HT-44 noise on SSB
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HT-44 - one last query for ideas before moving on.? ?The problem is a lot of "noise" within the audio passband when in LSB and USB modes.? I previously had thought the problem was in the mic amplifier stages, but it seems that's not true.? The nature of the noise is best seen in the attached spectrum pictures.? These were all taken from a tap on the dummy load with the HT-44 adjusted for about 10 watts output.? The first two photos show the output in CW and AM mode.? The second and third show LSB and USB mode.? The noise appears in the active sideband and shifts to the other sideband when changing from LSB to USB or vice versa.? The mic gain is set to 3-4 with somewhere around 5 mv sine wave audio input at 1400 Hz.? Before discovering the noise, I made one SSB QSO and the report was excellent audio but with some noise in the background.
What's been done so far:? All new NOS tubes.? Replaced .01 and .001 caps in the area of V1, V2, V3, V4.? That was done after finding one defective disc cap in the audio chain output.? Biased off the audio chain by using a jumper between the bias line and the first mic amp stage as is done for CW mode.? The noise remained, so I assume that the problem is in the balanced modulator stage.? ?Looked at the output of T2 with the spectrum scope and the noise is present there, so ruled out a problem in subsequent RF stages.
The balanced modulator section is at least working as I can minimize the unwanted sideband fairly well, and CW and AM modes don't look bad.? I suppose it's possible that there's a noisy resistor or capacitor and will look more closely at that in the next few days.? Years ago I ran into a difficult problem in an HA-1 keyer and it turned out to be a poor ground through one of the riveted tube sockets.? Hopefully this isn't that problem. Here's how the output looks on CW and AM.?? The horizontal lines are 10 dB, so pretty clean down to 50 dB or so.? Now for SSB: LSB.? Carrier point under the red M, LSB to the left, USB to the right.? Audio input was 1400 Hz sine wave USB.? Noise shifted with selected sideband. Ideas? 73, Floyd - K8AC |
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Floyd What type of disc capacitor did you use as a replacement?? See: It might be worth the effort to use metalized film caps in place of the ceramics for use in the audio stages. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, April 23, 2024 at 01:29:42 PM CDT, Floyd - K8AC <floydsense@...> wrote:
HT-44 - one last query for ideas before moving on.? ?The problem is a lot of "noise" within the audio passband when in LSB and USB modes.? I previously had thought the problem was in the mic amplifier stages, but it seems that's not true.? The nature of the noise is best seen in the attached spectrum pictures.? These were all taken from a tap on the dummy load with the HT-44 adjusted for about 10 watts output.? The first two photos show the output in CW and AM mode.? The second and third show LSB and USB mode.? The noise appears in the active sideband and shifts to the other sideband when changing from LSB to USB or vice versa.? The mic gain is set to 3-4 with somewhere around 5 mv sine wave audio input at 1400 Hz.? Before discovering the noise, I made one SSB QSO and the report was excellent audio but with some noise in the background.
What's been done so far:? All new NOS tubes.? Replaced .01 and .001 caps in the area of V1, V2, V3, V4.? That was done after finding one defective disc cap in the audio chain output.? Biased off the audio chain by using a jumper between the bias line and the first mic amp stage as is done for CW mode.? The noise remained, so I assume that the problem is in the balanced modulator stage.? ?Looked at the output of T2 with the spectrum scope and the noise is present there, so ruled out a problem in subsequent RF stages.
The balanced modulator section is at least working as I can minimize the unwanted sideband fairly well, and CW and AM modes don't look bad.? I suppose it's possible that there's a noisy resistor or capacitor and will look more closely at that in the next few days.? Years ago I ran into a difficult problem in an HA-1 keyer and it turned out to be a poor ground through one of the riveted tube sockets.? Hopefully this isn't that problem.
Here's how the output looks on CW and AM.?? The horizontal lines are 10 dB, so pretty clean down to 50 dB or so.? Now for SSB: LSB.? Carrier point under the red M, LSB to the left, USB to the right.? Audio input was 1400 Hz sine wave USB.? Noise shifted with selected sideband. Ideas? 73, Floyd - K8AC |
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From:[email protected] <[email protected]> on behalf of Jim Whartenby via groups.io <old_radio@...>
Sent:?Tuesday, April 23, 2024 2:03 PM To:[email protected] <[email protected]> Subject:?Re: [HallicraftersRadios] HT-44 noise on SSB ?
