I've had several linear amplifiers through the years but for me, this one really stands out. I picked up a non-working ETO Alpha PA-77 for a song a few weeks ago from a local Ham. The price was "really right" so I couldn't pass it up. I had no idea what was wrong with it other than the previous owner said it "wouldn't key".
I found out while doing some research (and troubleshooting) to this PA-77, that it was the predecessor to the famous Alpha 77 DX. The PA-77 is physically smaller than the 77 DX, but like the DX it uses a single 8877 (3CX1500A7) tube. I also found out that the PA-77 is NOT a candidate for a conversion to two 8877's (like the Alpha 77SX). Partly because of the smaller physical dimensions and also because the transformer doesn't have enough "oomph" to power a pair of tubes.
After disassembling the PA-77, I cleaned up the keying relays, realigned the top lid interlock switch, cleaned what I could, put it back together and I'll be damned, she worked perfect (for awhile anyway).
That's when the fun started. I noticed the blower motor starting capacitor was leaking when I had it apart but figured that would be a project for another day. Well a week or so after running it hard chasing DX, the blower motor starting cap decided to go boom. Not a firecracker boom, more like an M-80 boom! Damn near knocked me out of my chair when it went. Nothing like a LOUD boom and heavy black smoke in the shack. Needless to say, that capacitor is oil filled and man oh man, did it made a mess.
I disassembled the PA-77 again and I really didn't like what I saw this time. The filament wiring to the tube was in very near proximity to a couple high wattage resistors in the blower circuit (to cut the blower speed) and they got so hot the filament wiring started on fire. So a blown starting capacitor, smoked resistors and cooked filament wiring were the next repairs. But what caused the resistors to get so damn hot? After a little more troubleshooting, it appeared the blower motor was failing when it got warm. The longer the blower ran, the more current it was drawing until finally it was acting like a short.
Here's a few pictures of the damage (click to enlarge):
After repairing the filament wiring and removing the blower series resistor modifications (some previous owner had done), I also removed the blower motor and hooked it directly to 120V. Nothing but a "grunt". After taking a few hours to clean up the oil from the blown oil filled starting capacitor, I then went on the hunt for a new blower motor. Yeah, right! Trying to find a blower motor for a 40 year old amplifier is not the easiest thing to do.
I figured I'd start right at RF Concepts and see if they had any legacy blower motors laying around. Molly is a great gal and has ALWAYS been good to me, maybe I'd get lucky?
Here's the almost immediate reply I got from from Molly at RF Concepts:
Update for Case #13988 - "Blower PA-77"
Sorry to let you know but we don't have that motor in stock anymore. We tried some years back to get blowers for the 77 and the 77D and 77DX/SX series of amplifiers and ran into quotes in excess of $1,000 with minimum orders as well. You may be able to find a used blower yourself by searching on the Internet or if the blower is very noisy, you may be able to have the bearings re-worked and then replace them into the blower.
No luck at RF Concepts, so off I went to our local Grainger store to see if I could match something up. After an hour at the counter with a great Grainger salesperson (didn't get his name), I decided on a Grainger #6NZP8 motor. Used in HVAC applications, the 3.3", Shaded Pole PSC, 5/16" Shaft, 1/10 HP, 3,000 RPM, CCW/CW ball bearing motor would probably work with some modification to the rear panel of the Alpha. Plus it was only $75. It would take a few days to get the motor into the store, but I was in no hurry. I put the Henry Radio 2K-4 amplifier in-line and I figured it would take me a few days to clean up the oil in the PA-77 anyway. I ordered it and went back to work cleaning the amp.
Mr. New Blower Motor comes in a few days later and it took about 30 minutes to "fit" the PA-77 to the motor. I "elongated" the blower mounting holes on the rear panel to fit the 2" bolt pattern of the new motor (old motor was 1 7/8") with a small round file and cut down the motor studs to not interfere with the squirrel cage. Here's a few pictures of the "stretched" hole pattern and new blower motor installation:
So how does it work you ask? Well, I hooked everything up and fired the ETO Alpha PA-77 up and NO BLOWER. Seems the 6NZP8 is too much motor for the Alpha blower circuitry. As soon as you put power to it, it faulted the overload solenoid (OFF button lights amber). Hmmmm, 120V at blower, check. Blower motor on staight 120V, check. Put it inline and it faults the solenoid.
Have you ever had too much of something? The rated current draw of the new 1/10HP motor is 1.2 amps. I don't know what the Alpha blower circuit faults at, but I'm sure it was way lower than 1.2A (maybe 300 or 400ma?). Hindsight is always 20/20, but I wasn't going to order and wait another few days on a lesser current drawing motor to come in, so I ran over to Radio Shack and picked up a $10, 120VAC relay (90ma). I ran a #18 wire up to one of the fused mains direct to the blower, and then connected the Alpha blower circuitry to the other side of the relay. I added a little Radio Shack "Hook and Loop" faster (Velcro) to hold it down. Here's a pictures of that addition:
And now with the new relay and motor installed, the amplifier works perfectly. I'm surprised at how quiet the new motor is, and I especially like the fact that the new blower REALLY blows some air over the 8877. I still may put a variable temperature thermostat in circuit at sometime in the future but for now it is what it is, and "it" works. A 40 year old amplifier that is working well is a good thing. If I had to do it again, I probably would have tried the Grainger 3,000 RPM, .5A 6ZNP6 motor first. And as always in my "projects", a tip of the black hat to Keith (K0KE) for his technical expertise and direction!