[Hammarlund] More questions HX-50

[Kenneth G. Gordon]

On 8 Jun 2014 at 19:44, Robert Sauvan wrote:

> 
> Hi all,
> 
> I have been digging into the cluster of weird wiring in my HX-50 that I 
> have been inquiring about. What I have found is the original owner has 
> added 470K 1/2 watt resistors in parallel with the diodes at the power 
> supply. C104, C105, C109, and C110. What would be the purpose of adding 
> those? Also, during his rewiring, I noticed 2 types of diodes. 1 type 
> looks like a little torpedo, the other type looks like a very very small 
> resistor but is all black and no markings. Im guessing 2 of them are the 
> original diodes and 2 are replacements. Anyone possibly thinking the 
> same? Also, the schematic lists these diodes as "Diode, Silicon". If I 
> were to replace all 4 of the diodes, what would I use? There does not 
> seem to be any type of rating or spec of any kind labeled in the 
> schematic. Let me say, the radio is working fine, this modification just 
> bugs the heck out of me cuz it is a butcher job. Do I dare take these 
> resistors out of circuit.

OK. Here's the story: at the time that HX-50 was made, silicon diodes were 
not being able to be manufactured to very close specs, as they are now.

It was very common practice, and actually necessary, to parallel each diode 
in a string of them with at least 470K ohm resistors in order to equalize the 
voltages across them. If you DIDN'T do this, you would find some diodes 
would have voltage across them that was in excess of the voltage they were 
capable of withstanding.

This was due to the VERY variable internal resistances of those diodes.

However, at least within the last 20 years, diodes are made to such close 
specifications that those equalizing resistors are no longer needed.

If that HX-50 (drifty fifty) was mine, I would simply replace those diodes with 
modern 1KV or 2KV at 1 amp diodes and completely dispense with the 
resistors.

They would no longer be needed.

Ken W7EKB