Heathkit Apache

[w5rkl]

I have a problem with my Apache that I can't seem to find the source and cause.

With the Apache powered on and in standby, it operates as it should. Keying in TUNE position
produces good output at a low level as it should. Switching to OPERATE and keying the transmitter,
the finals produce about 120 watts into a 50 ohm dummy load. So far so good.

Switching to fone and keying the transmitter, the modulator's idle current is at 50ma and plate current
is around 225ma and audio sounds clean. However, after a short period of fone operation I can hear
what sounds like a very short but irregulator "tick" noise that appears to be coming from the left
front of the transmitter. This "tick" sound causes the modulators plate current to drop to zero each
time it occurs. The tick sound is very short, on the order of a half a second and when the noise stops
the modulators idle plate current returns to 50 ma. The problem occurs after the transmitters been
on for 10 or so. It doesn't always happen but when it does occur, the problem is irregular in its
occurance. Sounds like "tick...tick..tick...tick.tick...tick" and each time the plate current drops
to zero and returns when the noise stops.

I have turned the mic gain down to zero but the noise is still there and the mod idle current still drops
to zero. Since the EL-34's modulator's plate voltage is obtained from the HV supply and only in AM mode this also affects the final plate current as well.

In CW mode, I don't hear the noise. This leads me to believe the problem is in the modulator, maybe
a modulation transformer getting ready to give up the ghost or has already done so. The other
possibility is arcing in the mod tubes.

I've tested all the tubes and they all test good for what that's worth. I even thought it might be
arcing in the final cage due to dust and dirt but it doesn't appear that is the problem, at least for now.

Unfortunately I do not have a spare set of mod tubes to compare if this is the source of the problem.
Has anyone experience an Apache mod transformer failure while operating and if so what symptoms
did you experience? I'm stumped as to the source of the problem. I'm hoping it's not the mod
transformer or mod driver transformer as I don't have a spare and they are difficult to come by these
days. I have seen them on eBay back before this problem occurred.  Any suggestions?

Thanks and have a great day.

73's
Mike W5RKL

[Ken - K2UPI]

Hi Mike:

Have you checked the DPDT relay & circuit that feeds the EL-34 screens from the L.V (5V4) supply?
The relay is located in the power supply compartment on the rear chassis apron.  Also, the .1 mf
by-pass cap from one or both (pin 4) of the EL-34's?

73 es Good luck,     Ken/K2UPI

[Pete, WA2CWA]

Pull both mod tubes out and then test on transmit to see if it happens. Since you said, "it appears to be left front", check the the mode switch wiring for poor solder connections. loose switch contacts on the ceramic wafers, or a carbon tract that has been burnt into the ceramic between contacts. Also check the clamper tube, its wiring, and for proper adjustment of this circuitry.

For those who are fearless, pull the chassis out and stand it on one end. With a dummy load, fire it up on AM. Darken the room, and with hands in pockets, carefully visually survey the underside of the chassis for arcs. Likewise, inspect the lighted tubes, finals, clamp and OB2's, and modulators for any perceived internal arcs. Also, with power off, inspect the ceramic sockets for the modulator tubes, both top and bottom, for any carbon tracts.

If experienced, try using a long insulated non-conducting "diddle stick" which can sometimes escalate finding the problem if it's loose contacts or poorly soldered connections.

[w5rkl]

Thanks for the suggestions. I did a thorough visual check in the final, modulator, and power supply,
found no traces of any arcing. Checked the bandswitch in the final cage and found no evidence of
arcing.

Not finding anything, I decided to recheck all the tubes. Everything checked fine "except" the 12AX7.
One section had no filament yet the other side lite up fine. Duh, I guess I didn't check the bad side,
may have gotten distracted. Replaced it with a good one, yep I tested both sections of the replacement
so as not to make the same mistake twice.

Put everything back together, fired it up, and it tuned up nicely. I ran the load control through its
travel a couple of time before firing it up. The transmitter loaded up fine as always with 110 watts
into a dumy load at 200 mils plate current with load control at 10 o'clock. Had a short QSO with a
local station and I didn't hear the "ticking" sound. I don't know if the bad 12AX7 was the cause of
the "ticking" sound or not, only further testing will indicate it. However, the local station I was
chatting with said he had a very similiar "ticking" sound in his Apache. He said, like me, it took him
a long time before he found the problem. It turned out to be a leaky HV bypass .001 2KV mica
capacitor. He found it when it blew... Once replaced the sound went away and his Apache worked
fine. I have a couple of 6KV .001ufd caps here so I may replace the mica bypass cap for "insurance"
in the event that cap is leaky. It does not test shorted with a VTVM but at voltage it may be
breaking down and causing that ticking sound. Since the cap is in the final cage and the final is
at the left front, stands to reason that "may" be the culprit.

Will let ya all know what happens after replacement. Again, thanks for the suggestions, really
appreciated.

73's
Mike W5RKL

[Pete, WA2CWA]

If you're still blowing air into the final compartment, I would suggest changing it to a suck-out arrangement. Blowing air into the compartment brings in dust over time. Compound that with some humidity, and or, if you smoke, can turn those "dust bunnies" into problems for the final compartment hardware.

[w5rkl]

I changed the .001ufd bypass cap to a .001ufd 6KV disc cap plus I swapped out the original fan with
a much larger 120VAC muffin fan mounted to pull air out of the final cage. Unfortunately, the original top
vent holes are too small to create much air flow. I have some perforated aluminum sheeting that I can
fabricate a new top cover which should allow more air flow. The perforated aluminum is thinner which
allows more clearance between the top of the fan and the cabinet lip. I had to do this with my
Marauder when its cooling fan bit the dust.

I haven't heard the "ticking" noise yet, only time on the air will prove whether the bypass cap was the
cause of the problem or not.

Thanks for all the help and have a great day.

Mike W5RKL

[Pete, WA2CWA]

I removed the top cover from the final cage thirty years ago. Have an aluminum strip, about one inch in width, that straddles one hole of the cage at upper left and one at lower right. Mounted the fan to the strip.

[WU2D]

Mike,

I will start working on mine again in a month or so.

Last time I worked on it, I temporarily velcro-attached a fan. A standard half thickness muffin fan fits and it sucks air out of the cage nicely. I have not completed the modulator so this will have to wait for a real AM torture test.

Mike WU2D 

[w5rkl]

I want to thank everyone who has contributed suggestions concerning my Apache. They were all very much appreciated.

I have replaced the final amplifiers B+ bypass capacitor. The original cap, a .001ufd 2KV mica, simply
looked poor. Replaced it with a new .001ufd 6KV disc cap. Cleaned the EL-34's screen voltage relay
but that didn't help the fluctuation EL-34 idling current. Might need to replace the darn relay.

I have also considered replacing both EL-34's with Mullard tubes. The EL-34's in there
now consist of one Russian "Electro Harmonics" EL-34 and one Yugoslavian EL-34. The Yugoslavian
tube tests very good but the Russina tube test just inside the "OK" reading, a bit less then
the Yugoslavian tube.

I also cleaned the EL-34's bias pot but that didn't help the fluctuating EL-34's idle current. Might need
to replace the pot with a new one.

I've had it on the air a few times but not enough to test it thoroughly. By the way, the modulator
has had the W3SCC Apache audio mod done to it. If replacing the tubes, pot, and relay don't fix the
problem, I'll have to look at the mod itself to see if there's a bad solder connection, failing cap etc.
If nothing else, it will be fun..

Mike W5RKL