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[W2MV]

Initially, I suspected my IC-7610, but I ran a jumper directly from the transceiver to the antenna cable and the problem disappeared. I'm now thinking it's the T/R vacuum relay in my Alpha 78.

Signals drastically drop by maybe 30-40 dB (on all bands), but then after I transmit just a single CW dit, received Signals return to their normal levels. The return to normal levels may last a second or two, sometimes much longer.
It seems to have gotten worse since yesterday. Now, it takes several CW transmitted dits sometimes to bring received signals back.
Sometimes it doesn't work a all. Very frustrating and now renders operating useless.

Does anyone agree with my initial assessment that the vacuum T/R relay is likely at fault?

73s

Alan, W2MV

Transmitting seems normal so far, at least.

[W1QJ]

Make sure the ptt isn’t problematic if not it’s time for a new input or vac relay.  If it’s original it’s pretty old by now.  I’ve had to replace many vacuum  relays on customers amps.

[W2MV]

Thanks. What's interesting is that if I engage the PTT without transmitting (out of band so no RF output), that doesn't bring the received signals back...only when it seems, there's some RF.

[K4WH]

I would agree that the input or Vac relay may be gone.  But  .... do you have another rig to connect the Alpha up to  ?? Try that if you do.   OR  ........  get a long RCA to RCA cable and short out the unconnected end with a clip lead or even a screw driver to key the amp, and transmit.  Just on the outside chance it is RF getting back into the Icom.  Weird but it can happen.

You didn't say if you were:
Using the 78 in the bandpass or manual mode
All bands  ??
What type of antenna

Might be good to give Dick Byrd an email or call.  N4UQ  He is always glad to answer questions.

I had 2 brand new 3cpx800's here and just had Dick do the conversion on my 78 plus a fine tooth servicing.  Still a great amp. 

[KH6AQ]

The symptoms do point to a dirty T/R relay. You might be able to electrically clean the relay. This works with small signal relays (<2 A) but I have never tried it on a high current or vacuum relay. I cleaned a transceiver T/R relay this weekend toggling S9 + 60 dB (50 mV) into the RX port. I suspect it will work to temporarily clean the Alpha T/R relay and confirm/disconfirm your relay diagnosis. If confirmed, the relay should be replaced.

To clean the relay a DC current is passed between the contacts as it is toggled. The T/R relay K5 (page 27) has a DC path from the RF INPUT to the RF OUTPUT.

Relay contact cleaning procedure:

Disconnect the transceiver and the antenna from the amp.
Connect a 12V (>5A) power supply (-) to the amp GND.
Connect a 12V tungsten filament lamp in series with the power supply (+). A 5-10 watt lamp will do.
Connect the power supply (+) to the amp RF INPUT.
Repeatedly GND the amp RELAY jack to toggle the T/R relay.

The T/R relay is an ITT RJ1A or a Kilovac HC-1. Alpha is out of these relays at this time. Digikey and Newark show the HC-1 as out-of-stock with an 18 week lead time. Max Gain Systems (MGS) shows the Kilovac HC-1 as out-of-stock.


Manual/schematic

https://www.alpharfsystems.com/wp-content/uploads/2016/05/Alpha_78_Operating_and_Technical_Manual.pdf


Alpha parts  https://www.alpharfsystems.com/?ec_store=78-parts

[W2MV]

Thanks everyone for your responses and suggestions.
Yes, I'm planning to do some more testing. I have a second rig/ Icom 7410 I can try...with a "SEND" relay extension cable. Let's see what happens.

I actually have a spare, NOS HC-1 relay if it comes to that.

When I have the amp open, will also try to determine why "bandpass" mode on 40M has never worked. Maybe cap plates shorting?

Additionally, on 10M CW I can't get more than about 500W output without the grid current going too high. On 10M SSB about 800 watts. The other bands don't have this issue.

[AE0Q]

The best way to run the 78 is to tune up in the CW position to about 500w output, then switch to it to SSB (higher plate voltage) and continue your tuneup to whatever output you desire.

The CW / SSB HV settings were made for the old USA FCC power rules in effect when these amps were designed, which were 1.0 kw DC INput on CW (600w out)or 2.0 kw PEP INput on SSB (1200w out). The ratio of the low to high HV is such that if you tuned up at the CW limit using the lower HV (CW), you could then just switch to SSB and the amp would be tuned up correctly for the higher SSB limit.

It is better for the amp to run the higher HV all the time no matter WHAT mode you are using.  That keeps the grid and plate current low. Watch the grid current, NEVER exceed 50ma per tube (as stated in the manual).

It is normal to have slightly lower output on 10m than the lower bands.

[VE7RF]

I'm betting it's the NC contact on the INPUT relay.  For the vac output relay,  Max gain systems has plenty of gigavac GH-1 vac relays, in both 12 vdc coils...and also 26.5 vdc.   Coils are field replaceable.  The gigavac uses  metric threads on it's threaded base... non issue, since they come with the mating locking nut.   Interestingly enough, the gigavac GH-1 hi pot tests to slightly > 7.5 kv..which is way more than the other 2 x versions. 

The story I got years ago was alpha switched from the Jennings RJ1A  to the more robust Kilovac HC-1.  The HC-1 has more robust internal components / armature etc... and will last longer..esp if qsk cw is used.

[W2MV]

Thanks again to everyone for their response.

The problem was found last night, after several hours of diagnosis....mainly the process of elimination.

The culprit was an SPDT antenna switch. The resistance between the two switch ports was in the megohm range! I opened the switch and internally nothing was obviously fried, but I replaced it with a spare and now everything is good.

BTW it was an Opek CX-201; the new one is a better quality (I believe ) Daiwa switch. The old switch lasted about 10 years, handling a KW.