Apache problem

[w5rkl]

Hello everyone,

Recently, after tuning up then placing the tune / operate switch in operate, I keyed my Apache. Normally the plate current runs around 225ma with about 110 watts output. However, this time, when keyed, I heard a noise and my Apaches plate current dropped to 100 ma and the plate voltage was all over the place, very erratic. I immediately shut it down and checked the modulator tubes. As suspected, one of the 6CA7/EL-34's were shorted. I had a used spare, not same manufacture. I replaced the shorted tube and ran a test into a dummy load. With the new tube in place, the plate current and voltage settled down to normal. I tuned up in CW and did some testing with a straight key. The note heard in a seperate receiver sounded very good, clean, no hum so I switched to AM with the front panel gain turned completely counterclock wise. Still connected to a dummy load, meter in modulation position, I keyed the transmitter. Still everything appeared ok but when I started to advance the front panel gain control. I noticed that if I advanced the front panel gain control passed 8 o'clock the modulation current meter position would peg the meter. I quickly reduced the gain back to zero, completely counterclock wise, and the meter returned to zero.

From these initial test, I do not suspect the main modulation transformer because if the main modulation transformer was bad the meter readings would not be normal in any mode. I do suspect however, the driver tube, 12BY7, and possibly the driver transformer. The driver tube does test bad, initially test good but drifts rapidly down to bad on the tube tester. I don't have a spare 12BY7 so I have to wait until I get one to test my theroy, at least with the tube. Unfortunately, I don't have a spare driver transformer. If the driver transformer turns out to be bad I guess I'll have it rewound since they are a bit difficult to find. I do not have any resistance specs on the driver transformer primaries and secondaries so I'll only be looking for shorts between primary and secondary windings at this point.

Any comments and/or suggestions?

73's,

Mike
W5RKL

[WA3VJB]

Mike is the replacement EL34 known to be good ?
I don't see that you tested it on its own merit as well as whether it has about the same reading as the remaining one from the earlier pair.
While you're ordering tubes I believe I'd invest in some EL34. They're all over the place these days for the guitar crowd, but a matched set, new, will start you fresh along with that 'BY7 you need.

[W3SLK]

Also, take a real close look at the sheilding in your mic cable. Both inside and outside of the unit. It almost sounds like you are getting some feedback or RF in the speech amp. If the different tubes don't fix it, I would make this my next step.

[w5rkl]

As for the used replaced EL-34, yes it is a known good tube. The set I had in the Apache prior to the problem were a matched pair of EL-34's from Electro-harmonix. I seem to recall reading an article about early EL-34 failures from a specific manufacture. However, I can't find the article to verify if it's Electro-harmonix or not.

As for RF getting into the audio, that might be a possibility but at this stage I don't believe it is. However, I am not ruling out that as a possible problem.

There are a number of sources for EL-34's so sources, all with varying prices.

At this stage in the troubleshooting, I'm looking at the basic's, tube failure that caused the inital problem in the first place. As for the reason the modulation current pegs above 8 o'clock, that's because the 12BY7 is bad causing excessive drive to be placed on the grids of the EL-34.  However, if, after replacing the 12BY7, the problem still exists then power supply components, electrolytics and diodes, and the driver transformer will be the next components to check.

I believe the problem started with the 12BY7 audio driver failing. This failure in the 12BY7 caused an excessive amount of  drive to be put onto the grids of the EL-34s. This caused the EL-34 grid current to soar beyond its maximum value, shorting one of the EL-34s. Although the matched set of EL-34 were not that old, I suspect the one that failed was weak or it took the brunt of the surge causing it to short internally. Of course, without a replacement 12BY7, that's only a theory at this point. If, after replacing the 12BY7 with a new tube the problem goes away then I can say my theory is correct.

Anyway, regardless of the frustration factor, it's always fun as well as it's an excellent learning experience, repairing these old rigs. It's not the failures that's an issue, it's lifting that 105lb monster from the cabinet to the bench that's really a pain in the butt!!

73's,

Mike
W5RKL

[W3SLK]

Mike I feel your pain having been there and done that, (including same problem). Remember that a large amount of these were kits, (about 99.9%). Keep in mind that it is not uncommon for cold solder joints to manifestate problems 20+ years after they were built. As for the back/ass breaking problem of hauling it from bench to work table, I purchase a roll-a-round tool box from Sears that has a power strip built into it. If you wanted, you could do your repairs on the tool box. I chose to use it as a cart to transfer them. Slide off of the operating table to the cart; slide from cart to work bench with minimal effort. Work smarter, not harder!

