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Old 29th Jul 2018, 3:45 pm   #1
paulsherwin
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Default Roberts R606 MB VHF distortion

I'm completely flummoxed by a fault on an R606MB. FM reception becomes distorted and sibilant when the signal is strong. Pushing in the telescopic aerial to reduce the signal strength eliminates the distortion.

I can't find anything wrong with it, and don't understand how these symptoms could be caused given the way FM works. All the RF/IF voltages seem plausible. The discriminator electrolytic tested OK but I changed it anyway, ditto the discriminator diodes. No improvement. I changed the BF194/5s, not specifically to fix this fault but because I now always do it anyway as a precaution. No improvement.

I'm sure I must be missing something obvious. Would anybody like to point me in the right direction? The radio is working well apart from this fault.
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Old 29th Jul 2018, 4:19 pm   #2
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Default Re: Roberts R606 MB VHF distortion

Has the IF alignment been touched, the manual states Max output with best symmetry, not just peaked also the IF frequency depends on which ceramic filter is used.

Is the fault still present with the AFC switch off?
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Old 29th Jul 2018, 5:03 pm   #3
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Default Re: Roberts R606 MB VHF distortion

I haven't touched the alignment, and there's no sign of twiddling. I don't think the AFC switch makes any difference but I'll check again this evening.

It sounds for all the world as if something is overloading and clipping, but I can't imagine what or how. The distortion is quite subtle but easy to hear with some programme material. It definitely disappears with the aerial retracted.
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Old 29th Jul 2018, 5:13 pm   #4
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Default Re: Roberts R606 MB VHF distortion

Only other thought is that the strong signal is beating in the tuner section with another station and producing intermodulation.
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Old 29th Jul 2018, 5:22 pm   #5
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Default Re: Roberts R606 MB VHF distortion

Quote:
Originally Posted by paulsherwin View Post
I haven't touched the alignment, and there's no sign of twiddling. I don't think the AFC switch makes any difference but I'll check again this evening.
If the AFC is off, what about the AGC? It is possible to overload an IF stage and even on FM that will produce distortion.
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Old 29th Jul 2018, 5:26 pm   #6
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Default Re: Roberts R606 MB VHF distortion

I couldn’t see AGC on VHF/FM, but could have missed it, first thing I looked for but then again, I am not as quick finding these things has I used to be.
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Old 29th Jul 2018, 5:31 pm   #7
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Default Re: Roberts R606 MB VHF distortion

I can't see AGC on FM either. If it's present, I don't understand how it works.
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Old 29th Jul 2018, 5:40 pm   #8
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Default Re: Roberts R606 MB VHF distortion

I've not got the circuit, it was just a thought.
How about low power, dropping more with a strong signal, causing AF distortion?
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Old 29th Jul 2018, 5:46 pm   #9
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Default Re: Roberts R606 MB VHF distortion

No, this is definitely an RF/IF problem. When you push the aerial in, the sound doesn't get quieter - it just becomes cleaner.

I did say that this is a perplexing fault
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Old 29th Jul 2018, 5:49 pm   #10
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Default Re: Roberts R606 MB VHF distortion

It might be the bias out either in the tuner or TR4?

Lawrence.
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Old 29th Jul 2018, 5:54 pm   #11
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Default Re: Roberts R606 MB VHF distortion

The AF section is fine. I suspect you're right about a bias problem somewhere in the IF, though I've never encountered anything similar before.
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Old 29th Jul 2018, 6:01 pm   #12
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Default Re: Roberts R606 MB VHF distortion

There is a diode across the L1, the collector tuned circuit for the VHF RF amp, now is that there to damp L1 under strong signals and reduce the gain? It is not used as a varicap, the tuning is mechanical.
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Old 29th Jul 2018, 6:15 pm   #13
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Default Re: Roberts R606 MB VHF distortion

Grrr, that's inside the tuner head module, and I don't want to open that up if I don't have to. Does anybody else think that's a suspect?
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Old 29th Jul 2018, 6:17 pm   #14
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Default Re: Roberts R606 MB VHF distortion

I threw that in for discussion, not sure I am correct. I would just reduce the aerial size and accept it rather than dive in the tuner.
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Old 29th Jul 2018, 6:21 pm   #15
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Default Re: Roberts R606 MB VHF distortion

There will probably be a couple of BF19xs in there too, which are always suspect
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Old 29th Jul 2018, 7:29 pm   #16
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Default Re: Roberts R606 MB VHF distortion

What a mine of information on this site, see VHF mixer oscillator section paragraph 4.

https://www.vintage-radio.com/repair..._vhf-sets.html
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Old 29th Jul 2018, 7:52 pm   #17
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Default Re: Roberts R606 MB VHF distortion

Interesting. It looks as if I'll have to get stuck into the module. Oh joy. Thanks to all who have commented, particularly Frank.
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Old 29th Jul 2018, 8:22 pm   #18
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Default Re: Roberts R606 MB VHF distortion

Those modules are too small for my hands and eyes these days, hope it is straightforward.
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