The PHILCO Phorum
Scott SLRM - Printable Version

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Scott SLRM - mikethedruid - 04-01-2020

I am getting ready to start working on my Scott SLRM radio. You may remember my posting about acquiring some months ago. I have been reading up on it on the net. I have also carefully read the manual for it. This has led me to have the following question, since the manual, and what I have read on ARF postings seem to be at odds. The postings on ARF seem to insist that I should get an isolation transformer to connect the unit to power or it can cause damage to the receiver. The manual instead says, when using AC, to connect the ungrounded side of the power source to pin 1, which would mean to connect the grounded side to pin 3, of the input socket, nothing about an isolation transformer. This also implies that it assumes that one of the power leads will be grounded, as is usual, as well as the actual ground which will be connected to pin 2. My question is, do I really need the isolation transformer or not? I know it wouldn't hurt anything, but is another expense, and not a small one. From AES the one I would need costs about $61 plus shipping. Any advice, especially from any of you who are familiar with this radio, will be appreciated. Below are the schematic, as well as the pertinent sections of the manual.

[Image: https://66.media.tumblr.com/0a29c49e1b2c615e36bcd5b54df3f562/e9fc7a7d28e36913-63/s1280x1920/2c4bfdcbc151fea96cddc563e8f7a6eb1d20d31e.jpg]

[Image: https://66.media.tumblr.com/bfc4c9be16faede75d07e4cb3c837bb2/26e5b992c09fc46e-c1/s1280x1920/d71d5eb2204a3b751dd43448cfb5359923e7cef9.jpg]

[Image: https://66.media.tumblr.com/02a33b074349e80a2926e195b60a8a60/26e5b992c09fc46e-79/s640x960/66a3c1375f895ed10eacb06f3824e088da416ed1.jpg]

[Image: https://66.media.tumblr.com/e25c57ab12e7370ec3956256f34d8f78/26e5b992c09fc46e-3e/s1280x1920/f53085373e2791949c331712ae2198fd6e9ca248.jpg]

Thanks for any help you can give.

RE: Scott SLRM - Too poor to retire - 04-01-2020

An isolation transformer is for YOUR safety, not necessarily the radio or TV's. The last thing you want to do is accidentally touch a ground source on your bench, and a hot or live source in the unit you are working on. If you keep caution as your top priority (one hand in your pocket while taking live voltage measurements, for example; not standing in water  Icon_crazy, things like that), one might go without an isolation transformer. 
Dan

RE: Scott SLRM - mikethedruid - 04-01-2020

Actually, Dan, I've never used one in over half a century of this kind of work. One just learns to be careful, especially after a few times being zapped. No, in this case it is because of some problem with the circuit which can cause damage to the set, blow the pilot light and such according to a posting on ARF, although I really can't see why, especially in light of what the manual says. The guy on ARF implies it must be used all the time, basically, installed as a permanent part of the unit. Here is a link to the ARF thread, scroll down to the posting by Amptech Systems. https://www.antiqueradios.com/forums/viewtopic.php?t=112732

RE: Scott SLRM - rfeenstra - 04-01-2020

Notice the power plug is 3 prong polarized, grounding the chassis to house ground.  I assume your ac power cord is 3 wire.  If so, you do not have a hot chassis.  I listen to mine regularly without an isolation transformer.  Meter measurements of the chassis confirm that the chassis at ground potential.

Edit: I tooka couple of measurements on mine with the AC ground pin floating. No AC on the chassis but there was some DC. As I watched, the DC slowly came down. I grounded the chassis for a second and the chassis then remained at near 0 volts. Reception was actually quieter with the ground attached.

RE: Scott SLRM - Eliot Ness - 04-01-2020

I think rfeenstra answered your question, but below is a link to his restoration thread you might find helpful:

https://www.philcoradio.com/phorum/showthread.php?tid=19823

RE: Scott SLRM - rfeenstra - 04-01-2020

Mike, do you have the complete "Instruction Book" for the SLR-M?  I see you have at least part of it.

RE: Scott SLRM - Too poor to retire - 04-01-2020

My apologies. I certainly didn't intend on mis-leading the OP. Thank you for quantifying the reason!
Dan

RE: Scott SLRM - mikethedruid - 04-01-2020

Thanks rfeenstra and John. You have confirmed what I was thinking myself. I'm not sure why the guy on ARF said what he did. Maybe there was some problem with his house wiring, reversed plug would be my guess. Had it been me, after reading the manual, that would have been the first thing I checked, before I did any mods to the circuitry, heck, even before I plugged the d&mn thing in !. It does say, in the manual, to check the wiring of the circuit to make sure that the UNGROUNDED line goes to pin #1. LOL ! Maybe some people just look at schematics and don't bother to thoroughly read the manual. I learned LONG AGO to read the instructions before I did anything else.