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Heathkit DX-40 Raspy Raspy CW note




 
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Author Topic: Heathkit DX-40 Raspy Raspy CW note  (Read 1151 times)
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w5rkl
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« on: February 25, 2018, 03:17:38 PM »


I was testing my DX-40 today and notice the CW note is vary raspy. This problem occurs with a crystal of VFO (HG-10).
I don't power the VFO from the DX-40, the VFO has its own separate power supply.

I noticed if I remove the chassis from the cabinet the raspy CW note disappears.
Put the chassis back into the cabinet and the raspy CW note returns.

I swapped out the final tube, no change.
I haven't swapped 6CL6s yet.

I already changed the filter capacitors.

This was not a problem last year. This problem popped up after sitting on the shelf for a year.

I'm going to clean all the tube sockets to see if that helps.

When the FINAL TUNING control is adjust out of resonance, high Plate current, the note changes to a clean note.
Dip the PLATE with the FINAL TUNING control and raspy note returns.

The receiver is a Heathkit SB-303 and Kenwood TS-520S


73
Mike W5RKL
www.w5rkl.com
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W1ITT
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« Reply #1 on: February 25, 2018, 03:38:42 PM »

Take a close look at the resistor in the plate parasitic choke.  Those old carbon resistors sometimes open up with age.  The presence or lack of the cabinet will change the capacitance up there.  It wouldn't make much difference at HF, but up around 100 megacycles (it's an old radio..) where the parasitics live, the capacitance change begins to be significant.  While in there, I'd look at how the plate tuning and loading variables are grounded to the chassis.  Loosen the screws and then snug them up again to break any oxide or dissimilar metal corrosion.  If you had a spectrum analyzer, to see it, it's possible that you are making RF at more than one spot on the dial.
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #2 on: February 25, 2018, 03:58:47 PM »

Hi Mike -

Sounds like you've rounded up the usual suspects so far.

I would agree with TTT's summary.  If that doesn't fix it then try some of these:  


The cabinet on/off fix means the rig is unstable and probably on the verge of taking off.

Resonance means there is maximum RF current flowing in the plate tank circuit possibly aggravating poor connections. Look for poor connections in the C1 and C2 rotor contacts, bandswitch, etc.   There are several amps of circulating RF current flowing within C1-L1-C2 depending on power and Q.

Does it happen on other bands?  Listen on a receiver and tune around looking for parasitics for clues.  Use a short 1' wire antenna.

And another technique that rarely fails to find the culprit(s):   Load the rig up with a full carrier output and recreate the raspy sound. Then, using a fiberglass or plastic rod, push, poke and jab all connections on top and bottom of the chassis.  Especially concentrate on the pi-network.  If that does not show the problem, you might put the rig in a dark room at night, and tune the cap to produce raspy/ clean and look for any visual arcing. Using a mirror can help.

Jumper in a new test capacitor, like the plate choke bypass, the plate coupling cap, the grid bypass cap, etc., looking for bad ones. Is the neutralizing cap, if any, OK and working?

The last thing to try is shaking, warping and tapping the chassis in general with the rig running.  Careful... :-)

Let us know.

T



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w5rkl
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« Reply #3 on: February 25, 2018, 05:13:43 PM »

I tried all the suggestion, I even replaced the Parasitic suppressor with one from a Kenwood Final Amp running 6146s.
I had the Kenwood Final Amplifier as a learning tool so it missing a parasitic suppressor is not a problem.

I removed the front panel and cleaned behind the panel and along the front of the chassis apron with contact cleaner.
I cleaned the Final Tuning and Loading capacitor shaft to frame connections.

None of that fixed the problem.

I've been looking for a complete restoration project, is probably it. I'm not in hurry to fix the DX-40 so restoring it from
ground up will be fun. I've fully restored a number of old Heathkit rigs (see my website for pictures of my fully restored Heathkit
AT1, AR3, and HR-10B). At least I don't have to worry about the DX-40 having a bad power transformer like many DX-40s have.

Thanks guys for all the suggestions/help, I appreciate it very much.

73
Mike W5RKL
www.w5rkl.com

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K1JJ
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« Reply #4 on: February 25, 2018, 06:43:12 PM »

OK Mike -

Let's not give up yet... :-)

Try loading the grid down and see if the buzz goes away.  Do this by hanging a .001 cap in series with a 500 ohm to 1K carbon resistor from the final tube's grid pin to ground.   It will be a little harder to drive, but will provide a clue.  This lowers the stage gain and will possibly eliminate RF instability questions.

When the buzz is going, what do you hear on the receiver when you tune around?

T
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Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
w5rkl
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« Reply #5 on: February 25, 2018, 10:16:31 PM »

Tom,

Although I have considered performing a full restoration on the DX-40, I haven't given up all
possibilities just yet.

