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Author Topic: Kenwood TS-520 transmitter problems  (Read 21490 times)
KD4ACG
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« on: November 16, 2011, 12:57:39 PM »

Before I put my TS-520 (original) into storage about 15 years ago, the radio's transmitter had nearly sputtered to nothing. Having been an apartment dweller for most of the time since, I didn't do much to try and find/correct the problem. Now that I have a house, and space (albeit deed restricted) to run some antennas, I really want to get this radio back to life, alongside my more modern rig.

The short description of the problem, is that it doesn't transmit.

The specifics of the problem are this: I can't get the plate current down to anywhere near normal levels. The needle slams against the right side of the meter. If there was nothing there to stop it, it would probably spin. Nothing I've tried from the front or side panel, changes the reading.

Voltage is about 950, and steady.

Using the dummy load on my MFJ 989C, I can't seem to get any output power from the rig...or at least not enough to make the meter budge on the tuner. I do notice, however, that I can hear the tone that the radio generates while tuning, on my Yaesu rig, sitting next to the TS-520. So the radio is at least trying to generate a signal, but there's nothing to drive it.

I took it out of storage a few years ago, and took another look at it. I put two new 6146B tubes in it, with no change. I don't remember whether or not I also tried swapping the 12BY7A. Having been unsuccessful, it went back into storage for another few years.

Now that I'm in a house, and the radio is out of storage, I went through and gave all of the pots/switches a much-needed cleaning. Years in storage certainly took their toll, but at least that much is back to usable condition. Still some possible dirt in the VFO, causing some instability there. But that's another issue.

Can anyone suggest where to go from here? This is the first radio I bought when I first got my Novice ticket, and I'd love to resurrect it.

Thanks for your help.

73 de Jason, KD4ACG
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N4NYY
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Posts: 5202




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« Reply #1 on: November 16, 2011, 05:12:26 PM »

If that radio is  over 30 years old, you may need to recap it. You may have bad tubes, too.

There is a guy that sells them on ebay: http://www.ebay.com/itm/Kenwood-TS-520-HF-Transceiver-Capacitor-Replacement-Kit-/130598717322?pt=LH_DefaultDomain_0&hash=item1e684a478a

You may have other problems, but this is cheap insurance anyway, if you can do the works. Hell, you may be able to order the caps from Mouser, since he has the complete list anyway.

I am not familiar with the TS-520, but the Kenwood TS-850 was notorious for poor quality leaking caps. And that was modern solid state rig that was not nearly as old.

Some others may chime in, too.

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NO2A
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« Reply #2 on: November 16, 2011, 08:00:51 PM »

The 950v seems way too high. If I remember right the idle hv should be in the 800`s,key down in the 700`s. I`d replace the driver tube,as they can go south before the 6146`s. It most likely does need a recapping. If you can hear any kind of output on cw and it`s not a pure cw tone,the caps are shot. It`s possible the finals are still good but try another driver first.
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ZS5WC
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« Reply #3 on: November 16, 2011, 11:50:05 PM »

 Smiley
The IP meter voltage is developed in the cathode circuit -there is probably a cathode resistor shunt that is open circuit--thus the high meter reading, and no amplification--basically the cathodes will be at HV potential.
Measure the cathode shunt and cathode potential--beware of the HT!..
I think they were 10Ohm , 5w--If they are open--make sure the tubes and screen / grid bias is OK, as there resistors were used as 'Sarificial" resistors in case of an problem-tubes, bias or otherwise.

73 de William
ZS4L / ZS5WC
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AH6RR
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« Reply #4 on: November 17, 2011, 07:19:40 AM »

950V is fine for the plate so it does not need recapping since it would not change anything in that circuit. Change the 12BY7A tube and as William stated check the Final board R1 and R2 they are 10 Ohm at 1W and replace if needed. That should get you back on the air.

Roland AH6RR
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KD4ACG
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« Reply #5 on: November 17, 2011, 07:25:25 PM »

Thanks to all for your responses.

