I recapped a PT-10 and it seems to work except for blowing the pilot light after 3-4 minutes after turning on. When the first blew, I chalked it off to it being 50-60 years old, but it did the same with a fresh one.
The rectifier is a 35Z3, so there isn't a problem with the center tap filament issue. I have traced the heater circuit and don't see anything out of the ordinary. The pilot light holder appears in fine shape, no corrosion, etc.
The 40 ohm resistor in parallel with the pilot lamp measures 42 ohms, so that should be ok.
Only thing odd I have noticed is that the set only draws 0.2 amps, which seems low compared to other sets I have worked on. They usually run 0.4 - 0.6 amps when on.
Any thoughts on this?
Thanks,
John
Hi John and welcome to the Phorum,
Several things come to mind. Bad candohm resistor heating and shorting to the chassis, bad piece of rubber wire shorting to the chassis, bad insulation on the lamp socket, and too much current flowing thru the hv circuit.
GL de N3GTE
Looking at the schematic for similar models, PT2, PT6, and PT12, I would carefully trace all the filament wiring which is a series circuit. I would also look at both sections of the candohm resistor, or, better yet, just go ahead and replace it using a terminal strip and two resistors, an 80 ohm of at least 5 watts, preferably more, and a 40 ohm of 3 watts or more. You could buy 3 40 ohm (or 39 ohm) 3 watt or more resistors and use 2 in series for the 80 ohm section. Several places carry them, here's a link to get you started,
https://www.justradios.com/
[url=https://www.justradios.com/][/url]
I considered whether the candohm might be acting up, although the two sections (40 and 80 ohm) measure 42 and 84 ohms when cold. I wondered if something was happening when it got some current on it. That is where I will try first, as I've read other places that these can be problematic, but thought that since they measured in tolerance that it would be ok.
I looked at the filament string and it looks ok, wasn't touched in the cap replacement. Also have looked for any other shorts to the chassis and solder blobs, but haven't found any. The lamp socket and wiring also look fine.
I'll order the power resistors and report back if the candohm was the culprit.
John
If you use a calculator to determine the actual wattage for the cadohm multiply by 3 and round upward. Else the resistor will get very hot. Like melt the solder off the lugs hot.
Yeah, Terry. JustRadios.com carries 5 watt 40 ohm resistors for 59 cents each, or you can also find them on eBay, which, with the $20 minimum order on JustRadios might be cheaper, The draw is about 0.25 amps, so 0.25 x 0.25 x 40 = 2.5, I figure 5 watt will be OK, although it is only twice the wattage being dissipated. 10 watt power resistors are bigger, and would need to be mounted to the chassis. that could also be done. eBay also has those.
https://www.ebay.com/itm/qty-3-RESISTOR-...7869!US!-1
https://www.ebay.com/itm/US-Stock-5x-40-...7869!US!-1
I wondered about that as the size of candohm body looks to me like it would be about heat a 10 watt chassis mount would dissipate. I wondered if there was a wattage rating on the candohm back in the day. Riveting them to the chassis no doubt bled off some of the heat.
I know that cement resistors get really hot. I don't like to use them, but I guess they have their place.
John
OK, people, I looked at the data sheets for the tubes involved. The heater current is supposed to be 0.15amps. That is the current that should be running through this series circuit. Each of the 40 ohm resistors will dissipate 0.9 watts! That means that 3 watt resistors will work OK, being more than 3 times the wattage they will dissipate, 5 watt will be way more than sufficient, and 10 watt would be major overkill. Actually, the one with the bulb across it will dissipate even less because the bulb will be dissipating a lot of it. So, as I said in the beginning, 3 40 ohm, 3 watt resistors on a terminal strip should work fine.
Well, back with the candohm update. I replaced it with two 5 watt resistors, and it doesn't seem to get too warm. I decided to replace the 35Z3 rectifier since the voltage was a hair over 6 volts across the pilot light when I was putting in 100 volts with the variac. Now it shows 4.7 volts across the pilot light after putting in a new rectifier tube when I crank variac up to 120 volts.
I let it play for about 30 minutes and no problems. I guess the rectifier must have had a issue, but all is good now. I wonder how one would track down this problem if a usually problematic candohom wasn't involved.
Time for working on the case and making it look nice.
John