30L-1 advise needed

[W9WLS]

I have a Vintage Collins 30L-1 amp. I would like to run behind a Icom 7600 transceiver.
The 30L runs fine behind my (also vintage) TS-520 (most likely due to the 520's pi-network tuning) but when run behind the 7600, the 7600 fold's back power out rather sharply.

After putting in a new set of "Taylor 811's and re-tuning the input's on the 30L-1 I can get almost full output on the 30L-1 on all band's except 75 / 80 (presents an almost infinite SWR to the 7600 on 80) , on the other band's, the loading control barely get's off the CCW stop before the power output is reduced.
All input capacitors have been checked and are well within spec. , ALC has been tweaked and checked.

Any good thought's , recommendations, or experience would be appreciated here.

[koØm]

Quick suggestion, try some different lengths of coax in between the radio and the amp, see if the symptoms change.

.

[W9WLS]

Sorry Mel, I forgot to mention that I had tried the 20.5' length that is recommended in the original Collins manual (and later dropped to a 4' length) , no change.
The 30L-1 is a tuned input , not a more commonly found grounded grid amp.
I have tried using the auto tuner in the 7600 , no good there either.
One of the locales here suggested a manual tuner between the 7600 and amp. but that didn't help either.

[N8YX]

Disconnect the thing from AC mains, open it up and remove the tubes. Next, place a resistor equivalent to their in circuit impedance from the wiper side of the input network bandswitch to ground. You want to simulate the loading on the input circuit.

Lastly, terminate the input connector with 50 ohms then place an antenna analyzer or network analyzer across the termination resistor and sweep across various HF amateur band frequencies. Do this for each range covered. What do you see as far as load changes or return SWR?

[NQ6U]

Fred probably assumed you know this but I'm gonna say it anyhow: discharge the power supply caps first!

[W9WLS]

Actually Fred,, I have done this procedure (or something very near) early on using an MFJ-259 and a foot switch, don't remember what I found at the time (has been two week's ago now) but I didn't see anything that looked good and tweaking the input coils made very little change, this is what lead me to the "Live Fire" tuning which got me this far.

Carl: Collins built in some spring loaded mechanical "Auto-Grounding" to the 30L-1's , if you remove almost any of the inner shielding these short the HV to ground (I double check anyway, been hit a few times by other supply's).

It will be several day's before I can back track on this now but may try Fred's approach again just to see if I missed something.

Tnx , John

[W9WLS]

It's ALIVE !
After a bit of research, A friend put me on to Glen K9STH who is a technical adviser on the CCA net, traded several e-mail's with Glen on this and after poking around a bit (with the help of a grandson doing the lifting) found 2 very funky disk capacitors in the input circuit.
After digging around the junk box (the whole garage) we found suitable replacement's and got them installed , returned from 75 down and everything is coming into play pretty well (15 meters still a bit strange but workable) .
Would still like to find a replacement slug for the 10 meter coil but am looking into that .

Thanks to all for your thought's and advise !

Happy New Year , John