EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

I built this cabinet in 1979 when I first acquired the components.

Comment by Ken Carr on September 11, 2013 at 1:43pm

Nice way to use a chassis-only radio. I'll bet it sounds great!

Comment by Tom Jardine on September 11, 2013 at 1:57pm

Thanks, Ken. It sounds pretty good after spending a lot of time going over all the circuits. It's a little bass heavy on FM, the loudness compensation cannot be switch out, but I can tame it with the bass control. I listen to it often, especially on short wave at night. The three tube magnetic preamp that is part of it, is not as shown on the schematics. It has two inputs, one for 33-45 rpm, and the other for a 78 player.

Comment by Timothy Murphy on November 21, 2013 at 9:33pm

Tom, I have one of these with a misbehaving auto-tuner. The motor wants to run but does not. Manual tuning turns freely. I am going to dive into it next year for a full recap, but I'd like to study the mechanicals first. I cannot find any really useful guide to the Collins auto-tuner adjustment and repair. I have the blow-up engineering drawing. It looks identical to a GM TH700R automatic trans to me (eg., incomprehensible). Can you suggest some materials for me to read? Thanks

  

Comment by Tom Jardine on November 21, 2013 at 10:15pm

Timothy, when you say it wants to run, do you mean you can hear it humming or trying to go? The motor has a thermal switch that opens the motor when overheated. My motor would run, but cut out almost immediately upon starting. The thermal switch was opening for no reason. I bypassed the thermal switch (after taking the motor apart) and it works fine. I had a dog of a time removing the motor though. I'll post a picture of the motor disassembled.

Comment by Timothy Murphy on November 22, 2013 at 8:41pm

Hi Tom. I do not have the tuner on the bench right now so I'm working from memory. I would press a button at the far end of the keyboard and the motor would run the pointer all the way to the end of the dial and not stop. Then I'd have to turn the tuner off. Or, I might press a key in the middle and I could see the large gear nudge, but it would not turn.

     Thank you for the thermal switch suggestion. Would the switch make a noise when it operated. Does it self-reset? How long is a cycle?

     I understand what you say about removing the Collins assembly for bench work. The entire chassis has a level of interlocking mechanical design similar to a watch. It is not for Alzheimer's patients. I will be using my camera a lot.

Comment by Timothy Murphy on November 22, 2013 at 8:46pm

I just saw the photo. Thank you. I notice the bronze bearing and woven wire cover. This is nearly aircraft quality. Nice.

Comment by Tom Jardine on November 22, 2013 at 9:55pm

The thermal switch should open the circuit if the motor overheats and close the circuit again when cooled. It doesn't sound like you have that problem. On my radio, if I press the far right button (manual tuning) when I have previously used the automatic tuner function, the motor will drive the pointer all the way to the right and then back to the left and stop. You need to be pressing the manual button for the entire process until the motor stops. It is a very complicated little mechanism. Fortunately my Berkshire didn't have any problems with that except for the motor issue.

Comment by Norman S Braithwaite on November 22, 2013 at 10:44pm

The push buttons on the Berkshire must be held down until the station has been reached.  For the manual function, the push button must be held down until the Autotune has cleared.  The Collins Autotune was designed for aircraft use and gave the allied forces a great advantage during WWII (quick, accurate transmitter tune for preset frequencies rather than tuning up several circuits before transmitting - communication could be completed before the enemy could locate the new frequency).  The RCA Berkshire is the only civilian radio ever built using the Collins Autotune.  I doubt that many changes were made to the Autotune for the small production required for fulfilling the RCA Berkshire requirement.

Norman

Comment by Tom Jardine on November 23, 2013 at 10:37am

Timothy, is the motor engaged all the time now or trying to run continuously? Underneath the main tuning knob cap, see what appears to be a screw head. That must be tightened all the time except to mechanically reprogram the stations selected by the autotune mechanism. If that is loose while the motor is running for any reason, the autotune mechanism is thrown out of whack. 

Comment by Timothy Murphy on November 23, 2013 at 3:32pm

Gentlemen:

   That makes 3 very good suggestions. I did not know about the thermal limit switch but it is probably not my problem. I did not know that the keys needed to be held in place and the literature does not mention this fact anywhere. The lack of a latching relay mechanism might have been a clue to me had I thought it through. The tuning knob set screw is in the literature, I am aware of it and I will reread that.

   Speaking again of literature, have you got a suggestion Tom, where to find printed info on the Collins in this set? I have been chasing this for about a year Norman, as you know. The military tech manual was for a very different Collins unit. 

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