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Old 9th Sep 2017, 11:50 pm   #21
Ambientnoise
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Default Re: Mains RFI Filtering

Hi Adam, your conclusions do seem correct. I would certainly look at the transmitter to see if it is down on power. Can you scope the output ? Although connecting a scope will alter conditions in itself, it should verify if the tx is producing the correct output. I think the SSTRAN has links to tune the antenna in line with the freq setting so are these all correct ? It maybe worth altering the freq to use another antenna setting in case there is a problem with the tuning components. On my setup, if the antenna is within say 1.5m of mains cabling it then hums. To ask a daft question, are you sure it is not a hum loop in the source or the SSTRAN not liking an open input ? I guess you are testing with a shorted input ?

Rgds

Ken
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Old 10th Sep 2017, 12:47 am   #22
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Default Re: Mains RFI Filtering

Quote:
Originally Posted by Ambientnoise View Post
Although connecting a scope will alter conditions in itself
Yes that is true especially if the point the scope is connected to is a high impedance tuned part of the circuit.

(To avoid these sorts of dilemmas I have organised all of my pantry transmitters with a 50R output impedance. So to test them I simply use a standard 50R dummy load and attach the scope to that and there is negligible effect, or even with the 50R antenna connected there is negligible effect. Of course it then requires an impedance matching network in the base of the loop antenna to make the antenna 50R input, but these are easy with a toroid core and a capacitor to neutralize the reactance of the coupling circuit to the lower loop turn at the operating frequency. The other advantages of making pantry transmitters with 50R outputs are; you can run the signal to the loop antenna anywhere you like with 50R coax and find the better place for the antenna and you can very easily verify the RF power output of your transmitter into dummy loads etc. So far I seem to be the only person doing this for pantry TX's, but it has saved me a lot of headaches and it makes it much easier to get the loop antenna right too, I just design/adjust that for 50R input Z at the transmission frequency)
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Old 10th Sep 2017, 8:59 pm   #23
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Default Re: Mains RFI Filtering

I had a look at the TX today, went over all component placements, all installed correctly. Any less than ideal looking solder joints I re-flowed, no issues found. I did some voltage checks on the output stage, all looked fine except the emitters of Q4 + Q5 (final output stage in differential switching pair) showed 1.9v instead of 2.25v, which was more of a difference than other voltages. I checked all related resistors and found all very close to spec. My gut feeling is either my unit is fine and my house wiring less than ideal, or maybe I have a marginal transistor.

Once again I had a play with the location of the TX and the antenna. I did find surprisingly, a lower S5 setting than specified in the manual for my frequency gave the biggest peak, so I saw almost an extra volt peak when tuning the antenna. This has made the TX more listenable downstairs, but the hum is still very clear.

Im afraid it is beyond my wit to scope the output and understand how to equate that to RF power - I consider myself an "amateur" radio amateur , and am still learning when it comes to RF
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Old 10th Sep 2017, 10:00 pm   #24
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Default Re: Mains RFI Filtering

I assume you are using a wire and not a loop as the antenna ?

I don't know a lot about the SSTRAN, but looking on the net it appears likely to have a system with series tuned inductor network on the output in a attempt to load that into a 2m long whip or wire. There in lies a big problem, even if the wire was 5m or 10m long it is too small a fraction of the wavelength to be of great use as a radiator (so its not making good use of the available output power) and likely its not resonating too.

If you can wind a loop with at least 5 or 6 turns, say 1m in diameter with a tap at one turn to feed in your signal from the SSTRAN (possibly also bypassing its output network), and tune that with a variable capacitor (any 360pF radio cap will do) to get it to resonate at your transmission frequency you will get better results, especially for ferrite antenna radios, but as I mentioned further up in the thread you might have to add ferrite external antennas to some radios if the mains borne interference is objectionable.

(to get reasonable transmission one needs to accelerate electric charges in the antenna wire, but in a open wire at these frequencies it is difficult unless its very long, even with loading coils, but with a loop the circuit is closed and the current , inside the loop, at resonance is high, and this is one of the reasons why the loop works better than a wire)

Last edited by Argus25; 10th Sep 2017 at 10:14 pm. Reason: add info
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Old 11th Sep 2017, 9:31 am   #25
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Default Re: Mains RFI Filtering

Hi Adam, mostly run out of suggestions I am afraid though it would be good if you could borrow another SSTRAN or if someone with one could comment on your achieved output levels. The output level looks like it depends on the current through R8 so it is worth checking the voltage there. S7 is open I presume? You are welcome to borrow my unit (not an SSTRAN) at WB in December.

Ken
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Old 11th Sep 2017, 10:31 am   #26
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Default Re: Mains RFI Filtering

Thanks for the replies, some interesting ideas to explore. I may well have a go at making a loading coil for this tx and see what improvements can be made, and critically whether I can get enough coverage, without TOO much. As I wouldn't have space for the recommended base loaded antenna, perhaps with a more modest antenna I can limit the output that way.

