[Collins] 32S-3 problem
Dr. Gerald N. Johnson
geraldj at netins.net
Fri Dec 12 12:28:32 EST 2014
Mouser has 68K 3 watt and 5 watt metal film and wire wound that will fit.
www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0wszxZ1z0wui7&Keyword=68k+3+watt&FS=True
Mouser also has 33K 5 watt metal film and wire wound that will be
slightly more compact, but less original, though like the 32S-3A.
www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0wszxZ1z0wui7&Keyword=33k+3+watt&FS=True
Metal film or wire wound and with a greater power rating will solve that
relay tube problem virtually permanently. The originals are bound to
fail, faster with modern line voltage raising the low B+ voltage. Those
two carbon composition resistors were not the best choice for that circuit.
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
On 12/12/2014 11:00 AM, Al Parker wrote:
> 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
>
>
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