I just spent an entire evening chasing a 'ghost'... sort of.

I have a nice Arvin 664a.
Yesterday, after having set it aside for a few weeks, I decided to pick iy up again and finish re-doing this set.
All I had gotten to complete previously was to replace the power cord, two paper caps and to replace & redress a few old cracked rubber wires.
The loop antenna had been removed to make it easier to work on as it un-plugs with a 3 pronged plug very easily.
I couldn't remember where I left-off when I set it aside a few weeks ago so I powered it up again with caution.

Well as soon as it started to come alive there was a loud .. hum, or motorboating.
The nature of the hum was such that if there was a signal present, the audio voices were heavily distorted with this noise, almost like they were gargling as they spoke and so bad as to make the speech almost un-intelligible.

So I suspected that the cause was the old 150 volt, dual 20/40uf filter cap can which mounts on a phenolic insulator isolated from the chassis. (just a reasonable hunch)
But when I looked at the ripple on the B+ with my scope it didn't seem too severe if at all ...but what the heck.. I replaced both sections anyway. But...Still I had a very loud hum afterwards.
I have the Arvin plugged into my isolation transformer and my scope is plugged into the regular AC wall outlet through a GFI, but the 3rd leg, ground-pin is floating...( which I had done before I bought the isolation transformer to help avoid the GFI popping and flying sparks with AC/DC sets.)
Anyway ....as I clipped my scope's ground lead to the Arvin chassis I noticed that the hum instantly disappeared. ...How weird I thought.
So...I tried to see if there was any potential voltage difference between thier chassis's and their was nothing measurable.
The Arvin uses a floating B- ground as well. I thought there was a leak to the chassis from B- but there was nothing that I could measure. Nothing above a few milivolts either AC or DC and when un-plugged I measured no discernable omage leaks from the B- to the chassis.

I also found that even though the hum was disturbingly overpowering the audio... as soon as my bare fingers touched the chassis, the hum would disappear and stay gone!! At least until I turned it on again a few minuets later.

I was a bit bewildered by all this!

But I decided to plod along re-capping and keeping this mystery in the front of my brain and thinking about it at every step....
So..I figured there must just be another bad paper cap somwhere, ...maybe...and it should just be a matter of time locating it as I test and re-cap step by step.

but..HAH hah!!! no such luck for me.

( as I removed each cap one by one I'd test the radio again and measure the old cap. Every single cap was good and measured perfectly within tolerance. But it was replaced anyway and of course... there was no change in that overpowering motorboating hum sound.)

I had to do quite a lot of work as there were many cracked rubber wires to replace as well. I also was dealing with another two mysteries thay popped up along the way now...
1.)A really weird "total silence" that would come and go.
2.) And the B+ would also just dissapear too from time to time.

Well the disappearing B+ was identified as being caused by the filament string shutting off now and then for no apparent reason that I could ever find for certain.

Each time I'd start looking for a break in the filament chain it would start working again. I figured there was a loose wire in that string somewhere or a bad 35Z5 maybe. ( this radio doesn't use the 6.3 volt lamp tap as it instead uses a 110v ac lamp)
So... I changed each tube one by one yet there was no conclusion.
So I plodded along again just waiting for the filaments to die again. Then it went "silent" again.. but this time while the tubes were still lit. What now??

Well...in retrospect THIS one particular symptom, right here,... "OUGHT" to have led me a little closer to my motorboating hum solution... but I was not putting 2 and 2 together at that point very well as I was just perplexed with the silence.

So..While probing wires around and tapping and picking around at things under there with a plastic stylus ( a great way to uncover problems by the way)I noticed some intermittent coming from the 1st IF can wires.
So...I ripped it out, especially since there were several cracked wires needing replacing leading in there anyway. And inside that can I discovered that there is a resistor in there that connects to one of the leads coming out that was totally un-soldered!! The connection was purely mechanical with small loops at the ends of each lead that were inter-looping with each other and no solder!! Loose! Just flopping around hanging there making conection mostly... but very loosly.
Well that was the cause of the "silence" okay..... but now I was back to that darned motorboating hum again.

Finally the filaments quit again and this time I was able to trace it before they came back on. I located an open filament in the first RF tube. A 12SK7GT.

[ BTW... I just said that the 1st RF tube is a "12SK7GT" and you might want to note that this is another clue to the motorboating problem hidden here ]

However, because of my lack of years of experience with these old sets, I would not suspect anything... as I did not...lol.

I had another 12SK7GT here in the drawer and so I replaced it, but I was a bit worried that it might blow again if there was a wiring problem causing the filament to have blown open somehow.

But I really double checked the entire filament circuit and it was in perfect shape... so I flipped on the power and it worked fine but the hum was still there!!

