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Palomar 300a transformer

Tokin

The Man With No Shoes
Oct 31, 2019
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Oh, boy! 4th of July?

Chernobyl?

You wouldn't just pour water into the radiator, new gas into the tank and drive a 1975 barn-find car full of original parts very far before the first thing breaks.

Putting it back into service without releasing the magic smoke first requires more than just plugging it in. That amplifier is famous for clobbering a radio's final transistor when the relays become worn. First or second key, usually.

Filter capacitors tend to cause collateral damage when they break down. Even if an electrolytic capacitor got replaced when it was ten years old, the "new" one is over thirty. Much cheaper to replace them before they create additional damage.

Time for a YT vid "How NOT to care for and feed a 45 year-old linear".

That radiator hose looked okay before it exploded.

73

I warned him about using a variac to slowly bring the unit up to voltage first time, i did i did. Relays as you said can also be an issue. The relay replacements we got from you work flawlessly, thank you.
From what i understand this is a dad/son thing where dad had a 300a years ago and wants another to school his son. Unfortunately they didnt realize the amp was missing the supply before buying it. Im always suspicious of a 2 piece amp thats missing the transformer.
I've filled him in on my experience with my 300a and what i did and and how much was spent to make it reliable. Ive just about got him to contact Carl and order a pill amp. I have a feeling the 300a may need more than theyre hoping for. Its doubtfull the seller took the time to pull and number the tubes or even pack some bubble wrap between them so tubes making the cross country journey to the PNW are fairly slim.
 
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nomadradio

Analog Retentive
Apr 3, 2005
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My experience with the 300A only goes back 45 years, but any electronic device that old will have multiple issues. And if only one of them goes unrepaired, the radio, the amplifier or both can be damaged.

Easily.

I call it "cockroach theory". If you can see one, there will always be more of them you don't see.

73
 

Tokin

The Man With No Shoes
Oct 31, 2019
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My experience with the 300A only goes back 45 years, but any electronic device that old will have multiple issues. And if only one of them goes unrepaired, the radio, the amplifier or both can be damaged.

Easily.

I call it "cockroach theory". If you can see one, there will always be more of them you don't see.

73
Lol, my experience with the 300a goes back only 30 years, older man, hehehe...
 
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Jay Mojave

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Nov 17, 2015
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Yeah... The 300A Palomar has a great looking chassie chrome cover makes it look really great.

Many years ago at the TRW swap meet in Los Angles I say a modified Palomar chassie converted using two 4CX250B tubes and a bigger external power supply. It wasn't cheap and I decided to pass it up, but then the vain in my brain said get it! I turned around and it was being bought.

So food for thought that chassie could be used for other modern tubes as 4CX250, 4CX350, 4CX800 and such. Especially using a external power supply allowing the chassie to hold the RF Deck. Back to "I want the meter to go all the way over to the peg"

Jay in the Great Mojave Desert
 

Rick Miller

Member
Apr 1, 2015
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How timely, I haven't logged in for a while. I was just given a 300a without the transformer. For now it's a shelf decoration, but I'll be looking for a transformer soon myself...
 

Jay Mojave

Sr. Member
Nov 17, 2015
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Hello Rick and All: Ok good deal good for you. As Nomadradio has said that the Power Transformer Needs to be verified as too how much High Voltage (HV) it puts out, no more than 280 VAC, (280 x 2.82) as its feed to a voltage doubler and then the power supply capacitors ending in 786 Volts DC that is feed to the tubes.

I have a Palomar 300A Transformer that has 440 Volts AC that when feed to the voltage doubler and capacitors with 1240 Volts that explode the two 40 dollar capacitors with ease, ah the smell of exploding capacitors in the morning! A full wave bridge was wired so that the 440 VAC would now make 620 VDC and not explode the amp parts.

I am hoping that my Palomar 300A Transformer is a fluke and some one put the wrong transformer in its place. Meaning all the Palomar 300A are wired the same for 280 volts out of the power transformer. I contacted the chief engineer at Palomar asking if he could send any documentation on the Palomar 300A amps and what different configurations the 300A amps might have been made in. NO answer as of yet. But don't hold your breath. Good Luck.

The Palomar 300A amps are not a simple amp to fix especially after some ding dong worked on it. Be advised.

Jay in the Great Mojave Desert
 

nomadradio

Analog Retentive
Apr 3, 2005
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The factory sent out one version of the 300A with a small rectangular epoxy-block bridge rectifier on the HV circuit board. It was rated at 1500 PIV, plenty enough for that transformer.

Trouble is, the highest voltage rating we could find for that kind of part on the replacement market is 1000 Volts. Those would routinely break down before too long. Just not a high enough rating.

Ended up using eight 1kv rectifier diodes to repair those 300A power supplies. Two of the generic 1kV diodes in series would hold up just fine in each of the bridge circuit's four legs. Ends up looking a bit clumsy, but it works.

The change to the full-wave doubler circuit with four single diodes came along later, but I wasn't keeping careful track as to when. Just learned to go by what we see under the amplifier's chassis.

73
 

Jay Mojave

Sr. Member
Nov 17, 2015
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Hello Nomadradio and All: Ok great post verifying that there was two different Palomar 300A configurations, for the power supply circuit. Great stuff.

My Palomar 300A amp has the voltage doubler and a 440 VAC output transformer, thats why when got it off e-pay the power supply caps were toast and the diodes, and the 30 amp fuse on the amp was blown open. This with the transformer HV output wired directly to the diodes and capacitors bypassing the R4 relay switch function. The center relay on the circuit board is total toast another great moment in amp repair. Yeah clue less screw driver experts exist all right, not a good thing.

I will wire the new 1000PIV diodes two per leg, in a full wave bridge circuit. Allowing a safe HV level.

Still no input from the Chief Engineer from Palomar, crickets heard in the back ground....

The diagram on CBtricks.com See: palomar_300a_sch.gif (1003×786) (cbtricks.com)
shows 4 relays I am not seeing it on my 300A Amp, but it does show 6 relay contacts, and does have 3 relays each having 2 relay contacts.

Anyway anyone connecting a unknown Palomar 300A transformer to the 300A Amp should have a Tech check it out first.

Jay in the Great Mojave Desert
 

ChanceTran

New Member
Oct 18, 2021
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Hi....this model has the transformer in an outside bureau with an associating link. Discovering a substitution transformer has demonstrated more troublesome than I expected. Each of the 300-330V transformers are under 500mA. Most are around 20mA. On the off chance that anybody has a proposed merchant I would see the value in it.
 

Jay Mojave

Sr. Member
Nov 17, 2015
332
632
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Hello All: Here's a good video on the Palomar 300A linear amp. SEE:

Palomar 300A's keep pulling me back in..... - YouTube

This video explains the bridge power supply circuit that is used with 440 volt power transformer. And the use of biasing the tubes to cut off (I would think) in stby mode.
And that the relay function of R4 was not used, that turns off the HV power supply but is shown on the CBTricks diagram. Good stuff.

Jay in the Great Mojave Desert
 
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