[ARC5] Suggestions Needed

Sat Feb 20 00:26:03 EST 2016

Howdy Brian, nice to hear from you. An open OPT was suggested by someone as a reason for the audio to suddenly give a click and disappear while I was aligning one of my Rx's. I have a few spare OPT's and a few larger ones from some older AM radios (beautiful workmanship from 1930's.). I had the lead break off one of the neon bulbs, so that was why I was looking for one. I'm covered for racks at the moment, as I have one single and a double for the Rx's I use most, and have a triple remote tuner for them. What I don't have is a tuning cable for those three receivers, and not too sure where to get them or how to build them up. Thanks for the help so far. Talk again soon.....Joel KB0FPT 

----- Original Message -----

From: "Brian" <brianclarke01 at optusnet.com.au> 
To: joelroberts001 at comcast.net, ARC5 at mailman.qth.net 
Sent: Friday, February 19, 2016 8:05:27 PM 
Subject: Re: [ARC5] Suggestions Needed 

Hello Joel, 
In all my experience of Command-style receivers, I have not encountered a failed OPT or the neon bulb across part of the primary. 
Almost all neon bulbs strike at about 65 V - so any two-legged facsimile you can find will do. I suspect the main purpose was to prevent damage to the OPT if the headphones got disconnected. 
Why do you want replacement OPTs? If you just want cosmetically acceptable lookalikes, then keep scrounging at hamfests or on screwbay for gutted Rx chasses that still have the OPT. If it’s because you have acquired ‘scaled-down’ Rxs and you want them to work but are not concerned for looks, then either use an outboard OPT or fasten one on the rear deck where the dynamotor was. The 12A6 requires a 7000 Ohm load; the secondary can match whatever you choose to drive. I can supply a suitable, non-original 7000 : 4 Ohm OPT in a small Rola case, and I may have a 7000 : 600 Ohm one as well. 
A line output transformer (LOPT) is not a good substitute for a several reasons: 

    * LOPTs are designed for AC only, no DC 
    * maximum input is usually 100 mW 
    * frequency response is much wider than required for communications, like 20 Hz to 20 kHz, so the LOPT will be physically very large. 
Another possibility is to consider the Rx as a tuner and instead of using the 12A6, replace it with a 12SK7 or 12SJ7, or even a 12C5 or 12J5 to get a high impedance output to feed to a good quality AF amplifier; the beauty of this is that if you keep the original heater wiring, these tubes require exactly the same power as the 12A6, ie, 12V6 @ 150 mA. However, if you want to feed 600 Ohm lines around your shack, balanced and floating, then use the tube substitute I suggested earlier and an LOPT fed via a good quality, ie, no DC leakage, isolating capacitor. 
If you have several Rxs that you want to have on at the same time (as tuners), and you want to mount them in a 2-Rx or 3-Rx stall mounting, then add a mixing resistor after the isolating capacitor inside each Rx chassis. Then you need make no changes to the multi-stall mounting. I have a 3-Rx stall mounting available ... 
A further refinement to the multi-Rx tuner arrangement is to open the rear box of the multi-stall mount and connect all the Rx middle pins (B+) together. One dynamotor on one of the Rxs will then be more than sufficient for at least 3 Rxs. And then you have that lovely authentic dynamotor sound as background. If you do choose to open the rear box, and your LOPTs are too big to fit inside Rx chasses, telecomms isolating transformers (usually 600 : 600 Ohm, 300 Hz to 3 kHz) will drop in nicely. 
73 de Brian, VK2GCE. 
On Saturday, February 20, 2016 11:11 AM, you said: 
Good evening guys, I need some help on finding substitutions. Ken suggested asking the group. I need replacements for the Neon Voltage Limiters for the AF Transformer, and also need subs for the transformers, not necessarily NOS. No Rad Shacks left so will need to hunt Mouser or All Electronics. Whatever help you can give would be great.... Joel KB0FPT 

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