ETO Alpha 86 HF Linear Amplifier Mod Page

Converting the Blow Out prone PIN Diodes to Vacuum Relays


Other Interesting Experiences and Notes Regarding the Repair of a DEAD Alpha 86 !!!!!!

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*First Off*, Interesting Experiences and Notes Regarding the Repair of a DEAD Alpha 86


OK, Here's the start of my Alpha 86 story. I talked a friend of mine into buying a "Pristine" Alpha 86 off of the internet (From here on referred to as A-86).

The original owner said it was in perfect condition, EXCEPT... "The PIN Diodes Were Blown", and he would supply new ones from Alpha. (more on that below!)

Fine, the unit arrived and ONE transmit UM2106 Xmit PIN Diode came with..... Hmmmmmm.... Makes me wonder!?!?! Did the original owner know what he was talking about? What made him think the PIN diode(s) were fried, since he only bought one of them!?! So  I tested the PIN diodes that were on the T/R board, they actually tested as GOOD! So now I'm even more flummoxed! I pull my fully functional Alpha 91B out of line and insert the A-86. When I key down my rig..... NOTHING, it's as if the A-86 is NOT getting the key down signal from the rig via the Relay/Key-In port.

I contacted the seller and asked, "What exactly were the conditions when the unit failed, What made you think it was the Xmit PIN Diodes, and do you have the original docs. for it?" He came back with some bizarre, vague, rambling response. I tried to clarify his reply and he just went "off the meter" in defensive mode about, "What am I implying", and "I'm not a tech.", and a whole bunch of other histrionics.... OK, fine, I get the picture. He had a dead A-86, had No Clue what was wrong with it, "assumed" it was the PIN diodes, since that was a common issue, and sold it under the claim that ... "This is a beautiful, pristine unit, worked 100%, but those darned PIN diodes went out! Just replace those, and you're in fat city!" OK, Shame on me! I bought his line, and it was a big, fat line of S**T...... So much for integrity in the ham community! If he had just said, It's dead, and I have no clue why, but I'm guessing it's PIN diodes, that would have been one thing, but his presentation was a bit "skewed", shall we say.... OK, enough of that BS.... ANYWAY......

I download the schematics from BAMA, when I compare the schems to my unit (SN 6-08089208), I discover that neither the T/R board, or especially the control board, AT ALL match the schems!!!! When I look at my boards I see hand written stickers on them that say "Rev. B" Where the downloaded schems seem to refer to a set of Rev. E boards! CRAPOLA! This A-86 must have been some early unit, or Prototype that got sold to the public! Some of the boards even have a sticker on them with the original tech/engineer's name on it, for instance, the "Display Board" has a hand written sticker on it that reads.. "Jeff B."... :-) Cute! 

(Note, 12/27/09, I just discovered that the A-86 was mfg'd between 12/87 and 10/89 and only 274 units were ever made. So, does that mean this unit was made in 08/89, and was unit #208, based on the noted serial number?? IF that's the case, then it's a "late model unit" and I wonder why the odd-ball Rev. B boards??? Then again I could be completely misinterpreting the SN scheme too!)

Since the PIN Diodes test OK, and it seems I'm not getting a key down on transmit, I start to look at the control board. See the Rev E schem below: (Note my penciled in changes to match my Rev B board)


On My Rev. B unit There is NO D3 (1N4002) as noted above. The U1A NAND gate is Actually U4B... But, here's the thing.... With no simile to D3 How is the key down signal EVER getting to the pins 5 and 6 of NAND gate U4B and thus causing a relay closure on the control board????

After buzzing out the board several times I found that there is Absolutely NO connection between U4B's input and the Key-In port..... SO WTF!?!?!?!

I even went so far as to solder in a 1N4002 Between the noted points, and still, no joy!!!

I contacted Alpha (now a division of RF Concepts), they say they've never seen this control board, have no schem for it, and can't help me troubleshoot it!

Unless, of course, I send it in for the "normal hourly service rate"!!! Screw That! They even were trying to tell me that my T/R board was some aftermarket version that they had no knowledge of, even though it has ETO-Alpha stamped into the artwork... SO, the BL is I'm on my own with this one!

