Beginning a 37-620 Restoration

Beginning a 37-620 Restoration - Printable Version

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Beginning a 37-620 Restoration - glyphmon - 03-28-2026

N00b Edit Already!: If this is a feature, could a moderator please move this to the Electronic Repair forum, since that's where I intended it to go and where it belongs?

I'm new to the forum, and new to this kind of restoration. I've got a pretty solid IT background and basic understanding of electronic components, but this will be my most in depth attempt at soldering and replacing components, so I beg your patience. I'll probably ask a lot of questions, but I promise to try to search for the answers first.

The radio I'm working on was owned by my grandfather. My father's memory of the radio was listening to the address announcing the attack on Pearl Harbor. He would have been 15 at the time. I had the radio serviced by a short-lived local repair shop that dealt in vintage/classic audio equipment about 20 years ago. To my knowledge, tubes were tested and replaced as necessary. They also replaced the electrolytics (I may want to verify some of the choices he made for mfd ratings with folks here later), and it looks like maybe one of the other capacitors. Most of the paper capacitors appear to remain which I know will need to be replaced.

The radio, at least up until the point that I removed the chassis from the cabinet did work, but I would say to an extent. I could tune in a few AM radio stations, but not much, and practically nothing on shortwave. Also, there was a fair amount of hiss/hum, and the noise did seem to increase with amount of time run. Doing a deeper dive inside makes it clear that there's a lot of brittle points with some wires basically snapping off where soldered. The insulation on the wires running to the speaker and field coil are in really rough shape, so I'm very tempted to be heavy handed in replacing wires as I go.

I gather from comments I've read that this is one of the PITA sets to work on because of the cramped nature of the chassis, so I get it that this being my first attempt will be a bit of trial by fire. I'm good with that. I have downloaded the section on the 37-620 from the library here and have been trying to mesh my understanding of what's in the radio with what's in the schematics and the diagram. Enough introduction.

Any case, my preliminary questions are just a few as I get ready to replace a lot of capacitors.

1 - This is a "Run 2" 37-620 apparently, because the "Double Bakelite Condenser" 3793-DG moved to the rear of the chassis. The thing in its place, an 8318-SU Bakelite looks the same, but replaces what appears to have been a standard paper 30-4380. What's the consensus on replacing the 8318-SU?

2 - Related to the above, one of the things I puzzled over for so long with the 8318-SU was that, though I read only lugs 1 and 3 are what are used in the capacitor, lug 2 has a few things connected to it as well. Am I correct in assuming that the middle lug is simply being used as a convenient place to bind other common components that is not electrically connected to the internal capacitor? If that's the case and I should replace the 8318-SU, I should be fine just to remove it from the mix and connect the other pieces together from the middle lug?

3 - Two of the things connected to one lug on the 8318-SU is another capacitor (part 56, a 30-4112) AND a short wire, the other end of which I'm not sure if I broke off in my prodding, or may have already been disconnected. I suspect it may be a connection to the tone control aspect of the power switch. (I certainly couldn't tell if the tone control had any effect on the stations that I could tune into.) On the power switch, there are two lugs, adjacent, and currently occupied that look to run to the rear of the chassis and tie into the power system as expected. There appear to be at least two others tighter to the chassis front and oriented further upward that have solder on them, but no wire connected. Thoughts? Excerpt from the schematic is attached.

4 - I'm making my own map as I go along to match up connections as I swap stuff out since I feel like this is going to require a pretty extensive disassembly to reach the deep capacitors. Mounted to the side of the chassis on the opposite end as the power transformer, there is a long, non-conducting bar with a number of metal attachment points (guessing these may also be called lugs). Not sure the term for this long piece, but it appears to be just a place to tie components and wires together. On the very first location, to which resistor 46 and capacitor 47 are jointly attached, there is a wire, I believe originally blue, now faded, which, according the the schematics, is supposed to run to the "ON" position of the power switch, however, after a splice to a different section and color of wire, it chases down to a mount point on the chassis where the power transformer bolts down. The way I read the schematic is when the connection is to the chassis, this is shown as ground. This is more of just a verification that this is correct, still, seems odd that there's a splice with a different piece of wire, and that it needed to run so far around the chassis to attach. Excerpt from schematic attached.

