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Old 30th Apr 2024, 12:50 am   #21
Julesomega
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Default Re: Lowe HF- series rotary encoder problems.

I have been working on and reworking the HF-125, progressively losing confidence in my sanity and in my understanding of how things work. After decoupling the 5V supply to the optical encoder, it was working satisfactorily in the 'test bed' arrangement so I added an additional 220 Ohms for decoupling right on the encoder and fitted everything back in the case, changing all the electrolytics on the supply lines as a sensible precaution¹. The squiggling was now so extreme that trying to tune a clean AM signal immediately results in so much noise hash for about 10kHz either side of the carrier that you can't hear the modulation. This is worse above 12MHz. A few hours had the ME and toggle sw mounted outside the cabinet, with the OE remaining mounted through the front panel. Tuning by the mechanical encoder was almost without artefacts until I rotated the optical encoder which had only its supply connected, whereupon the fixed frequency squiggled up and down with each cycle of the inoperative OE.

Before decoupling the encoder, a 'scope showed pulses of around 2mV and 2µs on the supply line, but these had now been eliminated. There was only one rational explanation left, magnetic coupling into the PLO. This makes sense if you look at the layout of the PLL oscillator section, which is immediately to the rear of the encoder. The OE projects further towards the oscillator coils than the ME originally did. I knew these Toko S18 series inductors were susceptible to alternating magnetic fields in this circuit as when I first got the rx I found that nearby mains tfmrs would cause hum when using synchronous demod; hence the mod of increasing the turns and fitting aluminium cores. I have an ideal little audio receiver for testing hearing-aid loops but I gave up looking for it after an hour and listened with a small portable radio which found some clicking on LW.

Magnetic coupling will be worse in the test-bed situation, where the encoder outputs are via great loops of connecting wire, and must explain the slight artefacts even from the ME. A temporary screen using a section of tin can gave a slight improvement on the higher bands, which disappeared again when the screen was grounded through yet another length of wire. It seems so unlikely that a pulse of a few mA could cause the problem, but we are dealing with an oscillator running at 60 or 70MHz. Now that I have a credible explanation I can start making changes to restore acceptable tuning. Things should be better when the flying leads are removed. I'm not sure how to add effective magnetic screening, maybe the three coils can be moved behind a steel plate and oriented for least pickup. Maybe the loop bandwidth should be increased to track the pulses - I need to find out what changes were made for the later HF-250E using an OE.


¹ I couldn't get sensible readings of capacity and ESR in circuit, but after removal all the original caps were found to be in closer tolerance than the BNIBag replacements!
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Old 30th Apr 2024, 2:48 pm   #22
Cruisin Marine
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Default Re: Lowe HF- series rotary encoder problems.

There were screening cans made for the TOKO s18,mc107 and mc111 coils, Cirkit and Bonex sold them.
I dunno' if there is room to fit them in there though.
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Old 30th Apr 2024, 4:49 pm   #23
frsimen
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Default Re: Lowe HF- series rotary encoder problems.

Hello Julian,

I'm not sure if the photo is the current arrangement of the wiring. If it is, you have a very big loop around those sensitive inductors formed by the black wire. That will easily induce a lot of noise into the VCO circuitry.

Try bunching the supply wires to the encoder together and move them as close to the front panel as you can. That will reduce the amount of coupling into the more sensitive parts of the circuit by keeping the loop area as small as you can make it.

The HF225 is very sensitive to external fields too.

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Old 2nd May 2024, 12:35 am   #24
Julesomega
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Default Re: Lowe HF- series rotary encoder problems.

Having decommissioned the testbed mode, radiation from the leads to the encoder can be discounted. The leads are now tidy and the 5V supply is decoupled right on the encoder, as shown in the photo. A scrap sheet of mild steel between the encoder and oscillator coils for magnetic screening gives a further reduction in the tuning perturbations, but is very susceptible to hum and the slightest mechanical vibration, so the screen will have to be cut smaller, carefully grounded, and rigidly fixed.

I am now enjoying listening across the bands, and have been rejoicing in the reasonable signal levels to be heard on 1458kHz, with the nearest 5kW tx having moved a bit further away today

The remaining work to be done is to turn the receiver into a PLL loop bandwidth test bed. Analysing loop bandwidth is beyond me, but the determining components are R78 for bandwidth and R79 for stability. Reducing R78 to increase the bandwidth will make the PLO less susceptible to induced magnetic pulses; reducing the value by a factor of 10 has already given a noticeable improvement so I'm hoping a little more work fresh in the morning will completely restore smooth tuning for CW.

The optical encoder, at 64 cycles per revolution, gives a slightly finer tuning rate; it is very smooth and is specified for something like 30 million revolutions so I shan't be worrying about wearing it out. Others have pointed out that the display illumination is too dim on these Lowes so I changed the old yellow LEDs to modern warm white 120° types: the improvement is just what's needed.
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Old 19th May 2024, 9:36 pm   #25
Nick_B333
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Default Re: Lowe HF- series rotary encoder problems.

If I can ask a slightly o/t (but still HF-125 specific) question - was the issue with the AM sync detector detailed here ever solved? I have recently acquired a HF-125 with the same symptoms. My HF-225 doesn't have this issue, but then the detector uses SL6440 not the SL1640.
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Old 21st May 2024, 8:38 pm   #26
Julesomega
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Default Re: Lowe HF- series rotary encoder problems.

Nick, the unexpected solution was described on page 2 of that thread!

The optical encoder conversion was going so nicely until I did something, I haven't found out yet what it was, as I've been diverted by other things.
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Old 21st May 2024, 9:50 pm   #27
Nick_B333
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Default Re: Lowe HF- series rotary encoder problems.

Quote:
Originally Posted by Julesomega View Post
Nick, the unexpected solution was described on page 2 of that thread!
Thanks Julian. It wasn't clear to me that you'd actually cured it when you mentioned you had mis-directed the mute pin, but you obviously did. FM and the mute appear to be working on mine so I was concentrating on the SL1640 that I'd got enough carrier and signal going in and that the switch between pins 1 and 2 of the 4066 hadn't gone high resistance. I'll check it again when I've got a chance to grab a signal generator.

Good luck with the encoder replacement. Mine was faulty rotating anti-clockwise but it seems to have sorted itself after a few days of use.
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