[Retro Ron]

Hi Gang,
I just finished working on a Philco Model 5 Packard car radio. That set was quite a piece of engineering and a dream to work on. So now I am now working on this 1956 Chevy manual-tune radio model 987366. I re-capped it and B+ voltages were right on. It was playing away on the bench when suddenly I heard the vibrator frequency drop and the current started to spike. I cut the power and started checking things out. Everything seemed fine so I pulled the vibrator/rectifier and supplied 245V directly to pin 8 of OZ4 socket with my bench power supply. The radio plays great. Current draw is about 40mA but I don't have any idea what it should be. Does that sound reasonable?

Thanks,
Ron

[BILL]

I say the problem is in the power supply , did you replace the filters? and I assume you've changed the buffer , check for a shorted 0Z4 tube socket, and the power transformer that one is not all that easy to replace, it is sodered in to the chassie, and is the 0Z4 any good ? I no longer use original 0Z4's, You will have to check thing out with power on your power upply as the B voltage with jump,watch the tube socket for arcking, check for waxs heating on the transformer, sometimes the vibrator hash choke ,has enamal wire can get bumped and short to chassie

[Retro Ron]

Hi Bill,
Thanks for the help. All caps (except mica) have been replaced including the filters and buffer. The power transformer has been replaced in the past. I too generally gut 0Z4s and wire a pair of 1N4007s in the base but this time it checks out. I may still do it anyway. The socket looks clean but I will check for signs of arcing. Prior to the failure, plate voltages looked correct but I was having a heat-related problem where the audio would turn to static and then cut out after a couple minutes of operation like tube bias was going into cut-off. However, using the external power supply, the problem has mysteriously disappeared. I did manage to trace the problem back as far as the plate of the IF amp before the vibrator went south. Seemed like it was in the front end somewhere. I want to use a new solid state vibrator but don't want to cook it with excessive current. Guess I'm going to have to start lifting/testing all the resistors. I may try injecting AC into the transformer primary for a short duration.

Update: I momentarily powered the radio from the variac by applying about 15VAC into the vibrator socket and it worked fine. There are no carbon tracks or resistance or other apparent signs of arcing in the socket. I suspect the vibrator was just ready to die. A new solid state vibrator is now on order.

Thanks,
Ron

[codefox1]

Somehow I remember there is a paper capacitor in the vibrator power supply circuit that is specified as a 1600 V.D.C. for these old vibrator radios. A 630 volt replacement will not work (for long.) It's gotta be 40 years since I found out the hard way this is so. If I had to fix one of these things today, I would also replace the 0Z4 with diodes as has been suggested already.

But then again, my 1949 Plymouth radio played just fine when I bought it for $40 in 1969, and still was good when I sold it in 1972 -3 hard college years later and still able to climb Pike's Peak. It had a venerable 0Z4 and a 6 volt battery, and an indestructable flat head 6 motor. I paid $8.00 for a rebuilt clutch and about $20 for a set of relined brakes (all 4 wheels) somewhere along the way, and $60 or so for a set of new tires, (all 4) along the way, and that was about it.

Don't get me started, lest I tell you about the 1957 Turnpike Cruiser Convertable (Mercury) that replaced it . That one cost $95.00.

[Retro Ron]

You are correct. The buffer capacitor is generally rated for much higher voltage. In a pinch, I have used two 630V capacitors in series observing the laws of parallel capacitors. The more I experiment with this radio, the more I am convinced it was just time for the vibrator to die. It had probably been operated with a bum buffer capacitor and leaky filters for just too long. Plus the vibrator has a nasty dent in the side which although shouldn't have affected its operation, is rather unsightly.

Regards,
Ron

[Texasrocker]

Best to replace all those old vibrators with new s.s. types always. I keep a few of the old mechanical type vibrators here, ( with cases removed), for initial test after radios are recapped 1st. The points in the vibrators are always "flaky" at best, and sometimes need abit of help getting started, even with clean contacts.
No wonder all the older vehicles dash-boards were made of thicker-steel! You could give em a good "thump" with your hand, and keep the radio playing!