Pulling Hair With Zenith Transoceanic 6T40!!! - Videokarma.org TV - Video

Tubejunke · #1 08-29-2015, 04:01 AM

I really didn't want to post about not being able (thus far) to get a darned radio up to speed, but I seem to be going nowhere fast on this thing. It is a bit different than anything I have tackled before with it's little 1VDC and a 3VDC tube compliment utilizing directly heated filaments for cathode.

Essentially it is a dead set in all appearance, but that's because nearly all voltages are just low and so far I've missed the mark on why. For example the 3V4 audio output tube calls for 94V on the plate and I think I saw 53 or so. I've checked all of the Mohm resistors and found nothing way off. I swapped out the Selenium rectifier out of a spare I have and nothing changed. The two large wattage resistors right beside the rectifier are within specs. I found one leaky and way off value section of the electrolytic cap can; subbed that without paralleling across it, and I found two other paper caps leaky.

Of course out of all the paper caps two were bumblebees and a third .1mfd was some brown cased crap that may or may not have came new with the radio. The rest are those very hard and most often good white caps which I really don't suspect as causing any of my woes here. Perhaps the rectifier out of the other set that I subbed in was bad too. I found that they are quite different to check with a DVOM than a modern diode like a 1N4007. I should have used a VTVM while I had them both out! Duh!

Anyway, I can see that it is probably just ONE little thing that is keeping this nice old radio silent and I know that there are some that specialize in these, so it shouldn't be hard to come up with some ideas of problems common to these radios.

This is going to be a beautiful and nearly mint condition set when I do get it straight. My spare is the brown leather model that are much more expensive and hard to find, so I should be putting the hopefully working chassis in that, but frankly I like the black with gold trim much better. I think I will sell the leather case empty and keep the spare chassis.

Username1 · #2 08-29-2015, 07:22 AM

If this is a valid schematic --> http://www.mbzponton.org/n2awa/radio...600_schema.jpg
Check the power select switch, voltage at the rectifier, voltage at the 560 ohm resistor,
then follow it through the rotator switch..... Gotta be a simple loss somewhere.....

.

Tubejunke · #3 08-29-2015, 10:12 AM

Squirrel Boy thanks! That schematic is much more detailed than the Zenith schematic that came with the radio. Maybe I can come up with something soon now. I'm a bit confused in that I thought that the power select switch and rotator switch are one in the same.

Username1 · #4 08-29-2015, 12:03 PM

Quote:

Originally Posted by Tubejunke

Squirrel Boy thanks! That schematic is much more detailed than the Zenith schematic that came with the radio. Maybe I can come up with something soon now. I'm a bit confused in that I thought that the power select switch and rotator switch are one in the same.

I have no idea, I have never seen one before.... The site where I got the
schematic seems to think there are some mistakes in some schematics
floating around... I did not look into that..... You should verify this IS the
one that goes with your radio.....

Good Luck.

.

Electronic M · #5 08-29-2015, 05:00 PM

I would not have used a selenium to replace a selenium.....Too high a chance of replacing a bad part with a bad part or even a good part with a bad one.

Tubejunke · #6 08-30-2015, 02:34 AM

Quote:

Originally Posted by Electronic M

I would not have used a selenium to replace a selenium.....Too high a chance of replacing a bad part with a bad part or even a good part with a bad one.

Yeah, it was a bit of a hasty move, but I simply used what I had on hand. Thus far I have never removed a good selenium just to pop in a modern silicon diode and whatever resistance is needed.

On Squirrel Boy's advice I checked everything he mentioned and came up still missing about 40V at the output of the rectifier. Here is where better troubleshooting and/or better equipment comes into play. Following the power supply circuitry I double checked the remaining .047mfd 600VDC bumblebee that I was stupid enough to leave in as it tested no leakage under load last week. Now there was strong leakage at about 100V. I replaced that reluctantly as I had the other with the last of my equivalent value @ 1Kv. They were for a 40s RCA TV that I have, but oh well for now.

I was still missing the voltage! So next up the line of fire was the 60mfd section of the multi section lytic. All I had was an 80mfd so I jumped that in and my 100V at the output of the rectifier was there; and then some. Still never a sound out of that old speaker!! I need to get a cap that is the right value first off as I know from reading at least that these sets are more critical on voltage tolerance than a plain A5.

But next I will test what was a good tube compliment. I hope still so, butt through getting out bad caps I saw the filaments glowing in a lit room and that's not good for 1.4V tubes. Maybe I will get lucky and find maybe one bad one which would be enough to keep the set silent.

jr_tech · #7 08-30-2015, 03:39 PM

Quote:

Originally Posted by Tubejunke

But next I will test what was a good tube compliment. I hope still so, butt through getting out bad caps I saw the filaments glowing in a lit room and that's not good for 1.4V tubes.

Hope that you did not blow the 1L6... these tubes are scarce and expensive.

http://www.zenithtrans-oceanic.porta....co.uk/1l6.htm

http://www.ebay.com/sch/i.html?_from...+tube&_sacat=0

Since you found one section of the electrolytic to be bad, I would replace the other sections as well... if the "D" section of C-1 (200 uf) is shorted or is very leaky, excessive filament voltage would be applied to V-5 (3V4). C-39 (.1 uf) is also between the filament string and ground, and if shorted or very leaky, excessive filament voltage would be applied to V-5 (3V4), V-3 (second 1U4) and V-2 (1L6).

jr

Electronic M · #8 08-30-2015, 04:38 PM

I would sub a common cheap 1R5 (IIRC that is the sub for the 1L6) it will only oscilate on the AM broadcast band, though.....Still I'd rather be stuck with only AM until I have it working right than blow a 1L6 if in your position.....