Floyd
What type of disc capacitor did you use as a replacement?? See:
It might be worth the effort to use metalized film caps in place of the ceramics for use in the audio stages.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
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On Tue, Apr 23, 2024 at 03:03 PM, Jim Whartenby wrote:
I used polypropylene film caps when replacing the discs.? There really was no change in the noise compared to the original caps.? All of the replaced caps had broken ceramic where the leads entered the body and Walt previously mentioned that failed caps often had that characteristic.? I think it's safe to say that any modern cap is going to be at least as good as the ones used 62 years ago.?? |
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On Tue, Apr 23, 2024 at 02:38 PM, W7WRX wrote:
Yes, I originally went through the unit using Walts HT-44 Repair Manual.? I have no idea what you mean by spec AN.? I readjusted the carrier null and unwanted sideband suppression again after taking the screen captures I posted.? The carrier null is greater than 50 dB and both the LSB and SSB suppression shows greater than 50 dB.? I suspect both of those could be improved if the prominent noise weren't there.? I was using the spectrum scope on my IC-7610.? The screen can be captured to an SD card by just pressing a button and then I edited out the unnecessary areas of the screen with a photo editor.? An oscilloscope probe connected to the receive-only antenna inputs works great for seeing what's going on in the transmitter or other receiver.?? 73, K8AC |
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Floyd I see that the noise is about 40 dB down in the SA displays.? What is the spec for the HT44? Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, April 23, 2024 at 04:16:20 PM CDT, Floyd - K8AC <floydsense@...> wrote:
On Tue, Apr 23, 2024 at 03:03 PM, Jim Whartenby wrote: I used polypropylene film caps when replacing the discs.? There really was no change in the noise compared to the original caps.? All of the replaced caps had broken ceramic where the leads entered the body and Walt previously mentioned that failed caps often had that characteristic.? I think it's safe to say that any modern cap is going to be at least as good as the ones used 62 years ago.?? |
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On Tue, Apr 23, 2024 at 06:18 PM, Jim Whartenby wrote:
In the HT-44 manual, specs page, there is no information on that.? They list carrier suppression, unwanted sideband rejection, thrid and fifth order distortion products and unwanted beat output, that's it.? I've restored a total of 15 old transmitters (this is first Hallicrafters) and I've never seen a transmitted noise floor this high.?? |
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Floyd,
I don't own an HT-44 but looking at the manual, it uses AALC like some other Hallicrafters gear.? Possibly part of what you're experiencing is a low-threshold AALC.? Temporarily disabling it may help in localizing the noise source. From the manual: "An outstanding feature of the HT-44 is the amplified Automatic Audio Level Control (AALC). The AALC circuitry virtually eliminates splatter commonly caused by excessive audio gain and its?
resultant 'flat-topping."" This is accomplished by providing up to 12 DB of compression..." Paul, W9AC |
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On Tue, Apr 23, 2024 at 10:09 PM, Paul Christensen wrote:
I don't own an HT-44 but looking at the manual, it uses AALC like some other Hallicrafters gear.? Possibly part of what you're experiencing is a low-threshold AALC.? Temporarily disabling it may help in localizing the noise source. From the manual:Interesting idea - thanks, Paul.? Will look into that. 73, Floyd - K8AC |
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Well, the AALC idea didn't pan out.? I disconnected one end of the diodes and also removed the plate supply from the AALC amplifier stage.? No change whatever.? Also played with the mic? connector lead and there was no difference in the noise level or pattern with both ends of the cable grounded vs. just one end.
Floyd |
开云体育
How and where are you sampling when you see the noise.
Walt Cates, WD0GOF
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From: [email protected] <[email protected]> on behalf of Floyd - K8AC <floydsense@...>
Sent: Friday, April 26, 2024 2:23 PM To: [email protected] <[email protected]> Subject: Re: [HallicraftersRadios] HT-44 noise on SSB ?
Well, the AALC idea didn't pan out.? I disconnected one end of the diodes and also removed the plate supply from the AALC amplifier stage.? No change whatever.? Also played with the mic? connector lead and there was no difference in the noise level or
pattern with both ends of the cable grounded vs. just one end.
Floyd |
开云体育
he is talking about the shield on the cable.
Walt Cates, WD0GOF
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From: [email protected] <[email protected]> on behalf of Richard Knoppow <1oldlens1@...>
Sent: Friday, April 26, 2024 2:35 PM To: [email protected] <[email protected]> Subject: Re: [HallicraftersRadios] HT-44 noise on SSB ?
