[Collins] problem with Collins 30L1

[Dr. Gerald N. Johnson]

On Mon, 2006-07-10 at 08:12 -0500, fkamp at comcast.net wrote:
> I am trying to interface a 30L1 to an ICOM 737.
> 
> I got the -170 volt antenna relay line taken care of with a 
> relay.  No connection to the ALC.
> 
> With 70 watts RF input to the amp, I get no output.  Well, 
> maybe just the barest of meter needle movement of the watt 
> meter on the 0-200 watt scale.  The barest of movement does 
>   have a barest of peaks when the plate tank capacitor is 
> tuned to frequency.  No indication of plate current and the 
> 811As all have -30vdc on the grids.  (I was expecting the 
> grid voltage to be zero on transmit but I don't know what it 
> should be on a working amp.)

Grid voltage should be virtually zero. There's the relay current and
that 2 K shunt resistor (probably 1K across 120 volts DC for a current
of 120 milliamps through 39 ohms. Maybe 4.7 volts negative. 30 volts
negative will keep the 811s cut off.

So either the 39 ohm resistor (R28) has gone up in value or your relay
circuit has too much resistance when there's 120 milliamps flowing.
> 
> I have checked the following:
> 
> HV at 1800vdc as indicated on the panel meter
> 
> plate rf choke good
> 
> All fuses good including the 1 inch #20 wire from the
> filament ct to ground.
> 
> Bias supply running at -170vdc
> 
> Service bulletins 1,2,and 3 mods installed.
> 
> Service bulletin 3 puts a 2k 7w resistor across the antenna 
> relay coil, adds a 39 ohm in series with the relay line 
> (which is switched to ground) and changes R9 in the bias 
> supply from 82ohm to 47ohm.
> 
> All resistors have been checked for value.
> 
> All capacitors have been tested for leakage and shorts.
> 
> All diodes have been tested and none are shorted or open.
> 
> Currently running a brand new matched set of 811s purchased 
> from Ameritron.  The amp did the same thing on the old set 
> of RCA 811As.
> 
> I am hoping that this is a common problem on these amps. 
> Something easy to fix.  The only fix I can think of is to 
> wire the grid bias circuit so that grids are at -170 during 
> receive and at zero volts during transmit, but that would 
> mean changing the circuit from what shows on the official 
> schematic.

The original circuit should accomplish that unless. My guess is that the
update was not wired correctly.
> 
> Any help will be appreciated,
> Frank Kamp
> K5DKZ

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer