Truetone D 2237B ....SMD?

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Truetone D 2237B ....SMD?

Post: # 24870Post electra225 »

My old faithful Truetone radio that I redid the cabinet on has failed me. About a week ago, the rectifier went out while it was running. Yesterday, all of a sudden, it started the crashing noise in the speaker indicative of SMD. It does it cold and warm, instantly after I turn it on. I'm going to remove the chassis from the cabinet, then use my signal tracer that has a "noise" setting to see which IF can is the culprit, if any. This will be my first foray into the repair of SMD. I could get lucky and find something else, so there is always hope..... ;) :oops: :roll:
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Re: Truetone D 2237B ....SMD?

Post: # 24877Post electra225 »

I pulled this thing apart this afternoon. I used the noise feature on my old Heathkit signal tracer. This feature sends a relatively high voltage thru the probe to the IF transformer. I connected one side of the signal tracer to one half of the IF transformer, the other side to the other half. The idea is to send voltage thru the IF transformer to detect noise generated by the migrating silver mica. Both transformers passed this test. There was no positive voltage on any grid following an IF can. Both transformers checked okay in a resistance test. It played about two hours out of the cabinet, sounded as good as always. I put the chassis back into the cabinet. It didn't run five minutes until it made the lightning crashing noise in the speaker. When the crashing starts, the signal slowly fades until nothing can be heard in the speaker. If I whack on the second IF transformer with a screwdriver handle, it makes noise in the speaker and sometimes the signal will slowly return. I have the schematic, but it doesn't give any values for the caps in the IF cans. 100pf is a common value, so I'll start with that value. The only thing I know to do is to just guess, treat it for silver mica migration, then hope for the best. At least, at that point, I'll be relatively certain that if it doesn't work right, SMD is not the problem.
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Re: Truetone D 2237B ....SMD?

Post: # 24882Post William »

Didn't you have a Zenith that did the same thing. Out of the cabinet it played just fine, back in the cabinet it died within 5 minutes. I can't remember, did you ever get that sorted?

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Re: Truetone D 2237B ....SMD?

Post: # 24884Post electra225 »

I did. That was one of the sets I repainted. That radio had lots of issues, lots of little things that combined to make it not work very well. It had a bad tube, was badly out of alignment and I forget what all else I had to do. It will never be my best-performing radio, but it works acceptably at this point, or at least it did the last time I used it....
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Re: Truetone D 2237B ....SMD?

Post: # 24914Post electra225 »

I pulled the chassis out of the cabinet on this radio. I had three VTVM's connected to it along with the signal tracer. I was monitoring B+ at the rectifier cathode, and then checking for DC voltage appearing on grids following IF transformers. I never noticed anything amiss. It played for about six hours, never skipping a beat. There was a tube shield on the 12BA6 IF amp that was made of cardboard wrapped with foil. That tube shield did not fit well and made poor contact, INTERMITTENTLY, with the chassis. This would cause oscillation and noise. I found a good metal tube shield in my stash then replaced the cardboard shield. That seemed to cure that condition, but I can't say if that was the cause of the crashing sound and fading. I still suspect SMD, but I don't have any diagnostic evidence to support that.
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Re: Truetone D 2237B ....SMD?

Post: # 24917Post TC Chris »

As to SMD, that is the repair to tackle last, when nothing else works, right? But if you do, give us details. I've got my parents' old GE Musaphonic clock radio from the the early or mid 1950s that's suffering from SMD. I've read accounts of repair jobs but haven't tried one yet.

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Re: Truetone D 2237B ....SMD?

Post: # 24932Post hermitcrab »

I get nervous messing around those IF coils ...the wires are like hairs to my eyes ...
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Re: Truetone D 2237B ....SMD?

Post: # 24933Post electra225 »

I tried every troubleshooting procedure I heard of or read. No test proved conclusive. I don't see anything else that is cause for concern. The set has been recapped. I have a resistor or two that are not perfect as to value, but they are not in positions that would cause the noise in the speaker. This thing acts like SMD acts and sounds on videos. So, in desperation, I pulled the second IF transformer. Very tedious work, nerve racking work. Like Elton, I'd rather not mess around in an IF can, but my dependable old radio won't work now, so I can't hurt it and I might learn something performing this repair. Keep your fingers crossed.... ;) ;)
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Re: Truetone D 2237B ....SMD?

Post: # 24934Post electra225 »

Bummer.....The second IF transformer does NOT have SMD. Now I have a mess..... :oops:

No harm done so far, however. There is some migration, but not enough to cause a problem. My problem now is that the two capacitors in the mica thingy are different sizes, different values. One is physically smaller than the other. I rolled the dice as a learning experience. It's too early to say whether I have killed a good radio or not. So, my question is, how critical are the values of the mica caps in those IF cans? All I have on hand is 100pf. I got a supply in years ago when I decided it would be a good idea to be ready in case I had a radio with SMD. It was my understanding that the values were not critical, could be tuned in by adjusting the trimmers in the IF cans. What say ye, those wiser than I?

