TBS-50 Restoration Questions

[k7zmb]

Hello!

I am currently in the process of restoring an (original model) TBS-50. It is such a beautiful rig, out of respect for its integrity, I am attempting to use NOS where possible, or repair out of spec components such as the electrolytic. I do have a couple of questions, however, which I was hoping someone here might have some experience with.

R6 is listed as a 2W 1k resistor, which has drifted to about 1.4k. At around 1.5" long, I've never seen anything like this resistor. Is it a carbon composite, or something else? What am I looking at, and what are the odds of being able to find a replacement of the same type? I would hate to resort to just using something with a different physical appearance...

Right next to it on the schematics, are R7 and R8, which the schematics note are two 2W 5k resistors. In reality, it appears to be a single 10k (tests as 11.1k) resistor of a type I've never seen before. It is a 3.5" long, flat silver bar riveted in place. Fortunately it is still good, but I'm absolutely intrigued by this thing. What is it?

Thank you in advance for any insights! I look forward to reporting my progress in these forums!

-Robert, K7ZMB

P.S. I don't suppose anyone would have a replacement male power connector for the TBS-50 laying around? The key is snapped off mine.

[KA0HCP]

I have two HW TS50D's waiting restoration.  Just looked at the first one.  I don't see any components like you described.   I'll pull the other one out and also the manual with schematic.

Unless otherwise specified in the manual/schematic I would expect all resistors to be carbon composite.  That was how they made them in the 40's and 50's.

The "Power Connector" is an ordinary Octal type socket.  You can get them for about $5 from:

Antique Electronic Supply in Phoenix
www.tubesandmore.com/

or  Radio Daze in  upstate NY  (they are  moving websites, you may have look between one and the other, or call them)
old website:   http://www.radiodaze.com/
new website   http://www.radiosupply.com/ 

both are great vendors!

You have set yourself a tough task!  73, B

[k7zmb]

Thank you for the reply! I've ordered from both places, but somehow missed those plugs. They look ideal, and I was able to grab a few of the capacitors I will need to build the power supply.

I've attached a picture of the resistors, which will probably help with identification! I can't even begin to imagine that R7+R8 is a carbon composite. I didn't even recognize it as a resistor at first!

[KA0HCP]

Very intriguing picture.   Looking at the schematic, R7 and R8 are essentially a voltage divider.   It looks like that device uses the metal body as both ground and as a heat sink against the chassis and then has two tabs for each side of the divider.    Pretty neat little package.

R6 is typical of resistors of this age and older.  It is a 2W resistor, so to provide for adequate cooling, the cylinder is longer.  Much longer than is typical nowadays.  Modern materials are just much more efficient as resistors and conducting heat; making them smaller.

BTW, what model do you have?  B, C, D?

cheers, bill

[KM1H]

R-6 is a common carbon comp used in the roughly 1940-50 time frame, I run across them in radios all the time. You can hide a modern resistor behind it in parallel or even hollow out that one. Depending on the circuit it may even be OK as is.

Almost forgot, R7/8 is a wirewound also rather common in the 30's. Maybe they got a good deal on both resistors.

I ran a TBS-50D mobile in a 49 Ford for a few years in the mid-late 50's.

Carl

[k4kyv]

Those resistors are all very common from the 30s-50s era.  R7/R8 is a wirewound resistor, resistance wire wound over some kind of composition material, wrapped in paper insulation, with the metal case serving as a heat sink.  R6 is a carbon composition resistor.  Depending on what its function is in the circuit, the drift in value may not have a significant effect on performance of the rig.  Nearly all carbon comp resistors from that era have drifted at least that much. Measure the voltages at the tube pins.  If the voltages are off less than 20% it probably won't have any effect at all.  If they are off less than 50%, it may or may not cause any significant deterioration in performance; chances are, it won't.  If not, I'd leave it alone even if it is out of tolerance. Sometimes, even 100% off nominal value won't hurt anything.

[k7zmb]

Very intriguing picture.   Looking at the schematic, R7 and R8 are essentially a voltage divider.   It looks like that device uses the metal body as both ground and as a heat sink against the chassis and then has two tabs for each side of the divider.    Pretty neat little package.

Are you certain that is the case? When I look at the schematic, it appears that when switch SW3-F is set for 10m, the 10k made up by R7+R8 is simply bypassed, leaving the 1k for R6 in the circuit. When switched to 2m or 6m, all three resistances are bypassed. The schematic doesn't show R7 and R8 electrically connected directly to ground. I assumed it was riveted in place for heat dissipation, not as an actual electrical connection... Just when I thought I understood that section of circuit!  

I'm pretty new to all of this, so I'm easily confused at this point! Just ask my elmer, who I am sure is reading this slapping his forehead!  

R6 is typical of resistors of this age and older.  It is a 2W resistor, so to provide for adequate cooling, the cylinder is longer.  Much longer than is typical nowadays.  Modern materials are just much more efficient as resistors and conducting heat; making them smaller.

It doesn't look like the other 2W resistors, which is what threw me. I need to try to keep in mind that Harvey-Wells probably just used whatever they had on hand and/or got a good deal on at any given time. Heck, whatever ham owned this radio before me may have replaced it with whatever they had on hand as well, so it might not even be original to the rig!

BTW, what model do you have?  B, C, D?

cheers, bill

None of the above.   It's the first release. When they released the CW-only "B" model, they seem to have re-released the original model as "C" and the "A" model as "D". That's one of the things that attracted me to it; models this early seem pretty rare! I've attached a picture of the front panel.

-Robert, K7ZMB

[k7zmb]

I ran a TBS-50D mobile in a 49 Ford for a few years in the mid-late 50's.

I saw the instructions on "mobile installation" in the documentation, and giggled like a school girl! If it wasn't for all of the RF noise in my Toyota Prius, I fear the temptation would be too great for me! 

What did you use for the receiver?

-Robert, K7ZMB

[KA0HCP]

Robert, you have corrected me on R7/[.  Thus proving my memory isn't long enough to go from the shack on ground floor to the computer on the second floor. 

Bill

p.s.  Harvey Wells wasn't the only company to be completely pragmatic on component selection.   I'm re-capping a National NC-173 which was manufactured for about 4 years, circa 47-50, and have seen photos of three different variations of component types. 

[KM1H]

What did you use for the receiver?

A Gonset converter into the BC radio. It was a losing job to get that car RF quiet, the spark plug wires were solid copper; resistor plugs and wires werent available yet. I tried the snap-on plug resistors but with the high compression heads all it did was cause misfiring.

I added a noise limiter to the car radio and a lot of filtering under the hood which made operation tolerable.

Harvey Wells wasn't the only company to be completely pragmatic on component selection.   I'm re-capping a National NC-173 which was manufactured for about 4 years, circa 47-50, and have seen photos of three different variations of component types.

 

Those were transition years between high quality and the cheapest parts possible brought about by new owners and their own management team. I have a first run 1946 NC-240D that has all resistors well within spec and a 1948 version that needed almost all replaced.

Carl

Carl