Step by Step Restoration: GE T-106C, Let's go for it!

.......................DATE CODES................!

The vast majority of electronic components use some sort of date coding. It was a way (and still is) for manufacturers to keep track of inventory and quickly isolate a "batch" that had issues if a problem was found.

Time to play detective.

Most date codes use a 4 digit format. In the 1950s, they used a 3 digit format.
First part of the number is the last digit or digits of the year. The last two digits are the week of the year. So in essence, you read the code backwards.

Here's the tricky part. Sometimes you have to disect this number from a bunch of other numbers like the electronic manufacturer's code, the in-house part number, etc.
As I said before, in the 1950s, they only used 3 digits, so you only had one digit for the year. So if the first number was "7" this meant 1957. I guess the manufacturers realized they better get there act together when the 1960s rolled around as there would have been confusion if something was needed to be identified as being made in 1957 or 1967. Thus, the 4 digit code was born.

Time to put on your detective hat and let's look at some of the parts in this radio. I've made it easier for you by indicating where you need to look


Let's take a look at the filter capacitor we removed....



There's the date code--the last digits in the sequence!

The "240" is the EIA code. (Electronic Industry Association identifier). This shows where the part came from.

Here's a neat list to see the codes to find out who provided parts in your next project:
http://www.triodeel.com/eiacode.htm

Let's look at some other parts in this radio to see the date codes....



Looks like we are starting to see a trend here with our audio output transformer.....

Let's check another.....



The audio coupling unit's date code is definitely painting the picture for us.....


Well, I don't know about you, but I'd say it's official...this radio was not manufactured in 1957, in fact wasn't even built in the 1950s. We have a vintage 1963 General Electric model T-106C. Just think, it will be celebrating it's 50th birthday this year! We have a vintage Kennedy era radio here, not an Eisenhower era radio.

Now, see how helpful date codes can be when used with a little detective work?!

Stay tuned........

Thanks for taking the time to do this. Very informative!

Great job very imformative! Thanks for doing this...

A big story in a small radio. Good work!

No takers? C'mon, no one wants to even wager a guess? I'll give you some more time to think about the quiz question posted above


Quiz time: Do you know why a clock-radio would have a filter capacitor(s) with greater capacity (resulting in more filtering of the pulsating DC out of the rectifier tube)?? This is quite typical. Post a reply if you know and we'll see who gets it first and with the correct answer

Would this be to preclude any interference of the line ac that the clocks' synchronous motor needs to keep the correct time?

That's a good guess, but I'm sorry that isn't it.....

Any others?

Seems to me it's because of the hum problem because of the ac clock motor being so close.
Just my .02.
Bill Cahill

I'll reveal the answer Friday or Saturday....

I'll give you a *hint*, think hum, but it's not clock mechanism related.....

Okay, time to reveal the answer to our quiz, thank you to those that made an effort to venture a guess...

The question was why did manufacturers typically bump up the size of the filter capacitors in clock-radios? What was the point of larger capacity to filter the DC?

Answer: The vast majority of clock-radios were used in bedrooms, typically on night stands--the key being in close proximity to the person or persons in bed. If the radio was played at lower volume levels--which was typically done, especially if the clock mechanism had a "sleep switch", a person listening or trying to fall asleep would find any hum or lack of clarity in the audio annoying. By increasing the filtering of the pulsating DC from the rectifier tube, the smoother cleaner DC meant less noticiable hum at lower volume levels. Typically simple table radios (like the one we are working on) could get away with less filtering as they were turned-up (volume wise) for music/entertainment/news etc. in a room/workshop etc.

Now you know.

ah very interesting....

Then, pray tell, is it that some radios if you increase the filtering on the rectifier, does it get worse?
Bill Cahill

Hmmm. Another question....
Why is it that typically, RCA used 80 50 filtering, but, say a GE can get by with 50 30 mf filtering?
Bill Cahill

Bill Cahill wrote:Then, pray tell, is it that some radios if you increase the filtering on the rectifier, does it get worse?
Bill Cahill

Bill, not sure if I understand...What gets worse? Increase the filtering, the "cleaner" the sound....

If you are asking about the load on the rectifier, 80uF is about the max I've seen directly connected to rectifier's output (I'm talking 35W4's here) WITHOUT using some sort of current limiting resistor.

Bill Cahill wrote:Hmmm. Another question....
Why is it that typically, RCA used 80 50 filtering, but, say a GE can get by with 50 30 mf filtering?
Bill Cahill

It all depends on the manufacturer and what they are trying to accomplish. It seems Zenith made a decision a long time ago to use dual 80uF / 40uF capacitors in all their AM and AM-Clock table radios. Kept it the same across the aboard....Simplicity? Wanted a superior sounding product? Got a GOOD price break on ultra-large quantity of 80/40's?

