My latest acquisition at the Collins Radio Center is the last of the great ones, the 20V-3. It is one of the last broadcast transmitters that Collins Radio manufactured before that division was sold off to Continental Radio Company. The transmitter is in the 1000 watt output class and uses 4-400A tubes in the final and modulator. The color scheme is light creme and gray. Unlike the earlier 20V-2, the cabinet of the 20V-3 lacks the curved front corners and the meters are recessed, etc. The 20V-3 is definitely not in the Art-Delco tradition as the 20V-1 and 20V-2 units.
This is my third broadcast transmitter I have restored. A few years ago, I acquired a 1947 RCA BTA-250L that uses a pair of 810.s for RF finals, modulated with 828s. . I sold the transmitter several years ago after it sat in my small garage in one corner. The new owner bought it to be his backup transmitter at an AM station somewhere near Clearfield, PA I would hate to think that it round up at a deserted strip mine site which are common in that area of our state.
The next heavy metal transmitter that I acquired is a real classic, a Western Electric 451A-1 that is also in the 250 watt class. It was first put into service in 1947 at WCRO in Johnstown, PA and then ultimately became their backup transmitter to their primary transmitter, a 1000 watt RCA BTA-1R, that uses 4-400A tubes in the final. Today, this unit stands next to the Collins and is pictured on the front cover but you have to see both units upfront to appreciate their quality and formidability.
Before ever getting into a restoration project of any commercial transmitter, do make sure that you are in no hurry. This is not a project that you rush into and rush out of! Don.t expect that your newly acquired broadcast transmitter is a .plug and play. unit, at least not from my experience. However, once in a while you will get a pristine unit that the station just wants to get rid of. Good luck!
About two months ago, I negotiated a deal for this Collins 20V-3, which arrived at the Collins Radio Center in a small van. The transmitter was on its side with its .guts. removed (transformers and filter capacitors). Unloading this behemoth took three husky firemen, one studio engineer, and your editor . I think that I was more in the way than helpful! Believe it or not, the unit was slid on the top of a 30K-1 cabinet with a protective blanket, of course. I am in the process of refinishing this cabinet so it was no big deal to use it as a support for the 20V-3. Once the 20V-3 was slide from the back of the van on to the cabinet inside the garage, we tilted it up in the air. No easy task, I can assure you. That 30K-1 cabinet certainly came in handy and escaped without even a dent or bend in the metal. I would conservatively estimate the weight at this point was about 700 lbs. The 20V-3 weighs in at nearly 1200 pounds and once loaded with all its iron, I can tell you it becomes a permanent fixture. The plate transformer alone must weigh well over 100 lbs.
Every heavy metal transmitter needs a good cleaning and this 20V-3 was not exception. No doubt it had been collecting dust/dirt for at least thirty years or more. After many bottles of ammonia, which I still consider an effective cleaner, lots of rags and paper towels, not to mention hours and hours of time consumed, I was now ready to delve into the electrical/mechanical inspection and repair.
I do not recommend using a spray cleaner like 409 or Simple Green, etc. in combination with a garden hose for a real .dunking.. Unless, of course, you feel lucky not to ruin any components, particularly, unspotted transformers, etc. that despise getting dowsed with a garden hose, although I know some folks that have gotten away with it!
While poking around during the cleaning process, you may discover that some of the wiring needs replaced . as was true in the 20V-3. My inspection revealed that I would have to replace several pieces of high voltage cables that were defective. It seems that some of the wiring had been come to rest against some large 200 watt resistors, used to reduce the high voltage in the low power position. The result was that over the years the heat from the resistors had melted the insulation exposing the wire to, no doubt, produce a sensational fireworks inside the unit. In particular, I replaced the high voltage cables going to the high voltage rectifiers from their sockets and 4-400A modulator plate leads because the insulation was cracked or brittle from all that tube heat. In and around the 4-400A.s and power resistors in the upper level of the cabinet on the 20V-3 is a good place to inspect any high voltage wiring.
Finding new high voltage wire was not problem as a local neon sign shop had just the right black high voltage cable that I was looking for. The original yellow terminal ends were installed on the cable ends. Fortunately, aside from the high voltage wiring, all other control cables, etc. were in perfect condition.
Next, in no particular order, is to refinish/resurface the transformers, capacitors, etc. before they are remounted inside the transmitter. Most if not all of the transformers were cleaned of some surface rust and repainted using a Krylon spray paint in a light gray color. I mask off any identification labels so as to preserve the specification markings, etc. Try to keep everything original as they say. Of course, the outside the cabinet is not perfect after being jostled about that resulted in some of that beautiful surface being marred. I will not do any paint touchup until I get the time and patience to use an air brush. Getting the correct shade of paint will not be that difficult since I can take a small bracket from the door fan assembly to the paint department at Lowes or Home Depot for a computer match that will be fairly close to the original shade of factory paint.
Now it is time to get out the squirt can and white grease to apply some lubricant to all moving mechanical surfaces, particularly, the tuning mechanisms. But you know that so I will not elaborate except to say that a little oil on moving surfaces will free up those sticky tuning capacitors, etc.
Somewhere on the .punch list. is a reminder to burnish the relay contacts, check the emission of the small tubes in the exciter and audio stages, loose nuts and bolts, cracked insulators, oiling if you can the blower motors, replacing those old black coupling capacitors in the modulator deck of the 20V-3, etc.
Now the moment of truth, apply power and hope for the best. Filaments lit but no high voltage. Let me elaborate on what I discovered.
Someone had not installed a jumper in the interlock circuit which prevented the high voltage relay from energizing. A quick check of the schematic and a jumper was installed. Then, I discovered that I was not getting screen voltage to the 4-400A in the RF amplifier. One of the two 20K/165 watt screen dropping resistors was open. A trip to the junk box produced a new 200 watt resistor that did the trick.
Then I discovered an oscillation in the audio stage which was traced to the audio input stages that use two 6SJ7 tubes. Removing the tubes, stopped the modulation current meter to settle down. What I think is the problem is those old .black beauty. coupling capacitors that are leaky. I will replace them with .orange drop. capacitors. This will be done in the next several days. Hopefully, that should cure the problem but I am thinking that RF may be getting into the audio input line that is just floating at this point.
I have only scratched the surface, as they say, in relating my experiences or should I say .trials and tribulations. in restoring the 20V-3. I still have some cosmetic items to complete before I am entirely satisfied. Perhaps, there will be a postscript to this story so stay tuned.