[Ken Kidd]

Hi all, Since this is for a positive ground system (52 Plymouth), when I go to hookup my dc power supply to test it I believe I should connect the red positive lead to the chassis and the black negative lead to the input wire on the radio that has the inline 14A fuse. Is this correct? Do not want to buy another SS vibrator!!! These are polarity sensitive!! Thanks, Ken Kidd

[Radioroslyn]

Yes if the radio is set up for a positive ground system.
Terry

[BILL]

Has your radio been rebuilt? At lest a new buffer! I never test a auto radio with a transistor vibrator, make GOOD and sure it's OK first before you use the transistor vibrator, I have dozens of auto radio show up in my shop with burn up transistor vibrator ,because the owner thought that was all that was wrong with the radio.

[Ken Kidd]

Hi all, Yes the radio has been rebuilt. All paper caps, electrolytics and out of tolerance resisters have been replaced. Transformers and coils have been checked for continuity. Not sure what you mean is the radio designed for a positive ground system? Thought this particular model was specifically designed for positive ground vehicles??? How would I know? Have the old vibrator-it states 6V positive ground on it. Ken

[BILL]

Your radio with the original mechanical vibrator will work on positive or negitive ground, if it had a sychronous vibrator it would be polarity sensitive,
Again I'd get that old vibrator working to test your radio with , buzz the old vibrator on AC in series with a 40 watt light bulb

[Ken Kidd]

Thanks for the info Bill, Will try that 1st-Ken

[Ken Kidd]

Hi all, Have powered up the radio and all I get is a loud type buzzing, sometimes get a station, sometimes not like it's getting grounded out. Rechecked everything and have not found anything wrong. Did a continuity test with no power and found out that when the power leads are hooked up both are grounded to the chassis??? Not good. Checked the wire that has the inline fuse on it and it goes to a bracket that is bolted to the chassis. Looked at this piece a lot closer and found that there is a piece of mica between the bracket and the plate that the incoming wire and the wire going to the circuit are soldered to. On the back of the bracket is stamped .0003 500vDC. Think this is my problem. It's marked M7 on the schematic but there is no mention of M7 in the parts list. There must be a break in the mica sheet?? Can I use any type of thin mica and does anyone know what the purpose of M7 is. Apparently the whole unit should not be grounded to the chassis. Any info would be appreciated-Ken

[BILL]

You've check your filters? One lead is for the lamp on the dial , , 7m ,sounds like the spark plate GM radio use a thin piece of paper ,it's there to filter 6volt line , if it's not shorted it shouldn't give you any problems,
I once bought out an old radio shop that did auto radio work ,in with all parts equipment,and radios was a 8 tube 55 Ford radio with "JUNK" written on it ,I worked on it since I've usually won't let an auto radio beat me, what I found was that it had had a shorten buffer and as usual some one kept trying to keep playing by replacing the vibrator ,what happen vibrator hash burned up,there was burn marks even on the chassie! The shop I bought out had tried to repair the radio only using the wrong vibrator hash, the radio had a BAD buzzing sound ,really bad, the shop didn't know what to do ,must have told the customer radio was unrepairable .I found the right vibrator hash repaired it , today that radio is in a 55 Ford in Porto Rico

[BILL]

I've been thinking about your radio, I've worked on 52 Plymouth radios, they are a two piece, you remove them unbolting the center section of the dash, speaker grill, and then they come right out , nice design.
Some of these 812 had dynamic field speakers ,if you have replaced the speaker with a PM type make sure your not using the 6 volt line, this will give a bad buzz, find the output transformer secondary that goes to the voice coil and the other side to ground ,as I remember. the power supply output chassie is where the dynamic field terminal is, on the back side ,stay away from it if your using a PM.
Lastly if you got the chassie on the bench it two pieces try a jumper from the ground side of the RF chassie to the ground side of the power supply chassie

[Ken Kidd]

Thanks all for the input. I'm going to retrace my repair steps this weekend. It may have been something I did. The buffer was replaced but will check it again. The speaker is a PM but original to the radio. Did not find anything wrong there. That M7 part checks out OK with the chassis removed. Going to put power to the radio with it not mounted all together. Will jumper the 2 chassis's. Still scratching my head!! ken Kidd