With a hamfest coming up in about two weeks, I'll be able to get my hands on a new driver tube. In the meantime, I'll check the resistors. I'll post my progress when there's something to report.
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KE4DRN
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« Reply #6 on: November 17, 2011, 08:55:00 PM »

hi Jason,

No need to attempt to recap the radio unless you
have a confirmed failure on a cap, yes the radio is old,
however, most of the time the HV cap(s) will open.

Is the driver filament lighted? 
I believe there is a transistor that drives the driver tube,
worth checking.

you can use metal film or metal oxide to replace the carbon 1W
comp resistors if they are bad, please DO NOT upsize the wattage,
that can cause more problems, including damage to the winding
in the power transformer.

join other kenwood hybrid fans here

http://groups.yahoo.com/group/TS-520_820_530_830/

schematics in pdf no cost

http://www.n6wk.com/kenwood/

73 james
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ZS5WC
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« Reply #7 on: November 17, 2011, 11:18:23 PM »

 Smiley

If you test those resistors and find BOTH open , obviously something external caused both tubes to turn on hard, and blow the resistors. As a previous poster stated do NOT upsize those resistors!.
If you look at the final board-there is a 100pF DC blocking cap that isolates the 12by7 anode potential to the grids of the 6146's. IF this cap is leaky, this will certainly be the root cause your problem.
(the leakage could be in the region of Mega ohms.)
It is always a good rule to replace this cap anyway, with the correct HV silver mica cap.

Test the bias potentials on both the control grid and screen grid-they are given on the schematic diagram.
Just watch those plate caps-do not short them or touch them!. 

Let us know how you get on and good luck!.
73 de William
ZS4L / ZS5WC
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N4NYY
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« Reply #8 on: November 18, 2011, 05:50:30 AM »

Quote
No need to attempt to recap the radio unless you
have a confirmed failure on a cap, yes the radio is old,

I disagree. May not be the cause of the problem, but the key words are "storage for 15 years". Being that is a 30 year old radio, I would be almost sure he did not fire that up slowly with a Variac. He likely gave it full AC all at once. Those caps are likely on borrowed time.

For the price tag of under $30, it is cheap insurance.
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KD4ACG
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« Reply #9 on: November 18, 2011, 12:25:48 PM »

Here's the latest:

I checked the two 10-ohm resistors, and found them to both be defective. I replaced them (10-ohm, 1-watt, no more, no less), and it did bring the plate readings back down to earth. But before I could check and see if I was finally getting measurable output power, they both blew again...together...violently.

So the good news, is that the resistors did their job to protect the rest of the radio. The bad news, is that there's something else causing them to fail.

So I'll need to get two more resistors, and I'll also locate, check, and replace the 100pf capacitor that William suggested.

Even though I can't use it to transmit, I've been testing the radio's receiver. I notice that it doesn't seem to have as sensitive of a receiver as it used to. Few, if any, signals, will register on the S-meter. Compared to my other radio, it does seem like a weaker receiver, and not simply a "stingy" meter. So, a full, or at least partial, recapping may still be in the radio's future. In the WWV position, however, the receiver is much more responsive. But I'm taking this one step at a time--transmitter first.

More to come.
« Last Edit: November 18, 2011, 12:27:55 PM by KD4ACG » Logged
KE4DRN
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Posts: 3746




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« Reply #10 on: November 18, 2011, 02:21:57 PM »

hi,

the 520 has lots of wirewrap connections, to replace all of
the electrolytics is a lot of work to remove all the pc boards.

the pc board traces can be easily damaged by excess heat
from removing and replacing the caps.

many of us on the yahoo group I referenced operate
and repair these fine radios and only replace what is a
known fault, otherwise you may create a new fault and
end up masking the original problem.

best to get the radio operational first and if he wants to
recap the entire radio then at least he will have one in
working condition before any recap takes place.

exception is the HV filter caps, bleeder resistors and bypass caps.

73 james
« Last Edit: November 18, 2011, 02:58:37 PM by KE4DRN » Logged
N4NYY
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Posts: 5202




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« Reply #11 on: November 18, 2011, 03:03:37 PM »

Quote
best to get the radio operational first and if he wants to
recap the entire radio then at least he will have one in
working condition before any recap takes place.

