Troubles with an old car radio.

[madmike17]

It is a negative ground. I believe most positive ground cars were imports.

[old_coot88]

All the 6V Chrysler product cars i ever dealt with were positive ground also, though there may well have been exceptions.

[dieseljeep]

Originally Posted by old_coot88

All the 6V Chrysler product cars i ever dealt with were positive ground also, though there may well have been exceptions.

6 volt Fords as well.

[madmike17]

The truck is actually my fathers, and I will check with him tommorow. I am also going to test it tommorow when the battery is charged again. Thanks for the help so far!

[madmike17]

Turns out, the truck is positive ground. with the transformer diconnected, I read 307 volts dc to ground. When I reconnected the circuit, the vibrator didnt even run. After a lot of fooling around with it it finally began to run. Unfortunately, the radio does not work. A hum the same frequency as the vibrator came through the speaker, but nothing else. As i turned the tuning cap, there were crackles and pops from the cap itself. It sounded like high voltage arcing between the plates. Should the rotors have hv on them? I know the stator is directly connected to ground. The vibrator really didnt want to start, I had to tap on it and shake it to make it start. I think I am in over my head. Now what?

[old_coot88]

Originally Posted by madmike17

Turns out, the truck is positive ground. with the transformer disconnected, I read 307 volts dc to ground.

Now that's with C34 still in-circuit, right? And you're getting 307V positive to ground across C34?

When I reconnected the circuit, the vibrator didnt even run.

Sounds like something's shorted out on the B+ line, like maybe filter cap C29. Did you replace it after it 'spilled its guts' from reversed polarity?

After a lot of fooling around with it it finally began to run. Unfortunately, the radio does not work. A hum the same frequency as the vibrator came through the speaker, but nothing else. As i turned the tuning cap, there were crackles and pops from the cap itself. It sounded like high voltage arcing between the plates. Should the rotors have hv on them?

The tuning cap shouldn't have any signifigant voltage on it, maybe just a few volts max.

I know the stator is directly connected to ground.

Normally the tuning cap rotors are at ground and the stators are the 'hot' side of the cap (unless you've a got a very unusual style of tuning cap).

The vibrator really didnt want to start, I had to tap on it and shake it to make it start. I think I am in over my head. Now what?

You might try disconnecting what you did before, leaving just C34 in-circuit, and see if the vib will start up easily. If so, it suggests a heavy short in something 'downstream' (like in C29) lugging the supply down and stalling the vib.

[old_coot88]

Addendum:

BTW, what are you disconnecting? The break should be at either side of L6, the RF choke directly downstream from C34. That'll be a good opportunity to check for a B+ short 'downstream'.

[madmike17]

The 307 volts was positive in relation to ground. It was measured with the .1mfd cap connected to the secondary center tap and nothing else. I did replace the electrolytic that died. I didn't think to check the actual voltage on the tuning cap, but I'm pretty sure it was sparking. I actually found that the rotor was rubbing, so I bent one of the plates over a little. I made a mistake in typing, the rotor is actually connected to ground not the stator. When I disconnected the hv circuit again at said point, I got 267 vdc. This may have been due to the tubes being in and drawing heater current, while last time they were not. Once again I had to tap on the vibrator to get it started.

[madmike17]

Also a side note: do you think there is a place where Ken-Rad ST style tubes can be found? I would like to have them all match the originals. The 6B8G and 6N7G are replacements.

[bob91343]

I assume you have replaced all electrolytic capacitors. If not, do that before any more struggling.

[madmike17]

The only capacitors that havent been replaced are three very small value micas that I couldnt find replacements for.

[DavGoodlin]

Madmike
Those vibrators with mechanical contacts are trouble. There is a place in florida that makes them exact for your unit in a solid state version. I restored a negative ground 53 Chevrolet radio. A total recap was needed also. According to Justradios.com, mica caps rarely go bad.
http://www.radiodaze.com

Dave G