Question about R320 on a Tram D201A

[husker]

I was tracing back my issues over lost receive on a 201a I had sitting in my shack for a while. Seems R320 is pretty much smoked...what would cause that resistor to fail (it appears to have gotten very hot). It looks like it had been replaced at one time, the power side has been reworked as well (new BA brd). I really would like to know what may have happened so when I replace it the same thing doesn't happen again.

I was just wonder if anyone (Sandbagger) can give me a clue where to go to find the reason why R320 over heated and failed.

thanks

[2600]

R320 and R319 are in series. They serve to drop 400 Volts DC down to about 100 Volts on the plate (pin 6) of V302B, the manual (VFO) oscillator.

They are not big enough, and run too hot. We use a 5-Watt wirewound resistor to replace each of these. The larger surface area reduces the temperature rise. Nothing special about what kind of resistor to use, but we avoid the carbon-composition type resistor installed at the factory. We have used a 25 k resistor in the past. The value is not incredibly critical, within about 10 percent of the original 44k circuit resistance that the two parts add to the circuit as a series pair.

They get replaced as a pair. If one of them has outlived its "use-by" date, the other one can't be in much better shape.

73

[husker]

Interesting, that would make sense. Both do look a tad over heated, but 320 is just a shell. LOL, the colors are gone

Thank you!!!!!!

[2600]

That suggests that the resistance of those two resistors changed with age. The major drawback of that kind of resistor. The one that drifted to a higher resistance would get more than half of the voltage. Since the current is the same through two resistor in series, the one with the higher resistance would also dissipate more its one-half share of the power. Power is voltage times current.

Don't see those two resistors age with that kind of visible difference between them too often. They tend to deteriorate more or less equally.

Our routine list of parts to replace for a "100,000-mile 40-year tuneup" includes a double fistful of those carbon-comp resistors. The ones that have caused repeated breakdown headaches over the years get changed no matter how nice them may look.

A look around that circuit board will probably reveal a few more that look tired.

73

[husker]

well, my project Tram came back alive..kinda lol. So I have minimal AM audio (had to turn the volume all the way up) but SSB has good audio.
There is ZERO carrier on both AM and SSB. I am keeping at it ..

[Sandbagger]

Nov 18, 2017 2:12:56 GMT -5 husker said:

well, my project Tram came back alive..kinda lol. So I have minimal AM audio (had to turn the volume all the way up) but SSB has good audio.
There is ZERO carrier on both AM and SSB. I am keeping at it ..

There are probably more bad resistors and caps to track down. There should be carrier on AM but never on SSB. SSB only puts out power with modulation.

[2600]

It's not just the years, it's the miles, too.

A 1977 car that won't start might get running if you fix just one simple fault, like a corroded battery cable.

But the list of repair/maintenance details you'll discover once you get the motor to start will tend to be pretty long after 40 years. Good chance it won't run, stop, steer and shift perfectly after four decades.

And that's the reason we adopted the 100,000-mile tuneup as a strategy. To pre-empt faults that we know to expect by changing a fixed list of things that always fail, sooner or later. Whether those parts test bad today, or not.

The alternative is what I call "electronic whack-a-mole". Swatting individual failures of electrolytic capacitors and overheated resistors, one by one as they occur.

Probably a more-educational experience, if what you want is to learn troubleshooting technique.

But for a commercial shop it's a better bet to take the "sure thing" route rather than have a customer's radio mimic a yo-yo. Up one day, down the next.

73

[Sandbagger]

Nov 18, 2017 12:32:42 GMT -5 2600 said:

It's not just the years, it's the miles, too.

A 1977 car that won't start might get running if you fix just one simple fault, like a corroded battery cable.

But the list of repair/maintenance details you'll discover once you get the motor to start will tend to be pretty long after 40 years. Good chance it won't run, stop, steer and shift perfectly after four decades.

And that's the reason we adopted the 100,000-mile tuneup as a strategy. To pre-empt faults that we know to expect by changing a fixed list of things that always fail, sooner or later. Whether those parts test bad today, or not.

The alternative is what I call "electronic whack-a-mole". Swatting individual failures of electrolytic capacitors and overheated resistors, one by one as they occur.

Probably a more-educational experience, if what you want is to learn troubleshooting technique.

But for a commercial shop it's a better bet to take the "sure thing" route rather than have a customer's radio mimic a yo-yo. Up one day, down the next.

73

Agreed. The D201 has a much higher list of age/heat/current related part failures than most, and the "scorched earth" 100,000 mile overhaul approach is probably the best bet if reliability and enjoyability is the ultimate goal. Of course that's also the most expensive way to go, which is why most hobbyist radio guys end up going down the "whack-a-mole" road. But a shop has a reputation to uphold, and no one wants repeat calls for incomplete repairs. Bottom line, if you want a reliable D201 that performs well day after day, prepare to invest.

