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 Post subject: Ramsey FM transmitter noise
PostPosted: Apr Wed 19, 2017 4:21 am 
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Location: Orange County, NY
OK--so about a year back I decided I wanted to join many intrepid souls here and transmit a signal over FM to the whole house. I really wanted to build this myself but not necessarily DIY the entire unit, so checked around. It turned out there was a kit that fit the bill and I went for the Ramsey FM-25B transmitter. This was about a year and some change ago, thus before Ramsey announced it was getting out the kit business. As it turned out, this was the most complex kit I ever built! It was also a very enjoyable build over a few days & I thought the board and component quality quite good. Also, it worked on completion, which is probably the ultimate result for a kit.

I put the transmitter into use immediately and noticed excellent results in stereo to all of my receivers, after minor adjustment of input levels. The issue after a year of use: this got noisy. I didn't really notice it at first but over time I was noting a distinct noise during idle: sort of a chug-chug-chug with a lot of high frequency noise. I would describe this further but instead, here's an audio sample. Bear in mind, this is with no signal applied; during transmission this is harder to hear however I note to self that this noise is always happening while the instrument is on:

http://www.drbenstock.com/XmtrNoise.m4a

You might have to turn this up a little to hear the sound sample, but it's there. So: is this par for the course for any FM transmitter, typical for this transmitter only or does this unit need adjusting? If the last, what am I looking at? I can post the schematic if that helps.

Thanks--I figure there is enough FM transmitter building going on that someone has seen this issue previously.

Captainquad
Paul

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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Wed 19, 2017 5:12 am 
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Paul

Sounds like motorboating. An electrolytic cap may have gone bad?

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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Wed 19, 2017 8:11 am 
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Do you get this same noise if your audio source is completely disconnected from the transmitter?


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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Thu 20, 2017 3:11 am 
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Bob--same noise with audio input connected or disconnected. No difference whatsoever.

Norm--perhaps. That would be a pretty quick failure rate as I just assembled this kit about 9 months ago, but such things do happen. What are the most likely suspect caps: power supply or somewhere in the audio chain?

Thanks

Captainquad
Paul

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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Thu 20, 2017 3:20 am 
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Paul

Would expect power supply or other bypass cap. For test try paralleling each cap to see if noise stops.

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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Fri 21, 2017 4:43 am 
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Location: pensacola fl
May be a cap in the pll filter loop. These are small caps in value and voltage but theyre important. There might be two caps there one with a resistor in series with it going to ground. Depending on the phase detector and vco circuit chip may be 1 or 2 uf at 16 volts or less and could even be tant type. They can develop hi esr and this will cause this.


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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Sun 23, 2017 9:07 pm 
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Norm--so parallel with same value to see if performance improves with the assumption that the original's value is off?

Audioman--I've put a copy of the schematic here--would I be looking for passive parts in the PLL chain off pin 7 on the mother chip or elsewhere? http://www.drbenstock.com/SchematicFM25B.jpeg Plenty of small-value caps in this baby, mostly ceramics and plastics. I have an ESR meter that I can put to good use to bug out the suspects if I know where to look. I've been pretty pleased with the performance of this unit--as noted, it worked right off the workbench, and that is a BIG recommendation--but the noise has been annoying and I don't think this is native to this circuit. I avoided the cheaper Ramsey FM transmitter because of reports of disappointing performance, so I expect a little more here.

Thanks

Paul

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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Sun 23, 2017 11:15 pm 
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Paul

An electrolytic cap may have a bad seal, dried out and lost capacity. Most likely not shorted so paralleling with a replacement would be the easiest test.

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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Mon 24, 2017 1:10 am 
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I see a number of .01 and .1uF capacitors. I think it likely they are ceramic capacitors, either disc type or chip type. There have been production lots of these that can develop dendrite growth inside them with voltage applied. Over time the dendrites allow intermittent shorts, thus causing a host of possible problems depending on where they are in circuitry. Phase lock loops can intermittently shift frequency, voltage distribution lines with bypasses can cause audible pops, clicks etc. It may be a good idea to hang an oscilloscope on some of the important lines and watch for intermittent noise or voltage fluctuations. Have a set of new good quality capacitors on hand to replace them.

The thing about dendrite growth is that they heal after they short and then begin to grow again. They are almost impossible to see with the naked eye. It takes a microscope to see them after a part has been sliced open. Most technicians and restorers do not have all the equipment to do the examination of the internal area of capacitors to find such an intermittent problem.

I saw this happen years ago on telecommunications equipment. It nearly drove one of our engineers to despair before we found the cause. I am not saying this is the problem in this case, but it could be.

Joe


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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Apr Mon 24, 2017 10:01 pm 
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Location: pensacola fl
This is a fairly complex loop circuit. C21, C20, C14, C8, C6,C5, C13 would all be my firs suspects after power supply caps. The Electrolytics are in positions that if they are noisy or leaky then the parallel test may not work and caps with high esr can be noisy. A good cap tester can tell you about the esr but I would replace them if I had them on hand. C19 (not used) would normally be the loop filter however the designer decided to use a more elaborate filter with gain based on Q2 and Q3 and components. This filter should perform much better than a single capacitor could but more parts to check. The odds are the transistors are fine but a capacitor not so lucky. The other capacitors are on the control line (VT) and can therefore have an effect on the line's behavior so don't change the values ot the capacitors as time constants can be affected. C21 filters the bias for the varactor diodes. C6, C8 are prime suspects as these set the loop filter up along with the resistors. The resistors could drift of course but not usually in this type of gear as the load is minimal on them and unless they had a deffect they will test on value. Wouldn't hurt to test though. C20 carries the modulation into the varactor modulator so it can cause trouble but you can disconnect it and replace it with a close value for test. If it has leakage then noise will be generated. C14 is for high frequency noise suppression such as rf and other noise that the previous filter may miss. C5 is on the output of the phase detector and if leaky will upset the pll loop.

Hope this helps you out.


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 Post subject: Re: Ramsey FM transmitter noise
PostPosted: Jan Sun 28, 2018 10:45 pm 
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OK, it's been a while since I rang in on this one and I have had very sporadic time to concentrate on this. A few months back I did crack it open and test the caps flagged by Audioman. Everything tests within reason for ESR and, where I was able to check, value (my meter doesn't test some of the very low values involved here.) So I checked the parts supply, found I had a few of the necessary values & set about substitution one by one. First up was C13 & what do you know, bingo! The first thing I note is that the pilot light stays on instead of flickering and the motorboating noise has ceased. I'll give it a good burn-in before pronouncing the cure but so far so good.

Thanks to ALL who chimed in here and specifically Audioman for IDing the most likely suspects. I think I may have replaced a ceramic with a film cap, not so sure, and not sure what, if any, difference that might make to the circuit outside of the clear increase in performance.

Thanks

CaptQ
Paul


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