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Author Topic: TS-850 became deaf and no RF output power  (Read 5029 times)
VA3UT
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Posts: 11




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« on: June 08, 2013, 11:22:43 AM »

Hi,

My TS-850 worked flawlessly until last night, however I found that it became deaf with no output power this morning when I was about to join my club QSO.  When I press AT TUNE button, it shows SWR to the max with no RF power.  No signal but I can barely hear very strong local AM broadcast station.  All displays, buttons and knobs seem working just fine, so there must be something blocking the signal path.

It doesn't seem that the PA has any issue.  There must be an issue with signal path affecting both transmit and receive signal.  When I put it on CW and transmit, I hear sidetone, but when I put it on SSB and Monitor, I hear nothing.  I suspect the RF board.  Has anyone had any such issue before?

Thanks.

Chris  AD1AD
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AC5UP
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« Reply #1 on: June 08, 2013, 11:35:23 AM »

Hmmmmmmmmmmmmmmmmm........ What could possibly be common to both TX and RX?  What one fault would make the SWR shoot for the moon and the RX go deef?

If you disconnected the antenna from a properly working TS-850 you'd have the same symptoms.

So, if it were mine, I'd give the antenna changeover (relay) circuit the hairy eyeball. If you still have white noise in the RX that tells you the IF strip is OK and might suggest the heterodyne oscillators & mixer circuits are still good. The high SWR tells me the TX driver and PA sections are still trying. Given the age of the set there could be more than one issue, but I'd definitely start with the T/R relay circuit.
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KE3WD
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« Reply #2 on: June 08, 2013, 12:15:57 PM »

Yes, I'd be looking at the T/R relay also. 

But before even doing that, I'd be looking at the load, which would be the antenna and all else in between antenna and rig. 

A bad or broken antenna connection could also be the problem. 

So the very first thing I'd do is to disconnect coax at back of radio and connect my known good coax jumper to dummy load and check the rig's operation there. 

If that proved okay on the Transmit side, then I'd stick any random length piece of wire into the center of the rig's pl-259 antenna connector, drape the wire out across the floor a few feet and with the mic disconnected to prevent transmit, I'd take a listen to the receiver. 

Finally, if still no joy, I'd be pulling the covers off the rig and locate the T/R relay, and, while still in that receive test mode with the short wire for receive antenna, I'd tap on the T/R relay with an insulated spudger tool, wood pencil, or plastic screwdriver handle while listening to see if the receive popped in and out from my mechanical disturbances.  If it did, I'd either be cleaning the relay contacts with pieces of printer paper cut to fit and wetted with deoxit, or be removing said relay from its board and trying to open it to reveal the contacts for the same kind of cleaning, or perhaps would be ordering a replacement relay if it is one of those tiny plastic hermetically sealed jobbies, which I don't think you will find in the '850. 


73
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AC5UP
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« Reply #3 on: June 08, 2013, 01:51:17 PM »

A fault with the antenna is definitely possible, but I didn't go there first as it has been my experience that even a broken antenna will still hear something... If only what leaks through the feedline. The station selection might be Slim Pickens and weak, but should (?) do more than barely hear a strong local BCB signal as described.

Note to VA3UT:  A quick & dirty check would be to unscrew the outer ring from the PL-259 then pull the plug out slightly. If the receiver comes to life with only the center pin making contact that tells you it's an antenna problem.
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W8JX
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« Reply #4 on: June 08, 2013, 04:20:34 PM »

I think first thing I would try is to bypass tuner
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KA5IPF
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« Reply #5 on: June 09, 2013, 08:43:17 PM »

On the back of the SO239 is a short piece of wire to connect to the PCB. They break, look there first.

What is the current draw in CW TX mode? That will answer your question. High current will point to the jumper, low current opens up a lot of possibilities.

The TR relay is a sealed unit and on the unobtanium list, be very careful around it. The tuner is not inline in rx, tx only, so won't affect rx.

Clif
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VA3UT
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« Reply #6 on: June 18, 2013, 07:10:12 AM »

Thanks, everyone.

Based on all the evidences and symptoms, it was from a faulty DDS chip on the CAR board.  I decided to sell it as a part radio as I couldn't find the part.  On eBay there is a CAR board for $299 but it's not worth paying that much as the radio itself is worth about $500.

Best 73,

Chris
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