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Marantz 2275 ratio detector problems
Marantz 2275 ratio detector problems
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Old 11th November 2017, 01:53 AM   #1
anatech is offline anatech  Canada
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Marantz 2275 ratio detector problems
Unhappy Marantz 2275 ratio detector problems

Hi Everyone,

I am working on a Marantz 2275 receiver and I'm having a difficult time with the FM detector, which is a ratio detector. The problem is that if I centre the centre tune meter, I get no audio at all. The point of maximum audio is deflected to the right about 20 or so. So, I have replaced the following: Detector transformer, Germanium diodes (matched), all resistors and 200 pF capacitors. I matched each pair of parts as I did measure large differences between them. It was just a poke in the dark.

The signal on H208 and the transformer is symmetrical and I even replaced the IF filters to eliminate those as a possible problem. The signal was monitored with the scope probe in the air near to the circuit, not connected directly. Through every part change (done one at a time, or a pair) there was no change in the circuit behaviour. When the receiver is off, the centre tune meter is centred. That was check #1 in fact. I have also attempted to tune the tuner front end IF transformer. Another test I did was to sweep the IF with a 2 MHz range and use a spectrum analyser at the last filter. The IF response is correct and centred slightly high, about 10.717 MHz. The detector transformer came from a 2220 I have that worked properly, and I also tried a transformer from a 2325 scrap unit. It too worked properly before it was written off.

Any help would really be appreciated. This doesn't make any sense to me at all. I have aligned many Marantz FM sections over the years without problems.

Thanks, Chris
"Just because you can, doesn't mean you should" my Wife
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Old 13th November 2017, 10:21 AM   #2
DF96 is offline DF96  England
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Join Date: May 2007
Ratio detectors almost always have an electrolytic cap across the output from the two diodes. If this cap goes low in value, or very leaky, then you get strange results. In a valve FM radio this is the first component in the detector to change, as the heat dries it out.

The actual value of the electrolytic does not matter too much so use whatever you have to hand. Too small and you get no audio at centre tune; too big and the receiver won't cope with changing signal strength.
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