They disavow ALL knowledge of this unit! So much for Legacy Customer Service..... I hope they do Better with the $10K units they are currently selling... If I pay as much for an Amp as I do for a CAR, I really hope they can give me some adequate, on-line help!!!! 

Anyway, this is where I'm at... No one seems to be able to shed any light on the issue, and I have a beautiful A-86 on the bench that is making a great door stop!


Mike Hess - N3EAQ

For the Actual A-86 Owner - Tim - N3VNG (Posted 12/07/2009)

Update 12/12/2009 - Well, what do you know!!!! I found someone on EBay selling MicroSemi UM2110 PIN Diodes for $19.95 per unit!!!  What makes this special is that Alpha/R.F. Concepts will sell you a replacement PIN diode for the A-86 (A MicroSemi UM2106) for $145.00 per unit !!!!!! The difference between the 2 units is that the 2106 is 600V PIV and the 2110 is 1000V PIV... I'll take the 2110 please! ESPECIALLY at $20 compared to $145 !!!! 

It seems that everything you can get from Alpha/R.F. Concepts comes at a few 100% mark-up! These PIN diodes are 1 example. If you'd like an A-86 T/R board with Vac Relays instead of PIN Diodes, they will gladly charge you $500.00 for a new T/R board. Mind you, If you follow the instructions below, you can buy the parts, and DIY for about $100.00!

It's REALLY SAD! I was, until about 2 weeks ago, a died in the wool, no holds barred, unshakeable, Alpha Man. I'd have recommended any amp they ever made to anyone, anytime, anywhere! After my recent exchange with them, and complete lack of assistance with this A-86 I'm working on, they've gone from Gods, to peon's in my eyes! Next time out I'll look at a QRO or Acom as my next amp.... Who needs a $10K amp that you have to beg for help with! I already own a BMW, I know what It means to spend a fortune and get crappy service!

Anyway, I'm going to drop in the UM2110s when they get here, and see what that does for the A-86. I'm betting it still won't work, and I'm back to the WTF is up with this control board issue, but we'll wait and see! At least it'll save me the Vac Relay Mod, for no perfectly good reason.

Stay tuned!


Update 12/17/2009 - I installed the NEW PIN diodes noted above... Nothing!!  But then I noticed something interesting....  Mind you, I'm at the point that I'm into the HV BEWARE, Only Seasoned techs go here mode! I've got the unit disassembled on the bench, instrumentation applied, interlocks defeated, sitting on 1 hand, and moving slowly!!

I note that K1 on the T/R board is not energizing when the unit is powering up. A test shows that the 400 ohm coil is open, and a test with my bench power supply shows no closure on 24V power being applied... OK, at least I found 1 hard failure! So, a new Magnacraft W88X-8 Relay is on it's way! I also note that on the Rev. E schems there's a diode and cap protection set up between the 28V (K3+/- buss) and the relay coil (Snubber Ckt.). This is completely MISSING on this Rev. B board! So, that may have helped cause this relay go Tango Uniform. I'll add this circuit when I reinstall the new relay. See the graphic below.

More to come!  -- Mike -- N3EAQ



Update 12/24/2009 - The new Magnacraft W88X-8 Relay arrived and I replaced the toasted unit with the new. While I was at it I also added the diode ckt. noted above to the T/R Board. The old relay was attached to the ckt. board with a patch of double sided sticky tape. So, I removed the "sticky patch" and actually screwed the relay to the T/R board using the screw-down hole on the bottom of the relay, and the hole in the ckt. board that was obviously there for the relay to be screwed to. Why someone didn't just screw it down at the factory, I have no clue!

I applied 24V from my bench supply to pins 3 & 4 of J1 (K3-/+ above) and sure enough the relay energized, and all is well! I then reassembled the T/R into the main chassis and began my tests. The relay does now "kick-in" when you switch from stby to oper, Good 1st test! However, if you ground the relay port (Key In) as in Fig. 1 above, no joy. According to the manual, this should be a valid key-down method. (sans-RF). I hooked up a dummy load, and connected the RF from my Xcvr to the A-86 as well as the Xcvr's Relay Out connection.

