Unusual R-390A PTO TempCo Caps - UK Vintage Radio Repair and Restoration Discussion Forum

Outposter30 · 4th Feb 2021, 7:11 pm

I'm going through a Cosmos PTO which has been badly baked, and I found the 10pf TempCo caps are not what is normally found in a 390A PTO.

As far as I can tell, the PTO has never been opened, but I could be wrong. All the solder connections appear original, including the heater and thermostat wiring.

As can be seen in the picture, there are 2 small caps which have color dots on them without any other markings. One is marked with a single blue dot, and the other has a series of three dots, orange, yellow and red.

I'm assuming they are 10pf caps, since that is what the PTO schematics and parts list state, but can anyone tell me what the temperature compensation might be according to these dots...?

As can be seen, this PTO got so hot that the desiccant packages began to melt and run down the metal supports.

Any held would be greatly appreciated...TIA...Jordan in Canada

Outposter30 · 6th Feb 2021, 12:38 am

After further investigation, I did find evidence that someone had been inside the PTO at some point in time, which was further confirmed when I discovered there is a very faint number 3 on the cap with 3 dots, which required good lighting and a lot of magnification to even notice...!

I ordered several unsealed silver mica caps of the correct tempco for this and another Cosmos PTO I'm working on. Once they arrive I'll be checking these caps very closely after they are pulled...

QQVO6/40 · 6th Feb 2021, 3:39 am

Hello Jordan.

Do you have a copy of the "Pearls of Wisdom" documentation for the 390-A ?
There is 350 pages of info on the PTO alone.
Lots of bedtime reading and some very good information.
Well worth making up a jig to check the end point of the PTO.
These are not an easy set to get to grips with if you have not had a lot of experience with working on comms. receivers.
I would also suggest downloading copies of the original navy manuals for these radios.
Keep in mind that with the heater switch on the PTO runs at about 100deg inside constantly. It is not unusual for the thermostat to get stuck as well. as you have found out it can really cook things in there. Leave the heater switch in the off position as the 3 ovens are not needed for the sort of work we give these old beasties.
In service the heaters were needed to keep the RX on frequency as a lot of the time they were used for RTTY traffic and were not necessarily under constant human supervision.

Enough from me.
Cheers, Robert, VK2BNM.

Outposter30 · 6th Feb 2021, 12:06 pm

Hi Robert...Yes, I have a fair collection of 390/390A manuals and documentation I use for reference, along with the information available online. I always leave the Ovens switch in the OFF position...!

I got my first R-390 back in the late 70's, and I've had a dozen 390As or so since then, along with a ton of other tube and SS equipment...

I've never seen a PTO this badly cooked internally, and I hope to be able to restore it to proper operation.

More later....NNNN

4th Feb 2021, 7:11 pm	#1
Outposter30 Triode Join Date: Oct 2020 Location: Calgary, Alberta, Canada. Posts: 22	Unusual R-390A PTO TempCo Caps I'm going through a Cosmos PTO which has been badly baked, and I found the 10pf TempCo caps are not what is normally found in a 390A PTO. As far as I can tell, the PTO has never been opened, but I could be wrong. All the solder connections appear original, including the heater and thermostat wiring. As can be seen in the picture, there are 2 small caps which have color dots on them without any other markings. One is marked with a single blue dot, and the other has a series of three dots, orange, yellow and red. I'm assuming they are 10pf caps, since that is what the PTO schematics and parts list state, but can anyone tell me what the temperature compensation might be according to these dots...? As can be seen, this PTO got so hot that the desiccant packages began to melt and run down the metal supports. Any held would be greatly appreciated...TIA...Jordan in Canada Attached Thumbnails

6th Feb 2021, 12:38 am	#2
Outposter30 Triode Join Date: Oct 2020 Location: Calgary, Alberta, Canada. Posts: 22	Re: Unusual R-390A PTO TempCo Caps After further investigation, I did find evidence that someone had been inside the PTO at some point in time, which was further confirmed when I discovered there is a very faint number 3 on the cap with 3 dots, which required good lighting and a lot of magnification to even notice...! I ordered several unsealed silver mica caps of the correct tempco for this and another Cosmos PTO I'm working on. Once they arrive I'll be checking these caps very closely after they are pulled...

6th Feb 2021, 3:39 am	#3
QQVO6/40 Hexode Join Date: May 2009 Location: Newcastle, Australia Posts: 321	Re: Unusual R-390A PTO TempCo Caps Hello Jordan. Do you have a copy of the "Pearls of Wisdom" documentation for the 390-A ? There is 350 pages of info on the PTO alone. Lots of bedtime reading and some very good information. Well worth making up a jig to check the end point of the PTO. These are not an easy set to get to grips with if you have not had a lot of experience with working on comms. receivers. I would also suggest downloading copies of the original navy manuals for these radios. Keep in mind that with the heater switch on the PTO runs at about 100deg inside constantly. It is not unusual for the thermostat to get stuck as well. as you have found out it can really cook things in there. Leave the heater switch in the off position as the 3 ovens are not needed for the sort of work we give these old beasties. In service the heaters were needed to keep the RX on frequency as a lot of the time they were used for RTTY traffic and were not necessarily under constant human supervision. Enough from me. Cheers, Robert, VK2BNM.

6th Feb 2021, 12:06 pm	#4
Outposter30 Triode Join Date: Oct 2020 Location: Calgary, Alberta, Canada. Posts: 22	Re: Unusual R-390A PTO TempCo Caps Hi Robert...Yes, I have a fair collection of 390/390A manuals and documentation I use for reference, along with the information available online. I always leave the Ovens switch in the OFF position...! I got my first R-390 back in the late 70's, and I've had a dozen 390As or so since then, along with a ton of other tube and SS equipment... I've never seen a PTO this badly cooked internally, and I hope to be able to restore it to proper operation. More later....NNNN