Beginning the journey of restoring my 40-180 which was given to me by my grandparents years ago. As many know, the alignment procedure references the infamous loktal adapter that allows you to easily connect a VTVM for the procedure. I did notice instructions also mention you can connect the VTVM to the plate and socket (assuming ground) pins of one of the 41 tubes. Just curious if anyone had completed the procedure using this method? Also curious it it would also be feasible to just connect a VTVM across the voice coil? I’m not close to being ready to align the set yet, but thought I’d ask the question as I’ve read about a lot of different approaches from connecting leads to the appropriate pins of the 7C6 to simply “listening” for the best tone level from the signal generator.
Thanks!
Vince
Hi Vinny and welcome,
I would use a vtvm at the bottom of #43 that's the avc line. The voltage there is going to be negative (+probe to the chassis and - to #43) and tune for the most - voltage w/ generator output set at it's minimum.
Terry,
Thank you for the suggestion! Getting to the pins on the 7C6 underneath isn’t too simple with the push button assembly in the way. I’ll have to check this out as I assume the point at #43 is much easier to get at.
Thanks again!
Vince
What I've done on some of the 40/41 sets w/pushbuttons is remove the screws from the pb unit, I think there are four. Use some paper of cardboard to insulate the pb unit just flip it over. Some have the power switch integrated into that unit and others it's on the volume or tone control.
You maybe able to remove the unit add a couple of jumpers and make it work w/o it installed. Pretty sure I did it with a 40-150. I can't really imagine servicing one of those sets with the pb in the way and constantly moving them back and forth.
Terry,
This is all good stuff. I did a little exploring on my lunch hour today and looked under the chassis. Looks like there is a point on a terminal strip in the vicinity of the volume control which has a direct connection to what I believe is pin 6 of the 7C6 and the connection point you mentioned above (I need to look at it more closely tonight). Based on the schematic it appears this is plausible. If that’s the case, I have an easy connection point to where I need to be once I get to the point of aligning (definitely a ways off there). I found that the 1941 RMS Yearbook has a nice tube chart and pin diagrams for all of the tubes which will be very helpful as I test and troubleshoot as well.
This is all fun stuff, just a cool little puzzle that I get to put together and learn as I go!
Vinny
You have a good attitude towards this stuff, patience is a key radio repair virtue.
Paul
I was mistaken earlier. After closer inspection, I was one pin off, and it isn’t a straight path to the block I noted. I think I’m going to go with your suggestion of soldering in a short lead to pin 6 of 7C6 while I have the push button trimmers out of the way replacing the wax caps. Oh well, not a big deal.
I do love how you are supposed to leave the loop assembly in the cabinet for the RF alignment. I’ve got an idea for doing this while still keeping the chassis on the bench, but we will see when the time comes.
Another question for the group more out of curiosity. Has anyone had success with opening the broadcast loop assembly without destroying the cardboard cylinder? Mine is in really good shape with the exception of the completely rotted rubber coated leads. Looks like it is stapled to a wooden plate at the top and bottom, but wasn’t sure if it is also glued. At this point, looks like my options are either heat shrink around the existing wires or splice, solder, and heat shrink new wires to a portion of the existing ones. Was just curious what others have done with these loop assemblies.
Ok folks...looking for some more advice. I’m planning to replace the rubber coated wires in the 40-180 as 50% is cracked or brittle as expected. Any advice when it comes to the IF transformers? Am I good to use solid 20ga wire there as long as I keep the length and path the same as the original wires. The rubber coated wires are actually in good shape here, but if I’m going to replace the other rubber coated wires, I feel I should replace these too. Any tips or tricks to make this process go smoothly?
I'm no expert here but I have replaced the wire in many an IF transformer. I have usually used stranded wire. However, the frequency is not that high here so I don't think you would notice any difference in performance if you used solid. Just my opinion.
Most of the original rubber-covered wires were solid, not stranded. Exceptions are the IF transformer leads which are stranded as Rob (rfeenstra) said. I try to use new PVC insulated 20 gauge wires that match the colors of the originals. (The originals, I believe, were 22 gauge.) Do one wire at a time, take your time, this is a very long process.
Thank you all for the advice and tips so far...now onto another issue. Can you get replacement mica wafers for the tuning condenser compensators (21A and 21B)...specifically 21B? As you can see from the photo, there’s a piece broken from the edge and it’s cracked down the middle. It isn’t broken completely in two, but definitely cracked all the way across. The smaller one for 21A isn’t cracked but does have a small chip on one of the corners. Any help or advice is much appreciated!
Go to Google, type Mica Sheet, buy what you like and cut/drill to size.
I've had a lot of mica wafers broken in the corner like that. It usually hangs out past the capacitor plate and is subject to being broken. I've never bothered about it as long as the mica is keeping the movable plate of the trimmer from touching the stationary piece. Will probably work just fine with the crack as well. May change the dielectric constant slightly but compensated for by the adjustment. I vote for using it as is.
The mica pieces are in there to make sure the two plates of the trimmer remain insulated from each other. One can cut pieces from the thick, clear plastic used for clear packaging over many products these days. Just cut a piece that is the same size, or a tiny bit bigger than the mica, and drill an appropriate hole, and use that as an insulator between the two plates. It works OK.