POW KW+ keying circuit

[Trainman 8274]

Howdy,

I need a bit of help… I picked this amp up over a year ago.. It’s been setting a while so I decided to see if I could get it going… I replaced the big caps in it and a tube that was bad, but the keying/preamp board is blown up.. A friend recommended Nomads keying board, which I installed, but here’s the problem.. There’s no 12v DC in the amp.. It’s all AC… So, I can’t key the amp.. It acts like it might need a preamp, too.. So my choices are find a more suitable keying/preamp board, rebuild the original, which is pretty bad, or build a rectifier to convert 12v ac to 12v dc.. That would be the easiest since I have Nomads board already installed… Help please?

Also, there is no information on POW amps.. what little I have found says they are very similar to Maco amps.. I found the schematic for a Maco 750 and have been using that as a guide..

Thanks!

 

[Naysayer]

Never saw a POW amp but you say it's tube and all AC, even the relays? If true, are you willing to change out the relays to make them DC, too? Only AC relays in amps I saw were old amps like late 1950, early 1960 models and they had 115vac relays. So be it, not impossible.

Making DC for a relay is easy enough. Install a small CT transformer from Amazon or eBay with 1 piece FWB plus a capacitor and a terminal strip for mounting and connections you have nice smooth DC for anything.
What about your relays? if they run an AC you need to change them too. It snowballs like that, so you're in for more work.

Perhaps it would be better to keep it stock. Whoever designed the POW was a pro and good at doing it if they mass produced for commerce.

 

[linearone]

Use a footpedal.

 

[nomadradio]

Whoever made the POW amplifiers didn't care to publish any tech info. Don't think I've seen more than one of them up close, but only from the outside. No schematic has ever surfaced that I have seen nor heard of. I remember hearing that there was a connection to Maco.

They used 8950 or 2057 tubes, IIRC.

The heater supply is the quick and dirty source of AC to feed into a rectifier diode and a 1000uf filter cap. Shouldn't need a separate transformer.

73

 

[Trainman 8274]

Whoever made the POW amplifiers didn't care to publish any tech info. Don't think I've seen more than one of them up close, but only from the outside. No schematic has ever surfaced that I have seen nor heard of. I remember hearing that there was a connection to Maco.

They used 8950 or 2057 tubes, IIRC.

The heater supply is the quick and dirty source of AC to feed into a rectifier diode and a 1000uf filter cap. Shouldn't need a separate transformer.

73

Thanks Nomad! Do I need to change the relay? I hit it with 12v dc and it activated.. I was thinking about a rectifier.. I did pull power from the heater circuit, since that’s what it looks like the original builder did.. This one has M2057 tubes in it.. You are right about the lack of info.. I’ve seen a couple of completed listings for smaller ones and that’s it..

 

[Shadetree Mechanic]

Seems strange that the neutral from the power cord goes to the fuse holder?

 

[Trainman 8274]

Seems strange that the neutral from the power cord goes to the fuse holder?

I replaced the cord in a swinger 205 amp, recently. It was the same way.. AC vs DC.

 

[Shockwave]

If you can only find AC in this amplifier, you've found the problem with the original keying circuit. The amplifier must have DC to run its keying circuit.

As I took a moment to look at your picture, I can confirm the reason why you have lost dc. Look at the small gray electrolytic capacitor mounted to the terminal strip Behind The Rocker switches. It's positive terminal is snapped off of the anode side of the diode. Leaving Raw DC that will not operate any of the relays properly. Check that capacitor see if you can solder it back or if it needs to be replaced.

 

[Trainman 8274]

If you can only find AC in this amplifier, you've found the problem with the original keying circuit. The amplifier must have DC to run its keying circuit.

As I took a moment to look at your picture, I can confirm the reason why you have lost dc. Look at the small gray electrolytic capacitor mounted to the terminal strip Behind The Rocker switches. It's positive terminal is snapped off of the anode side of the diode. Leaving Raw DC that will not operate any of the relays properly. Check that capacitor see if you can solder it back or if it needs to be replaced.

Hi Shockwave,

Actually that cap let the smoke out.. I disconnected it to see if there were any other issues besides the keying circuit.. I have a replacement on order.. There is a diode between the heater circuit and where that cap is hooked up.. Would that and the cap rectify AC to DC? The big relay power comes from that same spot.. The small relay power came from the original keying circuit.

 

[Shockwave]

If the capacitor let the smoke out, there is a likelihood that the diode feeding it is also shorted. Check with your meter to make sure the diode only conducts in one direction before installing the new capacitor. The positive side of this capacitor is your DC output to run the relays.

 

[Shockwave]

One other thing I noticed is the RG-8 coax left its shield completely ungrounded. This is bad because it will radiate output power, right back into the input circuit the coax passes by. Ground both ends.

Also, do you know what happened to this amplifier? That keying board is so fried, I can't tell if that's lightning damage or, did someone stuff a KW into its input.

