My Collins 30L-1

© Copyright Alexander D. Schapira, 2007-2014, All rights reserved

With thanks to all who provided suggestions,
 I am providing this account of how I got my newly acquired 30L-1 on the air.
I hope this will be of some interest to other 30L-1 owners.

I bought this rig from from a fellow who posted it For Sale on the Collins reflector.  I picked it up at his place where he fired it up into a dummy load to check it out.  It seemed to deliver rated output, so I took it home despite some obvious cosmetic defects.


There were some discolored patches near the bottom left of the front panel.

Lower left Front Panel
The plastic trim ring was discolored.

Even though it had been powered up recently, I wanted to give it a good cleaning and inspection before putting it in service.  It needed it! I removed the four RCA 811A's, marked each one with which socket it was in, and put them in a safe place before proceeding further.

The line cord had obviously been previously replaced (with an inappropriate orange flat extension cord) .

The "Original" Line Cord
Note the "pre-existing" burned wires.

I replaced the line cord with 7 feet of black #14 3-wire cord.  It came as an unpleasant surprise that the AC plug did not fit through the hole on the rear panel. The plug had to be wired up after passing through the cabinet holes.

The new line cord connections

The fan before cleaning

The Relay

After the surface cleaning, the switches and pots got some DeOxit, the fan got some oil, and the relay contacts were treated with brown paper saturated with DeOxit. Finally, I cleaned the high voltage standoffs and plate caps.

I then began a close visual inspection which turned up something very unusual -- one of the filament wires was never properly soldered!!
Original Filament Wiring -- It Looked Okay!

Although the socket filament pin had the red glyptol marking, the wire to it was loose and pulled out cleanly.

Original Filament Wiring Wasn't Okay!

I reflowed the solder joint. I wonder if the rig always ran with only three 811A's or if the loose contact was sufficient to power its filament.

Not seeing anything else obviously wrong, I wrapped up the 30L-1, brought it up to the shack,
and connected it to my KWM-2, and Drake MN-2000 tuner.

Since the fellow I bought it from demo'd it into a dummy load, I felt okay powering it up with the KWM-2 and tuner.  It *seemed* to deliver about 600 watts into a dummy load okay (according to the meter on the Drake), but I observed three disturbing symptoms:
  1. When the KWM-2 is RECEIVING and I merely switched the 30L-1 AC power ON, the received signals were attenuated considerably.
  2. As above, i.e. RECEIVING and I merely switched the 30L-1 AC power ON, the wattmeter in the Drake MN-2000 (following the 30L-1, of course) showed about 5 watts output forward power!!!
  3. As above, i.e. RECEIVING and I merely switched the 30L-1 AC power ON, the TUNE meter deflected half scale negative.
A 30L-1 expert I met on the CCA Sunday net suggested that problem (1) is well known to be caused by a grid-to-filament short in one or more of the 811A's. I found this surprising, to say the least, since if the relay is is the inactive (unkeyed) state, the received signal should pass through the 30L-1 un-attenuated.

My expert also suggested that I ought to re-cap both the HV supply and bias supply since the caps are over 40 years old.  This I will do, and while I'm in there, I'll also replace the diodes with 1N4007's as he suggested.  He also suggested that the bias supply should deliver about 170 VDC (not 120 VDC shown on the schematic) and that the 10uF bias filter cap was under-spec'd at 150 V.  He also mentioned a number of other R's to check for tolerance.  Only after the power supplies are rebuilt can the real cause of the above problems (if they still exist) be determined.

It seemed to me the EXCESSIVE POWER SUPPLY RIPPLE (bad filter caps) could explain everything.  Say there was excessive 60Hz or 120Hz ripple on the 811A plates, and also modulating the bias supply.  The 811A's might not be cut off completely in RECEIVE mode causing them to deliver some output, problem (2).  This gets into the receiver causing the AGC to lower the gain causing the received signal to be attenuated, problem (1).  The TUNE meter circuit reacts to the spurious signal and deflects, problem (3). [I also considered that the 811A's might be oscillating, but I couldn't detect any spurious signal.]

Back down on the workbench,  I pulled the diode board and the capacitor board, both with original components.
Original Diode Board

30L-1     30L-1
Original Capacitor Board

Although the diodes tested okay, I replaced all sixteen of them (CR1 - CR16) with 1N4007's.
The resistors on the diode board were all well within spec.

Rebuilt Diode Board

I replaced the filter caps (C3, 5, 7, 9, 8, 12) with new 100uF/450V ones.

30L-1     30L-1
Rebuilt Capacitor Board

Turning to the bias supply,
Original Bias Supply

I found that R15, the 10K load resistor, read almost 19K.  It got replaced, as did C10, the 10uF/250V cap, with a 20uF/350V cap, and CR20, the diode, with another 1N4007.

Rebuilt Bias Supply

The two ten ohm, 2 watt resistors in parallel, R17, R18, from the HV power supply to the plate circuit were out of spec, so I replaced them also.

Finally, R28, the 39 ohm 1/2 watt resistor in the Antenna Relay line, on standoffs, was way out of spec, so I replaced it with a 39 ohm 1 watt resistor.

Original R28

Replaced R28

These repairs solved all three problems!
The 30L-1 is on the air.

As to cosmetics, I am not that finicky, but I did improve things considerably.  I cleaned the trim ring with AJAX souring powder, which completely removed the discoloration and left a near original finish. I carefully scraped off the hardened goo on the front panel, but some of the paint on the high points of the crinkle finish came off with the goo.  Perhaps I will try to retouch it.


    -Al Schapira, W2ADS

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Page last modified 04/17/2014