[Hammarlund] Hammarlund HX-500 Problems...

[Jack Harper]

Hello to the Hammarlund List...

I have been a member of the BoatAnchors List for 
the last 17-years (almost to the day - 25 May 
1996 - Tempus Fugit!) and posted on that List my 
trials and tribulations of beginning to bring a 
Hammarlund HX-500 transmitter back to pristine (hopefully) life.

A BA List member kindly listed that post on this 
Hammarlund List - which I just joined yesterday. 
After reading through some of the archives, I am 
delighted to be here amongst fellow Hammarlund Aficionados.


I posted that I was seeing several tube pin 
resistance measurements that are different from 
the published Hammarlund specs - six, in fact, 
out of, what, 175 pins or so, which is not bad at all...

I resolved two of those - caused by Hammarlund 
recommended modifications that were added to my 
transmitter. However, #3 did not make sense and I 
was convinced that the measured value is actually 
correct according to the HX-500 schematic and the 
actual wiring. A couple of Hammarlund List 
members agreed that the published number is wrong 
(the consensus is that Pin-8 of V6 6U8 should 
actually be about 2.3K rather than the published 350-ohms).


I also mentioned that the transmitter blows its 
6A fuse after a minute or two operation in 
STANDBY mode. Several people suggested that I 
check the power transformer, which I did last 
night by removing all the tubes and then 
connecting a 100-watt light bulb in series in the 
main power cord. The pilot lamps came on so I 
know that the transmitter was getting power -- 
and the 100-watt bulb had no glow at all, even 
with the Shack lights off - so, Good News, at 
least the power transformer appears to not be 
shorted etc (the light bulb, by the way, is good 
- I measured 10-ohms across that, which I think 
sounds about right).  This was the first time 
that I have applied power to the transmitter 
since I first got it and realized that it was blowing fuses several months ago.


The power supply is putting out the following measured (DVM) voltages:

Should Be           Actual Readings
---------------           --------------------------
-90 VDC                         -70 VDC (!?)
+270                             +383 (!?)
+780                             +760
+230                             +350 (!?)
+150                             +153

The HV is good, but the lower voltages are 
totally out of whack, which explains, perhaps, 
why the 6A fuse pops after about a minute in 
STANDBY -- +383 rather than +270 ?  These 
measurements were taken in STANDBY mode...


Someone, before me, hacked the power supply by 
adding the Hammarlund power supply modification 
with ugly orange wire, blobby solder joints, 
higher value than specified resistors in parallel 
- that sort of thing. I ripped all that out and 
replaced all of the electrolytics with new Mouser 
units, better wiring, with better (I hope - at 
least I twisted the wires together :) solder 
joints etc. There was also a resistor in the 
supply with a totally wrong value, which I replaced.

Anyway, after re-wiring the power supply with new 
components, the voltage measurements remained about the same as above.

Before I did the rewiring, I carefully traced out 
the power supply circuit that was in the 
transmitter when I got it and it met the power 
supply mods (other than a wrong resistor) that Hammarlund published.

The original power supply, when the HX-500 first 
came out, used a pair of 12AX4 rectifiers - which 
were changed to a single 5R4 and a solid state 
diode for the bias rectifier circuit to provide 
instant bias for the pair of 6146s. This also 
involved a different power transformer - I 
checked the stamped part number on the replaced transformer and it is correct.

I replaced all of the power supply electrolytics 
with new and left the original cans in place. I 
also checked all of the resistance measurements 
and they were good (other than the wrong one). I 
added a new terminal strip for the new axial 
electrolytics, which are all, of course, under 
the chassis. I did double the value of C164 and 
C165 to 80uuf (inductive input filter on the +780VDC HV).

Anyway, the fundamental problem with the supply 
is that it is putting out weird voltages. I have 
not yet checked the AC output voltages of the 
power transformer - though, surely those are not wrong...


Now that I have all the tubes out, I will try to 
check those on my tester either tonight or 
tomorrow night (I work 80-100 hrs/week as part of 
a tech startup, which chews up a lot of my time - 
I usually only get to work on Fun Things on 
Sunday afternoon - but, I got a bit less sleep 
last night :).  Then, I will see if I can resolve 
the other three strange tube socket pin 
resistance values - and then get to work on the 
weird power supply output voltages.


Question for the List:  Any thoughts as to why 
low voltage DC outputs of the power supply might 
be so cock-eyed:  +383 instead of +270; +350 instead of +230...?


I also notice that the Hammarlund power supply 
modification changed the supply output voltages as follows:

Original           Modification
-----------           -----------------

-100                     -90
-50                       -45
+350                    +270
+780                    +780
+300                    +230
+215                    +200
+150                    +150 (reg)

I find all that odd...


Even though I have been a ham since 1967, I am 
not really all that strong on diagnosing and 
correcting problems - Any advice and counsel 
greatly appreciated.  My hope with the HX-500 is 
to pair it, in great operating condition, with 
the R390A that I have as my primary operating 
station - along with the Hallicrafters TO Keyer :)

Regards to the List,

Jack, WØYJ
Evergreen, Colorado USA

ps - It is madly snowing like crazy here at 8,000-feet elevation  :)