Taa-Daa!! The unit keyed down, and everything on the front panel reads as normal! Looks like its working now!! I do notice that the HV reading on the display board is nowhere near the 3Kv that the manual is telling me it should read. So, I'm going to have to check if the meter/display, HV output, or the Manual are incorrect. Must be one of these! If I look at the schems it seems that the HV is 1250V, which is what the meter is reading, thus I suspect a typo in the manual. My 91B shows 3Kv on the HV meter, and perhaps older A-86's do too, But this one is reading ~1500v... More to investigate. There are 3 resistors on the HV board that control the HV output to the display/meter. A quick reading, and some calculation should tell me what's correct.

Also, I've noticed that the Voltage Regulator on the LV board is a LM317 where the schems. show a LM350. The 350 is a more stout regulator, and I'm going to replace the 317 with a 350 for prophylactic purposes. I'm also going to replace the incandescent dial illumination bulbs with 2x 10mm, High Output, Orange LEDs. This should give a pleasing "Amber Glow" to the dials, as well as longer life.

I've not spent much time looking over the RF Tank, but I'll go over it as well, while I wait for the LM350 and LEDs in the mail. Will let you know if I find anything.  -- Mike -- N3EAQ

Update 12/27/2009 - I've gone over the manual again and it seems that the HV meter should read about 1600 to 1900V when operational. I misread the manual originally mistaking the "Full Scale" value of the HV meter in section 4.3 as the nominal reading. It is clarified in section 4.4.4.

The orange LEDs have arrived, and the 2 dial illumination bulbs have now been replaced with these LEDs, also the pull down resistor on the control board, in my case R3, was replaced with a 4.7K resistor (read closer to 5K on the meter). (See Fig. 4) Can't test this out until the LM350T arrives, and I can reassemble the LVPS board. 

Took some time to look over the RF and Tank ckts of the A-86. I only found 2 items that gave me a bit of pause. First, I see there is no filament chokes installed on the 3CX400A7s (See D in Fig. 5 Below). The unit could probably benefit in the long term by having a pair installed. RF Parts sells a nice unit for $19.95ea, and I could easily drop in a pair, but I don't think I'm going to do so at this time. Second, I have a little question regarding the Plate Choke set-up. This looks a little dodgy to me. I have a little concern about VHF parasitics, relay arcing, and the use of disc caps in such a HV/RF environment, but again, I don't think I'm going to mess with this now, and may look at it later if it seems that issues are forming because of the current design/set-up. (See Fig. 3a & 3b)

I am going to recommend to the owner that he add the "optional" muffin fan to the unit to help with cooling, and positively pressurize the case.

So, pretty much just waiting to finish up the LVPS, put it all back together, give it the final test, and return it to N3VNG...  


FIGURE 3a &3b - Plate Choke



FIGURE 4 - LEDs & Pull Down Resistor

Update 1/3/2010 - I replaced the under strength LM317 voltage regulator in the Low Voltage Power Supply (LVPS) with the new and correct LM350. I put everything back together, buttoned up the case. (I even donated a few extra screws to help hold the sides of the case on! The original owner sent it sans about 14 case screws!).

I pulled out my 91B, and inserted the A-86 into the RF chain… Powered up, waited for the 3 minute warm-up to clear to STBY. Gave it 20W of tune-up RF from the K3 on 20 Meters into a well tuned antenna


The tuning indicator refused to give any indication to the left or right no matter how I tweaked the tune or load controls! Wha-Wha-Wha…..

The good news is that I was getting power out, and A LOT of it! as a matter of fact the A-86 was screaming that I was overdriving it with only 20W from the K3! I had to pull back the drive to 5W to make the A-86 happy! So, it seems that the tuning meter circuit is NOT WORKING. Not to fear, I already know what’s wrong. I’m virtually positive that the quad op-amp on the control board that takes the RF voltage sensed from the plate and cathode of the 3CX800A7s and does a comparison of the gain, is toast. That’s what drives the display board LEDs left and right for the tuning process as you adjust the tune and load controls. The RF voltage from the plates and cathodes are compared and the state of unity is displayed on the LEDs. If unity is achieved the LED in the center is lit, if out of balance one way or the other, the LEDs to the left or right light….. Thus, I’ll need to tear this thing down and replace that quad op-amp (in my case U3 on the control board).

