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Author Topic: How to check out a used TL-922A  (Read 3793 times)
KE4DRN
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Posts: 3710




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« Reply #15 on: February 07, 2007, 02:07:30 PM »

hi,

check this link for the Dim Bulb radio tester

http://antiqueradio.org/dimbulb.htm

Saves the transformer in your radio when fuses
are blowing.

73 james
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KC4RAN
Member

Posts: 17




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« Reply #16 on: February 07, 2007, 05:50:36 PM »

Well, I tested a bit further. Got 335K ohms between plate cap and ground, after the reading settled, using my little DVM. It took several seconds (30?) to settle at this reading. I'm guessing this is somewhat close to the expected 376K (47K x Cool from R13-R20 on the filter caps.

Tested from grid to filament on both tubes, can't find a short... both tubes exhibit similar resistances between the same pins as far as I can tell. I haven't compared what I'm recording vs Eimac specs, but it makes me feel better to know that I've at least located the first known bad part, and that the tubes don't seem to be shorted internally.

Now for the fun questions. I know that part of the suggested mods for this amplifier include putting caps in parallel with the one that it seems is blown (do the black marks give it away?) <big grin>. Since I know I have to replace one of them, do I go get a like replacement for the blown one, then add the suggested parts alongside them all, or do I replace them all with larger caps since I'm already having to replace one? I guess it comes down to the amount of work I'm willing to do... one solution has me remove existing parts in places where I wouldn't normally have to, the other has me just repair and replace, but the one cap will be slightly different than the others. No difference I guess, but any input is appreciated.

Any other obvious tests to perform? I don't have a capacitance meter, so I can't do anything other than visual or resistance checks on the caps.

Thanks in advance!
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KE4ZHN
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Posts: 138




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« Reply #17 on: February 08, 2007, 02:06:42 PM »

Dennis, Im no engineer by any means but it looks to me like you may have a shorted tube. Checking a 3-500z cold often times wont reveal the short but when the tube is hot and the internals expand thats when you get the short. Its almost always a filament to grid short. This will kill your bias supply/filament transformer if allowed to remain powered up for too long. Basically what happens is the bias supply and the filament supply are shorted to each other via the bad tube. Remember, the grid is grounded via the chokes and caps on each grid pin. Bad news! This will cause the primary fuses to blow because the filament/bias transformer has a short across it. If you turn the amp on with no tubes plugged in it should light up and run fine. Provided nothing else is shorted such as a pinched wire or something. As Steve said, be very careful of the anode leads and be sure not to let them touch anything inside! Looking at the pics you posted it appears that one of the bypass caps on a grid pin exploded. This is common in the SB 220 when a tube shorts and it acts exactly the same way your 922 does. Ive seen it before in plenty of SB 220`s. There is also a good possibility that the metering circuit of the amplifier may have been damaged. This is almost a guarantee if the meter switch was in the grid current position when the tube failed. The SB 220 and the 922 are very close in design except that the 922 is a bit tighter inside and in a nicer cabinet. I may be wrong but from the pics you posted and the symptoms you describe thats what I think is the problem. Good luck fixing the amp.

 73  KE4ZHN
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KC4RAN
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Posts: 17




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« Reply #18 on: February 09, 2007, 09:52:27 AM »

Thanks... I'll probably try taking both tubes out and test them individually in the 'not so bad' socket.

Is there a way to safely test for a shorted tube that won't result in more damage?

I guess I could place the suspected good tube in the suspected good socket and power up as a second test, and see how we go.

I know I need to replace that grid cap, and I'm considering changing them all with the same (higher capacity) caps instead of putting the addon caps in parallel (as suggested in the various mods).
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WB2WIK
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Posts: 20540




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« Reply #19 on: February 09, 2007, 11:01:14 AM »

You can't just use one larger value (capacitance) component to substitute for other smaller values in parallel -- that doesn't work for RF applications.  It probably works fine for filter capacitors in a power supply, but that's not the application you're discussing.

Capacitors each have a SRF (self resonant frequency) based on their capacitance, material, construction and lead or terminal length.  A 220 pF capacitor would have a much higher SRF than a 1000 pF capacitor of the same type.  Placing four 220 pF capacitors in parallel is very different from using one 880 pF capacitor.

Current sharing is another issue.  Capacitors in parallel will share current, distributed by their actual values; four 220 pF capacitors in parallel, for example, can handle four times the RF current of one 880 pF capacitor of similar construction.  There's another reason one cannot make arbitrary substitutions.

WB2WIK/6
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KC4RAN
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Posts: 17




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« Reply #20 on: February 09, 2007, 11:11:12 AM »

Glad I asked first. I'll pursue getting a like replacement for the cap that failed, and then look at performing no-tube and then single-tube testing.

Still wondering if there's a safe way to test for a shorted tube without damaging more components.

Thanks!
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WB2WIK
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Posts: 20540




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« Reply #21 on: February 09, 2007, 12:00:43 PM »

Re testing for a shorted tube without causing more damage, this is tricky.

You could try testing in the TL-922A using a variac on the plate transformer primary, and bringing up the plate voltage slowly.  Not so easy to do.