5 - I see renovatedradios.com is a source for reproduction parts, including the grommets. I'm aware of the three that mount the tuning part of the chassis to the rest which of course are all degraded. It appears that there were supposed to be rubber grommets (or something similar) between the chassis and the cabinet, through with the screws that fasten them together pass. Only one remains, but there are holes for the others.  Is it optimal to replace all of these (3 chassis + 4 cabinet) or do the ones beneath the chassis into the cabinet not really matter? I assume based on all of the electrical connections to the chassis itself, it is something that should be insulated from contact where possible.

6 - For the internal wiring that ultimately needs replacing, what specs should I be looking for in a replacement? Period-accurate isn't essential for me, though if there's something that preserves the aesthetic with contemporary quality, I'm good with that too. It looks there might be a couple different gauges in use, but not sure how essential that variation is. I'm guessing braided is more workable than solid.

7 - I'm going to probably replace the electrical cord as well - there's a bit of tape near the plug end and some frayed fabric outer insulation. That being said, I know there is the ground post on the rear of the chassis which the manual says should be connected to the nearest metal pipe or radiator - in the days of cast iron and copper pipes, for sure. It strikes me that this is ultimately the same ground that electrical systems try to duplicate with the earth ground rod and wire. Is it advisable to replace the cord with a three-wire (and three prong) replacement and internally connect the ground wire to the ground post on the rear of the chassis, or is that just crazy talk and I should stop working on this right away?

8 - I'm aware there's a separate forum here for the cabinet restorations BUT I figured I'd throw this out there in case a 37-620 tombstone owner is reading and/or answering. The Photo Finish isn't in absolutely destroyed but there are a lot of damaged patches. I know there's no good reproduction readily available for it. Would someone have (or be willing to take) and send in a message (or a reply) a high-resolution photo of the front of your 37-620 with sound Photo Finish? I'd be interested in trying to find a way to manipulate an image into something that could be printed professionally on a compatible medium.

I'm certain there will be more questions, but this is enough for a start.  I can get the best photos I can to further the answering of any of these questions as necessary. Thank you in advance for any wisdom imparted. This community looks like a tremendous resource.

RE: Beginning a 37-620 Restoration - morzh - 03-28-2026

Glyphmon,

Welcome!

Right now I have to run and others will chime in, but I have one request:

For better structuring, if you continue this topic as the restoration, would you please then do it in "Philco Electronic Restoration" partition?

Best

Mike

RE: Beginning a 37-620 Restoration - glyphmon - 03-28-2026

Mike,

I realized that as soon as I hit submit and went back to the main index of forums. I meant it to go there. Not sure if threads can be moved in this forum or now, but I could also re-post correctly and close this thread.

Best wishes,
Roy

RE: Beginning a 37-620 Restoration - RodB - 03-28-2026

Glyphmon, welcome to the Phorum. I'll let Mike take the reins since he was in the cue first and the rest of us will chime in with our thoughts and prayers. I currently have a 37-630 console chassis on my bench and since they are similar with minor differences I will take a look at what they are and report back. I obtained the radio for free and only decided to do the restoration because the cabinet is in such good shape. Mike will explain his technique for restuffing the bakelite blocks, listen to him, it works. Things to stress: verify the wiring with the schematic, take photos and put tags or tape on wires as you remove them.

RE: Beginning a 37-620 Restoration - morzh - 03-28-2026

I moved the thread.

RE: Beginning a 37-620 Restoration - morzh - 03-28-2026

A word of caution: try to fight the temptation of pluggin in ANY radio, even if you knew it all your life, without having done a full re-cap. All paper and electrolytic caps should go, the mica caps stay unless damaged.

#1. Replacing caps is just that. A cap is a cap. It is up to you, electrically (for this purposes) they are no different.

#2. When you do not know if a lug of a capacitor is just a solder post and has nothing inside connected, or it does, look at the wire coming to that lug from the inside. If it is there, it is connected, if there is nothing comint to the lug, it is a convenience solder post.

