[Collins] 32 s 1 self oscilation

[Dr. Gerald N. Johnson]

Clearly the voltages in charts and schematic were never proofread by a 
ham or engineer familiar with the circuits. The chart shows different 
voltages on the plates of the first mixer though they are connected by a 
coil, surely it has very little resistance. Then in the 
voltage/resistance chart one PA tube has -5.8 volts bias and the other 
has -58 volts bias. Obviously a typo but NOBODY looked. That's in a 7th 
edition 1960 dated manual that came with transmitter sn 2768. But its 
not changed in the 10 edition manual badged Rockwell-Collins with date 
1961 where the schematic says updated 10-27-61.

I know the tech writer who was updating the 1964 version of the 75S-3B/C 
manual wasn't smart enough to question such things. He thought he was 
and he described a circuit from his circuit analysis for the 821A-1 that 
was far afield from how it really worked, but he didn't know better. I 
rewrote 3 pages about that circuit when I proofed parts of that manual. 
I did rewrite the text for the S-3B/C manual but I didn't do anything 
with the schematic, parts list, or voltage chart. That probably should 
have been done by the support engineer but he was very part time on 
S-line support and working on other projects to justify his paycheck and 
didn't own or operate Collins ham gear.

With a rig open on the bench and scope or 410B in hand, it shouldn't be 
hard to come up with a new set of RF and audio voltages. My bench is way 
behind on projects this year, having a retina come loose has killed 
almost all bench progress for the last couple months. So far when 
working on a transmitter, I've been happy to measure PA grid current and 
output power and to see the drive run the plate current up high enough, 
I've not tried or desired to check individual stage gains.

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

On 4/24/2012 5:17 PM, Bob and Sue Jefferis wrote:
> Jerry, all,
>
> I believe the maintenance of voltage and resistance charts WAS consistent, at least through the S-line era. It was poor. I have found a number of anomalies in 32S-3 charts. For example, the plate voltages shown for 32S-3 V4, V5, and V6 simply don't make sense. The Voltage chart calls for 250 VDC. Measured voltages are in the range of 215 to 220 VDC (depending on power supply variation). If you look at the operating conditions of these tubes and bounce that against the voltage drop that should be seen across the parallel combination of R29 and R30, you should, and indeed, do see voltage drops in the range of 50 to 60 VDC, NOT 25 VDC.
>
> I have the same confidence level in the AC (RF) voltages shown in charts, A number of years ago I measured RF voltage levels from the 1st mixer grids up to the PA tube grids in TUNE and LOCK KEY modes with a scope and a 410B. All I have to do is find those old notes...
>
> 73, Bob, KF6BC