I have lost track here. Have you just shorted the mic connector to
see what happens? If you have does the noise continue? On 4/26/2024 12:23 PM, Floyd - K8AC wrote: Well, the AALC idea didn't pan out.? I disconnected one end of the-- Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
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On Fri, Apr 26, 2024 at 03:31 PM, waltcates wrote:
How and where are you sampling when you see the noise.The noise if being observed (visual and audible) on an external transceiver spectrum scope.? The signal is being sampled at a sampling port on the dummy load and is applied to the monitoring transceiver at a receive-only antenna port. Floyd |
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On Fri, Apr 26, 2024 at 03:35 PM, Richard Knoppow wrote:
I have lost track here. Have you just shorted the mic connector toYes, that was an early test.? Yes, the noise was still there.? I previously had biased off the first mic stage and the noise disappeared, so I concluded that it wasn't coming from the audio strip.? But, tonight I just removed the mic line at the grid of the second mic amp and the noise was still there.? Not sure why it wasn't there when the first stage was biased off.? Maybe I made an error.? At any rate, it now appears that the noise could be coming from the audio strip at the third mic amp or the audio modulator.? If I pull V5, the 12AT7 audio modulator, the noise is gone.? Next will examine the phase shift network between the third mic amp and the audio modulator.? I know the audio outputs from V5 are equal as I previously had a bad bypass cap in the output and one channel was way down.?? Flyd |
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While shorting the mic connector mic lead to ground didn't eliminate the noise, shorting the hot lead to ground for either of the coax runs to the mic level control kills the noise completely.? I installed new teflon coax runs from the audio stages to the mic level control and while that made a very minor but measurable decrease in the noise, it's still very strong.? The teflon coax makes it easier to undo ground connections if I do that. Sounds like either a bad ground connection through one of the riveted terminal strips, or maybe one of the shield connections might need to be lifted from ground, although they were all originally grounded.? If I go that route, any advice on which end to lift the ground from - one end of each coax goes to the front panel mic level pot and the other ends go to the audio stage tubes.
73, K8AC |
开云体育Floyd, The HT-44 is out of my ball park, but those rivets are everywhere. Why not try measuring the ohms from the socket [whatever pin needs the ground] to the chassis using a good meter for the job, and sharp, clean probes. That should check the contacts on both sides of the rivet. Likely an analog meter is best to detect likely erratic problems. ? From: [email protected] [mailto:[email protected]] On Behalf Of Floyd - K8AC
Sent: Saturday, May 4, 2024 7:20 PM To: [email protected] Subject: Re: [HallicraftersRadios] HT-44 noise on SSB ? While shorting the mic connector mic lead to ground didn't eliminate the noise, shorting the hot lead to ground for either of the coax runs to the mic level control kills the noise completely.? I installed new teflon coax runs from the audio stages to the mic level control and while that made a very minor but measurable decrease in the noise, it's still very strong.? The teflon coax makes it easier to undo ground connections if I do that. Sounds like either a bad ground connection through one of the riveted terminal strips, or maybe one of the shield connections might need to be lifted from ground, although they were all originally grounded.? If I go that route, any advice on which end to lift the ground from - one end of each coax goes to the front panel mic level pot and the other ends go to the audio stage tubes. _._,_._,_ -- don??? va3drl |
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No luck with rivet investigation.? I've done a couple of things that have made noticable improvement, but not solved the problem entirely.? I used this method of measurement to observe the effect of changes I've made:? LSB mode, mic gain fully clockwise, operation switch set to MOX, ground the PTT line at mic jack (no mic connected).? Before any changes, this resulted in an RF output of 12 watts on the external wattmeter (feeding a dummy load).? That's with the carrier null adjusted for minimum carrier level.??
The first change was to replace the original two coax lines running from the mic amplifier stages to the mic gain control on the front panel.? The original lines were tightly laced into the wire harness running along the right edge of the chassis (as seen from bottom with front panel facing you).? The result of this change was a reduction in the RF output from 12 watts to 8 watts.? The new coax runs are separated from the wire harness as much as possible.? Zip tying them together makes the run stiff enough that it stays in place pretty well. After reading about a similar problem in a different make of transmitter, I examined the routing of the AC wiring from the rear panel connector to the on/off switch.? Once again, the AC carrying wires were laced into the large wiring harness that's routed close to the high gain mic stages.? I replaced that arrangement with a new twisted pair running directly from the rear panel connector to the on/off switch.? This time when I tested the output level, it was only 4 watts.?? At this point, I'm wondering what the observed RF output level might be of other HT-44s, with the mic disconnected, but the mic gain turned up all the way.? Other HT-44 owners - please comment on what you see on your transmitter.?? 73, Floyd |
Floyd - K8AC
Jim Whartenby
waltcates
Paul Christensen
don Root