#4: it is called "Solder post / solder terminal strip". Sometimes there are posts that have a second terminal that could be riveted to the chassis for things soldered to it to be grounded.

#5. In general, the grommets under the tuning caps have two parts AZND sometimes they have a metal cylinder inside. The bottom part is needed as without it your top part does not act as good, since the screw now connects it to teh chassis and the whole thing becomes rigid. And, in some cases, where the tuning cap is wanted isolated, it is not connected by the screw.
I often use grommets from Home Depot, or order on Amazon.

#6. For most places anything from AWG22 to AWG18 is fine. The currents are minuscule. The sole exception is the filament wiring, which for some tubes could be in amps; I use AWG 20 to 18 tthere. Solid or stranded - matter of personal preference.

#7. By using the Grounded cord you improve safety. Sometimes, this GND is a bit noisier than the waterpipe as it goes to the bar to which other equipment is also connected. Try and you will see which is better.
This said, most radios will pull the stations without any explicit GND.

RE: Beginning a 37-620 Restoration - glyphmon - 03-29-2026

Thank you for the responses received so far. I've managed to free up the tuning module from the main chassis for some extra elbow room/compartmentalization of repairs. Lots of pictures and notes taken so far for reassembly.

A new question as I prepare to order some components. I went through and tested the resistors (all except one - see notes below) and none test at the ohms the parts list specifies (no surprise I'm sure). Results are below, a couple are somewhat close to spec, others way off. I've read that radios of this era were built to tolerate a variation from spec of up to 20% because of the quality/reliability of the components of the day; also that it's safer to have a little higher than rated than a little under. I'm inclined to replace them all given some of these readings, and given that I hope to never have to take this apart again. Thoughts?

Schematic No - Intended Rating - Measured value
6 - 61,000 ohms - 57,000 ohms (under by 6.6%)
16 - 51,000 ohms - 89,000 ohms (over by 74.5%)
17 - 20,000 ohms - 23,700 ohms (over by 18.5%)
19 - 10,000 ohms - 11,960 ohms (over by 19.6%)
29 - 70,000 ohms - 79,000 ohms (over by 12.9%)
30 - 32,000 ohms - 37,400 ohms (over by 16.9%)
33 - 1,000 ohms - 853 ohms (under by 14.7%)
34 - 400 ohms - 735 ohms (over by 83.8%) - this is the Bakelite, wraps around a paper capacitor that also needs replacing, any special guidance on this one?
39 - 51,000 ohms - 55,100 ohms (over by 8.0%) - might be safe to keep in place?
41 - 490,000 ohms - 500,000 ohms (over by 2.0%) - probably safe to keep in place?
44 - 1,000,000 ohms - 1,500,000 ohms (over by 50.0%)
46 - 51,000 ohms - 49,700 ohms (under by 2.5%)
50 - 1,000,000 ohms - 1,140,000 ohms (over by 14.0%)
53 - 1,000,000 ohms - 1,130,000 ohms (over by 13.0%)
55 - 490,000 ohms - 480,000 ohms (under by 2.0%)
58 - 1,000,000 ohms - 980,000 ohms (under by 2.0%)
64 - 9,000 ohms - could not find in chassis; I see it looks to be between electrolytics #62 and #18 electrically, and connected to a bunch of other things, but it's not on the diagram of the chassis; excerpt of schematic below

Aside from those questions, some follow up to the feedback I've received:

"All paper and electrolytic caps should go, the mica caps stay unless damaged." - Heard and on-board. Electrolytics already replaced, paper will all go for sure, micas can stay. What about the two "Semi-fixed" ones that are inside the tuning circuit? Which category do they fall under? Like the micas, their ratings are in pfd instead of mfd.

#1 - My question about the 8318-SU before was based in the fact that I didn't know the underlying technology inside the bakelite housing(s). Are they effectively equivalent to the paper caps in that sense? I see a lot of talk of "re-stuffing" so I presume they are most often included in re-capping. Follow up, now that I realize it however, I believe the capacitors that replace the electrolytics are polarized; the rest that we're dealing with should not be polarized, correct? So directionality in installation shouldn't matter going forward?