It seems out of the box the coils are very lossy, so without any changes very little of the 100mw has a chance of getting to the antenna. The loop antenna does sound very interesting Argus

Ambientnoise thanks for the kind offer, I think if I cant get what I need from the AMT3000, I may well put this downstairs and run something cheaper to cover the upstairs from the same audio source, maybe even the same frequency if theres no ill effects
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Old 11th Sep 2017, 12:34 pm   #27
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Default Re: Mains RFI Filtering

One thought has occurred to me, I'm curious if I can lash up the longest antenna wire possible up the side of the house, and see how well I can tune this throughout the whole range of inductor settings. The longer antenna might enable me to use lower inductance settings than those specified in the manual, and with less inductors connected there should be less signal loss, and I wouldn't need to modify the device either.

I must re-emphasise, with all this my aim is to simply cover my house adequately, anything further I consider unacceptable so I will be careful with these experiments, and at this stage they are just that.
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Old 11th Sep 2017, 2:34 pm   #28
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Default Re: Mains RFI Filtering

Hi Adam, fyi I have just lashed up the antenna level monitoring cct as per the SSTRAN and my unit (which uses the same config and currents as the SSTRAN output stage) gave over 17v with a 700 mm antenna.
Rgds

Ken
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Old 11th Sep 2017, 2:48 pm   #29
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Default Re: Mains RFI Filtering

My SSTRAN system feeds both a 5m aerial and to my RF distribution system (TV type with added MW) it is fed from a "DAB car adaptor" which does for the FM sets too. No hum at all on any sets. House wiring is bog standard. It must be something simple somewhere.
 
Old 12th Sep 2017, 11:43 am   #30
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Default Re: Mains RFI Filtering

@merlinmaxwell, I have to agree there is something wrong somewhere, though haven't been able to find it yet. The manual states you can place the TX ANYWHERE in a house and receive full coverage.

The voltage peak I get at the moment is 4.5v, which seems reasonable based on what other users of the AMT3000 have said. I'm operating it at 1503khz, and I'm told at that part of the band the voltage peak will be lower than other frequencies.

@Ken thanks for doing that check, your peak voltage sounds quite high, I think even with the base loaded antenna you may get 12-13v, perhaps your output stage has more power?

I will go over my board again, and may even replace the output transistors. I did see a post by Phil (SSTran guy) stating when operating above 1000khz you can remove R18 completely for some extra power, which I may try in addition to more creative antennas.

If anything I should be struggling to contain my broadcast, yet I'm struggling to get it round a 3 bed semi in the UK !
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Old 12th Sep 2017, 12:23 pm   #31
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Default Re: Mains RFI Filtering

Hi Adam, I am at about 1.25MHz, same current etc as SSTRAN. That ltp circuit is well published across the web. The voltage across R8 is well worth checking. That area of the circuit, controlling the tail current is the main area which I changed.

Rgds

Ken
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Old 12th Sep 2017, 3:14 pm   #32
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Default Re: Mains RFI Filtering

Thanks Ken. I'm going to have another look this evening when I'm back from work, I'll measure the voltage across R8 and see whats going on.

In general the assembly of this was flawless, with one small issue. There are 4 capacitors near the ICs, labelled "IC bypass capacitors" on the circuit diagram, they are each 0.1uf and seem to go from the 15v supply to ground. One of these, I could not solder for some reason, to the point where the circular piece of track underneath the PCB lifted through various attempts. In the end I carefully soldered this at the top of the PCB and confirmed the connection was good with a multimeter. But I don't see this as being relevant to this issue, but thought I'd mention. I consider myself pretty good with soldering, no idea why I had trouble on this one part of the board.
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Old 12th Sep 2017, 7:45 pm   #33
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Default Re: Mains RFI Filtering

I'm seeing 0.46 v across R8
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Old 12th Sep 2017, 9:13 pm   #34
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Default Re: Mains RFI Filtering

I am fairly sure that is correct and given R8 at 22R, it sets the tail current at around 20mA. Another idea bites the dust !

Ken
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Old 14th Sep 2017, 1:18 pm   #35
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Default Re: Mains RFI Filtering

yeah...I have emailed sstran for any suggestions, but I don't expect to get a reply. They're not known for their service, great product (has to be said) but like others I found after ordering I waited several months and didn't get a reply to messages until it was finally shipped.

Ill be taking out R18 and doing some experiments with different antennas setups. I've also ordered some ferrite rods to make some antenna boxes
for the downstairs sets, and as argus suggested might use some rod antennas to see if I can null out the hum that way
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Old 16th Sep 2017, 8:56 pm   #36
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Default Re: Mains RFI Filtering

Moved it again, took R18 out. In the new location I get 3v peak with the standard antenna and mains earth downstairs, overall better results, but upstairs now a little more noise than before.

One curious thing, with compression set to zero the modulation control does nothing whatsoever, which is contrary to what the manual states in the audio tuning section. Perhaps I am not getting full modulation hence the poor performance. I can't seem to over modulate and create distortion unless compression is up considerably
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Old 18th Sep 2017, 9:36 am   #37
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Default Re: Mains RFI Filtering

Had a dumb moment, my audio source wasn't loud enough on the last test.

In the new location it pretty much covers all the house but I'm definitely losing rf into my mains wiring, a cabin at the end of the garden has no coverage, until I move the set next to mains wiring. The other side of the wall the antenna runs up has power cables for my boiler parallel with the antenna. I think this will have to do for now, I will move it again at some point but I've had enough for now. It's knackering running up and down stairs and loft ladders!

I will also explore custom loading coils and loop antennas as suggested. Thanks all who chipped in
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