Beginning a 37-620 Restoration - Printable Version
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RE: Beginning a 37-620 Restoration - morzh - 04-21-2026
I left the file I derived the PNG from at work. Will incorporate that tomorrow
RE: Beginning a 37-620 Restoration - glyphmon - 04-21-2026
Mike,
Results:
First the Isotap seems to run a little high, so when it appears to be set for 120 V (100 V + 20 V on the two dials), it was actually outputting about 128 V, so that's just something I will keep in mind for the future. I backed it down one step for the test and got about 122 V. I accept that this is old equipment and may have quirks I need to work around. If I get much deeper down the rabbit hole, I will consider new equipment and separate functions (isolation vs variable transformer) as we've previously discussed.
V2: Measuring on both sides of the bulb while in circuit yields approximately 50 V.
V3: With the bulb still in circuit, measuring at the Bakelite where one wire from the cord attaches, and at the center lug of the power switch yields about 77 V. The only thing else in the circuit here is the 1A fuse before the power switch lug.
With the bulb out of the circuit, the same two points in the radio (V3) measure approximately 1 volt lower than the output of the Isotap.
As before, I wasn't really diagnosing the radio at the point of the video, but just testing different configurations for my own curiosity. Also, I wanted to record the bulb in operation for aesthetic purposes - I thought it looked nice in there.
Presently waiting on a lead for the IG-102 signal tester and an appropriate capacitor to begin the alignment.
Best wishes,
Roy
RE: Beginning a 37-620 Restoration - morzh - 04-21-2026
That's what I mean. You run your radio at 77V. Which should be some 40V higher, like 110-117V.
You no longer need that dim bulb and you will not be able to align your radio correctly with it.
Remove the bulb from circuit , plug the radio directly into the Variac, set it to whatever the nominal voltage there should be.
462ron - 462ron - 04-22-2026
Hi Roy, the Isotap is just a transformer with multiple taps (voltage settings) and so has no regulator. If your line voltage runs a little high so will the output. No big deal to compensate. +1 on what Mike said, after the set is confirmed to be working with no shorts, remove the dim bulb and then do your alignment with close to full line voltage!
Ron
RE: Beginning a 37-620 Restoration - glyphmon - 04-28-2026
Wanted to start on the alignment today, but now waiting on a couple of alligator clip leads for my multimeter (previously had just the stabby kinds) because I'll be darned if it isn't tight getting in to the plate and cathode pins on the 6F6G output tube in this chassis by itself, let alone while also trying to make the connection to the 6A8G from the signal generator.
While waiting, here are a couple questions that are keeping me on my toes. This radio seems a bit finicky in that, there are times that with the volume turned all the way down there can be some random static/noise come through. Not constant, and it varies all by itself sitting there, so logically, I would think there's something not 100% between the output and and the field coil. Where would you start chasing this noise down? I can't say for certain, but it seems to happen most frequently after making an end-to-end pass with the dial, though if it's in the audio output circuits with volume turned down, not sure why this would matter unless it's simply a time-in-operation/heat correlation.
The other thing that is problematic (and has been since before I started poking around so at least I know it's not my fault) is that at the lower end of the AM band it gets really noisy while adjusting the dial. This at least IS adjustable with the volume but it tends to try to blow my eardrums out if I forget to turn the volume WAY down. I see there are a few rotor blades (fins?) that look like they've abraded against the adjacent stator ones over time and they did get pretty close. I've tried to straighten that a little and it did improve the noise, I believe, so I suspect there's something still awry here. I don't have one of those ultrasonic baths and at this point in the game I'm not taking this thing back out to give it a bath even if I had one, but are there techniques you would suggest to dislodge any industrial-strength dust that is probably lurking inside this condenser, not to mention elsewhere? Canned compressed air doesn't seem to be cutting it. I don't SEE any other points where it looks like pieces that shouldn't be touching are.
And yes, the dim bulb has been removed - radio straight into Isotap.