#3 - Expanding on before, I've added a photo below to show the power/tone control. The two power wires go to the lugs at the back (black) part of the pot; at least two lugs appear further in stemming from the metallic section nearer the chassis front. The little wire dangling up and over, floating to nowhere comes off of lug 3 on the 8318-SU which as I mentioned before (on the schematic) looks like it's supposed to connect with part of this pot, but not sure if that's right and if so, which lug to connect to.

#4 - Just want to verify still if it makes sense that the blue wire (also seen in the photo above) running from the solder terminal strip at position 1 that's supposed to run to the "ON" position of the power switch in the schematics should be spliced and taped to a different wire that bolts down to the bottom right (as pictured) bolt holding the power transformer. It also is just weird to me that it's spliced to a different wire and wrapped in tape. I'm not sure if this was a modification after the fact since it doesn't seem like the way it would come from the factory, but it is definitely old tape. I presume this is one of the hot chassis setups so I guess it could be part of making that circuit.

#6 - Sounds like to be safe I could plan on using 18ga stranded for everything I might feel a need to replace, then. Separately, I noticed one wire that seemed to take a longer route than necessary between ends. As long as I'm not stuffing feet of wire into the chassis, if I find something that needs replacing, is there any difference if I run it one place vs another electrically speaking? With few exceptions, they're all single wires, so not likely a lot of twisting to get rid of EMI.

#7 - I think I'll forgo the idea of a grounded power cord. As a follow-up though, I've read that switching to a polarized plug is a good safety improvement which I'd like to pursue if that's the case. What would the correct way to wire that in? The current cord and schematic don't really give any indication one way or the other. I've also seen it suggested to add a fuse into the circuit for safety. Optimal siting electrically/on the schematic?

Thank you for your earlier answers, Mike. They are very helpful. I have no intent on powering anything on until we're solidly back together with all the possible replacements done. Rod, looking forward to hearing what you see in yours, especially as it may relate to questions 3 and 4 above.

It's amazing how cramped and exposed all these components were, perhaps even more amazing how well it worked AND that there weren't more electrical fires and electrocutions prior to solid state.

Best wishes,
Roy

(PS - unrelated, just picked up an RCA 6K3 this morning for $25 with a reasonably nice cabinet that will need a similar electronic overhaul, but that's a project for another day. Facebook Marketplace, all of about 9 miles away in rural Maine.)

RE: Beginning a 37-620 Restoration - morzh - 03-29-2026

1. So you have two bad resistors, the rest within 20% after 90 years, and you want to change them all because you are afraid that in your lifetime they will get much, much worse?
Well, electrically there is nothing wrong with it. Is it a waste of effort? I think, yes. But it is your effort and your time.

2. I think it is a mistake in the chassis diagram. What they show as the resistor #68, which is the ONLY resistor that is connected to the cap C62, is supposed to be the #64, 9K resistor. There are no other resistors there.
Wereas #68 in the parts list is listed as the Wave Switch Antenna, which it is obviously not.
The big resistor next to the transformer, white with red dot is the 9K resistor.

3. All trimmer/padder caps (inside or outside RF transformers) stay, unless physically damaged or rusted beyond repair.

4. All backelite block caps are paper caps. All go.

5. Tone control: trace it, it is a simple circuit. One wire goes to R46-C47 connection point (right bottom in the chassis diagram, they go to that solder strip), another to C56-C57 conenction point (they are right over the power transformer). There should be nothing else othter than chassis connection which is likely done through bolting it to the chassis, but could possibly be a wire.

6. Stick to running the wires the way they have been routed. While mostly not important, there are cases where the wire was intentionally routed away form an offending signal to reduce hum or noise.

RE: Beginning a 37-620 Restoration - DaleHCook - 03-29-2026

glyphmon Wrote: 6 - For the internal wiring that ultimately needs replacing, what specs should I be looking for in a replacement?

The replacement wire per se has been addressed well upstream. I try to stock both solid and stranded wire in all ten RMA colors, in gauges from 26 to 18.

