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Author Topic: All-American Five  (Read 23271 times)
KB1WSY
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« Reply #30 on: September 19, 2017, 01:13:02 PM »

3) In many - but by no means all, or even most - AC/DC "transformerless" designs, the chassis is NOT, repeat NOT, tied to one side of the power line.

This particular Admiral set came in two versions. The older version *does* have the chassis tied to one side of the power line, and that is the version I have. There is a later version whose model number has the suffix "UL" (Underwriters Lab version) in which the ground is "floating," isolated from the AC with a capacitor.

What I did when restoring the set is that I made absolutely sure that the power cord was fully unplugged from the AC power strip whenever I was working on the chassis. But I know exactly what you mean! If you are tired or have had a beer or two, it would be too easy to forget to do this!

73 de Martin, KB1WSY
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AA4PB
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« Reply #31 on: September 19, 2017, 01:49:24 PM »

Back in the late 1950's at the tv/radio shop we used to get a fair number of these AC/DC sets in for repair. We had an isolation transformer that was ALWAYS used to power a radio that we were working on. If you connected the ground clip of a signal tracer or other test equipment to the chassis that was plugged directly into an out let the wrong way, you were in for a fireworks display!
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Bob  AA4PB
Garrisonville, VA
KD0REQ
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« Reply #32 on: September 19, 2017, 02:05:04 PM »

we also had DC power in downtown Fargo into the 60s.

what seems a lifetime ago, when Sylvania owned the ECG parts line, they had a note in one of their counter handouts about flyback transistors blowing on or shortly after installation. turns out a snubber capacitor, a "safety part," was hosed also and needed to be replaced.

don't be shy about shotgunning wax/paper caps, it's the same thing... you could probably use anything close in an AA5 and never know the difference, but if it's acting like a different part and not a capacitor, squirrelly things are sure to happen. it isn't worth the labor and equipment to test them, when you can swap them out for a buck on today's market.
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KM1H
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« Reply #33 on: September 20, 2017, 03:34:21 PM »

Quote
This particular Admiral set came in two versions. The older version *does* have the chassis tied to one side of the power line, and that is the version I have. There is a later version whose model number has the suffix "UL" (Underwriters Lab version) in which the ground is "floating," isolated from the AC with a capacitor.

Im glad you corrected that misconception Martin. An experienced tech with those radios would not have made that mistake.
Earlier AC-DC sets before the RCA 1939 intro of the AA-5 tube set also suffered but it was not an across the board thing. Ive rewired my own collectibles and do it for customers.

Quote
And there's more!

As you say, if that coupling cap between the 12SQ7 plate and 50L6 grid is even slightly leaky - tens of megohms - it will upset the bias and cause the 50L6 to draw excessive plate current. The set will still work - but, over time, the high plate current can/will fry various components, such as the 50L6 cathode resistor, the 35Z5 rectifier, the 50L6 itself, etc. Worst of all, the most likely candidate for destruction is the audio output transformer.


Before all that happens the less negative bias due to paper cap leakage will eventually start with audio distortion plus AVC problems and eventually lead to G-K leakage and a resultant gas to further poison the tube leading to runaway element dissipation This is particularly common with the pentode IF/RF and audio output tubes and requires a good quality tube tester to find it. I use a Hickok 752A formally used by National Radio in the Service Dept where I worked in the 1963-69 time frame and have culled out many 6/7 and 12/14/25/28/35/45/50/117V heater versions that would be totally missed by all the entry level emission testers and a good many of the Radio/TV shop grade. Many if not all military testers do a good job of this, I also have a TV-7B/U and an I-177 and have compared all three over a wide range of tubes.

Carl
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KB1WSY
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« Reply #34 on: September 23, 2017, 08:34:47 AM »

Back in the late 1950's at the tv/radio shop we used to get a fair number of these AC/DC sets in for repair. We had an isolation transformer that was ALWAYS used to power a radio that we were working on. If you connected the ground clip of a signal tracer or other test equipment to the chassis that was plugged directly into an out let the wrong way, you were in for a fireworks display!

Since I have the service sheets for this set, I am thinking of checking the voltages. Given that the chassis is connected to the AC neutral line (or could be connected to the live line if there is a polarity mistake), I assume it would be a bad idea to use my VTVM even though that meter has only a two-wire AC cord (the meter's case is not connected to a third-pin AC ground). The risk of damaging the VTVM, not to mention shock risk from connecting its case (which is at the same potential as the floating  "ground" probe) to AC when measuring the radio voltages, is just too great. So I suppose the "safe" thing to do (well, the least-dangerous) is to use a VOM or DMM and be extra careful not to touch anything with my hands.... (Well, I am always careful when measuring voltages but in this case I need to be even more careful!).

Part of the reason I want to check the voltages is to make sure that the voltage being fed to the grid of the output tube is within normal range, i.e. that it not overdriving the tube. After all, when I restored the set I found that the old output transformer had an open-circuit primary so presumably it was overdriven in the past. I have changed the coupling capacitor, but it would be good to check that all is well.

73 de Martin, KB1WSY

« Last Edit: September 23, 2017, 08:38:45 AM by KB1WSY » Logged
AC2EU
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« Reply #35 on: September 23, 2017, 09:05:41 AM »

Yes a VTVM WOULD BE a BAD idea. However there SHOULD NOT be a connection between either leg of the AC to the metal chassis. That was "isolated", though in a very dangerous way with a paper foil cap which could leak or short.

the radio ground should be floating which does connect to an AC leg.
You can test the voltage more safely with a battery DMM , be be aware that that is exposed wall
AC in there.
We  use a current limited or fused isolation transformer  to work on hot chassis sets, which is still needed if you wanted to use a scope or generator on it.

