38-39 Vibrator Power Supply Replacement - Printable Version
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38-39 Vibrator Power Supply Replacement - Olskuul - 07-01-2017
Hey All. This farm radio uses a 6 volt vibrator power supply. The vibrator is worn out and I want to replace it with a clean, reliable power supply that's small enough to fit in the cabinet. It must have 6 volts @2 Amps and a B+. However, I haven't been able to determine from the schematic what the B+ voltage should be.
Can anyone help?
RE: 38-39 Vibrator Power Supply Replacement - morzh - 07-01-2017
From the plate voltages - about 120V.
RE: 38-39 Vibrator Power Supply Replacement - Arran - 07-01-2017
Somewhere I read about reconditioning a mechanical vibrator by connecting it to the power line in series with a 40 Watt light bulb. Usually what happens is that the hash capacitor fails and the contacts inside the vibrator burn up, sometimes you can repair it, and sometimes not, but don't even think about replacing the vibrator with a SS substitute without changing that capacitor first. This set uses a synchronous vibrator and those were troublesome devices, they not only produce a pseudo AC for the vibrator transformer but rectify the output as well. If you want to scrap the vibrator supply and run it off the AC power line, building a power supply shouldn't be too hard, though given that the set uses battery tubes 6 volts at 2 amps sounds high. The set has 6 tubes so they no doubt wired the filaments in a series-parallel arrangement, the schematic states 6 volts at 1.4 amps, and that includes the vibrator supply current as well. I would just treat this set like a 1930s farm battery radio without the vibrator supply and build a supply accordingly, half of the B+ supply is already in the radio, parts #45, 15B, and 15C, right after the vibrator supply.
Regards
Arran
http://www.nostalgiaair.org/pagesbymodel/249/M0013249.pdf
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-02-2017
Thanks for the info Arran.
AES offers a kit that provides the required voltages:
https://www.tubesandmore.com/products/kit-power-supply
What say you?
Using this, is B+ connected at 15B?
As for the A+, is 56 and 57 bypassed and 6 volts wired directly to the power switch?
RE: 38-39 Vibrator Power Supply Replacement - Arran - 07-02-2017
Yes; where choke coil #56 connects to the power switch looks like about the right place to disconnect the vibrator supply. You could also rewire that switch, if it is a double throw unit, so that one side is used to switch the B+ supply off and the other to shut the A+ supply off. I would consider building the new power supply where the vibrator supply lives, but by bypassing it instead you can have the option of rebuilding it later if you want to go back to original.
Regards
Arran
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-20-2017
I've decided to try and use the existing vibrator to supply the B+. I recapped it and it's working, providing about +125V. However, it's very dirty, injecting a strong hum and overrides any signal. On the scope, it looks similar to a square wave. I've installed new electrolytic filter caps and temporarily added more capacitance, but this noise persists. What could be causing this?
RE: 38-39 Vibrator Power Supply Replacement - Radioroslyn - 07-20-2017
I would suspect what you are hearing is vibrator hash this is rf noise generated from the contacts inside the vibrator arcing has they open and close. Some is normal. That is why there are those small chokes and .5mf caps in the p/s they constitute a rf filter against this noise.
If you have replaced the caps in this section of the radio then try connecting an earth ground to the chassis. If this does little or nothing you'll be need to replace the vibrator or open it up and cln/adj the contacts therein.
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-20-2017
OK, thanks Terry. I'll try each and report the results soon.
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-26-2017
I added a ground connection. No change. Tried adjusting the vibrator. No dice. So I purchased several NOS vibrators from Playthings and although one of them is quieter, the strong RF noise persists. I rechecked the vibrator components and wiring. All was in order, with the exception of #50 choke. According to the schematic, it should be .5 ohms, but measures 5 ohms. Is this cause for concern? The radio works, but the noise makes it unbearable.
RE: 38-39 Vibrator Power Supply Replacement - morzh - 07-26-2017
No, it won't drop much at these currents and won't dissipate much either. If it all works I would not fret it.
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-26-2017
What else can I try to eliminate this RF noise?
RE: 38-39 Vibrator Power Supply Replacement - morzh - 07-26-2017
Does your vib metal case connect to everything it is supposed to be connected to?
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-26-2017
Yes, it does. I'm making a few mods per a Philco change bulletin. These are supposed to help quiet some hum. I'm also changing the buffer cap from a mylar .1/600V to a .1/1500V Z5U ceramic. Perhaps there's been some spiking getting through.
RE: 38-39 Vibrator Power Supply Replacement - morzh - 07-26-2017
You could try ferrite beads on output wires.
RE: 38-39 Vibrator Power Supply Replacement - Olskuul - 07-28-2017
Loud buzzing persists, even after changes per the Philco bulletin. A portable transistor radio also picks up this buzz when placed near the vibrator assembly. O'scope shows a clean DC level and voltmeter measures around +125vdc, so this appears to be RF noise. I ordered some correct buffer capacitors, as the Z5U tolerance is unsuitable. But I don't believe this cap is the root cause. Any other suggestions to quiet this thing down? The radio seems to be working fine otherwise.