tecsun 660 mod, external loopstick? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 04:47:48 am
content: So the tecsun radio I have has a built in loopstick antenna.It has the option for a external HF antenna, but the stuff that comes off the loopstick does not have a port.If I were to mod it some how with a clever change over switch, to an external antenna port, would the ferrite antenna left in circuit have any effect on the circuit because of mutual inductance, if it is floating by the capacitance of a small switch?Not sure if there is room to do it, I figure probobly yes, perhaps needing a weird connector, but I don't know if the circuit will work right for the reason stated above.I would prefer to do a total bypass, not a 'piggyback' that I read about people doing by putting coils around the coil that exists and routing that out so the external coil couples the internal coil. I wanted to put a DPDT switch. The idea would be.. to test loopstick antenna designs with minimal other factors with the most simple equipment.

author: A.Z.
author_rank: Super Contributor
timestamp: January 30, 2021, 09:04:42 am
content: sounds like the PL-660 has indeed an ext antenna connectorhttps://ham.stackexchange.com/questions/5742/shortwave-antenna-listening-with-portable-receiver-like-tecsun-pl-660so, why you don't just use that to connect your external antenna ?otherwise you may just use a coupled antenna, since fiddling with the internal ferrite wiring isn't a good idea, it may and probably will cause a lot of issues due to longer wiring, in this case a "milk crate" antenna could probably be a good ideahttps://swling.com/blog/2017/06/how-to-build-a-milk-crate-am-broadcast-loop-antenna/

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 09:22:54 am
content: the external antenna only connects to the shortwave circuit. does it actually toggles a relay when you plug something in?

author: Calambres
author_rank: Frequent Contributor
timestamp: January 30, 2021, 09:54:01 am
content: There's a well known modification for this. It's for the 680 but will work in the 660 too:https://swling.com/blog/tag/tecsun-pl-680-modification/

author: A.Z.
author_rank: Super Contributor
timestamp: January 30, 2021, 10:25:48 am
content: Quote from: coppercone2 on January 30, 2021, 09:22:54 amI think it's just the jack plug which switches between the ext and the built in telescopic

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 10:33:55 am
content: What exactly are they doing here? So, when the external antenna is put in, it engages something that disconnects the built in antenna and connects the external antenna, but in this case you are just making a capactive connection so that the socket branches out to both the HF circuit and the MW circuit?In that case, it appears both antennas are running at the same time, so if you add a bypass switch, you can disconnect the internal antenna, but use the port that is normally there to interface the new antenna?So, this simplifies the modification to only requiring a switch.The capacitor is still some how disconnected by the radio from the circuit when there is nothing inserted, right? Otherwise that internal loopstick pickup would be going into the HF amplifier, right?The jack plug has some kind of mechanical gimmick in it that acts as a real switch, I think? Actually NOT familiar with those.It would seem that JUST that mod would put some kind of burden on the HF amplifier if done without the bypass switch.

author: A.Z.
author_rank: Super Contributor
timestamp: January 30, 2021, 10:38:31 am
content: check the pl 660 schematics, notice that the ext antenna by default only works on FM/SW while the ferrite always works in MW so they added a pickup capacitor which allows to use the ext antenna in MW too, adding a dpdt switch could then allow to exclude the ferrite, but I suspect this would have some side effects

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 10:42:58 am
content: what do you think these side effects would be? i am interested in the complex interactionwhat features or tests can be done to detect the expected side effects? I have alot of test equipment, perhaps I can direct connect a AWG to the radio and give it something that would likely highlight the problem. Are you just anticipating problems from the routing wires? Perhaps I can put a miniature relay in there and power it off the batteries to prevent RF rerouting.

author: A.Z.
author_rank: Super Contributor
timestamp: January 30, 2021, 01:03:04 pm
content: it was just a wild guess, but if the internal ferrite is connected as for most similar receiver, it's probably a direct part of the tank circuit, so mods will probably have side effects on it, can't predict them, no crystal ball here

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 08:44:10 pm
content: do you know what parameters typically are shared between the loopstick and the recievers front end to achieve things (I guess you mean tank circuit input filter? not sure what else it could have, notch filter and I guess anti aliasing LPF, but I know next to nothing about radios, the tank might fulfill both of those requirements) ? It is possible to measure the coil with a LCR meter to see Q, L, C, R , unfortunatly for me only to 100KHz, with the only poor mans LCR meter I have (de-5000).So perhaps if the coil in the radio is measured the replacement coil that are made could have some kind of guideline for (some?) of those parameters to maintain compatibility.Correct me if I am wrong but after the ladder filter in the schematic I believe the receiver coil is effectively ballasted from the rest of the circuit? Not really sure what the parts are after the 3 stage LC.

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 09:12:57 pm
content: nevermind, i got confused by the FM circuit, thought it was the LF circuit.I see it goes right into a FET. I don't think the FET will care, it isolates. The other part (MW I think) maybe.Don't really get that circuit. They buffer? it off the big part of the coil and feed it back into the coil as a power filter. Then you have another active circuit on 1 tap and then you have a 1mH coil and resistor, those I think are probobly not super tuned or anything.Why does it go off the antenna then go back on the antenna after it goes through a transistor? (2SK192). Are they loading it or something by putting a controllable resistance in parallel with it? It looks like VCC goes through Q2, R95, Antenna and biases the FET?So it does look like something very funny would happen if you disconnect the original AM antenna. But I think D3 isolates the tuned circuits from the antenna, so that means so long you know what that transistor is doing, you can put an external coil .Is this just a sensitivity adjustment? That is, the radio has DX Local and something else switch. Does it just make it harder to overload with that transistor engaged? I.e. at Local it is high Z but at DX (near a transmitter) it is at low Z burning energy?So the interesting thing is that Pin 4 of the loopstick is connected to ground by whatever transistor parameter is between gate and emitter and the capacitance of the coil?So if you disconnect it the transistor should be seen as a high impedance and the voltage will change with no real feedback as far as I can tell, but can you just ground pin 4 ?Can someone please teach what that transistor is for? I have never seen that. To me it seems like you can just put a potentiometer there and manually do what its doing.

author: A.Z.
author_rank: Super Contributor
timestamp: February 01, 2021, 03:42:00 pm
content: Quote from: Calambres on January 30, 2021, 09:54:01 amInteresting, found further pics herehttps://it9ybg.blogspot.com/2018/04/pl-660-modification-allows-external.htmlreferring to the circuit, if I'm not wrong, the capacitor is connected between the "hot" side of the ferrite stick winding and the ext antenna plug so it basically couples the ferrite with the external antenna allowing to use it on MW too, I just wonder how the receiver will behave on FM/SW after such a modification, since if you don't insert an external antenna, the telescopic whip will be coupled to the ferrite on FM/SW too

author: coppercone2
author_rank: Super Contributor
timestamp: February 01, 2021, 08:55:09 pm
content: but what about that transistor ?? why are you varying a resistance across the coil. I would expect a capacitor there. am i right with the dx/local mode thing?