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Old 12th Oct 2022, 12:07 pm   #1
Keith
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Default Lowe HF225 fault

I only use this receiver sporadically but lately it has developed a problem. On switch-on after a period of non use, the S-meter is initially hard over and there is no output. After a few minutes the meter slowly goes back down and signals are heard except that there is a now a warble on a locked AM signal in synchronous mode and some instability on the sideband modes. I wonder if anyone else come across this problem and can point me in the right direction?
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Old 12th Oct 2022, 4:41 pm   #2
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Default Re: Lowe HF225 fault

I've encountered the warbly sound and instability if the supply voltage has dropped below the level needed for the HF225's voltage regulators, so check that first. Next check that the internal +5V and +8V are correct.

The AGC voltage at pin 16 of the SL6700 should drop as signal levels increase. Your fault may point to something pulling the AGC line down at switch on. C86, Q9, Q10 or Q11 could be the culprits.

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Old 12th Oct 2022, 5:50 pm   #3
Keith
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Default Re: Lowe HF225 fault

Hi Paula,
Many thanks for your reply - it's given me somewhere to start! The long recovery period after switch on makes me think it might be an electrolytic so I'll check the supply rails first as you suggest.
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Old 24th Nov 2022, 5:27 pm   #4
Nick_G4IRX
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Default Re: Lowe HF225 fault

Wonder if you've resolved this Keith?

I've just picked up one of these, most impressed with the performance. There is a note out there by N9EWO suggesting that C156 (8V rail) and C157 (5v rail) should be replaced as they are prone to failure/leakage 100uF/10v were fitted but the suggestion was to replace them with 100uF/16v.

The AGC on the SL6700C is very sensitive to transients. I built a receiver based on this and the SL6440 30 years ago and after looking at the Lowe designs I tinkered around with the 6700 to try and improve the AGC and added some CFU455Gs between the IF stages. Still not up to HF225 performance though!
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Old 24th Nov 2022, 6:10 pm   #5
Keith
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Default Re: Lowe HF225 fault

Hi Nick,
No - still haven't got around to it, I'm afraid. I will though - as you say it's a good receiver. I can still use it for AM at the moment but I'd like to get the synchronous detectionworking properly again. I'll post on here with any progress. Thanks for the further info.
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Old 25th Nov 2022, 12:48 am   #6
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Default Re: Lowe HF225 fault

Keith - That sounds rather too similar to a problem I was having on an HF-125 - see the thread Lowe HF-125 synchronous AM detector
I didn't exactly solve it, it just solved itself! Maybe yours will need some proper work, but my notes may offer a starting point, and you'll see you can easily improve the performance of the filters.
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Old 8th Dec 2022, 3:28 pm   #7
Keith
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Default Re: Lowe HF225 fault

Hi Julian et al,
Sorry about the long delay in replying - other broken things kept jumping the queue!
I finally opened up the box and put a scope on the 5V and 8V regulated rails. Both spot on with nothing untoward - even at high sensitivity on AC. I replaced C156 and C157 anyway but no improvement in the warble on LSB, USB and Synchronous AM. When tuning down to zero beat on the latter, there is an audible glitch when the last digit changes. I'm fairly sure this used to be smooth. Anyway, sounds like more in depth work is needed. Any further suggestions would be appreciated.
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Old 8th Dec 2022, 6:58 pm   #8
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Default Re: Lowe HF225 fault

Hello Keith,

The glitch when the last digit changes is probably a feature of the design. My HF225 does the same thing. I expect it is caused by both the main VCO and the interpolation oscillator having to step at the same time, with the interpolation oscillator jumping 1kHz rather than the 8Hz or so of a normal step.

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