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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 4:01 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Good news !

Decided to go ahead and spend a few minutes on the tube with erratic filament resistance readings. After wrapping tube carefully in soft foam-rubber and placing base-up in old coffee-cup, used Hakko de-soldering gun to pull solder from filament pins and then carefully re-soldered. Cleaned pins thoroughly with Isopropyl alcohol and small pieces of paper napkin. Cold resistance now right at solid 2.5 Ohms along with the others, thankfully !

Also, removed the very intermittent and erratic filament power key-switch to disassemble and clean. As mentioned by Keith (Waterski), the heads of the two screws which held it together were potted in a hard, black substance which crumbled into a fine messy powder once chipped out carefully with tip of utility knife blade. Once apart, cleaned both the brass contact strips and beryllium copper "wiper assembly" with a special soft eraser designed for cleaning gold PCB "fingers" and then isopropyl alcohol. Applied a thin, even film of DeOxit Gold liquid to all internal electrical contacting surfaces. Was impressed by the design and quality of this very old switch which had Patent dates of FEB 28-11 & SEPT 8-17. After reassembly, it thankfully worked perfectly with solid continuity. Pictures below...

Stay tuned !

John


Attachments:
ARF THOMPSON FIL KEY-SW SCREW CVR.jpg
ARF THOMPSON FIL KEY-SW SCREW CVR.jpg [ 422.19 KiB | Viewed 730 times ]
ARF THOMPSON FIL KEY-SW PARTS CLEANED.jpg
ARF THOMPSON FIL KEY-SW PARTS CLEANED.jpg [ 153.87 KiB | Viewed 730 times ]
ARF THOMPSON FIL KEY-SW CMPLT.jpg
ARF THOMPSON FIL KEY-SW CMPLT.jpg [ 454.86 KiB | Viewed 730 times ]
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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 4:43 am 
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Joined: Sep Mon 18, 2017 2:23 am
Posts: 6192
Location: Plymouth, MI
Good work John.

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 6:00 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Thanks Dan !

This morning carefully cleaned all tube socket contact surfaces with special soft eraser we used years ago at work to clean gold edge-connector fingers on PCB's---picture of it below... Took some time to bring back shine. Then carefully vacuumed out all the residue. Will do the same for tips of tube pins before installing...

After reading several comments on-line about missing keys and broken power switches (owners didn't realize that they were designed to use a key and thought that toggles broken off !), decided to make a careful tracing of original key and add dimensions for those who might want to attempt making one. Quick & sloppy, but necessary info should be there. Feel free to share with others...

Next step is to inspect and tighten all mechanical connections and then verify all electrically while drawing a schematic.

A friend of mine who's focus is on these early sets is going to try and find an original, defective transformer that I can try and repair. If not too difficult, may simply unwind until I can find open, splice & insulate, then rewind... To do an initial test, though, will simply add load resistor to detector and couple to audio with capacitor as suggested earlier.

Found an old shielded computer monitor power transformer to use for building temporary B+ supply---am going to use voltage-doublers to get two 45V outputs which will be connected in series for 90V using tap for 45V. Have a well-regulated 5V supply for filaments and am simply going to use three "D" batteries for "C" supply...

Sure hope this true antique will work !!!

John


Attachments:
ARF RACE-CLEAN.jpg
ARF RACE-CLEAN.jpg [ 91.16 KiB | Viewed 710 times ]
ARF THOMP PWR KEY QUIK DETAIL SKTCH 7-6-20.jpg
ARF THOMP PWR KEY QUIK DETAIL SKTCH 7-6-20.jpg [ 519.31 KiB | Viewed 710 times ]
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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 7:18 pm 
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Joined: Mar Thu 24, 2011 1:29 am
Posts: 25
John,

There is not much that can go wrong with these early sets that cant be fixed, one way or another. I am sure it will work as well as any 20s radio when you are done.


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 9:05 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Thanks Mark !

Went ahead and adjusted the tension on tuning cap rotors and applied a couple tiny drops of DeOxit Gold liquid to rotor terminal contact points using "trusty hatpin". Rotated a few times and now have stable continuity throughout travel...

