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Puzzled Over 118MX - Printable Version

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Puzzled Over 118MX - Jamie - 01-22-2014

I recently restored a 118MX. I listened to it for several hours a day for a couple of weeks. It played perfectly. One day last week, it suddenly quit playing. The shadow meter was very narrow no matter where the dial was set. I could pick up one SW station, very faintly.

So, I pulled the tubes and checked them. No problem. Pulled the chassis and started going through resistors and generally eyeballing everything. I could see nothing wrong and everything checked out good on the DMM.

Today, I pulled the speaker out of the cabinet so I could get serious and check voltages and actually repair the radio. I turned it on... and it plays perfectly. Voltages are fine and nothing seems out of order. I've been listening to it for an hour now, but it is still on the bench.

I hate to reinstall everything in the cabinet, then have to go through this again. Any ideas of what could have caused it quit?

RE: Puzzled Over 118MX - Mondial - 01-22-2014

Check all the solder connections where you replaced caps, etc. It seems like an intermittent cold solder joint. Try wiggling the wires with an insulated tool while the radio is playing and see if you can recreate the problem. It really could be a bad connection anywhere in the RF or IF circuitry.

RE: Puzzled Over 118MX - Chuck Schwark - 01-22-2014

All all the resistors replaced? Even if the old one(s) checked ok, after heating up a bit, carbon comp. resistors can change value, especially the old types. How about that three-section resistor #61? Looks like higher wattage types there.

Could be a bad/cold solder joint anywhere - tube sockets, term strips, lugs on parts. Maybe a notorious Philco coil going open in RF section causing low gain on front end.

You need to make a chart of the tube pin voltages when it's mis-behaving to compare with "working good" condition. That will point you to the troubled area.

Chuck

RE: Puzzled Over 118MX - Jamie - 01-22-2014

It's been on the bench, playing perfectly, for several hours now. My fear is that, being upside down on the bench, there is a lot of air circulation and it wont get hot enough to reproduce the trouble. I poked and prodded with a wooden stick and found nothing. Mike sent me a PM that should help narrow down the search. I did leave several old dog bones in the radio, and my gut tells me that's where the problem lies. Chuck, I checked resistor #61 as soon as I first pulled the chassis, it checked out fine - but it had cooled off by then too.

I may have to put it back in the cabinet to get it hot enough. Or else, maybe cover it up with something on the bench? It's a heavy chassis and I'm no spring chicken... any ideas on that?

RE: Puzzled Over 118MX - jontz - 01-22-2014

If you think it is heat related I'd definitely cover it with an old towel that you don't particularly care for and let it heat up. If it is heat related you definitely don't want to be hauling that chassis in and out of the cabinet.

RE: Puzzled Over 118MX - morzh - 01-22-2014

When it is playing, with a medium-light-to-heavy long insulating object (colloquially called a "stick") start lightly tapping with it on everyhing that is suspicious: start from teh wires you moved why taking out the speaker and the objects they go to, and then everything you can think of. If it is intermittent, it is likely to appear while you are tapping.

If not..well, then wait till next time.

RE: Puzzled Over 118MX - Radioroslyn - 01-22-2014

You could give a heat gun a try too.
Terry

RE: Puzzled Over 118MX - Chuck Schwark - 01-22-2014

As far as resistor replacement goes on any antique radio from the 20's to the 60's, if it's an original resistor, it gets replaced.

In a 1935 radio, those resistors were not designed to last more than the "expected product lifetime." We are waaaaaay past that point....

Icon_wink
Chuck

RE: Puzzled Over 118MX - Arran - 01-23-2014

Another thing to watch for are bad solder joints on grid caps or flaky grid cap leads, those can cause all kinds of trouble. It's no wonder that most companies routed the grid leads under the chassis as soon as the second generation octal, like 6Sk7s, came out. In terms of the radial B.E.D code resistors, it isn't so much that they change value with heat so much as the lead's contact with the carbon rod becomes flaky, it's basically a wire wrapped around a carbon rod and then painted. In fact I think this is a large part of the reason why so many increase in value, not to mention the composition in the rod drifting.
Regards
Arran

RE: Puzzled Over 118MX - BrendaAnnD - 01-23-2014

Safer than just putting a towel over the radio, use something like a milk crate that's large enough to fit over the chassis, then put the towel on over THAT. Then you have no hot spots on the towel.

RE: Puzzled Over 118MX - TV MAN - 01-23-2014

Ahhhh - intermittants, dont ya love 'em? I like Brenda's idea of the milk crate and towel. I'd be concerned about a hot part against that towel also. I trick I used to use was to warm it up, either by letting it play of as someone else mentioned, or a heat gun. When it acts up, start tapping things to look for bad connections, and I often used component cooler/freeze spray. Also could be an intermittant switch contact. Seems tho, those things "know" when there is a technician present, and straighten up. Who knows, maybe you are on to the original problem as to why the radio was taken out of service many years ago...

RE: Puzzled Over 118MX - BrendaAnnD - 01-23-2014

Ah... Murphy's Law in regards to the service industry. The occurrence of a problem varies in direct reverse proportion to the distance from or proximity to a service technician.

RE: Puzzled Over 118MX - Jamie - 01-23-2014

Update - I tried tapping on everything with a wooden stick. No problems. So, using Brenda's milk crate idea, I put a cardboard box over the chassis (I know, I know, it's flammable) and let it play all morning. Once more, no problems. It is 12:52pm here now and the chassis and speaker are back in the cabinet.

I am sure it will quit playing now, so I can start all over. The only clue I got was, twice, the bulb for the shadow meter got a little dimmer. It only happened for an instant, and I could detect no shorts, nor could I recreate the condition. It never went out, nor would I even say it got dim. It just lost some brightness for a split second, twice. It is very hard to describe, and now I am even wondering if I imagined it. It is a brand new bulb. Perhaps this clue will give someone an idea? Thanks to everyone who has responded. You guys and gals are the best.

RE: Puzzled Over 118MX - morzh - 01-23-2014

If the light gets dimmer, could be a cold solder spot anywhere in the primary or the filament's secondary power transformer's winding. Could also be the cord or its soldering.
Oh! Could be the power switch.

RE: Puzzled Over 118MX - tab10672 - 01-23-2014

I have had tubes test fine and break down after playing awhile. Can you try spare tubes that you know are good from use?