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How to correct the tuning indicator to the center position? Auto tuning stops at best signal, station frequency is correct, but the tuning meter is off-center. This happens on all stations, so it is not one station drifting off. When the tuner is switched off, the indicator is at center position.
Thank you in advance for advice.
How to correct the tuning indicator to the center position? Auto tuning stops at best signal, station frequency is correct, but the tuning meter is off-center. This happens on all stations, so it is not one station drifting off. When the tuner is switched off, the indicator is at center position.
Thank you in advance for advice.
Hello MilanMilosevic,
I do have the same issue and I suppose you have read the Service Manual, page 43 : it says the meter can be adjusted mechanically after opening it. However, as your meter is centered when the tuner is off it is probably correctly adjusted.
As far as I know there is no electronic adjustment available for the B261, unlike in the B760. Unless someone else can help ?
Since the mechanical adjustment of the meter is OK (centered with B261 off), do NOT open it !
1) Check meter position at some frequency with no reception, unplug antenna.
What is the position?
2) The signal path of the B261 is almost identical with the B760, including the frequency discriminator driving the tuning meter. There is a section in the service manual describing how to adjust the discriminator (2 coils in a shielded can). Don't worry, there is absolutely no effect on audio with this discriminator misaligned, it just serves the meter.
3) In general with PLL Tuners, the reference oscillator (4.0 MHz for B261) might be slightly off. In this case you might have a small frequency offset in the "real" tuning. However, in most cases you would not notice anything because the PLL will just lock to the nearest point, which would be the seemingly correct setting. Unless the quarz is really bad or practically dead, the offset is usually very small. Since this is a quarz oscillator, the pulling range is generally fairly small. As far as I recall the B261 design has no pulling option included, in contrast to B760. This would usually be a small trimm capacitor in series with the quarz. Unless you have a precise frequency counter don't touch it.
Edit: I've checked the schematics, there IS a pulling trimmer with the B261 oscillator.
Zuletzt geändert von eckibear; 19.06.2014, 09:28.
Grund: Nachtrag:
I have this tuner since 2003 and the indicator has been off-center ever since. When I bought it, I performed deep cleaning and replacement of all elkos inside. The only alignments I did were +33V on the PSU and the tuning voltages with precision voltmeter.
I carefully read the B760 and B261 schematics and came to the conclusion that they share most of the design, apart from the microprocessor and tuning parts of course.
I bought my B261 new back in 1985 (expensive tuner then !) and it has been working perfectly since until the day, 10 years ago, I made a stupid mistake when fitting the "Antenna B" option, a wrong connection made my pride and joy completely mute. I had to bring it to the Revox Service outlet in France and when I got it back the tuning meter was off center exactly like MilanMilosevic's. I suppose they fiddled with the alignment, although it is perfectly working.
have exactly the same "problem"...never bouthered me much, though...well, it did in the beginning when i've been regarding the meter as the reference of a perfectly tuned station and not achieving the center even with the 12,5 KHz-step(s) - one click and the meter off-centeres slightly to the right.
With the meter positions shown in YOUR case my first guess would be a minor detuning of the discriminator coil doublet. The alignment is fairly straightforward following service manual chapter 4.6.
Here the quick fix, not perfect but practically good enough: Instead of the quiet FM test signal suggested you may as well tune to any normal FM station, since nowadays they all use well-controlled frequency standards. The LF modulation does not affect the meter alignment much at all, particulary anywhere near maximum indication (setting of coil FL1A). Try to find an isolated FM station to avoid interference in the wings of the filter while setting of FL1B, if needed at all.
Ideally you should make sure the PLL reference (4Mhz) has only marginal frequency offset (quarz tuned by C26). To be really confident you'll need a precise counter.
In general I strongly advise to use mechanically well-fitted, preferably soft (plastics) and non-magnetic tools when touching any of the coil ferrites. These are very brittle and some of them would even permanently(!) change their magnetic properties when exposed to (strongly) magnetized tools, the worst case being "handyman's magnetic screw driver"
With the B760 you will often observe a temperature shift of the meter detuning, the B261 is a little better. Part of this phenomenon can be attributed to the quarz oscillator temperature coefficient, also the filter coils are not that stable. So having the tuner loosely covered and warmed-up prior alignments is a good idea.
your help and effort are very much appreciated ...i just wish, i had an idea what you are talking about.
If i read the small print correctly, the meter is nearly a gimmick with little or no practical use other than being annoying (in case of Milan, Thierry and me at least)?.
As it doesn't affect the sound, i wont try to "fix" it, even with your excellent guidance - i'm very much aware of my limitations.
If i read the small print correctly, the meter is nearly a gimmick with little or no practical use other than being annoying (in case of Milan, Thierry and me at least)?
That's pretty much it. Just keep in mind there are other opinions out there, particularly among perfectionists and High-enders
@Borchee
OK, I must admit I like how you converted the B285 display into Studer backlit style.
@Eckibear
Can you please clarify the quick tuning meter fix you suggested:
- to use any station instead of FM generator and perform discriminator alignment procedure as per service manual, or
- do optical alignment by turning only FL1A coil?
@Eckibear
Can you please clarify the quick tuning meter fix you suggested:
- to use any station instead of FM generator and perform discriminator alignment procedure as per service manual, or
- do optical alignment by turning only FL1A coil?
- First, verify the meter itself is mechanically centered (B261 turned OFF).
- IF you only want to make the tuning meter look centered go ahead and align FL1A with the B261 tuned to any decent FM station. Keep in mind warm-up time as described above.
- Verify symmetry by detuning + and - one or more frequency steps, the meter should deviate symmetically.
- A somewhat better symmetry, especially with larger detunings, might be achieved by adjustment of FL1B. With no clean test signal (FM generator) at hand you should tune into a station with high signal and preferably with no adjacent stations within +/-200kHz or even more. The latter might be difficult these days. If in doubt chose a somewhat weaker station with no adjacent stations. Detune the B261 in multiple steps + and - and align meter symmetry with FL1B. There is a slight dependency between FL1A and FL1B alignment, repeat steps mentioned above.
In any case mark up the initial positions of the coils and perform only minor tweaks, usually the coil detuning is less than 1/4 turn.
To be precise, the IF filter section is also somewhat prone to detuning. However, alignment of this section is much more delicate and it WILL affect audio (distortion, selection). I strictly disrecommend to touch this section unless you have the full set of measurement equipment AND the knowledge needed.
zur Zeit besitze ich zwei B261, bei beiden ist die Tuning-Anzeige nach der automatischen Sendersuche nicht mittig.
Wenn ich nach erfolgter Sendersuche ( position Auto ) die 12,5 khz Taste drücke, wandert der Zeiger in die Mitte.
Soweit ich das im Internet nachlesen konnte, hat sich das Frequensraster der UKW-Sender geändert.
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, Eckibear...
...i just wish, i had an idea what you are talking about.
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