Hi all,

You might have seen my post about picking up this Zenith farm set in the 'September Finds and Losses' topic in the Clubhouse.

Zenith 6V27


I have been busy doing the electronic restoration, recapping and such. Now that I have finished that and have the radio playing very well, I decided to see what I could do about building a solid state vibrator. I like to play my radios often and I figured there is no sense in putting all those hours on my original Zenith vibrator made of 'unobtainium' so to speak.

So, I started out with this schematic posted by Bill Hutchinson a while back. I think Norm Leal may have had a hand in this too. Anyway, thanks to all who worked on this project.

Solid State Vibrator


While looking for a heat sink for the 2N3055's, I discovered this heat sink with the two tab mount transistors on it, in an old defunct switching power supply I had in the junk box. They are 2SC3039's, which are the equivalent of an ECG 379. They are 12 amp NPN silicon hv hi-speed switching transistors. I figured they would be a good choice for the task at hand and luckily the checked good too.

Heat Sink with two 2SC3039 transistors on board


I mounted the heat sink on an old six pin socket from a defunct 75 tube using a couple of 4-40 screws and nuts and proceeded to build the circuit. For the circuit, I cut to size a piece of a Radio Shack generic circuit board so that it would fit over the heat sink, being mounted on nylon standoffs. I built the circuit as drawn, except that I used the original values of 150 ohm for resistors R3 and R4. Bill stated that he had to change out R3 and R4 to 75 ohms to get enough base drive with the transistors he was using. Here is a pic of the circuit board.

Circuit Board


I then mounted the circuit board to the heat sink and wired up the transistors. Here is the final product.

Solid State Vibrator - Homebrew


I doubled checked my wiring three times, now there is confidence for ya! I then decided to give it a try, so I wired it up to a 25 vct transformer I had used for buck/boost circuits. Lo and behold, the darned thing actually worked!

Having had that success, I plugged my hombrew vibrator into the 6V27 and hit the switch. It worked great! The radio was playing well. I measured the dc voltage out of the supply at 170 volts with 6.3 volts dc input to the set. That is pretty close to spec. I think Zenith says the nominal hv should be 160 volts. The set was drawing about 3.5 amps, so I figured that was good enough. I think I measured about 3 amps or slightly over when running the OEM vibrator. I also tried setting the supply voltage as low as 5.5 volts and the homebrew vibrator still started reliably. I was pleased with that.

After the initial test of about a half hour, with no smoke or excessive heat, I decided to give the set a real test run. I just finished a 5.5 hour run on the bench. I could not feel any heating of the transformer in the set and the heat sink and switching transistors were barely warm. Only resistors R2 and R5 were warm to the touch, but not hot.

Anyway, I am quite happy with the way this circuit performs and it is pretty easy to duplicate. The nice thing is that since it is a plug-in unit, it can be unplugged and the OEM vibrator installed at any time. I hope someone finds this information useful. I would think this circuit could be useful in other vibrator operated sets.

BTW, in case you have never seen one of the 1936 Zenith dials lighted after it has been restored, take a look at this. They are pretty nice!

Restored 6V27 Dial - Lighted


Regards,

Ed

Wow, very nicely done!! Love that dial...

I like the vertical arrangement and use of socket for plug-in operation.
Clever work.

Great idea and very nice work ,Zenith also looks great !
now can you build me a few 6 volt 4 pin and a few 12 volt 3 pin vibrators ?

Good job, Ed! One of the neatest construction jobs I have seen recently.

Did you compare waveforms at the transformer between the old unit and the SS sub?

Have you tried the MOSFET unit which requires no heat sinks? The schematic for that one was in the photo gallery.

Great job! It's a lovely radio - I have one as well. Mine isn't a great DX'er (I don't have a proper antenna on it for optimal performance.)

The 2SC3039 transistors are very common in video arcade game switching power supplies. I usually keep a dozen of those transistors on hand to repair the video arcade power supplies.