Once I have the right value caps, I think the rest will be straight forward. And, yes, Chris, I have pictures..... ;) ;)
100_1239.JPG
100_1240.JPG
100_1241.JPG
100_1243.JPG
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Re: Truetone D 2237B ....SMD?

Post: # 24937Post electra225 »

I'm not hurt as bad as I had feared. According to what I read, 100pf caps are a good starting point as the transformers can be adjusted to work with the new caps. So my next step is to tin the parts I'm going to attach the caps to, then, very gingerly, solder the caps in place. I had intended to do both transformers, so all is well there. At this point, I can say with a straight face that I started with the wrong IF can. The primary must be the one with SMD. This one was easy to remove, so it's as good of a place to start as any. Stay tuned...... ;)
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Re: Truetone D 2237B ....SMD?

Post: # 24942Post TC Chris »

Where are the transformer coils? All I see is slugs.

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Re: Truetone D 2237B ....SMD?

Post: # 24943Post electra225 »

If you can see the little wires in the transformer, the coils must be inside those slugs.
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Re: Truetone D 2237B ....SMD?

Post: # 24944Post TC Chris »

Oh yeah... I just enlarged the photo and think I can see the coils.

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Re: Truetone D 2237B ....SMD?

Post: # 24945Post electra225 »

I messed up! I committed a rookie mistake. I had the SMD fixed, had the transformer all back in and wired up, then found my mistake. I can't adjust the lower slug from the bottom. I don't know whether the IF can will withstand another operation like it went thru. I may be in the market for an IF transformer. I can fix my mistake, but the heat may damage the fine wires. The plan was to repair the IF can, then see if I could get a signal at 455khz thru it, then do the first IF can. I may ruin a perfectly good radio, then find out that the crashing noise, the "unmistakeable signature of SMD" is all hogwash. Maybe there is another cause for the crashing noise? :oops: :oops: :cry: :cry:
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Re: Truetone D 2237B ....SMD?

Post: # 24946Post TC Chris »

You mean you can't get a tool on the slug from the bottom, or the slug is stuck? If you just can't reach it from the bottom, can't you go past the top slug with your tool and adjust from the top?

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Re: Truetone D 2237B ....SMD?

Post: # 24948Post electra225 »

Nope. This is one with two slugs that you can't access the bottom one from the top.
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Re: Truetone D 2237B ....SMD?

Post: # 24950Post TC Chris »

I just looked again and saw that the slugs use a screwdriver drive, not a hex that you can reach through. But isn't there a hole in the bottom?

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Re: Truetone D 2237B ....SMD?

Post: # 24953Post electra225 »

There was a hole in the bottom until I closed it! :oops: :cry: :roll:

Long story short. I'll get more into this little faux pas after I see the radio working. I can get a 455 khz IF signal thru the second IF transformer. I may not have to do anything else to it. The first IF can is infested with SMD for sure. I have pictures as proof and will share those directly. You should do both IF transformers when you fix SMD, so I haven't hurt anything. This is my first time doing it, and I got a little excited when I may not have been hurt yet. I have inquiries into getting a replacement 2nd IF transformer just in case. If I can get a good signal thru the first IF after I fix it, I might be okay. If I can have a working radio after this fumbling kluge, SMD won't be something I'll worry about doing in the future. I have a Motorola stereo with SMD and it has a printed circuit IF transformer. I may actually tackle it sometime in the future. Stand by..... :roll: :roll:
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Re: Truetone D 2237B ....SMD?

Post: # 24957Post electra225 »

I was doing okay until I lost B+ to the front end of the chassis. The RF amp, converter and half the IF amp have really low, 10 volts and under, B+. Running the circuit, there is good B+ to the plate of the IF amp. Pin 6, the screen grid has less than 20 volts on it, should have 50 or so. I put a new 33K ohm resistor in the B+ line, between the plate and screen grid. I am suspecting a cold solder joint or bad connection on pin 6. Nothing else much makes sense. I was aligning the IF when I lost the signal. I still get the IF signal from the grid of the IF amp to the speaker. This thing is being difficult. :oops: :cry: :roll:
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Re: Truetone D 2237B ....SMD?

Post: # 24963Post electra225 »

I lost B+ because a new 33K ohm resistor that still checks good is electrically dirty enough that it lowered B+ significantly enough to kill the front end of the radio. I chased my tail for two days trying to find a reason I didn't have B+ when the answer was being shouted from the rooftops. I saved all the old parts I removed. I bridged the new 33k with the old, drifted 33K. The old one restored B+. The radio plays, but with weak output. I still need to align the IF. I haven't messed with the oscillator or antenna adjustments, and won't until I am sure my SMD repair has been successful. This type of a fault keeps you humble..... ;) :oops: :cry: :roll: :lol:
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