This GE we're working on is simple and meant to be "cost effective". The 50/30 filter cap is quite common and of course found in this one as it get the job done. On GE's clock-radios, most of the same era used the same basic circuit, only they bumped up the 50uF section to 75uF to provide more filtering to the output transformer for the "cleaner" sound I talked about.

I know RCA and Zenith both did another trick for filtering, it is using a "tapped" primary in the audio output transformer in which the R-C circuit apart of the filtering from the rectifier tube is brought into this tap--this acts as a "hum bucking" device to clean-up the DC some more. Again, it depends on the engineering (or lack there of in some cases) and what a manufacturer wanted to do with parts on hand or how much $ they wanted to spend.

What I meant is some radios, and, for that matter, record players, that had a lower rating of filter on rect., if you increase microfarads of first filter, the hum actually gets louder.
Go figure. I wonder if it has anything to do with that better sets put an ac filter accross the rectifier tube. Usually a .05 from plate to cathode.
Bill Cahill

That's very interesting Bill. I've never run across that. I'll have to play around to see if I encounter that--do you have any specific models with a schematic to look at?

The plate to cathode bypass cap is on this radio, but I don't see how that would negate hum, my understanding is this is the same as paralleling a cap across a modern silicon diode--used as a noise shunt to allow transient noise spikes to ride across the cap...

Hmmm. Never heard of that. No, I't sbeen awhile since my last one. Next time I find one I'll definately let you know.
But, I do have one later Zenith am fm table tube radio in a wood case. I think that one uses low value filters. However, it did use a solid state rectifier.
Bill Cahill

This is a great project. Thanks. Good luck on your week long house project. We'll wait here until you get back. Good luck.
This is a great thread.
Bill Cahill

Just an update...have more pics to process which includes isolation transformer use and safety, a hand-drawn schematic of the power supply circuitry in this radio vs. the schematic available, and what appears to be a factory wiring mistake?!

We'll be keeping the tubes found inside as they all tested good (well emissions wise) for now....

Just need to find some break-away time from another major project for the picture processing.

Okay, time for some updates...

While the printed circuit assembly is out of the radio, it's time to do some "PM" (preventative maintenance)...

Printed circuits and vacuum tubes really weren't the greatest of combinations. Early boards were prone to failures from traces and connections lifting and breaking--and of course, the biggest influence--HEAT from the tubes. The two hottest running tubes in this circuit (temperature-wise) are the 50C5 output and the 35W4 rectifier. All this doesn't just warm the inside of the cabinet. The downside is the heat is also radiated into the solder connections and traces into the board. An experienced eye catches all the bad connections that have resulted from the heating and cooling cycles produced by the radio's years of operation.

We'll carefully check the trace-side of the board for anything that looks out of the ordinary and any bad connections that need to be touched-up with resoldering...

Here's an example of some sloppy factory work. There's actually a "tail" of solder that didn't completely melt and is loose--this is a short circuit waiting to happen. This will get removed.




Here's what I am talking about...this is VERY common in not only GE radios, but all radios that use printed circuit boards. Look carefully....

See how there appears to be a fine crack around the tube pin solder points. The radio may appear to operate fine, but if you don't take care of this, as the heat builds, these types of connections will cause nasty intermittent failures...



Of course, the hot output tube's socket pins were the worst, but I ended up resoldering ALL the socket pin connections. After 50 years, a little PM doesn't hurt at all.

*Here's a hint* What I do is take my desolder braid and desolder most of the factory "glob" that connects the pin to the traces. This leaves pretty much the pin and the trace without all the garbage (contamination) and old solder making another poor connection as you try to heat it up to apply the new solder.

Here's what it should look like afterwards...bright and shiny and uniform. No rosin joints, no "cold" (poor) joints etc..



Like many other GE's of this era, there was a LOT of solder joints to touch-up for reliable operation. But once it's down, it's that much less to worry about or cause grief in troubleshooting down the road.

Before applying power, we'll continue on to verify component values, look for any damage etc. It's a good idea to check the resistors to see if they have "drifted" in value. We'll be checking these in-circuit, so the basic idea is you don't want to see a resistance reading HIGHER than what the component indicates. If the resistor reads lower (less resistance) this may be due to tolerance, but more likely it's in parallel with another resistor or component which make is read lower. This is where the schematic comes in handy if you run into something questionable.

Let's check this output cathode resistor. It's a 150 ohm resistor. The multimeter is set to the proper scale and you read directly across the component...



Hmmm......this resistor reads LESS than it's stated value....



