[AVSorrell]

Hi Phorum,

I am restoring a 38-116, Code 121  which I picked up earlier in the month for $100.  I am fortunate to have a 38-116 code 125 parts chassis to pull from too (understanding the circuitry differences between the code 121 and 125).  The parts chassis is missing the on/off and magnetic tuning in/out knob as well as the dial mechanism but it includes the rest. I am in the process of checking the power transformer, audio interstage transformer (which appears bad from some leaked tar beneath it), the output transformer, and field coil for shorts and continuity.  I am also using my Hickock 605A to test the tubes. I will also be replacing all electrolytic and tubular caps and all out of tolerance resistors and testing all the other chokes, coils and transformers along the way.

Anyhow, although I am a novice, I have been in the hobby long enough to know the great resources available on the Phorum and ARF.  I have searched these boards and have compiled over 100 pages of commentary (from many of you!) on how to specifically repair and troubleshoot the 38-116.  I am very grateful for all the knowledge that has been shared online and am carefully reviewing it before I submit a question to the Phorum  - no need to ask something that has already been addressed.

I have gleaned a good deal of info on how to make the power supply safer and to better protect the 75+ year old tubes from power stresses. I was hoping that others whom have done this work before could double check my plan to make sure it is accurate. Here is what I would like to do (provided the transformers check out):  

1) install a polarized plug with hot leg connected to a fuse before the power switch.

2) install a fuse on the hot leg of the power cord before the power switch.  The radio draws 165 watts at 115 VAC (as listed on the blue label in the cabinet) so its amperage is 1.4.  I will plan to use a 1.5 amp slow-blow fuse to keep it as close to the draw as possible.

3) Replace the 2 .015 caps in the bakelite block (part 145) that were originally on each side of the line to chassis ground with 1 .01 uf X1/Y2 safety cap - this would be after the switch across the line. I will continue to utilize the existing bakelite block as a tie-point for neutral side of the AC line cord and the primaries of the power transformer.  

4) install a CL-90 in-rush current limiter after the power switch by adding a new 3 point terminal strip in an un-used existing hole on the chassis.  

Below is a rough diagram on how I plan to do it. The yellow highlight is the hot lead of the AC line and the pink is the neutral side. The purple shows ground connections.

Are there any issues with my plan? Did I leave anything out?  Thanks for your thoughts! -- Andy

[Mondial]

Don't see any diagram so it may not have posted. Your plan of restoration seems reasonable.

Are you planning on restuffing the original electrolytic cans? If so, it might be a good idea to use metallized polypropylene 630V film caps for the two 8 uF caps connected to the rectifier filament. These caps are subjected to very high ripple current, and modern miniature electrolytic 8 uF caps are not designed for such service. The film caps are available here:

https://www.tubesandmore.com/products/ca...ylene-fast

There is a posting on this site on how to rebuild the hardened rubber coupler for the tuning cap. They are usually hard as a rock, which makes the tuning stiff.

[Chris h]

For what ever it is worth, I found a small shaft coupler from McMaster Carr for around $25.  It has a urethane element which will remain flexible for many years.  (I kept the original for a later rebuild, if I choose to do so at a later date. ) This was on my 37-116 (code 122).

I also purchased a repop dial from Radio Daze. My originals were damaged beyond salvaging.  The quality and look are excellent. I am very pleased with it! Looks original, only slightly thicker material. It shouldn't be quite as fragile as the original, while still maintaining the same color and translucency as the original.

Good luck with your project.

[morzh]

I have also purchased the dial from RD for 37-116 and was not overly impressed: 1) the hub opening had to be widened as it did not fit the hub, 2) the silkscreen is not exactly the same, missing some red marks etc.

I hope for their sake they fixed these.

[AVSorrell]

Mondial - thanks for the advice - I will use those solen caps for the rectifier filament - as you are right they take a beating. Also thanks for the heads up on the rubber coupler - mine looks warped as well so I will need to do a rebuild to - I appreciated your post on the subject!

Chris - thanks for your input too - what was the Mcmaster Carr part # for the one you used? My dial is serviceable - multiple repaired cracks with invisible tape.

[Chris h]

Andy,
Sorry, but I tossed the recept, but it was easy to find on their web site. I think they had two couplers, one in stainless steel, the other in zinc plated steel. You should also confirm your fit and available space on your 38-116. I don't know if it's the same.

Regarding the dial scale, I was leary of the repop too, having heard that Radio Daze didn't quite have it right. But, as stated, was very impressed. The fit over the hub was perfect, and in comparing the new with the two damaged ones that I had, I couldn't find any flaws. I guess they took care of the problem.

The best original that I had, had tears near the edge which kept catching on the guide frame at the top.  I was able to manage that issue, when I really screwed the pooch by catching a sleeve on the dial when reaching across the chassis, breaking it real good!    Thus forcing the new one. Best of luck.

[AVSorrell]

Hi Group:

I am looking for a full-size template for the back of a 38-116 code 121 I am working on. Mine did not come with its back when i received it and I would like to recreate it. I think I recall Stephen M. mentioning that a Phorum member sent him a full-size template to recreate his in 2013 - but I cannot find the post about that. Anyhow, if someone happens to have this info handy I would really appreciate a share! I will also be happy to share my process once I recreate one. -- Andy

3/28/17 Edit: - I think a good resolution photo of one with the overall dimensions would work just as well if anyone happened to have that. -- Andy

[Mondial]

Your drawing is pretty close, but the .47 coupling cap connects to the wrong point. It should go to the point marked B instead.