 

[nomadradio]

I'll direct your attention to the small gray electrolytic capacitor just to the right of the two big blue HV filter caps. The rubber plug at the bottom of this capacitor is bulging outwards, and one of its two leads is severed. This is the filter capacitor for the 12-Volt relay power. And it's dead, Jim. Can't read its capacitance rating, but I'd guess around 1000uf at 25 Volts. Until this part is replaced you won't have enough DC voltage to close one of those relays, let alone both. I would suggest a 2200uf cap to replace it. My policy is to also replace that 1-Amp rectifier diode that feeds into it whenever a 40-plus year old filter cap fails alongside it. Cheap insurance. A 3-Amp part is overkill, and cheap insurance to boot, like 1N5400, 5401 on up to 5408. The higher numbers indicate higher steps of voltage rating. For a 12-Volt relay circuit the 50-Volt rated 1N5400 is just fine. Higher voltage-rated parts work just the same.

Found it humorous to spot my "KeyPNP" board we sell on fleabay. The scorch mark and crinkled paint next to it is impressive. There is a 4.7k 2-Watt resistor strung from that board to the radio coax jack. A tiny RF choke is visible also connected to the center pin of that jack. It looks a bit distressed. If that choke is shorted inside, the radio will see a high SWR whenever the amplifier is in line, even turned off.

And if it's an open circuit, the amplifier may refuse to show power. If so, a 22uH choke rated for at least a half Amp should replace it okay.

The two large HV filter caps are new, but there is more than one of the factory-original small gray filters. Leaving them in place is a ticking time bomb.

73

 

[Trainman 8274]

One other thing I noticed is the RG-8 coax left its shield completely ungrounded. This is bad because it will radiate output power, right back into the input circuit the coax passes by. Ground both ends.

Also, do you know what happened to this amplifier? That keying board is so fried, I can't tell if that's lightning damage or, did someone stuff a KW into its input.

Good morning Shockwave,

Thank you for all the good advice! I was wondering about the coax, also if you notice, there are red wires from the center pins of the 259 connectors to the relay. Shouldn't these be shielded or coax with shield grounded? In nomads post below he recommended replacing the diode and the inductor, so I'll be ordering more parts....

As for what happened to this one.. I don't know for sure. I bought it with a Browning Mk3 from the nephew of a silent key. The radio had a golden eagle mic, Pal VFO and I thought he was running this amp with it. The Mk3 dead keys at 4.5 watts and swings to around 12 on my meter. It needs some help, but does transmit and receive. I didn't know much about tube amps, but am learning. I haven't found much information about POW amps and those that I have asked just wanted to buy it.

 

[Trainman 8274]

I'll direct your attention to the small gray electrolytic capacitor just to the right of the two big blue HV filter caps. The rubber plug at the bottom of this capacitor is bulging outwards, and one of its two leads is severed. This is the filter capacitor for the 12-Volt relay power. And it's dead, Jim. Can't read its capacitance rating, but I'd guess around 1000uf at 25 Volts. Until this part is replaced you won't have enough DC voltage to close one of those relays, let alone both. I would suggest a 2200uf cap to replace it. My policy is to also replace that 1-Amp rectifier diode that feeds into it whenever a 40-plus year old filter cap fails alongside it. Cheap insurance. A 3-Amp part is overkill, and cheap insurance to boot, like 1N5400, 5401 on up to 5408. The higher numbers indicate higher steps of voltage rating. For a 12-Volt relay circuit the 50-Volt rated 1N5400 is just fine. Higher voltage-rated parts work just the same.

Found it humorous to spot my "KeyPNP" board we sell on fleabay. The scorch mark and crinkled paint next to it is impressive. There is a 4.7k 2-Watt resistor strung from that board to the radio coax jack. A tiny RF choke is visible also connected to the center pin of that jack. It looks a bit distressed. If that choke is shorted inside, the radio will see a high SWR whenever the amplifier is in line, even turned off.

And if it's an open circuit, the amplifier may refuse to show power. If so, a 22uH choke rated for at least a half Amp should replace it okay.

The two large HV filter caps are new, but there is more than one of the factory-original small gray filters. Leaving them in place is a ticking time bomb.

73

Good morning Nomad,

Thank you for the reply! I replaced the blue caps. One of the originals from 1978 let the smoke out. Yes the cap beside them is a 1000uf cap that puked its guts, too. It did that after replacing the big caps while I was testing. I will replace the diode going to that cap. I installed your keyPNP board. I posted a pic of the original keying board in a post above. It definitely let the smoke out in a spectacular way. I used an ohm meter to check the choke. It wasn't shorted, but does look bad. I put a nanovna to the input of the amp into a dummy load and was getting an swr of 1.8. Not great, but could be worse. I ordered the 1000uf cap and I the other two grey caps are 220uf. I ordered replacements for them too. I ordered a rectifier, but it looks like I wont need it. I just need to order the diode and a choke. Hopefully, that will get it running. I dont know what percentage the tubes are, but they all come on. I'm not real worried about that, but am curious to get it going. It had one bad one in it, when I bought it. I've replaced that one. Not a cheap endeavor for sure!

 

[linearone]

Not a cheap endeavor for sure!

This is why i cant find the heart to work on these things. I think it's absolutely amazing that guys are out there keeping all these old amplifiers running, but I will say that it's definitely a labor of love.