It's reading about 124 ohms as opposed to 150 ohms....HOWEVER, this is not a problem. Looking at our schematic and comparing it to our circuit (no output tube plugged in), nothing is in parallel with this resistor. So the resistor alone is causing the reading.....

Here's the scoop. This resistor has no 4th band (the tolerance band). This means it's rating could vary by up to 20% either way.

150 X .20 = 30

So this resistor has an acceptable range of 30 ohms either high or low. So 120 ohms at the low end, or 180 ohms at the high end. So 124 ohms is acceptable for our scenario, mathematically speaking.

After checking the other resistors, nothing was far enough out of tolerance to deem needing replacement.

The same was done with the disk capacitors in the circuit. You want them to read "open" as they block DC. High resistance or a short would be bad. The capacitors were okay.

As a last resistance check, I turn power switch "on" to the radio (remember we still don't have any power connected). With all of the tubes removed, I read an open (no resistance at all). This is what you want to see. A short across the incoming power would be BAD! We don't want to apply power to something that has a nasty short. Remember though, if you try this with the tubes in place, you will read a "short" as the tube heaters will appear as low resistance to the incoming power connection and "fool" your meter.



We'll continue on with some more schematic tidbits and some VERY important safety information on isolation transformers.

Stay tuned....

From your earlier post:

*Helpful hint*, many times a schematic won't be exact due to running manufacturing changes, or encompass multiple models, this schematic is based on a clock radio, so if you were paying close attention you would have notice what looks like a discrepancy between our radio and the schematic..........

This has occured on the last two projects I'm working on:

* The '1st Audio Plate Decoupler' for a Sonora was called out as a 6uF electrolytic on the SAMS schematic, but a 20 uF elec. was in place.
* On a Multivox 45 tube amp (which is more of an ongoing 'testbed' platform), one source schematic called out a 40/40/40 uF electrolytic can, but an original 40/40/10 was in place. The same is true for some of the other caps and resistors here and there.

With no knowledge of this particular clock radio, I will say the PCB looks like old school british layout.

Anyway, my 2 cents.

Continuing on with some circuit pointers...

For the newbies out there trying to make heads or tails of things, one thing to learn is the tube socket connections on the board vs. the schematic. It's important to remember how to view the connections--whether you are looking at the top of the tube socket, or the bottom (the connections side). On these miniature 7 pin socket connections, when you view the socket from the top view, and the index (the largest space between the socket connections) facing down, the pins are counted in a counter-clockwise fashion.



When you view the tube connections from the bottom side (the trace side in our case, or the wiring side on a steel chassis radio), the tube connections are counted in a clock-wise fashion. This is the same fashion the individual tube diagrams are given. "Index" at the bottom and pin #'s in a clock-wise manner.



The reason this is important to know is so there isn't confusion when finding a proper point if you need to check a resistance or voltage reading. Don't confuse pin #1 and #7, or your "counting" will be off.

An interesting discovery on this radio after tracing out the connections checking for any wiring errors...

At first glance I thought maybe it was a bad day at the Utica factory assembly line for someone...Per the schematic (and 99.9% of similar radios) the red wire from the audio output transformer connects to the cathode of the rectifier tube. The blue wire connects to the plate of the output tube. These are backwards on this radio!. Now, this will not cause an issue with the operation of the radio, but the phasing of the transformer would not be correct. I checked on a few similar GE radios and discovered that the index line seems to indicate the "blue" or "negative" connection corresponds with it, so I decided to leave the radio as-is. I believe the transformer was simply constructed wrong (wrong color lead wire installed) and then this was compensated by "reversing the color connections on the board". The stuff you find! And that's why I brought it up.



Let's take some time to show the difference between our radio and the "general" schematic depicted. After tracing out the connections, I have made the modifications to show the difference.





Obviously the radio we are working on doesn't have a clock mechanism, just a simple on/off switch apart of the volume control. Our radio uses a 50/30 uF electrolytic, not a 75/30 uF electrolytic. Our radio uses a .01 uF @ 1000 volts capacitor from rectifier plate to cathode as opposed to the .047 uF @ 600 volts line filter capacitor directly across the AC main in the original schematic. Both of the these capacitor configurations are responsible for "shunting" or providing a simple path for transient noise spikes (high voltage surges) coming through the power line into the radio.

There may be some other design changes, but for simplicity I thought I'd share the changes apart of the power supply area of the circuit for now.

The next thing that took place was a quick check of the tubes. My simple emissions-type tube tester comes in handy. All tubes tested "GOOD", but the real test will be in-circuit. The quick check just makes sure we don't have a dead radio thanks to burn-out tube or a tube that has a short in it.



Next we'll go over electrical supply basics and the importance of using an isolation transformer on an AC/DC radio so you don't get a nasty shock

Stay tuned...