HX108-2 AM radio kit problem

[Radio Scotland]

Sure most of you will be familiar with this kit. I've built two of these and they both have the same problem. And that is they oscillate on the bottom end of the band, From roughly 540-Khz down till the pointer end stops.

I can null it out by moving the aerial coil towards the battery compartment. But that makes the radio deaf on the bottom end of the band,

It's like the radio is over sensitive and it's causing it to burst into oscillation,


Any ideas ?


Cheers
Jay

[Jolly 7]

I have built one of these in the past. Mine suffers from some squealing but not at the lower range of the MW band. It works quite well otherwise. Looking at the technical specifications, the radio tunes from 525 to 1605 kHz. The IF of the radio is slightly unusual i.e. 465 kHz. I guess I was unable to align my radio properly. I do not have a scope and only used a DIY 455 kHz RF generator with audio tone. Your problem sounds like an alignment problem too. Which equipment did you use to align it ?

[Radio Scotland]

Originally Posted by Jolly 7

I have built one of these in the past. Mine suffers from some squealing but not at the lower range of the MW band. It works quite well otherwise. Looking at the technical specifications, the radio tunes from 525 to 1605 kHz. The IF of the radio is slightly unusual i.e. 465 kHz. I guess I was unable to align my radio properly. I do not have a scope and only used a DIY 455 kHz RF generator with audio tone. Your problem sounds like an alignment problem too. Which equipment did you use to align it ?

A signal generator set to 465 khz and a AF volt meter coupled to the speaker, Although i do have a scope, Just find it easier doing it with an analogue meter, It works fine otherwise, Good sensitivity


Cheers
jay

[Sideband]

Originally Posted by RadioScotland

A signal generator set to 465 khz and a RF volt meter coupled to the speaker,

That should be an AF voltmeter surely? Just in case it causes confusion.

I suppose the real question to ask is if anyone else has built one of these, do they have the same problem? It could just be the design of the radio. Maybe some extra decoupling will cure it. Is there a circuit? If so perhaps posting it will enable us to decide where the instability is occurring.

[Radio Scotland]

Originally Posted by Sideband

That should be an AF voltmeter surely? Just in case it causes confusion.

I suppose the real question to ask is if anyone else has built one of these, do they have the same problem? It could just be the design of the radio. Maybe some extra decoupling will cure it. Is there a circuit? If so perhaps posting it will enable us to decide where the instability is occurring.

Yeah An AF Volt meter.

Far as i know the forum only allows users to post portions of schematics, But the full schematic can be found online,

Apparently is badly designed and uses the cheapest of components, So problems are inevitable


Cheers
jay

[Jolly 7]

The circuit diagram that comes with this kit has errors. However, the author of LEAP, little electronic and arduino projects, allows free borrowing of the corrected diagram, herewith attached. Full credits and thanks to the author for the revised circuit diagram and excellent write up on this radio.

[Sideband]

Originally Posted by RadioScotland

Far as i know the forum only allows users to post portions of schematics,

Only if the circuit is available via the forum download service. Otherwise you can post the complete circuit

[Radio Scotland]

Originally Posted by Jolly 7

The circuit diagram that comes with this kit has errors. However, the author of LEAP, little electronic and arduino projects, allows free borrowing of the corrected diagram, herewith attached. Full credits and thanks to the author for the revised circuit diagram and excellent write up on this radio.

Cheers for that, Far as i can tell mine doesn't have those errors, Maybe they revised it to correct the mistakes, But just to be certain i'll check it again tomorrow,



Thanks for the info regarding uploading schematics sideband,



Regards
Jay

[Sideband]

It looks quite an old style type circuit....almost as if it has been converted from germanium PNP to silicon NPN. The use of transformer coupling went out in the 60's as far as I know.

Can't see anything fundamentally wrong so it might just be the general design layout of the PCB is questionable.

The use of two diodes in series across the oscillator and IF supply rail is interesting. They will clamp at approximately 1.4 volts so presumably the oscillator and I.Fs will be held at a reasonably stable voltage until the battery voltage drops quite low.

[Radio Scotland]

Originally Posted by Sideband

It looks quite an old style type circuit....almost as if it has been converted from germanium PNP to silicon NPN. The use of transformer coupling went out in the 60's as far as I know.

Can't see anything fundamentally wrong so it might just be the general design layout of the PCB is questionable.

Like the antenna being very close to the LO and IF Cans, But they're shielded.

[Sideband]

No probably not that. I was thinking more of PCB print runs. An I.F print run close to the mixer/oscillator or a print run from the collector of an I.F transistor running close to the base connection....anything that might cause feedback.

If it was close proximity of the aerial coil to the I.Fs you might be able to prove that if it's possible to unclip the ferrite rod and move it away (if the leads are long enough). Reversing the coil on the rod (if possible) might be worth a try in that case.

[Radio Scotland]

Originally Posted by Sideband

No probably not that. I was thinking more of PCB print runs. An I.F print run close to the mixer/oscillator or a print run from the collector of an I.F transistor running close to the base connection....anything that might cause feedback.

If it was close proximity of the aerial coil to the I.Fs you might be able to prove that if it's possible to unclip the ferrite rod and move it away (if the leads are long enough). Reversing the coil on the rod (if possible) might be worth a try in that case.

Was just thinking about doing that earlier funnily enough, Yeah the ferrite bar comes out and the leads are pretty long


Cheers

[Radio Scotland]

I Tested all the components before installing them and they're all in tolerance. So a faulty component being the cause is unlikely, I Also changed the tuning cap as the one supplied with the kit is junk, The problem was there before i changed the tuning cap, So thats not the cause either

[Radio Wrangler]

It might be squegging at the LF end.

David

[Radio Scotland]

Originally Posted by Radio Wrangler

squegging

I Thought that was a typo Never heard the word before. Had to google it. Its no big deal really as it only happens at the lower edge of the band, Annoying more than anything else. But i'd still like to get to the bottom of it.



Jay.

[Radio Wrangler]

It all depends on what you mean by 'oscillating'. There are quite a number of things it could be. the vocabulary has it's peculiarities, another one you'll come across is 'motor boating' which is a good description of the sound. This one is a low frequency oscillation of audio stages when battery impedances have gone high and the thing operates as a relaxation oscillator. In radar circles, interference is referred to as 'Fruit'

Squegging is really good if you can get it on a triple word score.....

David

[Radio Scotland]

Originally Posted by Radio Wrangler

It all depends on what you mean by 'oscillating'.
David

Short clip of the radio in action,
https://drive.google.com/file/d/1CxK...ew?usp=sharing

Remind me never to play scrabble with you Ha.

[Radio Wrangler]

Link seems to need a sign in, and comes up with security warnings.

David

[Radio Scotland]

Originally Posted by Radio Wrangler

Link seems to need a sign in, and comes up with security warnings.

David

Ok i'll see what i can do about it

[Radio Scotland]

https://www.youtube.com/watch?v=hTRn...ature=youtu.be