Step by step...

John


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 9:13 pm 
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Joined: Mar Thu 24, 2011 1:29 am
Posts: 25
[quote="Chas"]Thompson iron is known for fidelity, a replacement cannot "sound" like an original. I suggest setting an ARF Classified for the transformer. IMHO it should look a lot like the 2nd audio. Replacement windings must have a similar resistance if the plate resistance of the grid leak detector and its sensitivity are to be maintained.

The B+ bypass cap is fine, it only comes into play if a poor quality eliminator or light duty "B" batteries are used. Even then only when the eliminator is on...

If those 202's do go for sale test them well and do not make any claims for performance."As, Is, As shown" is advised. Do not do any International sale, they can go missing or be confiscated.[/quote]



Concerning the fidelity of the Thompson iron, this unit will likely be used with the horn transducer, in that case, I suspect that it would be difficult to hear any difference in "fidelity" between the transformers. Those 20's horn transducers are definitely the limiting factor to fidelity. I would not be concerned about the audio quality differences in the transformers. For what its worth, from my experiences with these horns.


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Mon 06, 2020 9:50 pm 
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Location: Plymouth, MI
There's a lot of misunderstanding about selling on Ebay and the Ebay guarantee. If you put in the description that the tubes tested good, any disclaimers are void under the Ebay guarantee. You must deliver a good tube to the buyer or they can send it back for a full refund. Sellers put those disclaimers in because they don't understand the Ebay guarantee or they are hoping buyers who don't know the rules won't return them and demand a full refund.
If the tubes test good on a good, calibrated tester, you should sell them as good tubes. You will get a lot more money for them as good tubes, and if you pack them properly they should be fine. It also helps to have photos of them being tested and passing as proof.

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Tue 07, 2020 8:06 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Dan---the radio and spare tubes aren't mine. Am refurbishing for an old friend... He'd have to make decision.

More progress this morning...

Tightened all mechanical connections and verified continuity throughout radio---all is now fine except for that defective 1st inter-stage transformer. Hope to get B+ PS built within the next couple days. If radio works decently with detector tube plate load resistor and capacitive coupling to 1st audio, will search through parts stash for possible transformers to try. If nothing suitable found, will contact owner to see if he wants to buy the P-T156 replacement which I've found is well recommended...

Also modified Freed-Eisemann NR-5 schematic to accurately show this circuit. Was surprised to find several significant differences and took awhile to complete and verify this. Once I clean this up in Paint, will share it here and with anyone interested... Neat how you can select either one or two stages of audio amplification by simply moving a good-looking nickel-plated brass plug between two 1/4" jacks on chassis panel ! Plate voltage to 2nd is disabled when only one stage is utilized conserving your B+ battery...

Plodding along...

John


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 4:16 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Threw together a quick B+ power supply this morning using a small shielded power transformer with center-tapped 36 volt (unloaded) output. Used two simple voltage doublers configured for each of the split outputs so I could get needed voltages. Output is just below 50 Vdc each leg with just under 100 Vdc across both (unloaded). Hopefully will provide enough current !

Almost ready to test !

John


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 4:35 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 3073
Location: Fenton, MI 48430
xrhonda91 wrote:
Threw together a quick B+ power supply this morning using a small shielded power transformer with center-tapped 36 volt (unloaded) output. Used two simple voltage doublers configured for each of the split outputs so I could get needed voltages. Output is just below 50 Vdc each leg with just under 100 Vdc across both (unloaded). Hopefully will provide enough current !

Almost ready to test !

John

12.5 ma with five 201A.

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 7:51 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Thanks Jim !

Got it working a couple hours ago !!!

The parts for B+ supply mentioned earlier were simply tacked-together across side of bench to test. Used 3-"D" batteries in holder for "C" supply. Couldn't find my spare 5V "brick" for filament "A" supply, so used an old adjustable Acopian set for 6 Volts.

To test before replacing open 1st interstage transformer, used brnhornet52's suggestion and added 22K resistor between detector plate and 45 volt source, then coupled plate to 1st audio grid with .1 uF cap. Also added a 100K resistor from "C-" supply for bias.