[If you look at Fig. 4 above you can see the RF voltage leads from the cathodes and plates coming into the control board. They are the golden coax cables you see between the LED and header connector on the edge of the control board itself]

Also, I see that there's not any indication of Plate current on the Ip display. I’ll need to look into that too. The signal for the Ip display comes from a sampling transformer on the High Voltage Supply Circuit board, is routed through the LVPS board, and then to the display board. That transformer (T2) could be messed up, or the signal path interrupted somewhere, but I’ll know better when I have it torn apart…. Again… :-(

That’s where we are. The amp “works”, but just can’t be tuned up by the “proper means”, i.e., the front panel tune meter, and I'm not getting an Ip reading, if it were wanted. (Which also probably means the Ip over current protection isn’t working either!)

Back to the bench we go..... I'm just happy the PIN diodes, T/R switching, LVPS, and other items I've touched so far ARE working!  

Update 1/12/2010 - I've replaced the LM358 Op-Amps... It didn't dawn on me until I started the replacement, but here's another deviation from the schematics! The schems show 2x Quad Op-Amps, the board has LM358s which are DUAL Op-Amps, and it has 4 of them. So obviously at some point the design changed from using 4 duals to 2 quads... I should have noticed this when I got the part number off the chip, since it was an 8 pin package, but it just didn't click at that point. At any rate the 2 Op-Amps are replaced that control the tuning meter. HOWEVER, the tuning meter STILL does not function! The Amber LED still sits dead center, and never moves left or right no matter where the tune or load is set. Soooooo.... either the plate or cathode sensing is not working (See B in Fig. 5 below), OR the Tube(s) are messed up.... The saga continues!

I did change the value of the LED pull down resistance to about 1K ohm and that brightened up the LEDs quite a bit. I also bought a VERY nice 230V solid aluminum muffin fan for $14.95 from Jameco electronic, and will mount that on the rear for added air flow once all the other issues are solved. (Note the $14.95 price as compared to the $300 Alpha wants for this same "upgrade"!)

FIGURE 5 - Under the 3CX800A7s


More to come as the beat goes on!

Update 1/21/2010 - Here's where I am today! I found out back around 1/12 that the whole "no tune indication" problem stemmed from the fact that those "gold" coax connectors that went from the Plate and cathode sensors to the control board, did NOT make very good contact in the header connector that plugged into the control board. i.e., the control board wasn't actually getting the signal! (see figure 4 or 5a for a look at the golden coax to header connections). So, I reinforced these connections and what do you know, the tune meter started to work! 

I put the amp on the air for about a week on 160 meters, and it worked 100%! Great! BUT, I'm still waiting for the doorknob caps to replace those silly disks (see fig 3a), and a filament choke to put in line to augment figure 5.... 

I got the doorknobs (2x 2000pf @ 7.5Kv) and the filament choke on 1/20/2010 from RF Parts. I pulled out the weak ass disks, fashioned a copper buss bar for the knobs, and a copper strip/buss fort the parasitic to plate choke to Cap connection. This buss connection and the copper buss for the knobs are a work of art if I must say so myself, and I DO say so myself! See Fig. 6 below! I finished it all up on 1/21/10, fired the rig up on 1900/160 meters and it works perfectly! Seems to tune better, and put out the juice easier too!

Next is to install the filament choke and aux muffin fan, and I'm done! I'll document that next.... Enjoy the pics of the current work that follows:

Here's a Collage of the Doorknob job, The front face with the LEDs, and the TR board with the PIN diodes and the new relay.

FIGURE 6.....





Until then....Mike - N3EAQ

01/23/2010: This should have been my final entry! I mounted the muffin fan, I installed the filament choke, and buttoned up the whole unit. Put it back in the operating position, fired it up, everything A-OK on 160, same for 80, went to 20 ..... No receive!?! Then it hit me! I use a secondary beverage antenna for receive on 80 & 160, which is where I've been doing all my testing. As soon as I went to 20 I started getting my receive RF through the A-86 and the T/R board (1.e. the receive PIN diodes and fuse)!

Obviously, one or the other is blown. Ugh, time to tear off the case once more. Sure enough, the receive fuse (F1) on the T/R board is blown! A little research shows that this is a 0.25A, 125V axial "Pico Fuse". I understand you can get these from Alpha still "for a price". I didn't even bother to look! I found someone selling them for $0,45 per unit, and bought 20 of them! I figure $9.00 is a small price to pay to have A LOT of SPARES on hand! As a side note, believe it or not Radio Shack (excuse me "The Shack"!) actually sells a 0.25A glass "Ultra fast acting, Buss Fuse"! You could run down to the local store and buy one in a pinch, make an adapter, and plug it in to get back on the air if you wanted. Since there's no rush here. I'll just wait for the pico fuse, and do a direct plug in without the hassle of needing to build an adapter.

