Collins 30L1 problem

[xe1yzy]

My Gurus...

After a nice "pop" noise  inside my 30L1 amplifier the output was gone, looking inside for the smoky parts I found 1 of the 4 ,47 ohms grid resistor roasted.

well I replace the resitor, replace the 4 811A, check the 2 power supplys, the relay, the rf input path, the HV in the plates, all diodes, and everything looks ok, but as soon as I push the PTT in the KWM2, the output goes to zero!.

I dont quite shure about this cathode driving amplifiers systems, but looks I lost the right input  impendance path somewhere, and maybe that is the problem.

What is the right bias voltage for a 811A?, and what is the maximun plate voltage?

any way, if someone are interested in a fancy Lighted dummy load call me  :mrgreen:

thanks in advance

[W2VW]

Do you get idling current when the amp is keyed without any drive?
If so then try it on a different band to see if it works there.

[K1JJ]

Key the amplifier and check for continuity from the SO-239 input through the relay and into the connection to the cathode above the bilfilar choke. There is a coupling capacitor in that path too that should be checked.

An easy way to do it is to key the KWM-2 and close/key the 30L-1 relay(s).  Put a scope probe on the cathode to look for RF drive there.

As Dave said, also look for idling current in the PA metering. That will also tell you a lot.

Normally there should be zero volts on the grids of a grounded grid RF amplifier. The HV should run around 1800V or so, depending upon AC line voltage in and your transformer primary tap configuration.

T

[xe1yzy]

Ok Guys...

Well, the idling current looks ok, I switch to other bands and same happens, Im also check the rf path from the so-239 to the cathode, I even replace the .01 coupling capacitor in the path to the cathodes.

This afternoon Im going to check with the scope the RF drive good idea T thanks, yes  mesure o volts in the grids when I key down the ptt and -140 or so when the amp is in stand-by position.

The plate voltage is arround 1900 Volts.

[WA1GFZ]

Check the output cap between the plate circuit and ouput pi network. It may be open.
Check for a short in the pi network tuning caps.

Put an SWR meter between the exciter and linear to see if there is a swr problem.
Does the 30l1 have rf feedback to the cathode circuit? Do not remember if this was the amplifier. It might be a third winding on the bifilar choke.
check for short to ground on the input circuit.  fc

[w3jn]

Pdero - do you mean the output of the KWM-2 drops to zero?

If so, I'd suspect the ALC circuit.  Try disconnecting the ALC line from the 30L1 to the KWM-2.  Output from the KWM-2 should increase.  If not look at the ALC rectifier in the 30L-1.

The 811As are zero-bias - grid voltage should be 0 at transmit and the high negative voltage you noted cuts them off completely for receive.

You had a parasitic oscillation, which is why the 47 ohm resistor popped.  High voltage at the ALC detector could have fried the germanium diode and/or associated capacitors in this circuit.

Good luck and 73 - John

[WA1GFZ]

I'd put my money on John's idea.
Use a 1N5711 in place of a 1N34 if it is bad.

[xe1yzy]

Pdero - do you mean the output of the KWM-2 drops to zero? Whit the 30L1 off, the kwm2 transmit, if I turn on the amplifier, when I push the PTT, the output of the kwm2 go to zero, no output

If so, I'd suspect the ALC circuit.  Try disconnecting the ALC line from the 30L1 to the KWM-2.  Output from the KWM-2 should increase.  If not look at the ALC rectifier in the 30L-1. Im already do that John, Im disconect the ALC cable, the ALC level in the kwm2 increase
The 811As are zero-bias - grid voltage should be 0 at transmit and the high negative voltage you noted cuts them off completely for receive.
Yes John, they go to " 0"  when push the PTT

You had a parasitic oscillation, which is why the 47 ohm resistor popped.  High voltage at the ALC detector could have fried the germanium diode and/or associated capacitors in this circuit. Ok, Im check the diodes and looks ok, but Im agree, for me looks like an ALC problem,  if I disconect the ALC circuit and test the amp?
Good luck and 73 - John

[xe1yzy]

Check the output cap between the plate circuit and ouput pi network. It may be open.
Check for a short in the pi network tuning caps.

Put an SWR meter between the exciter and linear to see if there is a swr problem.
Does the 30l1 have rf feedback to the cathode circuit? Do not remember if this was the amplifier. It might be a third winding on the bifilar choke.
check for short to ground on the input circuit.  fc

No they not have a rf feedback in the cathode circuit, at least not in the bifiliar choke, but its a good idea put a swr between the exiter and amp.and Im going to check also for shorts to ground in the input circuit

Thanks!

[w3jn]

ALso, Pedro, the 30L1 has a phase detector circuit (a diode in the cathode circuit of the 811s and another diode coming from the output circuit after the loading capacitor) which is what allows the zero-center tuning feature when the meter is in the "RF" (or "TUNE" position?  can't remember which).

If the 30L1 meter shows no output, but a SWR meter or scope on the output shows healthy output, something's bad in this circuit.  It's a couple diodes and a network of capacitors and resistors.  This happened to my 30L1.

Good luck and 73 _ John

[WA1GFZ]

maybe that was what I was thinking about.

Remember this is the result of the oscillation not the cause.

The resistor may have gone bad over time though.
Carbons usually increaes resistance over time making the suppressor
ineffective.

[xe1yzy]

ALso, Pedro, the 30L1 has a phase detector circuit (a diode in the cathode circuit of the 811s and another diode coming from the output circuit after the loading capacitor)

Im getting closer...