When tuning across the DX-40 signal I hear a rise in raspy sound with the chassis in the cabinet.
Remove the cabinet and tuning the receiver produces a rise in the CW beat note, no raspy sound.

In TUNE the final amp is shut down by grounding the final amp's screen and removing the final amp
plate voltage. Even then I can still hear a slight raspy note in the receiver. To be sure I'm going to
remove the final tube and test again to see if what I suspect is, I don't think the problem is in the final
amp but rather in one of the early stages. I suspect the final amp is simply amplifying the earlier stage
raspy RF signal.



73
Mike W5RKL
www.w5rkl.com



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K1JJ
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« Reply #6 on: February 25, 2018, 11:10:50 PM »

OK Mike -


If you think it's an earlier stage, then take the .001 / 1K resistor series combination and put it from the driver grid pin to ground and try it again.  Work your way back to the buffer grid doing the same test and see if the rasp disappears. This quick test will help you isolate an instability problem down to a stage - maybe.

Also, sampling the stage in/out chain with a scope might show you where the rasp starts.  

Try some .001 bypass caps where there is normal filtering for the grids/screens/ cathodes. Sometimes just a quick cap jumper across existing bypass caps can expose the problem.

Yes, a full restoration will probably fix it shotgun-approach... :-)

T
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Favorite Song - Trololo thru the years:  
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K4RT
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« Reply #7 on: February 26, 2018, 04:14:34 PM »

I’m not familiar at all with the DX-40. I had a similar issue with my DX-100B, which turned out to be a tube in the RF chain that was failing and did fail. I don’t recall now which tube.  Good luck.
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Brad K4RT
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« Reply #8 on: February 26, 2018, 04:22:17 PM »



  Being that this was a kit at some form in it's life...Start with loosening and then re-tightening all the screws and nuts in the unit. Pay particular attention to all the ground screws. The bolts that hold down tuning caps. The so239 in the rear; even the front panel controls that have the ground tabs, key jacks, terminal strips etc.  I have seen many if not all of the screws in a Heathkit can be tuned 1/4 turn anyway. They may have loosened over time with temperature/age or the original kit builder never really tightened the screws. They leave off the star washers that aid in the grounding because they can be difficult to install. These guys are un-neutralized, so any possible VHF ground path become important. Just a thought.

     Steve
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« Reply #9 on: February 26, 2018, 04:49:49 PM »

I would start by modifying the 6CL6 power feed and Keying circuit, schematic #5.


Phil - AC0OB


http://amfone.net/Amforum/index.php?topic=43002.0
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w5rkl
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« Reply #10 on: February 27, 2018, 05:34:23 PM »


Update.

I replaced a bad buffer screen resistor, R6 which is a 27K 1 watt. R6 measured 41K so it was way out of tolerance AND
one lead was not soldered. Fixed that by replacing R6 with a new 27K 2 watt (only had 2 watt spares).

Next I found during tune up, with the chassis in the cabinet, if I peaked the Grid to 3ma then in CW mode, adjusted Final control
for maximum dip, the transmit signal is rather distorted (very raspy). Tuning around the signal clearly indicates distortion on the
receive signal.

The Load control is not adjusted but rather left fully CCW. If I continue rotating the Plate control for a plate meter indication of 140ma,
the transmit signal cleans up quite a bit. If I rotate the Final control CCW resulting in a higher plate current indication, the distortion
increased. If I leave the Plate dip at maximum which indicates 90ma, adjusting the Load control CW results in an increase in plate
current but no decrease in transmit signal distortion. Leaving the Load control fully CCW and rotating the Final control past max dip
for 120 to 140ma indicated results in an improved transmit signal quality, significantly less distortion.

I could be wrong but since the DX-40 is not designed to have a neutralizing capacitor, it might be necessary to add one to tame the
final amplifier down. Any suggestions?

73
Mike W5RKL
www.w5rkl.com



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« Reply #11 on: February 28, 2018, 12:54:02 AM »


...Any suggestions?

73
Mike W5RKL
www.w5rkl.com


The DX-40 was not designed to present a clean CW pulse, but we can make some improvements:

1) Provide separate power supply feeds and Keybias to the 6CL6's and re-peak L2, as per schematic #1,

2) insert a 47 ohm 1 W CF resistor Rx* between C31 and the grid leak circuit, as per schematic #2,

3) Add Pi-Net capacitance as per schematic #3,


4) Insure you have a stable and well filtered power supply system as per schematics #4,5.


Phil - AC0OB


 

* DX-40 Improvement Schematics for AMPHONE.pdf (159.43 KB - downloaded 21 times.)
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