I have 3 Transoceanics. 2 are the mini tube version like yours (1 works, but I need to fix a wiring mistake with the lytic negatives), and 1 is the older loctal version. Once they reach my bench I plan to use 1R5s in them until all is right.

Titan1a · #9 08-30-2015, 05:06 PM

There is a solid-state replacement for the 1L6.

Tubejunke · #10 08-30-2015, 11:13 PM

Quote:

Originally Posted by jr_tech

Hope that you did not blow the 1L6... these tubes are scarce and expensive.

Since you found one section of the electrolytic to be bad, I would replace the other sections as well... if the "D" section of C-1 (200 uf) is shorted or is very leaky, excessive filament voltage would be applied to V-5 (3V4). C-39 (.1 uf) is also between the filament string and ground, and if shorted or very leaky, excessive filament voltage would be applied to V-5 (3V4), V-3 (second 1U4) and V-2 (1L6).

jr

I believe you hit the nail on the head in this statement. It's pretty much a list of what I have found bad minus the two .047mfd 600Vs. My original 1L6 was bad (low emissions) way before all of this. The set of tubes that I bought had the 1R5 which I haven't yet tested to see if I ruined it like the others.

I have so many radios that seem almost indestructible with little or no work it seems a bit strange that this Zenith is so riddled with bad caps and other problems (mostly related to caps). The chassis is clean enough to eat off of, but I know that doesn't mean much electrically except to me indicative that it was taken care of.

Tubejunke · #11 08-31-2015, 04:55 PM

OK, last night I got out the Hickok 2000A and tested the 1R5 and although there is still filament continuity, the emissions are about as low as they can get. Again, exactly as jr_tech described is what seems to have happened. I knew that the 1.4V filaments were easy to open up, but I didn't know that if they didn't that the tube would be ruined regardless.

I have learned a HARD lesson in why people that mess with these radios are so particular about the tube voltage. Now I guess I will have to find another tube compliment save for a 3V4 which I have an extra. Now that I have read that the 1R5 sub will allow only broadcast band, I am wondering if the whole project is a waste of time and money. The 1L6s are high priced and to me turning the Transoceanic concept and novelty into no more than an AM radio makes no sense.

I do like the thing so I guess the idea is to get the electronics right and make the 1L6 investment. Then you have a nice, sensitive shortwave radio. I wish there was a collector around my area. I could give him the valuable brown leather unit to get my chassis right WITH a fresh 1L6.....

Electronic M · #12 08-31-2015, 09:44 PM

I've heard that the cheaper Loctal converter tube used in the older loctal version of the chassis is electronically compatible, and can be made to work by using the base of a defunct 1R5 (or similar) and a loctal socket and wiring the two into an adapter.

Tubejunke · #13 09-01-2015, 02:19 AM

Quote:

Originally Posted by Electronic M

I've heard that the cheaper Loctal converter tube used in the older loctal version of the chassis is electronically compatible, and can be made to work by using the base of a defunct 1R5 (or similar) and a loctal socket and wiring the two into an adapter.

Your right. I think someone is making adapters for that very reason. I will have to look into that idea as again, there is no reason to bother with a Transoceanic if its not going to do what it was intended to do.

Also I have read that you are best off to run them on batteries, but I am dead set against purely battery operated sets. Someone is making plug in replacement batteries, but I'm sure they aren't cheap and I don't want to sink even more money into that when I should be able to use AC with no problem. I think it's simply a matter of getting everything dialed in perhaps in a much tighter tolerance than I am accustomed to.

We shall see & I will report any progress that I make; or not....

Electronic M · #14 09-01-2015, 01:04 PM

The battery idea is likely to protect against damaging the filaments (and power line RF noise). I've heard of people replacing the ballast with a SS regulator device to protect the filaments. There are also TVS diodes (AC version of a Zener diode), and if you put in one of the proper rating in parallel with the 1L6 filament it will still receive safe voltage even if something goes wrong enough to smoke other filaments in the string.

To read more about TVS diodes search 'Pilot TV-37 Transient Voltage Suppressor' they install them on the heaters of the rare CRTs in those sets to protect them.

Tubejunke · #15 10-18-2015, 12:48 AM

I officially dub myself a dumb as* tech on this thread to the whole world. It took the purchase of a whole other Transoceanic to figure it out. The only thing I was right about was my saying that I felt that one little thing was all that was holding me back. It usually is anyway.

So I bought this other unit and removed the chassis for inspection, cleaning and/or repair. I powered it up and actually worked on broadcast at something like 70 volts. Somewhere in my excitement I unplugged the headphone jack wiring and powered it back up and OH NO! It was dead just like the other one. What happened? What had I done? All I did was unplug that... OMG! Plug it back in and it's alive, it's alive! Great!

So then I remembered that I never took the jack out of the case of the first set! So I go and retrieve it; plug it in and,,, well you guys know the rest. Now the chassis did have bad electrolytics costing me several tubes and the wacky voltages.

What stinks sort of now is that I put the second chassis that worked and all original back in a nice, black case which is the one that I plan on keeping. I really should put the eventual recap back in the keeper and the original in a brown leather case that I have that is some kind rarity that collectors go bonkers over. That one I plan on selling once I get a high speed WWW connection and start an Ebay store to move some of this electric gold sitting around. It's crazy to me what the things go for when found in a rather ugly leather case. On top of that any damaged parts can't be subbed by other parts as the brown case bears all brown parts like the Wavemagnet antenna and even the knobs, which luckily I have. Unfortunately, my BROWN plastic face-plate looks melted right where it says Zenith, so that will detract from the value. I have a near mint face-plate, but it is black and silver. That just won't due! LOL! Maybe a brownie will show up on the bay.....