You could use a hipot tester (piece of test equipment, completely independent from the amplifier) and ramp up the voltage slowly.  This would be very safe for the tester, the tube and the operator since you can preset a current limit that could keep even a direct short circuit limited to very small energy.  But, it would require a hipot tester!

Back when Eimac manufactured 3-500Zs (years ago) you could send them a tube to have it tested.  But they don't do that anymore.  Possibly RF Parts is equipped to do this-?  Have you asked them?

WB2WIK/6
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KC4RAN
Member

Posts: 17




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« Reply #22 on: February 09, 2007, 12:20:58 PM »

I do have a variac around, I believe. I'll have to check if I still have it. I'm assuming I put the primary 110v input through the variac, and start at low voltage?

I have not inquired of RF parts if they will test tubes. That might be my best bet, especially if I have one good one and one bad. Maybe they can sell me a replacement tube that has a mu close to the good one.

Thanks again!
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KC4RAN
Member

Posts: 17




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« Reply #23 on: February 09, 2007, 12:27:04 PM »

Called RF parts, sounds like they do tube testing sometimes. I'll plan to do a no-tube test and make sure I don't have any other problems, then go forward with talking to them next week about testing both and possibly replacing one or both of the tubes.

73
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KT4I
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Posts: 2




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« Reply #24 on: February 13, 2007, 03:14:49 AM »

Hello,
I recently picked up a TL-922A also. Mine done the same as yours done... Turn it on and BUZZ... POP, blow fuses AGAIN!!
What I done is to remove the anode caps, replace the fuses and it fired up with the voltages on the CW and SSB settings within specs. So that told me that one or both the 3-500's were shorting out. Put in a set of good pulls, and it's purring along nice and happy now.
Also you might want to check the D2 Zener Diode. If that is blown, it could possible cause the filiment transformer to burn out.
Good luck,
Bill KT4I
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KC4RAN
Member

Posts: 17




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« Reply #25 on: February 13, 2007, 11:31:14 AM »

Thanks for the advice. If I remove the tube caps and power the amplifier on, I'll still have a failure if I have a filament-to-grid hot short, right?

I still haven't checked my grid meter components either. What components typically fail in that circuit when the tube shorts like this?

I called RF parts, and although it's not a service they advertise, they will do it when they have time to. I'm going to send one or both of my tubes to them, have them run tests, then probably pick up a new set of 3-500Gs. I'll probably sell the one good Eimac 3-500Z to someone with a one-tube amplifier.
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KT4I
Member

Posts: 2




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« Reply #26 on: February 13, 2007, 03:34:41 PM »

Yes, the short will still be there but removing the Plate Caps will open the short on the high voltage. Then, if it doesn't blow the fuses, it indicates that there is a short in one or both of the tubes. After turning the amp on with the Plate Caps removed, you should have approx 2200V on the CW setting and 3100V on the SSB setting.
If this all shows true, the replace the tubes and any of the other parts that you deem to be bad or weak. Don Kessler and/or Rich Measures are the guru's on the 922 repair and updates. They would be a good source of advise if the repairs are more than just the tubes.  
Again use safe practices when working with the amp.
73,
Bill KT4I
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WB6MMJ
Member

Posts: 54


WWW

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« Reply #27 on: February 17, 2007, 12:52:23 AM »

I have had two TL 922A amplifiers. One of them, when I got it, had a blown fuse in it. I traced it down to a shorted 3-500 tube which shorted because the Zener Diode, used for Bias, was bad. Just remember, BE CAREFUL when working with Amplifiers. It just takes one mistake and you are dead or burned. I have the scars, from my early days, to prove how dangerous amplifiers can be.  I have repaired quite a few of them and built three. Always unplug them and discharge them before putting your hands inside.
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W9GB
Member

Posts: 2580




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« Reply #28 on: February 24, 2007, 11:35:43 AM »

"It took several seconds (30?) to settle at this reading."
That sounds like a capacitor discharging (either through your DVM or the resistor).

DO BE CAREFUL, the HV supply on the TL-922 will deliver 2.2 Kilovolts DC ..... woudl like you to be around to use it!  :-)

Kenwood TL-922 schematic
http://www.somis.org/tl922wd.jpg

Richard L. Measures, AG6K wrote of improvements and changes to improve the overall operation of the Kenwood TL-922, you may find the comments of interest.
http://www.somis.org/QSK922.html
http://foxtango.org/amplifiers/AG6K.htm

Diagram
http://www.somis.org/D-a-07B.jpg

VE3VWA modifications to TL-922
http://ultra1.homeunix.com/tl922/index.htm

Best of luck with it!

w9gb
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KC4RAN
Member

Posts: 17




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« Reply #29 on: March 06, 2007, 10:20:04 AM »

Thanks for the concern and note of caution - I too would like to be around to use it. The amplifier had been off for a day or so, disconnected from main power, hooked up an earth ground, and I went in with my ground probe (one-handed) before I started measuring. I touched each and every piece I could, looking for sparks indicating residual charge somewhere. There was none, but I still let it settle for another few minutes before I went in with the probes.

Thanks for the links as well.
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