Recapped and re-resistored Philco PT2. Played very well for about a month, every day 10 hours, then went intermittent, then burned out two dial lamps before the 35Z3 rectifier finally shorted out.

I have read some other threads about 35Z3. Can I replace it with 1N4007, not as a test, but permanent? I see that it is recommended that the filament be replaced across pins 1 and 8 with a 250 ohm 10watt resistor. I don't understand the use of such a big resistor since the filament in the 35Z3 measures 32 ohms. And 10 watts seems sort of overkill.

Any help on this appreciated, it is a very nice radio, and I don't have a 35Z3.

Bill

Bill

Since 35Z3 is used in AC/DC radios you already know a resistor is needed in place of filament. The resistor has to drop 35 volts at 150ma. 35 X .15 = 5.25 watts. Even a 10 watt resistor will get hot.

A silicon diode can be used to replace the tube but you may still have a problem. What caused excessive current to be drawn burning out the lamp & tube? Since all caps were replaced may be a bad audio output tube? Usually 50L6GT or 50A5.

A 35Y4 can be used in place of 35Z3 if pin #4 doesn't have a connection at the socket.

http://www.nostalgiaair.org/PagesByMode ... 014125.pdf

Norm, thank you for your help.

The lamps went bad after the intermittent started. I guessed that the 35Z3 was just going bad anyway and caused the lamp failure. I did not test the 50L6GT. Will do.

I only have one spare loctal tube and it isn't used in this radio.

Thanks again,

Bill

Bill

The first place I searched for a price on 35Z3, $3.00.

http://www.hamtubes.com/tubes/list4.htm

Might ask in our classified section. Geno sells tubes at reasonable prices.


viewforum.php?f=15

That is the cold resistance. 35/.015 = 233 ohms, which is the resistance of the filament
once it reaches operating temperature.

Pete

Hi, Norm,

Tested the 50L6GT. Quite strong. My other question was why the 250 ohm resistor when the 35Z3 filament read 32 ohms? And, taking a page from replacement of a selenium rectifier, if I use a 1N4007, do I need a dropping resistor in series?

Thanks, Pete, while I was typing the above question, you answered it.

And norm, thanks for the tip about the tube source.

Bill

Edit: Thanks, Norm, for the schematic.

Did you replace R27, the Candohm resistor mounted on the chassis? If the 40 ohm section is intermittent it will blow out the pilot lamp.
Don

Hi, Don, Haven't heard from you since the Philips. After the lamps went, I replaced the Candohm, both sections. Now it is working with the 250 ohm resistor and 1N4007 diode. There is some squealing (heterodyning?) but it tunes stations. The squealing might be due to my having jumpered from the radio to the loop antenna in the case. I will clean up the wiring and install it in the case.

Can I just keep the 250 0hm resistor and the 1N4007? Should I check for voltage and current at various points? On the Kill-A- Watt, the line current is 220 ma with the dial lamp working now.

Thanks,

Bill

The 250 ohm heater resistor is OK, but you also need 100 to 125 ohms 5 watts resistor in series with the 1N4007 diode to limit the current spikes and keep the DC voltage same as original. It may be too high now.
Don

Thanks Don,

I will check voltages. When I recapped it, I recorded the voltages on the filters.

Bill

Bill

Should add a dropping resistor in series with the diode. May need 47 to 100 ohms? Higher voltage can cause squealing as you tune.

Thanks, Don and Norm, for advice on the dropping resistor. The B+ line on the schematic calls for 95 v., at the recap I had 83 v, but can't remember the line voltage. I will adjust B+ with a resistor.

I found a site where the Candohm is shown as 25 watts. I don't have any sand resistors that big. I will have to sort through a bunch of tubular ceramic resistors which I have.

Will check the voltage to correct the squealing.

Thanks again,

Bill

Try it with the sand ohm, and place it close to the chassis, to help pull off some heat.

I have one I have used in the kitchen for 12 years or so and have had no problems. As far as the squealing, I pulled mine out of the cabinet to relube the controls and tuner, and my set squealed afterwards. I had reversed the antenna leads when I resoldered them.

Thanks, Jim, I learned that the antenna is important when I recapped it. I like the look and sound of this radio.

It is all working now. The B+ is 88 v, and the dial lamp glows slightly dimmer, but cheerily still. The 250 ohm 10 w resistor gets at least as hot as the 35Z3 did. I used double stick thermal transfer tape and stuck it to the chassis. Unfortunately, it is close to where the power cord comes in and I may have to move the resistor further away.

Thanks for all who contributed, and feel free to continue. Contributors of knowledge make a job like this a lot more fun.

Bill

The 40 and 80 ohm resistors dissipate about 2 watts each, so 5 watt resistors will be sufficient. The B+ voltages seem low, maybe the electrolytic capacitors are old stock with too much ESR, which would also cause squealing.
Don

Don, thanks for your help. For the 40 ohm Candohm I found a 50 ohm 20 watt resistor which has an adjustable band and set it to 42 ohms. It is large, but there is room. It stays cool. I left the 80 ohm Candohm in place, if it opens, the radio will just quit.

The resistor that replaces the filament is 250 ohm 10 watt. I got a glimpse of 170 degrees F on the IR thermometer. I moved it away from the power cord, and fastened it to the chassis with thermal tape and metal clips. It warms the whole chassis, but the 35Z3 did the same thing, just from a different location.

When I put the series resistor in the 1N4007, the squealing went away. If I would use a slightly smaller resistor here, the B+ would rise. I just used 100 ohms, and figured the 88v is OK.

Bill

Update: The 35Z3 rises again.

After the first failure, I removed the dead 35Z3 and replaced the dial lamp. I put a 1N4007 in series with a 100 ohm 5 watt resistor in place of the 35Z3. I put a 250 ohm 10 watt resistor across the 35Z3 filament connections. I replaced the 40 ohm Candohm. The radio has played just fine in this configuration, several hours a day for 9 days, until today. Today I removed the parts from the 35Z3 socket and put a strong 35Z3 back in. Played it, using a 100vac bucking transformer, several hours for two sessions, and then it quit.

I tested the dial lamp and the 35Z3, both OK. I waited an hour, and turned it back on. The dial lamp came on for about 3 seconds, then blinked twice and went out. I cut the power. This was exactly the previous symptom it displayed just before the old 35Z3 and the dial lamp went to parts heaven, after several turn-ons.

I suspected the Czech E-cap filters I had previously installed, but it played on these for 9 days with the 1N4007 without a hitch. I have noticed with the 35Z3, the inrush brightness of the dial lamp is somewhat brighter than the inrush with the 1N4007. The 35Z3 and the lamp still test OK.

Any input welcome and appreciated,

Bill

Bill

Are tube filament lit? If not you may have an intermittent filament? Test each tube. Let the tube operate for a period of time to see if the filament goes out? Heat causes filament wire to expand. This can cause an intermittent.

Also check connections to your added 250 ohm resistor.

Norm, thanks for our quick and helpful reply. I will leave the tubes on the tester for a while to see. But after this symptom appears for the first time, on subsequent power up, the filaments are cold and it only takes 3 seconds to go into failure mode. It did play for several hours on the newer 35Z3 before it failed.

I took the 250 ohm resistor out when I put the new 35Z3 in. It was not installed when the failure occurred.

Thanks, feel free to go ahead with your analysis, I will reply in the morning, going for Zs now, O&O,

Bill