[Hammarlund] Hammarlund HX-500 Transmitter...

[Kenneth G. Gordon]

On 16 Sep 2013 at 14:24, Jack Harper wrote:

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

Good.

> 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.

Again, good. I would have done the same thing.

> Question:  Am I missing something here??

I doubt it. Such idiocy is common in many otherwise excellent rigs. If I were 
you, I would do what you have done and not worry about it.

> 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.

Before I (at least) could advise you on this issue, I would want to know what 
each of those voltages "does". 

However, the difference between 90 and 100, 45 and 50, 270 and 300, is 
10% or less. That between 215 and 230 is much less than 10%. I suspect 
the difference between any of those can be safely ignored, as can the +780 
VDC.  

But the difference between 270 and 350 is almost 30%. I would be very 
interested in what that one "does", and that one is the only one I would even 
consider worrying about. The rest I would completely ignore. The differences 
are of absolutely no consequence in a tube-type rig.

> 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.

But WHICH resistors, and what do they "do"?

> 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.

Actually, much Hammarlund documentation is problematic.
 
> 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.

Then fix those...once you understand why the voltages are different and 
what they "do". 

> 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 don't think that is a complete solution. I suspect that some of those higher 
voltages are a simple result of using a solid-state rectifier instead of a 
tube-type. In the particular circuit in which those are used, it may not make 
any difference. The first thing I would be concerned about would be voltage 
rating of filter caps. Perhaps the "new" power transformer has a big effect on 
those voltages too.

> 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.

I dunno. What do those specific voltages "do"?

> (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?).

Rule-de-thumbo: "100 ohms of bleeder per volt". So, at least 30 K in this 
case. 40 K is probably an excellent choice. I'd use a 10 watter. Hammarlund 
probably simply left it off the schematic. 

I suspect you are worrying a bit too much about the differences in voltage: all 
but one are within tolerances of other components, and even that one may 
still be within the tolerances of the other components in that particular circuit.

Personally, I don't see much of a problem with those voltages.

Kenneth G. Gordon W7EKB

"Courage is being scared to death but saddling up anyway."--- John   Wayne