Troubles with an old car radio. - Videokarma.org TV - Video

madmike17 · #1 10-26-2011, 09:08 PM

The high voltage section of my Truetone (made by Wells-Gardner) C11 car radio is not functioning. you can see on the schematic the area is marked by a dotted line. The transformer ohms out ok, but it is still not putting our any voltage at all. Vibrator is clearly vibrating, after being opened up and loosened up after many years of no use. I am not sure if the transformer is bad, or if the vibrator points are not conducting properly. Any ideas? It has been completely recapped, btw. Schematic: http://www.nostalgiaair.org/PagesByM...0/M0023530.pdf

madmike17 · #2 10-26-2011, 09:13 PM

Also, he vibrator currently installed is a radiart 5400. This may or may not even be the correct vibrator. I noticed one pin was cut off to make it fit the socket.

bob91343 · #3 10-26-2011, 10:30 PM

You should see a square wave on the transformer winding. You do have an oscilloscope, right? If not, get one.

It appears to use a self rectifying vibrator. They have a lot of contacts, any of which could not be operating. It also appears that two different vibrators may have been used. There is no indication of vibrator type on the diagram.

In a set that old, the buffer capacitor (the one across the secondary of the transformer) may be bad, which places a lot of stress on the contacts. That's the first thing to replace, along with the filter capacitors.

old_coot88 · #4 10-27-2011, 12:54 PM

That's called a synchronous vibrator. A second set of points does the job of rectifying the stepped-up voltage coming out of the transformer.
A radio using a synchronous viber is sensitive to supply voltage polarity. That is, if the radio is negative ground and you reverse the input polarity, the stepped-up voltage will also have reverse polarity. It'll be putting "B-minus" voltage on the filter caps instead of "B plus" voltage.

madmike17 · #5 10-27-2011, 04:04 PM

I wonder if I installed it backwards last time? It ran for about a minute, and faint crackles came through the speaker, but no sound. Then the 10 mfd cap let out its guts. I couldnt find a reason, I just figured it was defective. I also later found out that the audio interstage transformer was bad. If it is just the contacts, would a little filing help? Ill try to find a scope if I can.

old_coot88 · #6 10-27-2011, 05:46 PM

Quote:

Originally Posted by madmike17

I wonder if I installed it backwards last time?

It's doubtful the vib could be installed backward. But if the supply voltage to the radio is the wrong polarity, the filter caps will also get reversed polarity. This is true only with synchronous vibrators, not with radios using tube rectifiers (0Z4, 6X5 etc.). With tubes, the supply polarity doesn't make any difference.

Quote:

It ran for about a minute, and faint crackles came through the speaker, but no sound. Then the 10 mfd cap let out its guts.

If the 10mfd cap you're referring to is C34, it's right on the main B+ line from the transformer, and it would definitely 'let out its guts' under reverse ("B minus") polarity. Two more caps 'downstream' from it, C29a and C29b, would also get seriously cooked (unless C34 dead-shorted and spared them that fate).

Quote:

I couldnt find a reason, I just figured it was defective.

It sure sounds like reverse supply polarity. Had you installed it in a car when this happened, or when you had it on the bench?

Quote:

I also later found out that the audio interstage transformer was bad. If it is just the contacts, would a little filing help? Ill try to find a scope if I can.

If the audio interstage transformer open, or what?
The vibrator contacts might still be OK if you didn't run it that way too long.

madmike17 · #7 10-27-2011, 06:04 PM

It was C29B, and it tested ok despite all the fluids that came out of it on my $20 harbor freight multimeter. The transformer T6 had an open primary, which I used a capacitor to bypass as show on the philco website. I believe the technique was called rc coupling. It seems like it must be a problem with the vibrator contacts. Would filing them be of any help, or would they be damaged?

old_coot88 · #8 10-27-2011, 06:04 PM

Before attempting any filing, I would see how the vibrator runs with correct polarity. It may be OK.

madmike17 · #9 10-27-2011, 06:08 PM

The vibrator can be reversed in the socket for a different polarity Im pretty sure. It will be used with negative ground on a 1947 international pickup.

old_coot88 · #10 10-27-2011, 06:11 PM

Addendum to earlier post

As a side note,

there is a quick and easy way to convert one of these radios to run on reverse polarity. I used to do it with Buick (negative ground) radios for use in Chrysler product (positive ground) cars. All you do is swap the transformer's two HV secondary leads (leaving the center tap alone). And presto, the radio runs fine on reverse supply polarity.

Doggone, I never saw one before where the vibrator itself can be reversed. L'arn sumpthin new every day.

madmike17 · #11 10-27-2011, 06:21 PM

Its an 8 pin socket, the vibrator has 4 pins. there is an arrow on the chassis which can point to either a + or - marked on the vibrator. So to be clear: I should connect the pin connecting to the coil to 6 volts and the case to ground. The pulsed DC can be measured with a scope on the two larger pins. Is this what needs to be tested now?

old_coot88 · #12 10-27-2011, 06:36 PM

Quote:

Originally Posted by madmike17

Its an 8 pin socket, the vibrator has 4 pins. there is an arrow on the chassis which can point to either a + or - marked on the vibrator. So to be clear: I should connect the pin connecting to the coil to 6 volts and the case to ground. The pulsed DC can be measured with a scope on the two larger pins. Is this what needs to be tested now?

Here's what I would do FWIW: Put your meter across C34 and 'blip' power to the radio just long enough to see that correct polarity is appearing across C34. That'll confirm if supply polarity is correct. Then switch the meter to AC, power up the radio and see if you've got equal AC voltage from center tap to both ends of the HV secondary. If so, this'll pretty much confirm the vib is healthy and the points are good.
I always looked at filing points as the last resort.

Well son of a gun, upon blowing the schematic up to full size, C34 is actually .10 mfd, not a 10mfd electrolytic as i originally thought. That's why C34 didn't get cooked by reverse polarity and C29 did.

madmike17 · #13 10-27-2011, 06:58 PM

There is no voltage on either C34 or any part of the secondary. the schematic isnt really making sense to me right now though. It seems that the pin out of the vibrator is viewed from the top, rather than the prong side as it says based on looking at the vibrators actual internal wiring.

old_coot88 · #14 10-27-2011, 07:08 PM

Hm. Can you hear it buzzing at all with the radio powered up and the tubes lit?

madmike17 · #15 10-27-2011, 07:21 PM

Yes, the vibrator clearly runs. The only solution I can think of is to just turn the vibrator around. Could this be damaging if I'm wrong? What if i just disconnected the center tap to see if there is voltage when the vibrator is switched so that nothing could be harmed? Also, voltage off the center tap should be DC, right? It seems that if the vibrator was reversed, the result would be AC coming through because it is not cancelled out.