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N6WK
02-04-2011, 09:00 PM
I need to know what the replacement diodes are in Bridge supply of the Alpha 76A. The originals are RCC-1733. I think Steve, WB2WIK/6 knows. Does anyone?

Thanks,
Gordon

W9GB
02-04-2011, 09:11 PM
The RCC-1733 diodes were used in several Alpha amplifiers of that era (e.g. Alpha 374).
They should be ~ 3 KV @ 0.5 amp diodes. A higher voltage rating, such as 5 KV would work.

BTW, Alpha/RF Concepts should have those in stock !
http://www.rfconcepts.com/s.nl/sc.15/.f

Earlier QRZ Q&A on this subject.
http://forums.qrz.com/archive/index.php/t-80584.html

w9gb

KA4DPO
02-04-2011, 09:11 PM
Belay that, I was looking at the parts list for my old amp. D1-D4 in the 76 are 3000 PIV .5 amp diodes. How about 4 1n4007's in series for each diode.. That's 4Kv PIV at 1 amp, should be enough to handle it.

W9GB
02-04-2011, 09:17 PM
I'm pretty sure the stock part was a 1N4004 so a 1N4007 would be a good replacement.
The 1N400X diode are low forward drop and rated at 1 amp so they are a good choice.
John -

1N4004 are rated to 400 Volts; 1N4007 rated to 1,000 Volts.

Individually, they will not handle the High Voltage regulation used for the 8874 RF tubes -- 1,600 VDC (Low) to 2,400 VDC (High)

PHOTO attached (upper right are 4 diodes in the bridge)

w9gb

WB2WIK
02-04-2011, 09:24 PM
Belay that, I was looking at the parts list for my old amp. D1-D4 in the 76 are 3000 PIV .5 amp diodes. How about 4 1n4007's in series for each diode.. That's 4Kv PIV at 1 amp, should be enough to handle it.

The RCC1733 isn't a single junction diode: It's a high voltage rectifier stack from Rectifier Components Corporation, who I think is now out of business.

The "K2AW Silicon Alley" HV stack replacements, like 5kV at 1A units, should work as replacements. I have also used Semtech SCHS5000s, SCHS7500s and other similar HV assemblies.

KA4DPO
02-04-2011, 09:25 PM
John -

1N4004 are rated to 400 Volts; 1N4007 rated to 1,000 Volts.

Individually, they will not handle the High Voltage regulation used for the 8874 RF tubes (2,000 to 3,000 Volts)

w9gb

I realized that after I hit the send button. I think I fixed it for him...
By the way, here is another possibility but $65 bucks each.
http://www.surplussales.com/Semiconductors/Diodes-1.html

45XV246 15KV @ 1.2 amps. If these don't do it nothing will but I think building a stack from 1Kv diodes is a good way to go. Even if you double each arm to 6 diodes the forward drop will still only be 14 volts.

N6WK
02-04-2011, 09:31 PM
Well, maybe they aren't bad then. I checked them with my diode tester and they show open either way. I also ohmed them and same results. The problem I am having is that I show No HV on the meter, yet checking the circuit all works fine and when I place my meter on each side of the 180K resistor on the board, the HV meter goes up. So I am convinced there is a problem with the rectifiers.

WB2WIK
02-04-2011, 09:37 PM
Well, maybe they aren't bad then. I checked them with my diode tester and they show open either way. I also ohmed them and same results. The problem I am having is that I show No HV on the meter, yet checking the circuit all works fine and when I place my meter on each side of the 180K resistor on the board, the HV meter goes up. So I am convinced there is a problem with the rectifiers.

I'm not sure what the 180K resistor is about, but it's normal that HV rectifier stacks have too many junctions in series (in each one) to read "like a diode" using an Ohmmeter. The problem is the Ohmmeter or DVM voltage is too low to turn on all the series junctions -- it would take higher voltage to do that.

Anything is possible but usually when rectifier stacks like this fail, they become a short circuit (or close to a short circuit) and not an open circuit. An open is possible if a huge energy transient took them out, literally blowing it apart. But normal operational failures produce a near-short.

N6WK
02-05-2011, 12:12 AM
Thanks Steve, I found the problem and you are right. the DVM couldn't trigger the junctions.
Turns out, one of the 1 meg Resistors for the meter circuit was open. Could have sworn i checked them all and they were fine! oh well.

73,
Gordon

W9GB
02-05-2011, 12:42 AM
Gordon,

Alpha, at one time, had a service bulletin concerning one of the resistors on the control board for the 76A series.
I no longer have my notes or that Alpha 76A.

The resistor ( R118 ??) is in the middle left of the board and in the grid power supply circuit.
The resistor overheats (1/2 watt value and should have been a 1 watt value).