Thanks to all for your comments. It was a fun project.

Dennis- I did look at the waveform on the scope. It appears to have a longer duty cycle, about 50%, which probably explains the slightly increased current draw. However, I don't believe it is a problem, as there is no transformer heating to speak of. If I can find the schematic for the Mosfet circuit, I would probably give it a try, just to see what difference there is.

Cliff- My set is extremely sensitive. At night, I am receiving a station every 10 kc. This set is definitely more sensitive than the ac version using the 5619 chassis (6S27/6S52). I am quite impressed with its performance. This set has a really good set of tubes and a fresh tune-up, so maybe that helps some too.

Ed

Hi Ed, my name is Allan and I am restoring a Zenith farm radio and ran accross your Solid State vibrator. One question, on the schematic, am I looking at the bottom of the socket or the top.
You did a great job on the radio and the new vibrator.

Allan

I think it would be a classic piece because it is still loooking.

Allan,

It's been a while, but I believe that schematic is with the bottom side of the vibrator shown. Also, if you need more info on radio vibrators, you can download the 1945 Radiart Vibrator Replacement Handbook pdf file at the following link.

http://www.mcmlv.org/Archive/Radio/Radiart%201945%20Vibrator%20Replacement%20Handbook.pdf

Ed

Solid-state replacement vibrators of that type severely stress the transformer, with the potential for failure.
The reason being that they do not create the proper waveform.

A real vibrator holds the contacts closed for 70%-90% of the half cycle, then they open.
This open interval is critically important, as it allows the magnetic field to collapse.

The design of the system is such that the vibrator contacts will close on the next half-cycle when
the voltage induced by the new drive pulse matches the decaying voltage in the transformer, so the
overshoot is in the same direction as the newly-applied drive for the next half cycle.

What the solid-state vibrator does is hold the current through the primary at maximum, then
suddenly switch its polarity, causing a heavy current spike. The resulting overshoot may exceed
the insulation rating of the transformer windings, causing arc-over and subsequent failure.

Here's the correct drive waveform. The open intervals t₂ and t₄ are clearly visible.



- Leigh

Ref: Mallory Vibrator Data Book, First Edition, March 1947, page 11 Figure 6

It would be a snap to program a microcontroller to produce that switch timing. Drive a pair of logic-level mosfets. I bet you wouldn't even exceed the other circuit's component count.

Dave Wise

ARF member Tom Guest did exactly that, using a couple of IC chips driving a pair of power MOSFET's which connect directly to the vibrator transformer. They run stone cold, do not even need heat sinks. The schematic for that version of the solid state vibrator is somewhere in the ARF photo gallery. I have built a few of them and they worked perfectly with no failures.

I figured it had already been done. I'll poke around the gallery; I'd like to see how he regulated Vcc to the micro.

Dave

Not a micro. Tom did it with a cmos hex inverter with RC timing and RC integrators. JGrady also used a hex inverter, plus a 4013 dual flip-flop. A micro could do it with lower parts count (10 instead of 22), and even a tiny dirt-cheap one would be up to the task. But people shy away because of the programming. If several people ask, maybe I or another member who's comfortable with them could write a program.

Oh, and he used a 2931 LDO regulator as I thought.

Dave

(Edited to say "integrators" instead of "differentiators". Fingers outran my brain.)

Here's Tom's circuit:


John

I built one using a PIC12F675 microcontroller. Still in the process of testing but it looks good and only uses 9 parts total, plus I have an input on the micro that lets me set the dead time. No need to use large caps to set the frequency either....

I'm familiar with that one. Are you using a pot+ADC input to set the dead time?

Dave

Very good.

Are you changing the off time as a percentage of the nominal pulse width?

The transformer and buffer capacitor form a resonant circuit at the nominal operating frequency of the vibrator.
It will not tolerate much variation in that frequency (pulse repetition rate), preferably less than 2%.

Mechanical vibrators were quite accurate in this regard, just like a metronome.

- Leigh