Now most of the B- leads all meet and connected together at the 2 ground lugs of the now-disabled old, filter cap can. I saw a crack in the phenolic insulator there ...so even though I knew it should not matter, since the positive side is dead now... I still could not determine any other cause of this motorboating...so I decided to get all the B- leads off of there and join them together with a nice new, isolated phenolic terminal strip whose isolated metal foot I would solder onto the old cansiter lugs for support. Well I did this very nicely and re-dressed everything and made sure everything was soldered together properly and that the terminal strip itself was solidily soldered onto the chassis and also onto the old filter cap canister lugs for support.
Well when I powered up... all voltages were perfect and clean... but the hum was louder than ever. ARRRGGhh.

Now I have seen motorboating in the past caused by a missing or bad chassis isolation cap. But in this case I had just replaced the .1uf C9 with a nice new orange drop.
So I went on hunting.
While examining everything else under there I noticed what looked like a burnt or discolored 6.8 meg R-15.
I took it out and it measured 8 megs or so, and the mica cap C-6 (250pf) connected to it was also at least 150pf high... so I replaced them both ... on general principles since I could not see how they were any part of the hum problem.

Sure enough the hum was still there.!!
Everything under this chassis was replaced now with new components carefully properly installed and dressed for neatness and good sense...Yet nothing had made an improvement. What else was left, I thought??!!!

I was grasping at straws now... so I unsoldered the new termial strip up off of the chassis as it was bridging from the isolated dead old filter cap canister negative lug and the chassis. Now... I KNEW ...this should NOT matter because the entire cap can is dead!!! But I unsolderded the terminal strip from it anyway...and just let it hang up in the air off the chassis as a test. I wanted to see if the chasiss bridging to that old capacitor can lugs could somehow be causing all this.

Well as I turned it back on there was no hum!!!

But this was impossible I thought. So I took a test lead and jumpered the cahssis back to that capactor groung lug.. and I couldn't get the hum to return that way. So I re-soldered the terminal strip foot back onto the chassis and to that very same lug again and NOW...there still was no hum.

Well I was happy but totally baffled.

I had very little or no signal though because of the disconected antenna... so I tried to clip a lead to where the antenna connects... and there it was again... loud blasting motorboating.. but it stopped as I let go of the antenna lead.
I decided to connect the antenna properly by plugging it in correctly with its 3 pronged plug. Well the radio played just beautifully.. clean loud and clear.

That was at about 5 AM this morning after being up all night. So I just hit the sack and looked at it again today.
Well guess what???
As I was scrutinizing the circuit and reading the detailed notes on the print....There it was!!
ah Hah!... a warning note about motorboating oscillations.

QUOTE:
"Due to variations in tubes some sets which were equipped with 12SK7GT tubes may have a tendency to oscillate. This condition can ususlly be corrected by placing a shield on the 12SK7GT tube or replacing it with a 12sk7 metal tube."

So all last night I was chasing an oscillation tendency. That explains why when I touched the chassis it would stop. Or when the scope grounding clip was connected it stopped.

Now if you'll remember, a few paragraphs back I said that the "silence symptom" caused by the bad connection in the 1st IF can was a or should have been a clue to my problem? Well this is why. The oscillation had it's origins in the RF amp section which is in FRONT of that 1st IF can. That's why the there was total silence to the audio section. The hum was not getting past that point. So I ought to have concluded that the audio section and power circuits were not contributing to it and that it was originating in a prior stage! The ONLY priorstage... the 1st RF tube!!!
Why didn't I realize that??

Well maybe I've learned a little. hummmmm "maybe"

First of all to READ the notes on the prints!!!
also to remember that GT tubes will oscillate .. ( they put 'em in those metal-can-tubes for a good reason)
but also to learn to recognize the clues as they jump up at you and to consider them as possible indications to the solution.
Well I'm glad this problem is over now... lol
and especially with a good logical ending ...and not like a cliff-hanging mystery that you never get to understand.

Peter

Peter: Great job on getting the Arvin back in good working order. Like to read these tough repair adventures. We all have our fair share of some real dogs we probably shouldn't have bought but it wouldn't be fun for long if just replacing caps got every radio working well. You can really tell a tale! Congrats!

Richard

Thanks Richard:
Actually I enjoy the challange...When its finnaly done and working.. that's the best reward ever!
I used to work as a bench tech at HP's eastern regional repar center back in the early 70s. We only ever received the broken equipment from the local centers that nobody else could fix. So literally, every job was a puzzler. But when that's your primary job you get used to working at that level of complexity and it doesn't scare you.
Since the 70s I went into computer field service ... back when they were as big as closets....lol...
Then various systems integration jobs and then a little computer consulting business.... then photography ... and still some computer networking... now sort of retired...
Though I got my basic electronic training in 'tube radios' back in 1963 at Ft. Monmouth in the signal corps... I haven't been working with tubes ever much after that.. so anyway since I only started this hobby in Sept. I just don't have the depth of experience yet with these old radios... so I'm bound to be making many more dumb mistakes... probably even after I get some experience. lol

...but what else would I be doing?... it's a lot of fun... really.

There is an old DJ (83yo) in this NYC area, that plays old music (1930-40s) all night long. So I do my bench work listening to that till dawn.
...so I'm just an old guy... repairing old radios, listening to an old DJ playing old music... and it never gets old!