[Hammarlund] TX-500 problem...

[Jack Harper]

Greetings to the List -

I appreciate the several people that offered 
advice on the Hammarlund HX-500 transmitter that I am working on.

Next step is to carefully check the power 
transformer and the power supply filter chokes to 
see what's what. As I mentioned a few days ago, I 
did check the transformer with the classic 
100-watt light bulb in series with the mains 
technique with no glow, but several List Members 
pointed out that the method is not all that sensitive.

So, next step is to unsolder all of the 
transformer leads; check resistance to ground for 
each; and then power up the primary and check the 
output AC voltages on each lead.

I was hoping to have been able to do that over 
the weekend but, alas, I had to work a big chunk of Sat/Sun.


However, I did manage to carefully check the 
tubes (all 21 of them:) late last night - 
transconductance, shorts, gassy, filament, and 
all the rest - took three hours - and found seven tubes that need replacing:

One tube (V6) has an intermittent triode section 
- this 6CM7 tube is a dual triode and one triode 
section comes and goes as you thump the side of 
the tube - glad that I found that - would be hard 
to find back in the Transmitter. V6 is the 3rd 
Audio Modulator and, also, the VOX Amp.

Another pentode (V11 - 6CB6) is totally dead with 
no filament at all - that tube is a bandpass 
amplifier, which, undoubtedly, explains at least 
part of why the Transmitter has zero power output 
- nothing is getting to the final amplifiers.

Four tubes (V9, V10, V16/V17) are simply weak - 
including the two power amps - 6146 tubes (63% and 85% gm).

In addition, the 5R4GYB high voltage rectifier 
(V18) is now totally dead, which is interesting 
as it worked before I removed the tube from the 
Transmitter. I turned the tube upside-down, 
tapped it, and the large filament literally fell 
out of the vacuum tube body and rattled around 
inside the evacuated bulb like a dead 
incandescent light bulb :)  Obviously, simply a 
mechanically weak filament on the edge - I am glad that it failed.

Clearly, the Transmitter went through Hell during 
the UPS Ground shipping back last August - 
rattled around on a truck for a thousand miles - 
I am not too surprised to lose a couple of filaments...


These are, of course, lethal problems - but not 
all of the problems. I will continue to work on 
the Beast until I get all of the static 
resistance measurements on the tube socket pins 
correct - and then the DC voltages on the pins also correct.

However, I am glad to get the tube checking 
completed - I crossed that off the almost 
one-page list of things to do - and I am glad to 
almost certainly know why the Transmitter has no 
output power at all, when clearly it did for the 
seller on his YouTube video - which is still up 
at http://www.youtube.com/watch?v=ybJ1LsCU0xg


I did not get to check the power transformer and 
choke inductors - hopefully, tonight if I get home in time.


Question for the List:  On the YouTube video 
above of the HX-500 posted by the seller, the 
transmitter has bad keying (to my ear) - I think 
that is a classic chirp on the leading edge. Any 
thoughts as to what might cause that, which I 
clearly need to get rid of (CW is all that I do).

There *is* a Hammarlund recommended modification 
(#10) that replaces R132 (originally 220-ohm in 
the power supply) with a new 15-ohm resister ( a 
List Member explained to me why the mod should 
work). The Transmitter had a 500-ohm (totally out 
of spec) resister in for R132 when I received it 
- and as filmed in the YouTube.  I have replaced 
R132 (500-ohm) with a new 15-ohm resister and, 
hopefully, that will cure the chirp. However, any thoughts on the matter?

The Hammarlund modification says, "This chirp 
condition can be cured by improving the 
regulation of the power supply (low voltage 
sections) in accordance with the following 
instructions. (1) R132 currently a 220-ohm 
10-watt resistor should be replaced with a 15-ohm 
2-watt resistor. R137 currently 1500-ohms 10-watt 
should be replaced with a 300-ohm 10-watt 
resistor. If some chirp still persists, try 
replacement of the 6CB6 tubes in sockets V10 and V11."

The Transmitter, when I received it, had R137 
(also in the power supply) at 1500-ohms and I 
replaced that with a 750-ohm resistor (as per the 
power supply modification) - thus, mods on top of 
mods :)   I have NOT yet completed the second mod 
on top of the first mod -  R137 is still 
750-ohms, but then I have not yet been able to test, yet, for the chirp.

In addition, I point out that I measured V10 
yesterday as weak - that is one of the tubes to 
"replace" according to the Chirp Mod.  V11 is now 
totally dead and so I don't know its operating 
condition (weak?).  Perhaps, new V10/V11 and R137 
to 300-ohms will kill the chirp. Thoughts?


Again, I appreciate all of the healthy advice -

Regards to the List,

Jack, WØYJ
Evergreen , Colorado USA