The 27K connects to the same places as the primary did, and the coupling cap goes to the plate side.

[AVSorrell]

Hi Group:
I have an open primary on my audio interstage transformer - Philco part # 32-7865N.  The secondaries test fine. I am checking with Heybour to see what the cost would be to have one recreated as Gary does not have one on hand at PTOP. while I am waiting on heybour, I thought I would take a shot at Ron's modification B to the audio interstage transformer when the primary is bad. Below is my sketch with Mondial's corrections noted above (I deleted the original picture he references since it was not accurate):

   

I have also been talking with Gary at PTOP. I mentioned I was planning to do mod B and he didn't think that was the best route. Here is what he had to say:

"Not sure if that is the best way as it is a single plate to push-pull grid DRIVER circuit and not just a plain interstage audio coupling circuit." Gary suggests a DRIVER type transformer and says the spec's are not too critical on type if I am just trying to get it working.  I would like it to work the best it can given i don't have a spare audio interstage transformer (the primary was open on the parts set audio inter. transformer I have too).

What do you guys thinks?  Thanks for your help  - it is really appreciated! -- Andy

[Mondial]

Yes that looks correct now.

If you want to do it right, you really need to have a new transformer made, or find a good one from another chassis. It is not your usual driver transformer. Note that it has two separate secondaries with each driving its own 6L6.

 The reason for this is that negative feedback is applied from the opposite 6L6 plate to the bottom of each secondary through caps 115 and 119. Without separate isolated secondaries you cannot accomplish this, and the output stage will not work as designed, with higher distortion and reduced speaker damping.

Until you find the proper transformer, the resistor mod should work, but you will not get full output. You can also try replacing the resistor with a low current  audio choke of 20 Henries or more. This will give more drive and greater power output.

[AVSorrell]

Yes that looks correct now.

If you want to do it right, you really need to have a new transformer made, or find a good one from another chassis. It is not your usual driver transformer. Note that it has two separate secondaries with each driving its own 6L6.

 The reason for this is that negative feedback is applied from the opposite 6L6 plate to the bottom of each secondary through caps 115 and 119. Without separate isolated secondaries you cannot accomplish this, and the output stage will not work as designed, with higher distortion and reduced speaker damping.

Until you find the proper transformer, the resistor mod should work, but you will not get full output. You can also try replacing the resistor with a low current  audio choke of 20 Henries or more. This will give more drive and greater power output.

Thanks Mondial. I haven't heard from Heybour yet, but I hope they are able to recreate one for a reasonable cost. I was really hoping the parts set would have a good interstage transformer but same issue.

[Mondial]

If you have another parts chassis, there is a 20 H low current choke present in the power supply. It is marked 138 on the schematic.

If it is good, it will make a great replacement for the 27K mod resistor. Just connect it the same way as the resistor, between points A and B. It is not shown on your schematic, but point A should also connect to the B+ supply from cap 80B.

[AVSorrell]

Hi Phorum -

Mondial - thanks for the idea about using the choke. I have added the connection to the elctro cap on 80B. I have good news from Heybour whom I am checked with to recreate my audio interstage driver transformer (part number is 32-7865) in my 38-116, Code 121 which has an open primary. On the code 121 it is part # 117 and on the code 125 it is # 102 on the parts listing - so it works in codes 121 or 125.

Mondial has been very helpful as I developed a plan B if I could not find a transformer from a junker or one from Heybour. After talking with Heybour they will be able to recreate the transformer. They offer a price break for the additional transformers if I order more than one. So - do any of you need a audio interstage driver transformer like mine? If I order just the one I need, it is $50. If I want two or three the cost is $40 for the additional ones. So, if you need one, I can order it and you can pay me back the $40 - saving you $10 off the normal cost. Let me know ASAP as I will be placing order order tmw, Thursday 4/6.

Below are the specs I provides to them from a 1940 Philco parts listing:

audio interstage driver transformer that goes from the plate of a 6J5 tube to the grids of 2 6L6 tubes. (single plate to dual grid). Type 5 mounting style (I attached a photo showing the locations of the A, B, C measurements). Note that says: "Section # 1 and # 2 in series" and I attached a photo showing the winding diagram. I did not know the wire gauge, but I told them I thought perhaps # 40 or 41. The weight of it out of its shell was 14.7 oz. It has 6 wires coming from it.

Primary DC resistance: 400 ohms

secondary # 1 DC resistance: 175 ohms

secondary # 2 DC resistance: 150 ohms

Driver: one 6J5 tube

Output: two 6L6 tubes

Turn ratio: 1.2:1

Primary M.A.: 8

Size: A (height): 2 13/32" B: 2 7/8" C(coil width): 1 7/8" (references a picture I attached)

Mounting Holes: 2 3/8"

Core Width: 2"

[Radioroslyn]

Too bad the primary current isn't higher. If it was would probably be a good replacement for the sets with p/p triode w/ a 42 or 6F6 driver. Like a number of the 16 and 116 sets, 200X, 660, ect.

[Ron Ramirez]

...yes...but nevertheless...

where are youse guys gonna find another deal like this? If I had a 38-116 with a good driver transformer, I'd go for your deal, Andy, just to have a reasonably priced brand new spare!

I'm really surprised no one replied wanting to get in on that deal. I think $40 for a brand new custom wound replacement interstage is very reasonable.

...but I don't own a 38-116...else I would...