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Old 13th Sep 2021, 9:49 pm   #1
AlanC
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Default Yupiteru MVT-7200 off frequency

This is probably a long shot but does anyone have any service info for this 90's vintage scanning receiver? I've had mine from new and it still works fine but has drifted off frequency by about 4khz. So to tune a USB signal on 14.200 for example you have tune to 14.996 for example.

I have read on a forum that it is extremely similar to the MVT-7100 for which circuit diagrams and board layouts are available. Based on that information, there are many conversion stages, the first LO is synthesised and all others are crystal controlled or derived, in particular the first IF is either 227 or 592 MHz depending on which sub-band the radio is working on, and they get the wide coverage by switching these together with combinations of high side and low side injection of the first LO. The 1st IF is converted down to 45 MHz by mixing with an overtone crystal oscillator on 182MHz either directly or tripled to 547MHz and the resulting 2nd IF is mixed down again to either 10.7MHz for wideband modes or 455kHz for all else. The USB and LSB carrier oscillators are implemented using a ceramic resonator that is pulled in frequency by switching fixed C and trimmers into circuit.

I guess a starting point would be to accurately measure the error at different points in the range and figure out from that which oscillator is the likely candidate, though any thoughts are welcome!

Cheers
Alan
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Old 13th Sep 2021, 10:49 pm   #2
Andrew2
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Default Re: Yupiteru MVT-7200 off frequency

Is the error the same at all frequencies or is it small at low freq's and bigger at higher ones? If it increases with frequency then it's likely to be the PLL reference xtal that has wandered a bit.
If the error is the same across the range, then it's one of the later conversion oscs.
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Old 14th Sep 2021, 8:02 am   #3
AlanC
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Default Re: Yupiteru MVT-7200 off frequency

Hi Andrew,

Thanks for your reply.

Yes indeed. The total tuned range of 100kHz to 1650 MHz appears continuous to the user but internally there are 8 sub bands where the various internal conversion oscillators do different things and different RF filters are switched in and out. The actual first oscillator synthesiser VCO covers 592 to 1057MHz but only tunes part of that range within any one sub band. So if the error is coming from there (as is likely given that, in effect, the 12.8MHz reference crystal is being multiplied by anything up to 82 times, so that it would only take about 50Hz error in the frequency of this xtal to produce my 4kHz error) it should be possible to determine this with reference to the data.

So I think if I draw up a chart of 24 points - the low, middle and high end of each sub band - fortunately the frequencies are all given on the block diagram - and record the error at each point, this should tell us.

Of course this assumes that the 7200 and 7100 work in the same way and we don't know that.

Alan

Last edited by AlanC; 14th Sep 2021 at 8:09 am.
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Old 18th Sep 2021, 4:31 pm   #4
AlanC
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Default Re: Yupiteru MVT-7200 off frequency

Okay so to wrap this one up, here's what I found.

I first radiated the first IF of 592.3 MHz (sub band 1) at the set and found the error existed here - in narrow FM mode the signal peaked at 592.296. So the problem lay in the IF stages.

I then injected 45.1 MHz (2nd IF) and found that the -4 kHz error existed here too. Narrow FM peaked with the injected signal at 45.096 and USB gave a 1kHz audio tone with unmodulated carrier at 45.094, and LSB at 43.097. Slightly asymmetrical but that's another thing.

It was while I was playing with it in this configuration with this that I noticed that applying pressure to the sandwich of boards made the resolved audio tone in LSB mode jump around in frequency, and in fact the error could be made to disappear.

Now the 3rd conversion oscillator in narrowband modes is a crystal osc running at 44.645 MHz, which is trimmed with a varactor diode, whose DC voltage comes from a trimmer pot. I suspect it's either this pot or possibly a dry joint so I will reflow the solder round some of these components to see if that does it but the physical size of the smd stuff in this set is a bit daunting!

At least I know what I'm dealing with now.

Thanks.
Alan
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Old 18th Sep 2021, 7:04 pm   #5
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Default Re: Yupiteru MVT-7200 off frequency

It sounds like you are closing in on the problem Alan. I like getting to the bottom of these things, but like you I sometimes get cold feet when I see what I have to deal with!
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Old 20th Sep 2021, 5:07 pm   #6
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Default Re: Yupiteru MVT-7200 off frequency

If you want to monitor that oscillator while you poke and press things try using another receiver tuned close to the frequency of the oscillator and in CW or SSB mode - you will hear the oscillator as a steady tone which will change pitch whenever you poke anything sensitive, but bear in mind that touching parts of any RF oscillator will often detune it for the duration of the touch.
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Old 22nd Sep 2021, 8:29 am   #7
AlanC
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Default Re: Yupiteru MVT-7200 off frequency

Good suggestion, Sirius. One thing I noticed about this receiver is that the screening could be better to say the least; its sensitivity to the first IF radiated nearby is astonishing. In fact I would say that if you lived near a TV TX working on around 592MHz you would surely suffer interference from it. But I can 'see' the LO on my RTL-SDR setup so that works both ways! I'll spend a bit more time on it this weekend. Thanks
Alan
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