely no markings on the transformer's cabinet to distinguish one from the other. They look the same. Even on the inside, there is no marking to identify which transformer you have.

A mismatch of the low-voltage transformer to the 'bridge'-version RF deck gets you an amplifier that is perpetually on low side, no matter which way the High/Low switch is set.

A mismatch of the 560-Volt transformer to a RF deck with the doubler circuit gets you 1600 Volts.

Briefly.

Very briefly.

Just long enough to make the filter caps go POW!, or something like that.

With any luck, breaking the connection between the transformer's HV winding and the HV rectifiers will allow it to power up on standby, run the fan and the tube heaters. If so, your transformer is probably still good.

73
Click to expand...
NomadRadio, I want to thank you for this information regarding the transformers. I was recently given an old 300A with two power transformers. Had I not read this first I would have assumed they would both match the amp. Don't you know both of them had the higher voltage winding and neither one matched the old 300A with the doubler circuit. Some simple rewiring of the amplifier deck made it work with the newer transformers without having to make the over voltage mistake first. That saved at least the new rectifiers and caps I had just installed. Thanks!

One other thing I found with these transformers is they used different power cords even on the same voltage transformers. Some only have 16 gauge wire while others have 14. The primary and filament windings suffer from voltage drop though the 16 gauge wire. That not only reduces power output but it can also reduce tube life if the filament voltage to these oxide coated cathodes drops below 6.3 volts.
 
Last edited:
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Congratulations on NOT blowing up new filter caps.

You would think that after this many years I would have stumbled across the "skinny wire" problem by now.

But that's a detail I had missed. Might explain why some of these amplifiers would be anemic, never quite achieving the same power as another identical unit.

Had the notion of taking the transformer out of a Palomar 350Z and putting it into a cabinet with a cord/plug to fit a 300A.

At least that would make the 350Z good for something. If I use rotator wire with two fat leads, that should avoid the low heater-voltage hazard.

73
 

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