My caution is to replace only one wire at a time. Take plenty of photos so that a replacement wire can be dressed (aka routed or located) exactly as the original was dressed. Proper wire dress can be important in some circuits.

glyphmon Wrote: I went through and tested the resistors...

Did you do so with the resistors in-circuit or out-of-circuit? Testing in-circuit can result in measurements which are too low because of connections to parallel resistances. However, any resistor which measures too high in-circuit is almost certainly bad. I prefer to test all componrnts out-of-circuit by disconnecting one end, and then testing with a quality LCR meter (I own a DER EE DE-5000).

RE: Beginning a 37-620 Restoration - glyphmon - 03-29-2026

Mike,

Thanks for the insight about the probable error on the chassis diagram. Testing what was marked as #68 yields 18,730 ohms, instead of the 9,000 prescribed.

So that means that these four vary by over 20%:
16 - 51,000 ohms - 89,000 ohms (over by 74.5%)
34 - 400 ohms - 735 ohms (over by 83.8%)
44 - 1,000,000 ohms - 1,500,000 ohms (over by 50.0%)
64 - 9,000 ohms - 18,730 ohms (over by 108.1%)
are the only resistors that you would worry about replacing, correct?

I'm good with that. My thought to replace all was two-fold: probably not able to tell which ones varied from spec because they were degrading over time/use versus had been that way from the factory; desire to have it as close to originally designed as possible. That being said, if it is a waste of time/effort/money - and I DON'T have a desire to un-solder and re-solder things in cramped places if unnecessary, I'm perfectly happy to keep originals in place other than these four.

4 - found your guide on the Bakelites - will go from there. The pictures make that process much less daunting.

5 - the blue wire from the R46-C47 solder strip connection point is the one that runs to the chassis bolt at the corner of the transformer (spliced/taped along the way); the other short wire from the C56-C57 connection point is the one that's dangling, and is, I'm pretty sure the wire I've highlighted in red on the schematic below. I'm just not sure what the disconnected end is supposed to connect to in the chassis. What is that unmarked circle it's running to? I thought it would be part of the power/tone control pot but there are at least two vacant lugs I can see, and not sure that it was even connected before I opened it up.

[attachment=34422]

Thank you again for your time and patience.

Best wishes,
Roy

RE: Beginning a 37-620 Restoration - RodB - 03-29-2026

Hey Roy,

The variable mica caps inside the IF transformer cans can be left alone. Very few have problems and any issues are usually fixed with disassembly and cleaning. The mechanics of them are a sheet of mica separated by a static metal base and a spring loaded metal leaf. There's a hole in the center of all three to allow a screw to clamp the sandwich. As the screw tightens the capacitance between the metal plates gets higher and vice versa. Now you're tuning the circuit.

Bakelite blocks were made with a black tar-like substance that can be heated and removed in one solid lump. There's a procedure elsewhere in the Phorum. The caps in the bakelite blocks are for the most part paper caps. There are some bb that have caps in pico farads and I usually replace them with disc ceramic caps. The bb in the power transformer primary has paper caps and they have to be replaced with Y2 safety caps. You should add a fuse to that circuit and also rewire the caps so they are in the circuit only when the power switch is on.

It looks like you have the bug, as you indicated another purchase. I would suggest ordering the standard parts in quantities of 10 or more, saving 40% or more on unit cost.

I use 20 gauge wire for most of my wiring. I also have a supply of cloth covered wire salvaged from other chassis guttings that I use for external wiring like pilot lights. In almost all cases I usually have to replace the speaker cable and I use 18 gauge at least for the field coil.

I will check back shortly on your questions 3 & 4 when I get down to my bench.

RE: Beginning a 37-620 Restoration - glyphmon - 03-29-2026

Dale,

Thank you for the insight about in-circuit vs out-of-circuit testing. Admittedly, these were all in-circuit tests for convenience - quite a few were difficult to reach with the tester, let alone soldering iron. Mike has highlighted those that varied by greater than 20% as being of concern for him which I absolutely agree with. Certainly the ones that were approaching the 20% threshold I was nervous about in the first place. Would your suggestion then be that these others also be replaced? Given the margin of error with in-circuit testing, they seem like fair game once again.

