Alpha 76A behaving strangely.

[KC2TAU]

The Alpha 76A at our club station(W2SZ)has been a great performer up until recently when it has stopped outputting power. Here are the symptoms:

1) Moving either the tune or load control causes no change in the plate or grid current.
2) Increasing the drive from the radio(TS850)causes for an increase in plate current(I did not notice an appreciable change in grid current). Drive can be increased until maximum allowable plate current is reached.
3) This happens across all bands.
4) A watt meter was placed on the output of the amplifier and no output was noted. Zero output is also shown on the amplifier's built in output meter.

Any ideas?

[WB2WIK]

First I'd make sure the T-R relay is really switching properly and connecting the amplifier output to the antenna. Based on symptoms, it sounds like it's not.

If that checks out okay, carefully inspect the bandswitch. Bandswitches in the 374-76-78 family of amps (several models) were prone to failure and if a contact opens, that can cause the amp to be trying to deliver power into "nothing," because it will be an open circuit between the PA tank circuit and the T-R relay that follows.

[W8JI]

The Alpha 76A at our club station(W2SZ)has been a great performer up until recently when it has stopped outputting power. Here are the symptoms:

1) Moving either the tune or load control causes no change in the plate or grid current.
2) Increasing the drive from the radio(TS850)causes for an increase in plate current(I did not notice an appreciable change in grid current). Drive can be increased until maximum allowable plate current is reached.
3) This happens across all bands.
4) A watt meter was placed on the output of the amplifier and no output was noted. Zero output is also shown on the amplifier's built in output meter.

Any ideas?

For the amp to behave that way, the problem is at the anode of the tube. It can't be anywhere else. I'm positive of that, as long as your descriptions are accurate.

Since plate current is HIGH but unchanging, something is loading down the anode system with a very low impedance. This means either the plate tuning capacitor is shorted, the bandswitch is blown out, or the tank circuit just BEYOND the plate tuning cap but NOT past the loading cap has gone open (which is what a blown bandswitch or broken lead in the tank inductors will do).

Again to have high plate current, no output power, no ability to show resonance, it has to be a problem right around the anode of the tube where the tube is loaded with a low impedance. It cannot be an antenna relay or anything past the loading control, if your description is accurate.

If your description is wrong and the tube really does not have normal plate current or does show a change in grid current and plate current when the tuning cap hits resonance, then all bets are off. The problem would need to be restated.

In a club station amplifiers are often operated by some poorly skilled tuners, so it would not be unheard of for the amp to have been grossly mistuned and the bandswitch or plate tuning cap blown out. This happens with my amps with some 40-year licensed Hams who happen to not have a clue how to tune an amplifier. It has happened to me more than once with guest ops who are, regardless of age or time licensed, amplifier lids. :-)

73 Tom

[KM1H]

The plate blocking caps in that amp are a pair of .002 discs in parallel and they do crack as well as leak. If there is leakage a fuse would have blown when the HV hit the safety choke. The bypass caps at the base of the plate choke are also trouble areas as as side note.

The BS almost always arcs at the front wafer right side support from the top left contact to the support screw covered by a ceramic spacer and nylon washer. Clear as mud? If its gone it wont be hard to miss.
Alpha wants $300 for the wafer. It may also have arced the rear wafer and they used the $300 one and a progressively shorting one which is only $90. Its a dice roll which one you have. My 76PA (2nd station amp) has the cheap one and its the shiny metal panel version. There are also black panels and sticky panels:rolleyes:

I cant imagine a variable cap shorting at around 2000V as they didnt skimp on the spacing but that identical Centralab (Electroswitch now) wafer is used in a slew of Alphas and other brands with the same arcing problem. I place a Teflon sleeve over the wire and contact and build up a thick layer of Q Dope where the wafer, spacer and washer meet. Havent arced my 1986 LK-500ZC yet and Ive pushed my luck many times:eek: Probably replaced a good 30+ wafers for customers over the years.

Another thing to check while its open is the anode clips. They are cheap plated steel WW resistor clips that often corrode on the inside surface making an intermittent contact with the tubes that just keeps getting worse. Had a 76CA in here 2 months ago that was the worse Ive ever seen.

Carl
KM1H
Amplifier repair and 6M Conversions since 1964