BE SAFE!
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KB1WSY
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« Reply #36 on: September 23, 2017, 09:35:33 AM »

However there SHOULD NOT be a connection between either leg of the AC to the metal chassis. That was "isolated", though in a very dangerous way with a paper foil cap which could leak or short. ...the radio ground should be floating which does connect to an AC leg.

My set is an early AA5 that does *not* have the "floating ground." One side of the AC is indeed connected to the chassis. My model has the "plain" 5K1 chassis. There was a later version, UL5K1, which did have the floating ground. This is well explained on the schematic (see the text at the bottom).

Arguably, I should be modifying the set so that it *does* have a floating ground, i.e. I should convert it to match the "UL" (Underwriter Labs) version. Of course that is only slightly safer....



73 de Martin, KB1WSY
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AC2EU
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« Reply #37 on: September 23, 2017, 11:13:24 AM »

Wow that's REALLY dangerous.
Well, at least put a polarized cord on it and/or use it with a small isolation transformer.
It's amazing that they made stuff like that!
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KB1WSY
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« Reply #38 on: September 23, 2017, 11:30:34 AM »

Wow that's REALLY dangerous.

Yup, that's what I've been trying to say. I knew these sets were lethal but had never owned one, and was amazed at just how dangerous this one is. What makes it even worse is that the power switch is on the chassis side of the line, so that even if you connect that side of the power cord to Neutral, the Live side is permanently switched on (since it is a single-pole switch, mounted on the back of the volume pot). According to one source, such switches were typically wired in the chassis side of the circuit "to save money and avoid having a terminal strip."

With that configuration, you could get zapped if you touched just about any part of the heater chain, even when the set is switched off (and because the heater resistance is low when cold, it could be lethal). So I have made sure to unplug the AC cord from the power socket whenever working on this set!!!!

Well, at least put a polarized cord on it and/or use it with a small isolation transformer.

I have fitted a polarized cord. But of course that will not protect me if the house wiring is wrong. I live in a 90-year-old house with some very old wiring in some rooms (plenty of two-prong sockets still there). Fortunately, the shack/workshop has modern sockets that seem to be wired the right way round.

I don't own an isolation transformer but obviously should get one. Although, thinking about it, even that cannot eliminate all of the risk -- surely you can still get a shock if you make a path to ground?

On one site dedicated to fixing these radios, I found an intriguing safety tip. Find a cheap electric hair dryer (or a broken/used one in a junk shop). In recent years these have been fitted with a GFCI safety cutoff that is part of the power cord. Chop off the cord and fit it to the AA5.

(You can also buy a pre-made GFCI cord but those seem to be quite expensive.)

73 de Martin, KB1WSY
« Last Edit: September 23, 2017, 11:37:49 AM by KB1WSY » Logged
AC2EU
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« Reply #39 on: September 23, 2017, 11:35:54 AM »

there is no current path to ground with the isolated secondary.
You have to have a legitimate 3 prong service for GFI to work.
I wouldn't trust it in your house!
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KB1WSY
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« Reply #40 on: September 23, 2017, 11:39:12 AM »

there is no current path to ground with the isolated secondary.
You have to have a legitimate 3 prong service for GFI to work.
I wouldn't trust it in your house!

All these things are good to know. Thanks!

73 de Martin, KB1WSY
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KD1I
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« Reply #41 on: September 23, 2017, 04:34:53 PM »

AC2EU, respectfully, GFCI works just fine on 2 prong devices because they measure an imbalance between the hot and neutral currents and not a current on the ground wire. Picture that a fault condition can be between the hot line and yourself to ground such as a water pipe. In this case, there is no current flowing in the ground wire but a different current in the hot vs the neutral.    Very Best 73, Jim
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KB1WSY
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« Reply #42 on: September 23, 2017, 06:05:57 PM »

AC2EU, respectfully, GFCI works just fine on 2 prong devices because they measure an imbalance between the hot and neutral currents and not a current on the ground wire. Picture that a fault condition can be between the hot line and yourself to ground such as a water pipe. In this case, there is no current flowing in the ground wire but a different current in the hot vs the neutral.    Very Best 73, Jim

Interesting. FWIW Amazon sells *two-pin* AC plugs with built-in GFCI: https://www.amazon.com/OAONAN-Replacement-Interrupter-Protection-Conditioner/dp/B073PRH8H3/ref=pd_sbs_328_4?_encoding=UTF8&psc=1&refRID=NVXZD6BH9PEGS13NPB3X.

That sort of device is normally used on relatively high-power devices (hair dryers, pool pumps, air conditioners) but I assume that a GFCI would still be tripped by a low current, normally less than a lethal dose?

Really just an extra safety "crutch" in this case and by no means a fail-safe one. The radio is unsafe by design!

73 de Martin, KB1WSY
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AC2EU
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« Reply #43 on: September 23, 2017, 06:19:50 PM »

AC2EU, respectfully, GFCI works just fine on 2 prong devices because they measure an imbalance between the hot and neutral currents and not a current on the ground wire. Picture that a fault condition can be between the hot line and yourself to ground such as a water pipe. In this case, there is no current flowing in the ground wire but a different current in the hot vs the neutral.    Very Best 73, Jim

Makes sense,I stand corrected. Thank you!
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W9IQ
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« Reply #44 on: September 23, 2017, 07:21:18 PM »

But what will not work with a GFCI on a two wire circuit is the GFCI test button. The test button routes some current through the ground to test its efficacy. Probably not good to rely on an untested GFCI when you know the risks from the load are higher than normal.

- Glenn W9IQ

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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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