Strange things happening with my TS 950 SDX (and amp?)

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Voodoo Audiophile
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Strange things happening with my TS 950 SDX (and amp?)

Post by JJ2013 »

Hi y'all VooDoo Gurus and AudioPhiles!

This question may not "belong" in this forum but wanted to post this as there are many who really know the TS950 SDX so well 8)

Note, the amp I am using is the Ameritron AL-811H (I purchased a set of brand-new tubes that Ameritron said were a matched, neutralized set) and I had been using it without any trouble on SSB and CW for a couple of months.

Here's the sequence of events:

1. I was on the 10m band, SSB, amp on and went after a DX station who was listening 5 up (incidentally, I did make the DX!).

2. Engaged XIT function on the rig for + 5 kHz and called (MONITOR ON).

3. My audio sounded a little scratchy at first (I have NEVER had this happen before, amp ON or OFF while operating on SSB on 10 m or 20/15 m bands)
and then it started sounding progressively more and more terrible, almost sounded like a lot of RF was mixing with my audio).

4. My linear amp exhibited odd behavior like it was no longer tuned up properly (which it always is!).

5. Amp didn't amplify at all, it was like the amp had just completely disappeared.

6. My audio sounded more and more like RF with barely intelligible audio (MONITOR ON) in the cans of my headphones.

7. Decided to switch off the linear amp.

8. Audio returned to "normal" while using the MONITOR function.

9. Tried amp on 20 m and it did put out about 250 W (amp can put out approximately 600 W on peaks), but clearly there was a problem with
the amp now as well.

10. Went on 10 m again today (obviously, not using the amp at all now), but even now my audio sounded (not using XIT, just regular "simplex" QSO)
really bad, "scratchy," like I heard more RF than audio level frequencies......Tried 15 m for fun and audio sounded great as always on this band when using the MONITOR function and headphones.

11. Decided to put my trusty old Kenwood TS870 on the air and went on 10 m and audio sounded beautiful as always. I did NOT use the same audio chain (a lot of Behringer audio gear) as I did with the TS950SDX. But then again, I'd never had ANY problems with audio (and RFI??) using the exact same outboard gear and TS870 a few months earlier, before I decided to get a preowned TS 950 SDX.

Have been really satisfied with the 950 SDX and even the stock version (I am using the 4 kHz filter though) sounds better than the TS870 (I use the TS870 to monitor my audio in real-time when fiddling with the settings on the audio gear). Absolutely beautiful, the 950 SDX is : )))

I am really perplexed as to why this happened?! Could it have something to do with using the XIT function?? Surely NOT! I believe that this was the VERY first time I'd ever engaged the XIT function on the TS950 SDX and had never had any sort of "RFI" (if that is what it is....) issues to date.

Not sure if the local Kenwood technician who comes highly recommended can locate the problem based on my description above though but I don't have time (and probably not the competence nor instruments) to fix the rig!!

Awaiting a deluge of suggestions :-)


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Post by BillPaul »

I mentioned this in reply to your e-mail, but I'll repeat it here for the benefit of others.

There are a couple of well-known problems with the TS-950SDX that can result in distorted audio. Usually the problem is that the TX audio in single sideband modes sounds raspy or warbly. The severity of the problem can also vary from very mild to so bad that you sound like you're talking underwater. The two problems are:

1) Cold solder joints on the resistor blocks CP1, CP2, CP3 and CP4 on the PLL unit
2) Bad trimmer cap TC1 on VCO2 (mounted on AF unit)

Both of these issues can result in noise on one of the mixer signals used in the conversion paths for both RX and TX. The noise is translated into distortion in the transmitted and received audio. The distortion in receive may not be harder to notice due to the static you normally hear on the band. If you tune in somebody with a strong signal in USB or LSB mode and listen closely, you may be able to hear it more easily.

The fixes for the cold solder joints usually involve just reflowing the solder, though in some cases it's also recommended to install some jumper wires too, just to be extra sure. I did this to my radio not long after I got it because I started to notice the problem with TX audio distortion too.

Unfortunately those fixes didn't cure the problem. It didn't go away completely until I also replaced the trimmer cap in the VCO2 assembly. This is a 10pf trimmer. Obtaining a replacement is easy, but getting the VCO2 assembly out of the radio and taken apart is a bit of a chore.

The trimmer cap I used as a replacement was a Xicom part from Mouser which is no longer available. This looks like a reasonable alternative: ... itors.html

There is one other possible issue, though one of its symptoms is very easy to spot and you didn't mention it, so I'm not sure it's the problem here.

Inside the shielded box that contains the final unit, there is also the filter unit. There are a couple of switching diodes on this board which are responsible for controlling some of the relays in the antenna tuner. Sometimes these diodes can be damaged due to a strong RF or static event and become leaky (i.e. they may conduct in the wrong direction at lower voltages than they're supposed to). Once they become leaky, when the TX power is turned up high enough, they may pick up some of the RF and start to switch on when they're not supposed to. The result is that the relays in the tuner engage when they're not supposed. In some cases you can hear the relay chattering due to the diode switching on and off.

When this happened to my radio, the problem it caused was only evident on some bands, in my case 15 meters and up. It was most pronounced on 10 meters. What I did to provoke it was:

1) Turn off the automatic antenna tuner
2) Connect a dummy load
3) Put the radio on 28.400MHz
4) Set the radio to AM mode
5) Turn the power control all the way up and the carrier all the way down
6) Engage the transmitter
7) Slowly start turning the carrier control up to full power

In my case I found that once the radio reached about 100 watts of carrier I could hear a relay in the tuner assembly start to chatter. Once it got over 100 watts it would engage solidly and stay there until I stopped transmitting. The problem was not evident with the antenna tuner switched on.

The fix is to replace the diodes D11 and D12 on the filter unit. They're common-cathode dual diodes. Strictly speaking you can use two plain switching diodes with their cathode leads tied together to replace them, if you can't find exact replacements. The part I used for my rig was the NTE590. (Thanks to Mike WZ5Q for the skinny on this.)

These are the only things I can think of that would lead to TX audio distortion. The most common causes have been the first two problems (cold solder joints, bad trimmer cap). The next time that it happens, I would try putting the dummy load on the radio and seeing if the problem is still evident. If it is an RFI issue, it wouldn't happen with the dummy load, so if the problem persists with the dummy load connected, it's likely one of the issues I covered here.

-Bill, N1GPT

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Post by JJ2013 »

Hey Bill!

Very good of you to share this for everyone's benefit :D

Now, since emailing last, I'd used my two other rigs and have absolutely no RFI and amp appears to be working like it always had! It would seem that cold solder and/or comps gone bad may be the likely cause and I really appreciate your expertise on this!!

Now, I don't really have time get down and dig into the "guts" of the rig and will need to have it looked at by a competent techie - ideally a local to Dallas Fort Worth guy - and in fact a friend of mine recommended one! He is also about 20 miles away but is "ship only"! I am sure there is really good reason for such a policy but I am no going to ship the rig next door but rather drop it off and pick it up although I may not have a lot of choice : (

It just seems nuts to have to pay shipping both ways as it is not cheap either! I would prefer quality work and components rather than blowing money on shipping.

Should anyone know of a local to Dallas Fort Worth Kenwood technician I would be willing to drive out (even if it is 50 miles one way) rather than pop the 50 pound rig in the mail so to speak.....Please, let me know there is anyone out there who would accept a drop-off??

Cheers and thank you again Bill!


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