The original T/R relay is VERY slow Geoff.  Normally you have to crank in at least a 1/3 second delay in the transceiver to accommodate it (or Writelog).  The mod for that is quite complicated.  The open frame relay, which is toward the back rear if I remember correctly, is replaced by a circuit board that has a very fast closed relay and a lot of other parts on the board.  You buy it from Ameritron.  In addition, it runs on 24 VDC and there is only 12VDC in the amp.  So, Ameritron also modified the small metering circuits board mounted to the meter switch.  They changed the full wave rectifier that was there to make 12VDC to a voltage doubler to make 24.  It’s a messy mod.  I have done it twice – its not too bad if you buy the parts from Ameritron.  I may have the schematics here in FL if you want to try it, but the relay and relay board have to come from Ameritron. 

The caps are another story.  They are rated at 450V and if I remember there are 8.  That’s 3600V if everything is balanced.  The new caps from Ameritron are about ½ half high.  I glued a 1” dowel about 1” tall next to the HV board, and glued the new cap on top of the dowel.  Then wired it in along with another bleeder resistor.  Ameritron parts are amazingly cheap.  Then we tapped up the HV transformer to max, which depending on line voltage will put you up near 4 KV.  The amp really plays great at 4KV – lots of gain, and still very stable.  This was all done on the AL-1200 that’s at PJ4G.  Noah also had me set the blower to full high (a one wire move on a terminal board).  I didn’t notice any change in noise or air flow.

Hal 

AL-1200:

Splice into the capacitor bank and add another capacitor and shunt resistor.  I suggest buying them from Ameritron – they are cheap – exact replacements and easy to deal with.  Well, not exactly exact replacements.  The new caps are not as tall as the old caps.  I glued them onto a wooden dowel to make them stand high enough to easily splice in.  Make sure to route the transformer HV leads away from all this – they tend to droop right down on top of the cap bank.

Kick the blower up to the next to highest speed.

Tap the HV transformer for max.  You may see as much as 3.9KV.  Gain will be great. 

The original T/R circuit is a real dog.  You want the factory mod high speed T/R board and LV P/S mod that goes with it.  Contact Ameritron.  Its easy to install if you don’t already have it.  The old version T/R relay is open frame and takes forever to throw.

Questions?

GG

Yep Rick – I knew you had the theory okay.  In operation the resistors keep the stack balanced, and I failed to mention when I go through one of these I first look over all the solder joints and the resistors for any signs of trouble.  I then disconnect one end of the stack and measure each bleeder.  Anything at all suspicious should be tossed and replaced.  Most of these stacks fail from an open resistor.  That cap will short and then the whole string goes like firecrackers.  I hear it's exciting.  

Now, on power up – we have a complex set of variables that I don’t have all the parameters for.  The caps are a near dead short on start up and all that would limit current from a very stiff supply would be their ESR.  New and old ones might have different ESR.  But, we are not charging from anything like a stiff source.  Worst case is throwing the switch at the top of the waveform at which point either magnetic saturation or the windings’ resistance is limiting the current.  My best guess is the power transformer between its winding resistance and marginal magnetics is current limiting for a while – until the voltage is high enough on the stack for the bleeders to dominate the ESR.  I don’t have real numbers.  Also note, the caps are rated for something like+50% voltage for surge.  

I have never seen one blow personally but have heard the stories.  Don’t know if it happens at turn on or while running.  If you can find ten 450V caps with the same ESR that the best you can do but based on history probably overkill.

Best

Hal

31 March 2011

AL-1200 (1) was removed from service the week before last when the plate blocking caps were arcing so badly that any RF over 500W caused popping shorts across these caps and pulled down the HV, shorting with every dash and dot. W0CG removed them and cleaned today. See here for photo. These caps say HEC HT50, 500PF +-20% 9811, 7.5KVDC N4700. Appear to be the same in all three AL-1200s. One of the caps has a visible crack and darkened spot on the side of the body, running about 1/3 of the way from the tube side to the tuning cap side. Cleaned as well as possible and re-assembled. This cured corona problem, but there was still internal arcing at high power. On April 3 used a file to file off burned areas in the middle stator plates of the plate air variable, then carefully cleaned all dust and dirt. Amp seems fine at full power now.

31 March 2015

Placed this amp in service at Station 2 after the contest season. Working fine in a week of post-contest QSOs.

29 November 2015

Used for CQWW CW at Station 5 (40 meters). Popping noise from HV board area could not be found visually. Put it back in service. It started smelling hot, the popping stopped, then it stopped smelling hot. Will look inside after contest.

2 December 2015

W9NJY and W0CG opened amp and saw nothing whatsoever wrong. Amp placed in service at Station 1 for trial.

12 December 2015

Amp worked perfectly for about a week and then began smelling hot at Station 1. Swapped in AL-1200 (5) and will open up #1 when have time.

7 March 2016

K6AM researched and sees smoke coming from one electrolyic and at least one with burn marks on it. CGS211T450R4C3PH he says is the part number. 215 ufd 450 volt 1 3/8" OD.  4 3/8" Ht to top of terminals with screws out on 1/2 inch screw centers. We have several older caps and eight new ones in Box 9 in the EBR duct. Next trip try swapping some of these in to see if the problem will go away.

9 March 2016

K8ND ordered eight look-alike caps from Mallory via RF Parts Company. Click here for that invoice.

22 October 2016

W0CG replaced all eight HV caps with the new ones bought earlier this year from RF Parts Company. The bottom cap on the HV string was visibly burned as viewed from the top.

27 October 2016

Cracking noise intermittently from amp. Sounds metallic. Removed from service pending troubleshooting.

27 November 2017

Replaced the entire HV board using the eight new caps (large) that I bought for this amp last year and the HV diode board from the W9NJY bag from Box 9. N7IR did the replacement. There is still audible cracking. Next thing to try is to disconnect the HV secondary from the board and see if it still cracks. If so, the transformer is shot.

9 December 2017

Disconnected HV secondary of transformer from HV board and ran amp for 30 minutes. All normal, no popping noises. Reconnected HV supply and popping resumed. Looks like bad electrolytics, one of the new ones. This is not worth any more on-island troubleshooting time with only one more day left here. Order more HV boards and also study adding a 9th electrolytic and 50K equalizing resistor.

24 January 2018 E-mail from N7IR

 

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3 March 2018

W0CG disassembled the HV module in AL-1200 (1) that was popping. Tests with VTVM showed two of the eight equalizing resistors were bad (one open, the other around 36 M). Took the diode/resistor board that was part of the bad board removed from AL-1200 (4) that had blown up capacitors on it and tested the eight resistors. All eight looked good. Put the new large caps for AL-1200 (1) onto that board. This combination of the new large caps and a resistor/diode board worked, with no apparent popping. Will run amp further tomorrow, then put it on the air. Need to order a stock of the 50K equalizing resistors.

4 March 2018

The old original diode/resistor board from AL-1200 (1) has four of eight equalizing resistors bad.

25 November 2018

N7IR replaced all resistors and caps on the HV board and all is working perfectly now.

25 February 2019

Placed in service at Station 1. Used in ARRL SSB, working perfectly.