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Old 12th Oct 2022, 1:07 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, 5:41 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.

Paula
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Old 12th Oct 2022, 6:50 pm   #3
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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, 6:27 pm   #4
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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, 7:10 pm   #5
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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, 1:48 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, 4:28 pm   #7
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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, 7:58 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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Old 23rd Feb 2023, 11:45 am   #9
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Default Re: Lowe HF225 fault

I still have a problem with this receiver causing a rough note on CW, SSB and SAM signals. The instability is not enough to be noticed on AM mode but renders SSB, etc unintelligable. The usual suspect electolytics have all been replaced to no avail.

There are three oscillators involved in the system (LO, HET and CAR) and, clearly, instability in any one of these could produce this fault condition. However, I have isolated the problem to the 1st local oscillator by disconnecting the PLL derived source and substituting a stable input at the same frequency. At this point the problem disappears. CW note is pure and SSB etc signals are again clearly resolved (limited tuning is still possible via the HET oscillator).

My question is, how should I now proceed with diagnosing what is a complex microprocessor controlled PLL. My first thought is that I need to break the loop, possibly by feeding in an appropriate DC voltage to the varicap that controls the frequency. This would tell me if it was purely an analogue problem in the FET oscillator or was coming in via the digital system.

Obviously, if anyone else on the forum has come across this problem I would be very grateful to hear about their findings.
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Old 23rd Feb 2023, 6:24 pm   #10
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Default Re: Lowe HF225 fault

Hello Keith,

The designer of the HF225 has gone to some trouble to make sure that the supply to the first local oscillator circuity has low noise and ripple. I suggest you try a few more checks before you try driving the varicap diodes from an external supply.

1) Check that the voltage at the emitter of Q21 is close to 6.1 volts, assuming the 8V line is actually 8V. If you measure more than 6.1V on the emitter of Q21, check the voltage on pin 5 of Q25. That should be 6.1V too and is set by R77 and R78. If it is, either Q21 or Q25 could be faulty.

2) If you haven't already done so, replace C106 and C107. If C106 in particular is faulty, the ripple will not be improved by as much as it should.

It's best to use a battery for the external supply for your "break the loop test" to reduce the risk of introducing unwanted noise in via the tuning supply should you need to go along that path.

Paula
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Old 24th Feb 2023, 12:28 pm   #11
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Default Re: Lowe HF225 fault

Hi again, Paula,


Thanks for your reply. Hopefully I'll get to the bottom of this one day!


The capacitance multiplier seems to be working fine. The output on Q21 emitter is 5.95V with less than 5mV p/p ripple (input is 7.85V). I changed C106/107 anyway but with no effect. So it doesn't seem to be a supply voltage issue.


I have now tried opening opening the loop and supplying the varicap with an external stabilised bench PSU. The oscillator is pretty unstable in free running mode, of course, but I think I can still hear the rough note. I also tried substituting the -2V rail that is derived from the clock. No difference.



So I'm beginning to wonder if there's a problem with one of the pin diodes that do the oscillator range select. I might try disconnecting them (don't have any BA244's to hand) and seeing if the problem is still there on the low ranges.


Any further suggestions much appreciated.
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Old 24th Feb 2023, 6:33 pm   #12
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Default Re: Lowe HF225 fault

Hello Keith,

The ripple should be nowhere near as high as 5mV, I wonder if you have an earth loop with your test gear that is giving that result. A few microvolts would be rather nearer the mark.

There are a few other things you could try. Run the HF225 from a battery to see if the problem is still there. If it is, try moving the radio to another room. If the problem goes away when using a battery, check that you're not getting some earth current issues between the PSU and your signal source or an external earth. That could cause noise to be injected into the VCO control line.

Feeding the VCO control line from an external source will need that source to be exceptionally well smoothed and stable, hence my suggestion to use a battery for that test rather than an external supply. That's true also for the -2V line. You also need to use well shielded cables otherwise noise will be introduced.

I have assumed that your noise is 50/100Hz related. If it's a higher frequency, have you changed C115, the filter on the VCO control line? C128 and C129 are also worth considering for replacement if you haven't already done so.

I hope you manage to find the problem, the HF225 performs well when working properly.

Paula
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Old 24th Feb 2023, 7:30 pm   #13
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Default Re: Lowe HF225 fault

Hi again, Paula,

Thanks for your further suggestions. The fault appeared after a longish period of non-use. Previously the RX had been working fine. I have run the set from a stablised bench PSU (Farnell) but this made no difference.

However, I may have made a breakthrough. The fault is not present on all frequency ranges. The beat note becomes clean after the transition between 5.999 and 6.000MHz. This is a point where one of the PIN diodes is turned off. I need to check the other transition points.

The anodes of the PIN diodes seem to go negative when they switch off. This confused me for a while but I can only think that they are then just rectifying the RF from the oscillator.

I have tried lifting the anodes of all three diodes (at which point I can only use the lower frequencies, of course). The "warble" is still present, however. I'm beginning to wonder if it is actually frequency jitter at all. Problem is I don't have a receiver with a BFO that covers the LO range.

Not sure if I can send you a short audio file attached to a PM. Alternatively maybe I could put a short clip of the fault on Youtube for you to hear.
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Old 24th Feb 2023, 8:17 pm   #14
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Default Re: Lowe HF225 fault

Hello Keith,

The manual I'm looking at gives the transition frequency as 5MHz, but that may have changed for later models.

Only one change takes place, namely D35 is switched on. Try lifting D35, it could be the culprit.

Paula

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Old 27th Feb 2023, 5:24 pm   #15
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Default Re: Lowe HF225 fault

Hi Paula,


Still struggling with this problem - but making some progress I think.

Firstly, you were right about the transition frequency - it was 5MHz. However, lifting D35 did not remove the noise but it then continued above 5MHz.

In fact the noise is present in all the frequency ranges, gradually increasing all the way up to each transition after which point it drops dramatically, rising again until the next one. This corresponds with the tuning voltage - in my case between about 2.5V and 6V. So, I don't it's the pin diodes per se but has something to do with the voltage across the varicap.

I've looked carefully with a scope on 20mV/cm at noise on both the tuning voltage and the fixed -2V and can't see anything that would cause the effect. there seems to be no difference either side of the transition.

I'm beginning wonder whether a faulty varicap could introduce noise depending on the voltage across it. Unfortunately I dont have a spare.

I'm just about to send you a recording of the effect so would be interested in any comments,

Keith
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