A Palomar 300A in distress

[Dmans]

Final installment of the 300A refurb.

I finished the driver board repairs, wound the suppressor coils around a 47 ohm resistor(and used the ferrite beads) and installed the driver tube socket board in the chassis. Per a couple recommendations I added an auxiliary ground to the circuit board.

Below is the installed driver board installed in the chassis. In the upper right corner of the board you can see the braided ground strap soldered to the ground plane of the board.

Next was removal of the final tube socket board. This board also had cold solder joints around the tube socket pins. I re-soldered all of the pins and replaced the 22ga jumpers between the grid bias pins of the tubes (green 16ga wire on the tube socket side of the board) and replaced the filament jumpers for the series pairs (green 16ga wire on the solder side of the board).

The added ground braid is soldered to the upper right corner of the board in the photo below.

The picture below is after installation of the final tube socket board to the chassis. The area circled in red is where the ground braid attaches to the chassis. I added an external toothed washer under the braid and a flat washer on top of the braid to insure a good grounding point.

Below is the top of the driver tube board after installation. Again, the extra ground is circled in red.

Below is from the underside of the chassis with all of the boards installed and all of the wiring secured.

All that is left is install the tubes and test. Not sure of what the numbers on the tubes (other than type) mean but all 6 are the same date code(?) or lot #(?) or ???

So after tuning in the input SWR on the low and high settings (1.2:1 on both) behind the Cobra 139XLR, I was seeing 650 watts Pep on SSB on high and 175 watts Pep on SSB on low into a dummy load. Behind the TS440 with 80 watts drive, I was seeing 650 watts Pep on SSB on low into the dummy load.
Running the amp on the air, all reports were good. No more fuzzy audio when driven with the Kenwood or the 139XLR.

I believe this to be a successful repair and a good learning experience. Perhaps with this “refurbishment” this Palomar will last another 40+ years. Now at this point I believe I am into this amplifier for just over $150.00. Not bad for 1/2 a kilowatt!

73’s
David

 

[jessejamesdallas]

Been having problems with my 300A blowing rectifier diodes...When they go, it pop's the fuse...Had one pop about 3 months ago, had it replaced, then just the other day 2 more let go...my tech replaced the old ones with (I think he said) 1000v ones...not sure how many more original ones are left, and not sure what is making them blow. Amp works just fine, and both times the diodes went bad was when I was just listening and not talking...amp wasn't even all that hot either.

 

[Dmans]

JesseJamesDallas,
The 300A is not built with a current surge resistor (normally 3.9 - 10 ohms @ 5W) or a current surge choke (1 - 10 mH) in series with rectifier string, but that is what these components are supposed to do-protect the rectifiers. {The Skipper 300 has a "glitch" resistor in each leg of the transformer secondary. They are .5 ohms @ 10 watts} Although a definite pain, due to space constraints, it was for this reason that I replaced every diode in mine. All spike suppressing diodes on relay coils, 12v rectifier diodes and negative grid control diodes. And since it required removing all 4 circuit boards, I replaced all the resistors(small and large wattage) and electrolytic capacitors.

I have used the 1N5408 diodes in 5 recent amplifier refurbs without an issue (I have 3 more amps to do and am planning on using 1N5408 on those as well) . They are rated at 1000v and 3 amps. The original diodes in my 300A were 1N5406 rated at 600v and 3 amps.

It could be that another component is contributing to excess current through the rectifiers?? A filter capacitor comes to mind.

I am sure someone with much more experience than me can offer a suggestion for this issue.

Good Luck!

73's
David

 

[nomadradio]

The original 1N5400 3-Amp rectifier diode had a reverse-voltage breakdown rating of 50 Volts. The 1N5401 was the 100-Volt version, on up to the 600-Volt 1N5406, 800-Volt 1N5407 and the 1000-Volt 1N5408.

A manufacturer buying rectifiers ten, twenty thousand or more at a time will buy the cheapest one that does the job. Naturally the higher-voltage rating added a few cents to the sale price. A hundred thousand pennies makes this look worth the trouble. A repair shop buying a few hundred or less at a time probably stocks only the 1000-Volt 1N5408. It's a perfect substitute for any of the lower-voltage versions. Keeping just the one part on hand is just easier.

Oh, and 40-plus years ago, the price difference between voltage ratings was a lot more than a couple of pennies.