Best wishes,
Roy
RE: Beginning a 37-620 Restoration - morzh - 04-29-2026
A note about alligator clips: for tight spaces, grabber hook type is better.
As the ones below.
https://www.grainger.com/product/1RK15?gucid=N:N:PS:Paid:GGL:CSM-2295:KFV4LP:20800606:APZ_1&gclsrc=aw.ds&gad_source=1&gad_campaignid=22475795968
RE: Beginning a 37-620 Restoration - DaleHCook - 04-29-2026
morzh Wrote: A note about alligator clips: for tight spaces, grabber hook type is better.
I will second that. I have two sets of test leads to connect my signal generator to a chassis, one with small alligator clips, and one with E-Z-Hooks.
RE: Beginning a 37-620 Restoration - RodB - 04-29-2026
The tuning capacitor blades should be cleaned. After 80 years they have acquired a film of gunk plus whatever has been in the air; dust, volcanic ash, etc. People have used everything from BrakeKlean to WD40 to using the dishwasher. First, you need to use a solvent to take off the crud, I use 91% alcohol with a long bristle brush. If you want to use WD40 then come back with the alcohol or electronics spray cleaner and wash off the WD40. Clean the capacitor with the gang open. After it dries you can use an ohmmeter to test each section for shorts. If a section has a coil across it, it will probably measure a few ohms and you'll have to unsolder the wire to that section. Don't forget to lubricate the bearings on both ends.
BTW Don't use BrakeKlean. Even after using it then a bath in the dishwasher it smelled putrid and had to spend a couple days in the garage to dry out.
RE: Beginning a 37-620 Restoration - glyphmon - 05-02-2026
I have more cleaning to do on the tuning capacitor and surroundings, however, I managed to get some crud out and that seems to have made a significant improvement, so that's the start of a win.
I have a suspicion that the "working" IG-102 may not be as working as advertised. It may also be the case that I just don't know how to use it well, but I tried with a variety of configurations and couldn't get any change in readings or output with the radio. If someone who uses this signal tester has the patience to give me the step-by-step setup to use on the 37-620, whether the IF alignment, or the broadcast band, anything that would be able to really tell if the signal generator is actually working, that would be much appreciated.
In the case that this unit might be something else that I need to get fixed to be able to use, I am open to buying something new and entry level. Mike's advice that you know what you get when buying new is acknowledged here. That being said, I found Ron's old signal generator recommendation thread and while trying to avoid opening the rabbit's hole of what everyone prefers/has been using, I think the consensus that the models Ron had picked out were determined to be workable. One, a KKmoon, appears no longer to be generally available. The other (Koolertron) still is. Would it still be the consensus that this will suffice for aligning AM and FM tube radios?
Mike, I saw your protection jig thread for use with the newer signal generators. I've got a 22 ohm resistor, but no stock of diodes. Could I impose upon you to provide a link to (or exact part number for) a good diode of which I could order two to put together such a jig? I did end up creating an account with Mouser so I suspect it might be something I could order there. I can probably find the BNC ends, but if you wanted to provide links to those too, I wouldn't mind.
Lastly, I think there are still more demons to be excised as there is some appreciable, randomish static coming from the speaker even with the volume turned all the way down. If the volume were up, I could accept it is legitimate static, but with the volume off, I'm thinking something is awry. What are the things I should be checking and how should I do so to try to isolate where this noise is coming from?
Best wishes,
Roy
RE: Beginning a 37-620 Restoration - RodB - 05-03-2026
I'm assuming you don't have an oscilloscope to view the signal generator output. One way to test it is to attach a coil of wire to the output leads, crank the output power to max, then use a working radio tuned to, say, 1 MHz on the broadcast dial. Set the generator to same frequency with the loop near the radio and vary the signal gen frequency back and forth. Even if it's unmodulated you should hear a quiet spot (yes, The Sound of Silence) somewhere around 1 MHz, if it's modulated and working you should hear a tone. Next question: to use the signal generator reliability for alignment, how are you going to know what frequency it's actually producing?