17 - 20,000 ohms - 23,700 ohms (over by 18.5%)
19 - 10,000 ohms - 11,960 ohms (over by 19.6%)
29 - 70,000 ohms - 79,000 ohms (over by 12.9%)
30 - 32,000 ohms - 37,400 ohms (over by 16.9%)
50 - 1,000,000 ohms - 1,140,000 ohms (over by 14.0%)
53 - 1,000,000 ohms - 1,130,000 ohms (over by 13.0%)

These were certainly much closer to spec but still over:
39 - 51,000 ohms - 55,100 ohms (over by 8.0%)
41 - 490,000 ohms - 500,000 ohms (over by 2.0%)

Would you say that anything that tests under while in-circuit is likely fine as long as it's within the 20% disparity? Most seem fine but with this refined understanding, 33 gives me pause.
6 - 61,000 ohms - 57,000 ohms (under by 6.6%)
33 - 1,000 ohms - 853 ohms (under by 14.7%)
46 - 51,000 ohms - 49,700 ohms (under by 2.5%)
55 - 490,000 ohms - 480,000 ohms (under by 2.0%)
58 - 1,000,000 ohms - 980,000 ohms (under by 2.0%)

Best wishes,
Roy

RE: Beginning a 37-620 Restoration - glyphmon - 03-29-2026

Rod,

Thanks for the confirmation on several details. I found Mike's guide on redoing the bakelites.

Can you be a little more explicit on what to do to achieve this:
"You should add a fuse to that circuit and also rewire the caps so they are in the circuit only when the power switch is on."

I tried studying electronics with a course from Heathkit in the early 90s (when I was in grade school) but I think my parents over-estimated my ability at the time, and I never got back into it as much as I should have. I look forward to hearing what you see in your set as I'm not 100% convinced this power switch/tone control circuit is wired as intended and indicated by the schematics.

Also, can't afford, nor have space, nor time for a hobby bug to take hold right now. It was close, cheap, and not bad looking. Art deco floor models have a certain allure - well, some of them, anyway. If I get my workspace built in a decade or so, then I can get cracking with inventory.

Best wishes,
Roy

RE: Beginning a 37-620 Restoration - morzh - 03-29-2026

What Rod likely meant is, move the lead of the cap inside the 67, that connects to the switch, to the right of the switch. So if you pull the plug and touch the prongs, you won't get shock (it won't be too bad but you'll feel it). Another wasy of ensuring it won't happen is to add a fee hundred K's of 1M resistior in parallel.
A fuse goes in series with "Hot lead" or two fuses for both leads.
I don't do it, as I do not keep the radios plugged all the time, but if you do, it is a good idea.

RE: Beginning a 37-620 Restoration - DaleHCook - 03-29-2026

glyphmon Wrote: Would your suggestion then be that these others also be replaced?

Roy -

You could replace the ones which are over. I would always test them out of circuit first, but then, I am a professional, using professional equipment including a vacuum desoldering station, so it is likely easier for me to isolate and test components than it is for you.

When it somes to the ones which measure under, if it is easier for you, go ahead and replace them - replacements are neither rare nor expensive. You might want to test the old parts after they are pulled out to see what their out-of-circuit values are, compared to their in-circuit values.

I gather from your first post that you are new to soldering. Here are three important rules:

1) Too little heat or too much heat when soldering may lead to problems.

2) Always remove old solder before re-soldering. Many of us use desoldering braid or a pump-type solder removal tool like an Edsen Soldapullt. I suspect that fairly few of us have a vacuum desoldering station or a hand-held vacuum desoldering tool like a Hakko.

3) A good solder joint begins as a good mechanical joint. Use longnose or needlenose pliers to tightly wrap leads or wires around terminals of lugs before soldering them.

I have a fourth rule which I think works best. I only use lead-tin solder (mine have flux cores). Lead-free solder does not seem to work as well for me as does the lead-tin solder that the factory used.