Otherwise, all seems to be working well, all the repairs are holding, and all the modifications and doing their job as expected. Will add some final pics later...


This will likely be the last installment unless something else pops up!

Thanks for coming along for the ride! Hope it helped you somehow, Hope you found it informative, and enjoyable reading!

Feel free to comment if you wish. Best Regards. Mike Hess - N3EAQ - N3EAQ@N3EAQ.NET

NOW on to the VAC RELAY Procedure

VAC RELAY Procedure:

All you need for the Mod is:

*First, remove the Relay K1 from the T/R board. 

You must fabricate a bracket to hold the two Vacuum relays and mount it where K1 used to be.

I just used a piece of aluminum and carefully bent and drilled it to fit.

I used the original screw holes in the T/R board that were used to mount K1.

*Next, Remove the Old TX PIN diodes D1 and D2. Also remove D10 and D6.

*The Vacuum relays are wired so that the Normally closed contacts are wired together.

See Picture below :

The Center (Common) contact of the Vacuum relay on the right goes to the RF Input jack.

The Center (Common) contact of the Vacuum relay on the left goes to the RF Output jack.


FIGURE 1 - T/R BOARD - Before ANY Mods... "A" points to PIN Diodes in Cage - To Be REMOVED!


FIGURE 2 - T/R BOARD - After PIN Diodes are Removed... ALL Other Items  - To Be REMOVED!

*NOTE - The above T/R board is a picture from my "early unit A-86" You will note that it is a bit different from the below (Fig. 3) "later" unit!

FIGURE 3 - T/R BOARD - After Mod....


*The Cathode coax must be moved from the T/R board and placed on the Vacuum relay on the right NO terminal. This is the Input relay

*The coax on the T/R board from the Tank Coil must be moved from the T/R board to the Vacuum Relay on the left NO terminal. This is the Output relay

*The Coils of the vacuum relays are wired in Parallel. You get the +28 volts needed for them from pin 4 of connector J1 on the T/R board.

*The other side of the coils must be wired to the Control board behind the front panel.

Some mods need to be done to the control board:

*Pin 2 of connetor J1 on theT/ board is NO longer wired to the relays either.

So basically, you are disconnecting everything from the T/R board except the SWR circuitry and the +28 volts for the relay coils which all connect to the J1 connector. Nothing from J2 will any longer be needed on the T/R board.

*Now, run a wire from the Vacuum relay Coils through the wall into the transformer area and up to the Control board behind the front panel. I used tie wraps and secured it to the main loom running to the front.

*Add a snubber diode across each vacuum relay coil and a .01 cap to ground off each coil primary as well.

*You must remove resistors R15 (47) and R17 (100) from the Control Board.

* Please note, these part numbers appear to be Different on some amps, so check the picture and schematic below to be certain.* (see my experiences above in the preface!)

*Connect the wire from the Vacuum relay coils to the point where R 15 and R17 used to connect to K2 relay on the Control board.

This wire allows the Relay K2 on the Control board to now control the closing of the Vacuum relays whenever the amp is keyed.

FIGURE 4 - CONTROL BOARD - *NOTE - As you can see this is a Rev. E Board!!*

Compare to MY Rev. B ??? Control Board in Fig 5A Below!


FIGURE 5a - Mike's Rev. ??? Control Board


Ok, almost done. Only a couple more things to do :

* Remove the connection of PIN diode rev bias board AB103 from L9 in the tank circuit.

This board placed DC on the tank output to reverse bias the TX PIN diodes. You will not want DC on the tank when you mod the T/R system with vacuum relays. L9 is where you will connect the output relay for TX RF output. You can leave AB‐103 in place, just make sure it isn't connected to the tank anymore after the mod.

* I had to reroute the cathode coax to go between the 3CX800A7 tubes instead of around the back but that was easy.

This Procedure was described to me by N6WK, and I simply provided further editing!

The origin of the mod is from Matt KK5DR and ALL credit goes to the original engineer!