[ve6pg]

...MIKE ..DONT PANIC...THINK ABT STUFF THAT SHUD BE OBVIOUS....OK...UNDER THE RIG IS THE "RESTING CURRENT"POT OR AM...THEY ARE JUNK...I REPLACED MINE ON THE CHASSIS,NEAR THE 5VR.THE POT IS CRAP,AND NEEDS A GOOD REPLACEMENT...ASIDE FROM THAT,THE POT BEHIND THE KEY JACK,AGAIN IS JUNK..ALOT OF GUYS FEED THEIR AUDIO RIGHT INTO THIS AREA,BYPASSING THE POT..THE APACHE AUDIO AMP HAS A LOT OF GAIN,AND I MYSELF HAVE BLOWN EL34s,BECAUSE OF TOO MUCH AUDIO INPUT...SO,MY SUGGESTION IS TO CLEAN/REPLACE THE POT UNDER THE RIG,CLEAN/ REPLACE THE POT BEHIND THE KEY JACK,AND SET THE TRANSMITTER UP WITH THE AUDIO GAIN ON THE FRONT LEFT SIDE,AT ABT 9 O'CLOCK,AND THEN ADVANCING THE ONE BEHIND THE KEY JACK,FOR DECENT LEVEL OF AUDIO...THE POT UNDER THE RIG,SHUD BE SET FER ABT 40-50 mils,WITHOUT ANY AUDIO...I KNOW WHAT U R GOING THRU,JUST FINISHED MY APACHE,AND WERKING ON ANOTHER FER A FRIEND....LET ALL OF US KNOW HOW U R MAKING OUT...TIM....SK...

[w5rkl]

I've bought 2 new EL-34's and ordered a new 12BY7 so when they arrived, I'm going to see what happens. Hopefully when I install the new tubes the Apache will come back to life on AM. While I have it out of the case, I'm going to replace all the electrolytics and diodes so I don't have to go back in again.

I agree, the Apache's audio is quite strong at 100 watts of plate modulated audio. When they work and the original interstage caps have been changed to .01ufd orange drops, they sound really great on the air and pack a nice punch.

The solder joints on any Heathkit can be a real problem, especially if they're on a PC board.

The bias circuit for the EL-34's is one of the things I'm going to check. It's a problem area for sure. Those diode "bricks" are a pain and should be replaced. The Apache has 3 of them, one in the modulator bias circuit and 2 in the low voltage supply for the modulator.

Anyway, will keep ya all up to date on the progress. It's not a big deal since I still have a Ranger and Mohawk for AM. I use to use a Marauder on AM but it has limitations for AM, 20 watts with 100 ma plate current maximum. Presentlyl, it's sitting in the garage along with a few other boatanchor pieces.

73's,

Mike
W5RKL

[ve6pg]

MIKE...WHILE IT'S OUTTA THE BOX,PUT A ONE AMP FUSE IN LINE WITH THE 5R4s,AND THE 5V4...INSTEAD OF SMOKING THE EL34s,YOU WILL BLOW THIS FUSE FIRST,NOT MESSING UP THE TOOBS...WERKS FER ME...TIM...SK..

[w5rkl]

The Apache is alive and well, at least for now. Still need to replace some additional caps and a tube or two. I did replace some caps and a couple of resistors in the modulator section, this Apache's audio section has been modified more than just the 3 interstage caps. Set the modulator bias current to 50 ma, set the clamp tube as it was a little off allowing a bit too much plate current with drive control turned down to zero. Listened to the audio in a seperate receiver, sounds very good.

73's

Mike
W5RKL

[ve6pg]

..OK MIKE...I WAS ON DIS MORNING,OUR GROUP HANGS ON 3725,AM....SOME MEATHEAD WAS ON 3733,AND WAS COMPLAINING THAT I WAS SPLATTERING ALL OVER THE BAND...I CANT HELP IT IF MY ANTENNA SYSTEM IS VERY GOOD,WITH ABT 4 MILES OF RADIALS....HE WAS IN QSO WITH ANOTHER HAM,ON SSB,AND WAS GOING ON ABT MY HIGH POWER,AND OVERMODULATING MY TRANSMITTER...I WAS ON THE APACHE,WITH AUDIO MODS TO REDUCE THE HIGH END,(WHICH USUALLY CAUSES THE "SPLATTER"),BUT WITH A STRONG SIG FROM ME,AND HAVING 100 % MODULATION,I WAS THE BAD GUY..HE MENTIONED TO HIS PAL,THAT I DIDNT KNOW HOW TO OPERATE THE OLD GEAR,AND THAT I WAS 50/9 AT HIS QTH...THIS REALLY PISSED ME OFF...THIS GUY IS ABT 70 MILES FROM ME,NOT CLOSE BY,AND BECAUSE MY TRANSMITTER IS CLEAN,AND ANTENNA SYSTEM VERY GOOD,I STILL LOOKED AT AS SOME KIND OF NUISANCE ON THE BANDS....GO FIGURE...TIM....SK...