In that unit, the resistor was cracked -- and created an intermittent open. K9EWJ (sk) never did find the problem.
That amplifier also took one trip to Alpha in early 1980s -- and they missed it as well !
So, you might as well do a measurement and visual of the remaining components, while you have it open.

w9gb

w9gb

N6WK
02-05-2011, 12:49 AM
Thanks Greg, I will check all the components on that board and also look into the Alpha Service bulletins. The guy that dropped it off just bought it and it has bad 8874's in it. Ouch !
Maybe he will sell it to me and I'll convert it to a pair on 3CPX800A7 tubes

WB2WIK
02-05-2011, 01:02 AM
Thanks Greg, I will check all the components on that board and also look into the Alpha Service bulletins. The guy that dropped it off just bought it and it has bad 8874's in it. Ouch !
Maybe he will sell it to me and I'll convert it to a pair on 3CPX800A7 tubes

Cockpit error, no doubt. 8874s can last 30-40 years if you never draw excessive grid current and the blower or fan keeps working properly. My old Alpha 374 (first model, 3 x 8874s) still has its original 8874s working fine after 30 years.

Most of these old Alphas had untuned inputs and either a "high drive" or "low drive" option, which involved changing a jumper in the input circuit. Anyone who modified his for "low drive" usually popped the grids in the 8874s unless they had a QRP exciter and could never run more than 10W or so. Check that out.

N6WK
02-05-2011, 05:49 PM
I found the couplers between the Knobs and the Load and Plate cap have been slipping.
Does anyoneof you know, Are the Caps fully meshed at Zero or at 100 ?

WB2WIK
02-05-2011, 09:31 PM
In almost all amps the caps are fully meshed at "0" and fully open at "100."

W9GB
02-05-2011, 11:24 PM
The guy that dropped it off just bought it and it has bad 8874's in it. Ouch !
That is indeed very unfortunate -- I have only seen that with grid failure (no protection in factory 76A); ventilation failure; or operator error (abuse).

Maybe he will sell it to me and I'll convert it to a pair on 3CPX800A7 tubes.
IF this is the 2 tube model, then the 3CX800 conversion does make sense -- but these NOS tubes are expensive (~ $900).

IF it is the 3 tube model, its a coin toss.
Last time I price the 3CX400A7 / 8874 tube it was in the $400 price range.
Medical and broadcast pulls do exist.

Ken, KA0Y DIY home-bult his VHF EME amplifiers using a pair of 8874 tubes,
that is how common new and used these tubes were in 1970s and 1980s
http://www.ka0y.com/ham.htm

I would consider the grid overcurrent protection and QSK modifications.
Dick Byrd, N4UQ performs these modifications.
You might want to call Dick and see if he has some spare 8874 tubes from earlier conversions.
http://www.dickbyrd.com/

w9gb

N6WK
02-06-2011, 01:04 AM
In almost all amps the caps are fully meshed at "0" and fully open at "100."

I agree Steve, but there is always that rare exception and I think this might be one of them hi hi
when I put them both fully closed at ZERO, the tune numbers don't match the book but the Load does.
However, if I make the tune fully open at Zero and the load fully closed at Zero, the Numbers match the book perfectly on all bands. I'm leaving it this way since it matches the manual for tuning numbers.

N6WK
02-06-2011, 01:06 AM
I'll drop Dick an Email and see what he has available.
These failed because all the fins were packed full of Dirt when I opened it up, therefore having no cooling !!

Thanks for the tip.
73,
Gordon


That is indeed very unfortunate -- I have only seen that with grid failure (no protection in factory 76A); ventilation failure; or operator error (abuse).

IF this is the 2 tube model, then the 3CX800 conversion does make sense -- but these NOS tubes are expensive (~ $900).

IF it is the 3 tube model, its a coin toss.
Last time I price the 3CX400A7 / 8874 tube it was in the $400 price range.
Medical and broadcast pulls do exist.

Ken, KA0Y DIY home-bult his VHF EME amplifiers using a pair of 8874 tubes,
that is how common new and used these tubes were in 1970s and 1980s
http://www.ka0y.com/ham.htm

I would consider the grid overcurrent protection and QSK modifications.
Dick Byrd, N4UQ performs these modifications.
You might want to call Dick and see if he has some spare 8874 tubes from earlier conversions.
http://www.dickbyrd.com/

w9gb

WB2WIK
02-06-2011, 02:55 AM
I'll drop Dick an Email and see what he has available.
These failed because all the fins were packed full of Dirt when I opened it up, therefore having no cooling !!

Thanks for the tip.
73,
Gordon

Wow, that's unusual. Must have been used in a pretty dirty environment, and the owner didn't care to look and see what was going on. With any kind of amp, especially one using external anode tubes, it's mandatory to open them up annually and suck the dust out. With 8874s and similar, they can go in the dishwasher (no kidding) to clean them. I did that, and still do that, with similar tubes. Forcing hot water through the fins from the kitchen sink also works fine.

The dishwasher's easier. Any time I have dirty tubes I run them through the dishwasher. They come out clean and lemony smelling.

Sometimes the hot water and detergent will wash off the markings, though, so it pays to use a Sharpie permanent marker on them to label them in case you forget what they are.

W9GB
02-11-2011, 10:40 PM
These failed because all the fins were packed full of Dirt when I opened it up, therefore having no cooling !!
Alpha use to post photos (older web page design) of that exact problem from "clueless" Alpha owners --
who never performed annual maintenance / inspection. You really wonder -- how clean their radio room were -- or -- if they ever changed the oil in their automobiles !
==
Some good news.

W3JN has some "pulled" 8874 triodes from Quintron paging transmitters (a common usage for that tube in that era).
You might be able to get 3 good ones.

eBay Item number: 160543923214

Good luck!

w9gb