Last night I found a 2K 7W resitor across the relay open, the phase detector works, and if i measure the diodes with the multimeter in "diode" position they are ok.

I have high SWR between the exiter and amp when I push the PTT, -160V in grids in standby position, and -2 volts in TX, I have to replace first the 2k 7w resitor to see if I can have " 0 volts" in TX position.

Tnx

[xe1yzy]

I found the 2K ohms 7 watts resitor conected in parallel with the relay open, I only have a 2k 2watts resistor and im installed only for test the circuit.

1.-with the 30L1 in stand by position I have -160V, when I push the PTT -2V in the grids. also the HV reach the plates, 5V in the ALC circuit. and 2000 V in the HV P.S.

2.-Hi SWR when I push PTT, in all bands

3.- I dont see any wiggle in the amperage meter when I PTT the amp. or or loading sound in the p.s. ( off course the 4 tubes light up and Im also check the bifiliar choke), maybe its good idea to replace all the .01 caps in the cathode circuit!

4.-Dont have any short in the output pi tank. or path to ground, same in the rf input circuit. Even if I by pass the input circuit nothing happends

5.- I check the relay, and cables and all are ok, the path from rf input trough .01 coupling cap for the 811's grids are ok.

6.- I can adjust the "Tune" to zero indication in the meter.

I going to replace all the .01 caps in the cathode circuit, I dont have any other clue!

...but its a Collins right?  

Gracias por tu ayuda amigo!

[WA1GFZ]

Are you sure the exciter is working properly on the correct frequency?

[xe1yzy]

Are you sure the exciter is working properly on the correct frequency?

Yes, I even change the exiter! and check cables and wiring

Thanks, I keep on serching!

[w3jn]

Hmmm.... I put a reply up (I thought) but it doesn't show.

Pedro - I downloaded the manual from the CCA website.

I was wrong - the 47 ohm resistors are in the GRID circuit, not the plate parasitic supressors.  You had one of the 811As draw a ton of grid current.  Now then, looking at the signal path and seeing as you have correct bias cutoff voltage, we can discount L3 (grid choke) and C2 (grid choke bypass).

Your strategy of checking the .01s on the cathode is correct.  I'd also check the 220pF capacitors that are on each grid.  ALso, C1, the .01 capacitor between the grids and the input tank could be open.  Last, make sure the filament choke (L8) hasn't arced thru so its shorted to ground or something.

Last, 5 volts in the ALC might be a bit much.  You can adjust this resting ALC voltage by R16.  Best way to do this probably is with an oscilloscope, and give a few atomic HAAAYLOs and make sure the output of the linear isn't flat-topping.

73 John

[WA1GFZ]

Great,
I have not seen the 30L1 schematic in a while but I would check the TR relay to see if it is working. This would Key the bias and switch in the amplifier. Also check the contacts. Sometimes it just takes a little dirt between the contacts to stop things from working. High input swr and no
plate current sounds like a control problem.  fc

[WA1GFZ]

John,
I wonder if there was a grid to plate short or is there a grid to heater short
still in the rig??
One of my 4CX3000As has a random g1 to heater short and it does the same thing. HMMMMMMMMMMMM
Maybe remove 1 tube at a time and see if it comes up???

[w3jn]

Pedro said in a PM to me he put a new quad of 811As in it.  Worth checking the tubes, though.

[WA1GFZ]

Pedro,
Do you see any plate current when you key the linear?
John can tell you what is should be without drive.
If it is zero then look at the relays. I suppose the relay contacts with the resistor across it was for bias. Again lots of grid current as John said.
Grid could have shorted to plate. Check your bad tubs for a grid short for
a clue. Look into the tube to see if There is a floating grid wire. fc

[xe1yzy]

Your strategy of checking the .01s on the cathode is correct.  I'd also check the 220pF capacitors that are on each grid.  ALso, C1, the .01 capacitor between the grids and the input tank could be open.  Last, make sure the filament choke (L8) hasn't arced thru so its shorted to ground or something.

Ok John, Im going to replace all the 5 .01 caps,  and also the 220pf , Im already check the filament choke, they dont have any shorts or leakeage.

Thanks a lot!

[xe1yzy]

Pedro,
Do you see any plate current when you key the linear?
John can tell you what is should be without drive.
If it is zero then look at the relays. I suppose the relay contacts with the resistor across it was for bias. Again lots of grid current as John said.
Grid could have shorted to plate. Check your bad tubs for a grid short for
a clue. Look into the tube to see if There is a floating grid wire. fc

No I dont see any plate current when I key the linear, but the relay are ok, and the bias its ok (-160V)

Im install a new set of 811A, and they are tested with my beloved Hickok 534, tubes are ok

[WA1GFZ]

the bias is stuck in receive mode, It should go low close to 0 when you key the linear. This is why you have no plate current and may be the cause of high input SWR.  
Test the old tubes to see if one is shorted. Find the relay(s) and see if they pull in when you key the linear.

[xe1yzy]

the bias is stuck in receive mode, It should go low close to 0 when you key the linear. This is why you have no plate current and may be the cause of high input SWR.  
Test the old tubes to see if one is shorted. Find the relay(s) and see if they pull in when you key the linear.

Ok, no the bias are -160V in stand by, and -2V, and as John told you, I repalce a four new set.

I keep on searching!

[xe1yzy]

mmm... I am Stuck?

last night I change all the 220pf a .01pf capacitors, check one by one before installation, and nothing happends.

Only rest the bifiliar cathode choke, they dont have any short to ground, and have good continuity, but maybe is leaking the input RF  back to the P.S.

any Idea for test the bifiliar choke?

Gracias!