[husker]

lol, no doubt guys! It appears someone has already gone through this one one time already, but I don't think they went far enough.
What I meant about carrier on SSB was modulation driven, it was late and I was getting tired.

Is there a check list out there for the 100k mile tune up?

thanks guys, you are awesome!

Oh, I did find the original owner's "transmitter ID Card" taped to the inside of the top cover. His name was Felise Lisi from NY. I tried to see if I could contact him, but he passed in early 2000.

[husker]

Woah........there is NO R507 resistor?? Can anyone think of any reason why that would be removed? It's not jumped, just simply gone!

[2600]

The two 6BA6 sideband IF tubes each have the same two-resistor voltage divider feeding pin 6, the screen grid. A 27k 2W is marked R503 on V500, and R509 on V501. A 12k 1W. resistor goes from pin 6 to ground on both tubes. R502 on V500, R507 on V501. This steps down the 270 Volt receiver B+ down to about 65 Volts on pin 6 of each tube. The 12k resistors R502 and R507 routinely overheat and will look literally burned. In some cases they may change resistance value, sometimes down to a few hundred ohms, or fail as an open circuit.

Good chance that the missing resistor just looked bad. These parts have no effect on AM receive, only sideband. Our 100,000-mile list replaces all four of these with more-modern metal-film or carbon-film resistors, all rated for 2 WAtts. Wirewound parts are okay, since this is strictly a DC circuit with no RF current in it. Leaving out R507 could cause V501 to run too hot and fail prematurely, and/or overheat R509, which feeds B+ to the plate circuit of V501.

73

[husker]

Thank you for your response guys! Is that 100,000 mile replace list published anywhere?

thanks again

[2600]

Seems to me I posted it here somewhere a long time ago. Just one problem. It was written back in the 90s using parts we had bought by the hundreds from a surplus outfit. Some of the substitutions it listed have no rational meaning, except that we had many hundreds of them on hand. Those parts are now long gone, and the file should get updated to the stuff we buy from Mouser and such suppliers these days.

There should be a more up-to-date text file listing the parts on our Jurassic computer system. This would be the parts list we use to pull a bag of parts from stock to do that job. Takes some effort to port a Wordstar 4.0 file from it. Makes it unconditionally secure, but a pain to extract a file to the real world.

I'll have a look and see if it appears relevant enough to post.

73

[husker]

thank you very much! I can't thank all of you for all your help!!!

[husker]

Another question, I was reading that there should be no 10ohm 1/2 watt resistor directly behind the 6L6GC tube, if there is I should remove it and put a jumper there. The owners manual I have it only shows a jumper there....so should I remove this resistor? What good did it do in the first place? EDIT, never mind, I do have or DID have the fixed bias. I removed the yellow wire and jumped the trace cut.

[husker]

I did find these....are they any good?

Attachments:

TramTuneupPage2.pdf (131.29 KB)

TramTuneupPage1.pdf (297.28 KB)

[wildcat444]

Nov 21, 2017 17:34:10 GMT -5 husker said:

I did find these....are they any good?

Thank you for posting this, Husker! This is geared towards the D201A... should it also work the same for the D201?

[husker]

Aug 21, 2019 12:43:12 GMT -5 wildcat444 said:

Nov 21, 2017 17:34:10 GMT -5 husker said:

I did find these....are they any good?

Thank you for posting this, Husker! This is geared towards the D201A... should it also work the same for the D201?

Some, yes but not everyone of them. The A version has some different parts to it.

[wildcat444]

Well, that's too bad. I know there is a difference in the addition of the extra channels, but I guess I will have to look at the steps to see if I can make out which ones apply to the D201 (23 ch PCB version). I am about to take on a "100,000-mile 40-year tuneup"... well, actually it doesn't look like this was power up in decades and I'm not about to until I have at least replaced the electrolytic caps and hi wattage/cooked resistors. Looking for all the info I can find on guidance to do it right.

[bobcat4109]

Kind of late to the party (R320 issue) .... but the info is interesting.....  after this... I'll shut up!

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Agreed!!!! For sure!

I just looked up the specs on that tube (just ride with me on this one!)
The max plate current is 18mA. Doesn't sound like much.

BUT... if anything ever got that tube in a mind to pull 18mA, 18mA through a 22K would dissipate a little over 7 watts.

Will that ever happen? Probably not.

But it is reasonable to assume that, depending on the operating point, it might pull half of that 9mA.... and 9mA through a 22K ohm resistor still amounts to 1.78 watts and that is way too close for comfort.

I like the 5 watt solution.