I agree. I recap frequently on restoration, but usually only for a radio that is at least 30 years old or so, and not been used in ages. If it is used normally, than it might not be neccessary. Regarding the trace lifting, I never have a problem as I use a temp controlled iron and set if for the lowest possible setting that will work. The key is to only hold the iron as little as possible.
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ZS5WC
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« Reply #12 on: November 18, 2011, 10:45:52 PM »

 :)Well Jason , You are making headway at least!..
An easy way to check if the 100pf cap is leaky is to pull the tubes, and check on the grid pins of the sockets for DC potential. (No carrier or drive applied)
Any DC potential here would indicate a leaky cap.
Also the blowing of the resistors -does it happen before the heater switch has been turned on or after- If before it would indicate a faulty tube, if after it could indicate a faulty bias potential or one / two faulty tubes.
The best insurance is to pull the tubes and make sure all the screen / control grid potentials are 100% (and screen resistors), as well as the DC blocking cap-if you or a friend have an AVO tube tester test the tubes to make sure there are no internal shorts.
If your potentials are 100% , and you have spare GOOD tubes to install try them, and get the other tubes tested some time.
Regarding the lack of sensitivity--it is normally due to a dirty band switch or faulty bandswitch shaft coupler--so deoxit the band switch, make sure your attenuator switch is OUT (IF your rig is a 520S), and check the TX / RX relay. I assume your Drive/ preselector was peaked?.

73 de William
ZS4L / ZS5WC
« Last Edit: November 18, 2011, 11:22:24 PM by ZS5WC » Logged
W5RKL
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Posts: 1107




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« Reply #13 on: November 19, 2011, 06:22:17 AM »


After you replaced the cathode resistors, did you check the final tube's cathode current in transmit (meter switch set to IP)? If the cathode resistors failed after replacing with new resistors then more than likely the IP meter, in transmit, would peg full scale to the right indicating saturation current levels in the final tubes.

Measure the final tube bias voltage at the RF Unit's "FG" terminal. The voltage in receive should be approximately -101VDC (both final tubes biased to cut off), changing to approximately -65VDC in transmit. When measuring the "FG" terminal voltage, ensure the CAR control is set fully CCW. If the "FG" voltage drops significantly below -65VDC in transmit but returns to approximately -101VDC in receive, it's quite possible you have a problem in one or both of the final tubes.

If the measured RF Unit "FG" voltage is -101VDC in receive than the bias supply, -C on the Rectifier Unit, is not the source of your problem. Loss of -C bias voltage would result in both final tubes conducting to saturation levels even in receive which would cause the cathode resistors to overheat. Switching to transmit with the meter switch set to IP would result in the meter pegging full scale to the right. When this occurs the fuse typically blows rather quickly, providing the correct rating fuse is installed (6 amp at 120VAC, 4 amp at 240VAC primary power).

Leaky HV filter capacitors will not cause your cathode resistors to fail. Bad/Leaky HV filter capacitors will result a higher than normal reduction in HV when placing the 520 in transmit. If the capacitor failure is caused by an internal short within the capacitors, the fuse most likely will blow when power is applied to the transceiver, again providing the correct fuse rating is installed. The HV power supply in the 520 and all other Kenwood hybrids, uses a voltage doubling circuit where the HV filter capacitors are connected in "series" for a total capacitance of 50ufd at 1000VDC voltage rating
(100 x 100 / 100 + 100 = 50). Both capacitor's working voltage ratings adds (500 + 500 = 1000) when the capacitors are connected in series. The 470K 1/2 watt resistors across each HV filter capacitor are voltage equalizing resistors.

A HV level of 950VDC in receive/standby is normal for all of the Kenwood hybrids.

Do you have the 520 service manual which contains the schematic? If not then I do not recommend you attempt any repairs without the service manual and schematic. You can download the 520 service manual(s) at

www.n6wk.com/kenwood/

73s
Mike



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KD4ACG
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« Reply #14 on: November 25, 2011, 03:43:52 PM »

Just wanted to reassure everyone that I haven't abandoned this thread. I've put the project on hold until next weekend's hamfest. Hoping to get some of the needed parts there (such as new tubes, just to be sure).

Thanks to everyone for your advice so far. I'll keep you updated on my progress.

73 de Jason, KD4ACG
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