[Hammarlund] Hammarlund HX-500 Transmitter...

[Jack Harper]

Greetings to the List:

As I mentioned before, I am slowly bringing a 
Hammarlund HX-500 transmitter back to life.

My goal is for the HX-500 coupled with my R390A 
and the TO-Keyer to be the primary rig for the 
rest of my not so much remaining life - so, I am 
being careful and perhaps a bit overly fanatical 
about "getting it right" - and hope to also learn 
a thing or two along the way...


Questions:

(a) I am astonished to see that Hammarlund runs 
the +780VDC line from the power supply (5R4) up 
through the chassis to the 6146 pair simply 
through a hole in the chassis steel. The hole was 
drilled just slightly larger than the plain red 
hookup wire and so literally the only thing that 
keeps the +780VDC from the chassis is the old 
insulation on that wire - and that insulation is 
now wearing through (fan vibration?).

The interesting thing is that under the chassis, 
a very nice porcelain standoff with a lug 
supports the connection to the power supply 
filter choke. But, then the wire for the filtered 
HV just zooms up through that drilled hole with 
just a few thousandths of clearance around the insulation.

I have a spare HX-500 parts unit and checked that 
thinking the one that I am working on had been 
totally hacked - but, it is the same with that 
red wire just going through a hole in the steel.

I suppose that I am not a very brave soul and so 
just bought a used porcelain feed-through on eBay 
and plan to install that by drilling a 3/8" hole 
through the chassis for the feed-through with 
proper lugs top and bottom etc with the top inside the RF cage topside.

Question:  Am I missing something here??  Why 
would Hammarlund do that sort of thing on a HV 
line?  Just to save a buck at the orders of a 
"cost accountant"? Surely not... This certainly 
reinforces, by the way, the practice of 
automatically installing a modern 3-way power 
plug on any boatanchor. One never knows what evil might lurk under the hood...


(b) Hammarlund originally had a 6AL5 rectifier to 
generate the "bias voltages" (6146 et. al.) but 
came out with a Service Bulletin in about 1962 
that called to replace that tube with a solid 
state rectifier "to provide instant bias for the 
6146 tubes to prevent possible excessive cathode 
current at power-up" (waiting for the tube to 
warm). The same mod also called to replace the 
original pair of 12AX4 rectifiers (for HV) with a 
single 5R4, which required also a different power transformer.

My HX-500 had that modification made, which 
essentially meant rebuilding the power supply, 
which was done.  The correct power transformer 
(by specified stenciled part number) was also installed to replace the old.

The thing that has been throwing me is the fact 
that the original unmodified schematic had one 
set of power supply output voltages (there are 
seven:  -100, -50, +350, +780, +300, +215, and 
+150), but the Service Bulletin had a very 
different set of power supply output voltages 
(-90, -45, +270, +780, +230, +150). That does not 
make sense to me - nothing else in the 
transmitter was modified by the Service Bulletin.

To make matters worse, someone manually penciled 
in different power supply resistor values 
(dropping resistors) on the Service Bulletin 
schematic (the "new" schematic) - e.g., 750-ohms 
instead of the Hammarlund 15-ohms etc.

So, there are three sets of possible 
voltage/resistor values - two formal Hammarlund 
and one informal penciled in every copy of the 
Service Bulletin that I have ever seen.

The HX-500 that I have had the Service Bulletin 
mods made and the dropping resistor values are 
the ones penciled in (that penciled scanned 
schematic, by the way, appears to be the only one 
available anywhere on the Internet).

Problem is that the voltages coming out of my 
doubly modified power supply are wildly out of 
spec (which spec?) - e.g., +360VDC instead of +270VDC etc.

One thing that I have learned on this project is 
that I cannot trust the HX-500 documentation - 
especially, apparently, as the company began to come to an end.

I begin to believe that the Hammarlund published 
output voltages in the Service Bulletin are WRONG 
- why would Hammarlund change those output 
voltages when nothing else in the transmitter 
changed? In addition, whoever did the power 
supply mod in my HX-500 fiddled the dropping 
resistor values to meet the penciled in values - and gummed the thing up.

So, my theory is that I should get resistor 
values in the power supply that generate the 
voltages on the original Hammarlund schematic - 
and ignore the WRONG voltages specified in the 
Service Bulletin.  I have done that, though they 
are different from what Hammarlund wanted: e.g., 
68-ohms rather than the published 15-ohms - that sort of thing.

Question:  Does it make sense for me to get the 
output voltages specified in the original 
unmodified schematic? I believe the Service Bulletin numbers are bad.


(c) I notice that Hammarlund also left off a 
bleeder resistor on the +300VDC section of the 
power supply. I have checked the original 
schematic and do not see a path for that to bleed 
down at power down (i.e., maybe a resistor 
somewhere outside the power supply to ground). I 
would like to insert a bleeder (40K?). Does that 
make sense???  Why would Hammarlund leave off 
that bleeder? Again, an evil "cost accountant"...?


I appreciate any input -


Regards to the List -

Jack, WØYJ ("Friend to all Things Hammarlund")
Evergreen, Colorado USA

ps - By the way, you can see this transmitter 
running at 
http://www.youtube.com/watch?v=ybJ1LsCU0xg  (as published by the seller).