Replacing just one or two of the rectifiers in a 300A is false economy. They are cheap. The 300A that uses the lower-voltage HV transformer has two series-connected pairs. If those two diodes in a pair don't match, they may not divide the voltage equally. If one of the two gets more than its one-half share of the voltage this could make it fail. Any time a single diode fails, all four should be replaced with parts from the same batch. Changing them onesy-twosey can cause trouble. It's false economy for a part this cheap.

If yours has the higher-voltage transformer, it was built with a square-block rectifier bridge that had four wire leads on it. If you only use four 1N5408 diodes to replace that bridge when it fails, you *WILL* have random breakdown problems. That one-piece bridge was rated for 1500 Volts. Four 1000-Volt diodes just don't have sufficient rating to be reliable in that circuit. Takes eight of them, wired as four series-connected pairs. Pretty much the same as you find in any D&A Maverick or Phantom amplifier.

Last and probably most-likely cause of phantom "POOF!" rectifiers is old filter capacitors. If the two large ones are older than 20 years, they can break down, smack the rectifiers, and then seem to "heal" and work for a while again before repeating the problem. If they are original, and have a date code from the 1970s, they are almost-certainly the root of the trouble.

Could be other possibilities, like a tube that has contaminated "gassy" vacuum. But without knowing which of several 300A versions you have, those are my best guesses.

Oh, and yeah. Those four-digit numbers on Dman's tubes. Those are called an "open" date code. Two digits for the year, two digits for the week of that year, 01 to 52. These are found only on factory-original tubes that were shipped in bulk trays. Seeing those numbers tells you that tube was originally sold to a factory in bulk and was never packaged in a single-unit cardboard retail carton. Tubes that were distributed that way will have a two-letter date code. This kept the retail buyer from griping about how long a tube had been on the distibutor's shelf before he bought it. You could only tell how old the tube was if you had the "cheat sheet" to translate the letter code to the actual production week.

And the other "188-5" marking? The three digits "188" are the EIA (Electronics Industry Association) member number of the part's manufacturer. GE was 188, and the "-5" indicated the GE facility. In this case, their Owensboro, KY factory. A part made by Mallory had the EIA number "235", and RCA was 274.

I think. Had a list of those years ago. No telling how deep it's buried now.

73

 

[Dmans]

Thanks Nomad!!
Great information from your post as always.

73's
David

 

[Tweetybird]

Loud and clear on the white plastic capacitors. All electrolytics will be replaced minus the 2 main filter caps. They were replaced shortly before I acquired it.



There are 4 "cement" resistors (I call them) at the lower left. 2-27ohm 5 watt and 2-50ohm 10 watt. They will be replaced as well. I tend to replace resistors rated 2 watts and up. Can't be trusted after 40+ years.

I have replaced the old coax in some recent "upgrades" I have done for other folks recently. The coax that was pinching the 12vac wire from the transformer has a foam dielectric. Never have liked that stuff! In my experience it doesn't "play well with heat". I have used RG-313. It has a Teflon dielectric, is small and holds up to heat well. (A real space saver too!)

Some people may ask why I fool with these sweep tube amps. For me they are a great cheap learning tool. Yes, sweep tubes can be hard to come by, but I have found that if the tubes are treated right and not pushed hard, they will give good service. Not unlike a 70's car or truck with 100k miles, these amps will require a "tune-up". To me this means replace all electrolytics, all rectifier diodes, all higher wattage resistors, cleaning relay contacts, flux removal from traces as well as a close inspection and repair of questionable traces, resoldering high voltage traces, replacement coax jumpers and (sometimes) new SO-239 connectors too.

This particular amplifier kind of "fell into my hands" for $125.00 and was working great. I don't know of anywhere else you can get 1/2Kw for that kind of money. I believe it to be well worth the investment to "renew" this classic!

If my operation of a sweep tube amplifier offends anyone, please place me on your ignore list.

Nomad,
Thanks for the tips on this one. I last owned one of these in 1979 and I think it is a great classic piece to restore!

73's
David

I just bought one for 75.00 I need to fix

 

[Tweetybird]

Final installment of the 300A refurb.

I finished the driver board repairs, wound the suppressor coils around a 47 ohm resistor(and used the ferrite beads) and installed the driver tube socket board in the chassis. Per a couple recommendations I added an auxiliary ground to the circuit board.