RE: Beginning a 37-620 Restoration - morzh - 05-03-2026
Roy
https://philcoradio.com/phorum/showthread.php?tid=19209&highlight=zener
I used a SOT23 part so not to use two single diodes.
A dual Zener 5.6V with common Anode. Like this, 2BZX84C5V1
https://www.mouser.com/ProductDetail/Diotec-Semiconductor/2BZX84C5V1?qs=OlC7AqGiEDmwy8YzVcrnVw%3D%3D
Another part that could be used is a Back-to-Back TVS diode like say this one, SY205208DXC .
https://www.mouser.com/ProductDetail/Silergy/SY205208DXC?qs=sGAEpiMZZMsItbzKdwyIe0v7IbkW2XxcjZwLeg7KTXY%252BLqjp2vEDIg%3D%3D
Of course it makes little sense to order $0.25 part fro MOuser and pay $9 for shipping.
Things to keep in mind.
Regular Zeners have fairly large capacitance and will degrade the signal when it is of a fairly high frequency. It still is usable but will not be the same amplitude as low frequency.
To avoid that the TVS can be used, but the TVS is not made for constant clamping, it is a fairly high, but very short, energy pulse protection device. So while it will protect from a very short pule, it will not withstand prolonged exposure to clamping energy.
I use Zeners.
RE: Beginning a 37-620 Restoration - glyphmon - 08-06-2026
A quick question in preparation for me getting back to this chassis since I've been focusing on the RCA in between...
This radio has not had tube shields since I've been aware of it. Since these are all glass tubes, I assume it would be best to seek out shields for those tubes that have bases meant to hold them?
I suspect these are meant to be the rounded square type of shields in this radio? Are there varieties of them or are they all pretty standard that are that shape? I note they're probably not going to be shiny at this point in history. Any sources you would recommend pursuing? I do see a couple on eBay right now.
Best wishes,
Roy
RE: Beginning a 37-620 Restoration - morzh - 08-07-2026
If the radio does indeed have the shields' bases, then it was meant to operate with the shields installed, and even if it works without them, I am sure there were reasons for using those shields.
So my recommendation would be, yes, get the shields and put them in. Starting at the alignment phase.
Where to get...the usual suspects are:
- eBay
- local swapmeets
- the "classified" (Want to buy) message board, either in this Phorum or ARF, or ....
RE: Beginning a 37-620 Restoration - glyphmon - 08-07-2026
Mike,
Thanks for the confirmation - I was leaning in that direction. Found a set of 4 on Ebay for about $22 including shipping. Unfortunately, I don't think there are any meets within about 150 to 200 miles driving from my location (unless they're in Canada, in which case they're still pretty darn far).
Best wishes,
Roy
RE: Beginning a 37-620 Restoration - glyphmon - 09-10-2026
Okay, with the RCA 6K3 finished, back to this guy.
Tube shields are now in place on all tubes where there are receivers for the shields except for one (which needs a longer grid cap wire before I can put the shield in place).
This radio still has some gremlins. The two I'm going to focus on first are a touchy band switch making it very hard to actually use AM at least, if not the other bands as well. Obviously, this probably just comes down to some contact cleaner for the band switch, but this band switch is possibly dodgy anyway which I suspect may impact voltage measurements, so I haven't taken those yet, though I can try.
The other gremlin that I would like to solicit advice on is the following:
When the radio warms up after a minute or so, we get a static-like noise coming through which is NOT affected by the volume control. When I can get the band switch to play nice and tune into a station, I can control the volume of the station with the volume control and it DOES come through, but it is mixed with this separate noise that is unaffected by volume level. It is present, unchanged, even with the volume all the way down.
In case it is relevant, as a reminder, there is no tone control in this radio. The power switch was replaced with a non-original part that doesn't include it. The blue wire from the junction of C47 and R46 is permanently soldered to ground in the vicinity of the power transformer.
Best wishes,
Roy