[Collins] 32S-3 problem

[Al Parker]

Thanks, Jerry,
	Thanks for the detailed explanation.  I've found something I can put in 
for a test, but will hold out for something more durable for a permanent 
fix.
73,
Al, W8UT


On 12/12/2014 11:43 AM, Dr. Gerald N. Johnson wrote:
> 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