Below is the installed driver board installed in the chassis. In the upper right corner of the board you can see the braided ground strap soldered to the ground plane of the board.
View attachment 28365

Next was removal of the final tube socket board. This board also had cold solder joints around the tube socket pins. I re-soldered all of the pins and replaced the 22ga jumpers between the grid bias pins of the tubes (green 16ga wire on the tube socket side of the board) and replaced the filament jumpers for the series pairs (green 16ga wire on the solder side of the board).

The added ground braid is soldered to the upper right corner of the board in the photo below.

View attachment 28366

The picture below is after installation of the final tube socket board to the chassis. The area circled in red is where the ground braid attaches to the chassis. I added an external toothed washer under the braid and a flat washer on top of the braid to insure a good grounding point.

View attachment 28368

Below is the top of the driver tube board after installation. Again, the extra ground is circled in red.
View attachment 28369

Below is from the underside of the chassis with all of the boards installed and all of the wiring secured.

View attachment 28370

All that is left is install the tubes and test. Not sure of what the numbers on the tubes (other than type) mean but all 6 are the same date code(?) or lot #(?) or ???
View attachment 28371

So after tuning in the input SWR on the low and high settings (1.2:1 on both) behind the Cobra 139XLR, I was seeing 650 watts Pep on SSB on high and 175 watts Pep on SSB on low into a dummy load. Behind the TS440 with 80 watts drive, I was seeing 650 watts Pep on SSB on low into the dummy load.
Running the amp on the air, all reports were good. No more fuzzy audio when driven with the Kenwood or the 139XLR.

I believe this to be a successful repair and a good learning experience. Perhaps with this “refurbishment” this Palomar will last another 40+ years. Now at this point I believe I am into this amplifier for just over $150.00. Not bad for 1/2 a kilowatt!

73’s
David

Help me fix mine please paid 75.00

 

[Tweetybird]

Help me fix mine please paid 75.00

I actually have three palomar that need work two skippers 1 three hundred a

 

[Tweetybird]

I actually have three palomar that need work two skippers 1 three hundred a

I read where you said it's a good one to learn so here I am .

 

[Tweetybird]

If either or both of the white-plastic "R50" relays has gone bad, we have a drop-in solution for that one. NOS replacements pop up on fleaBay and tend to be pricey.

The date code on both is early 1976. They might both still work okay.

If they don't, cleaning only helps sometimes.

This is what we use.

They're on fleaBay.

73

 

[Tweetybird]

I want to replace those r 50 relays too. What are the ones you showo

If either or both of the white-plastic "R50" relays has gone bad, we have a drop-in solution for that one. NOS replacements pop up on fleaBay and tend to be pricey.

The date code on both is early 1976. They might both still work okay.

If they don't, cleaning only helps sometimes.

This is what we use.

They're on fleaBay.

73

Repairs have begun on my 300A. There is no easy way to access the circuit traces on the high or low voltage boards other than to remove them from the chassis and no easy way to remove one board at a time. Talk about cramming 5 lbs of s#^t in a 2 lb sack......
Since the trouble began with a pinched 12vac feed to the low voltage board, that is where I chose to start.
Below is a picture of the low voltage board after removal from the chassis.

View attachment 28094
View attachment 28095

I removed all components and wiring, cleaned up the crusty flux and installed all new components on the board including new wiring.

Below are pictures of both sides of the board after overhaul.

View attachment 28096
The small wires in the picture above were the original wires. I believe they are 22ga. I replaced them with 16ga with Teflon insulation. I am hopeful that replacement of the old RG-58 foam coax with RG-313 will make enough room for the larger wires

View attachment 28097

Stay tuned.
73’s
David

What watt are the 1k resistors on this board

 

[Mr PrEz]

Hello guys!

i have the Palomar 300a, I was using it like normal but i noticed the driver stop working, i opened and moved the anode wires a little bit and start working, but I ordered 2 new NOS 6LF6 ones, put them in and now the driver won’t turn on. I have the filling the disc capacitor is dead. Any suggestions?

 

[nomadradio]

Are the filaments lighting in the driver tubes?

That capacitor tends to show scorch marks and damage when it fails. It's probably not the problem.

73

 

[Mr PrEz]

I had 2 long 6LF6 working fine but I ordered the short ones now, nope the filaments are not lighting my friend. Low power works fine I get around 80 dead key 650 peak

Thanks!

 

[Tokin]

I have another member here refurbing one of my 300a's. Wind the chokes and just about finished and ready for testing