Not having the horn speaker here, used Jim's idea of using output transformer with regular speaker. Went through transformer stash and picked out the one with highest impedance impedance and connected...

Cleaned tips of tube pins with special eraser mentioned earlier before installing to ensure good electrical connection to freshly cleaned contacts.

Attached lead from antenna post to finger-stop on old 1920 W.E. dial candle-stick phone I installed on work-table to celebrate its 100th b-day earlier this year...

Made certain all connections secure, verified all voltages at terminal posts, made sure rheostats at minimum, and inserted key to turn on for first time in at least the 50 years friend has owned it. Slowly advanced the rheostats until I heard sound from speaker. Carefully rotated dials to where I thought our strongest local station might be, but nothing... Adjusted rheostats a bit higher, played with dials again and faintly heard signal. Adjusted other two and it thankfully came in strong and very clear !!! Was so excited, felt like dancing a jig ! Called the owner to let him hear it playing over the phone---he and his wife were also both excited ! He gave permission for me to purchase two P-T156 interstage transformers---one to replace the open unit and the other to have on hand if the original Thompson 2nd unit fails... Before cleaning work area, dialed the rheostats up further and was able to receive another local station. Am concerned about shortening the life of tubes, so try to keep both as low as possible...

Still need to find time for tedious task of cleaning up modified schematic, but plan to do it as soon as possible...

More to come...

John


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 10:14 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 642
Location: Bristol TN 37620
John.

Nice work. Really thrilling when one of the battery sets comes alive.


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 10:40 pm 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 1422
Location: Keystone Heights, FL, USA 32656
“...added a 100k resistor from -C for bias...”; bias to what?

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“Nothin’s worth nothin ‘till somebody wants it.”—Irv Metter


Last edited by jrehkopf on Jul Thu 09, 2020 10:57 pm, edited 1 time in total.

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 11:06 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 3073
Location: Fenton, MI 48430
Great to hear it's operating. I had same thrill with my Arborphone 27 I bought 10 days ago. All the Arborphone needed was four 201A tubes.

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Wed 08, 2020 11:57 pm 
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Joined: Sep Mon 18, 2017 2:23 am
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Location: Plymouth, MI
Glad to HEAR it John! :lol:

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Thu 09, 2020 12:17 am 
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jrehkopf wrote:
“...added a 100k resistor to -C for bias...”; bias to what?
Grid leak bias...

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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Thu 09, 2020 10:51 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Thanks Guys !

The P-T156 is on the way, so am looking forward to installing it and hearing what difference it makes. Also want to hear how it sounds with the "Music Master" horn speaker which came with radio when friend bought it around 50 years ago---just hope it's coil is still good ! Asked him to check it and let me know, but haven't heard back yet... It's a beauty appearance-wise !

John


Attachments:
ARF SPKR 4 T-N 7-9-20.jpg
ARF SPKR 4 T-N 7-9-20.jpg [ 462.76 KiB | Viewed 599 times ]
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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Thu 09, 2020 11:13 pm 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 1422
Location: Keystone Heights, FL, USA 32656
Chas wrote:
jrehkopf wrote:
“...added a 100k resistor from -C for bias...”; bias to what?
Grid leak bias...

I can’t picture this. Will wait for the schematic.

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“Nothin’s worth nothin ‘till somebody wants it.”—Irv Metter


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Fri 10, 2020 6:01 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6618
Location: Indianapolis, IN
Jeff,

I just temporarily added that 100K from grid of first audio to C- supply for testing since the interstage transformer secondary (which normally supplies path for bias) was open and was using capacitive coupling from detector output to see if radio would play. It will go away once new transformer installed...

John


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 Post subject: Re: 1925 Thompson V-50 Grandette Neutrodyne project...
PostPosted: Jul Fri 10, 2020 8:25 pm 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 1422
Location: Keystone Heights, FL, USA 32656
OK, got it. Thanks.

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“Nothin’s worth nothin ‘till somebody wants it.”—Irv Metter


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