[Collins] 32S-3 problem

[Dr. Gerald N. Johnson]

In the 32S3 the cathode bias resistors for the relay tube are two 68K 2 
watt carbon composition resistors from B+ (R89 and R112 in the 32S-1) in 
parallel. They are known to drift high in value with age and those are 
worked hard so it doesn't take so much age and they can have burned 
open. They put a positive bias on the cathode effectively making the 
grid bias negative to turn the tube off. It won't turn off with the grid 
the same voltage as the cathode. I see in the 32S3A military style 
manual that those two have been replaced with a single 34K 5 watt 
resistor, R140. I would find a tin oxide metal film power resistor, 
known these days as flame proof with a 33K 5 or 8 watt rating and 
install it in place of R89 and R112. I introduced that type of resistor 
to Collin high power transmitters in 1964. Phase discriminators used a 
pair of 2 watt carbon composition resistors and tended to drift with 
age. I tested a 3 watt metal film resistor at ten times rate power, it 
glowed red and the leads were hot enough to cut paper, but when cooled 
back to room temperature the resistance change was only in the 5th digit 
on the digital resistance bridge. The resistors survive fires, but when 
overheated that much they can ignite their surroundings.

It also won't hurt to check the resistor from cathode to ground though 
usually carbon composition resistors go up in value from heat and age, 
there might have been an IRC slipped in that goes down in value. 
Resistance checks from cathode to ground won't detect the 68K resistors 
going open, much less going higher in value.

73, Jerry, K0CQ, Technical Adviser to the Colins Radio Association.

On 12/12/2014 8:15 AM, Al Parker wrote:
> Hi folks,
> I'm working at a 32S-3 for a friend. Initial problem is that it "keys
> up" about 3-5 seconds after power up. The finals go into idle, but no
> further. I think I need to solve the key-up problem first.
> I've checked voltages and resistances in the vox/relay control area,
> can't find any problems. Checked tubes, even substituted for the relay
> actuator. Unplugging any tubes other than the relay actuator (6U8) tube
> will not un-key it. Grounding it's grid doesn't help. When switched to
> "tune" the finals' plate current goes to zero, but the relay doesn't
> drop out. (the PTT switch is off)
> It's been a while since I've worked on a 32S-x, so I keep wondering what
> I'm missing. Any thoughts would be greatly appreciated.
> thanks, 73,
>
> Al, W8UT
> www.boatanchors.org
> www.hammarlund.info
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