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Old 23rd Feb 2021, 2:59 pm   #1
john_morris_uk
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Default Etronic RA640. Not working on one waveband!

I’ve had an Etronic RA640 radio sitting in the workshop shelf for some time and thought I’d better get round to finishing it. I won’t bore you with the details of what I’ve done so far but suffice to say, the IF is all aligned and it’s working well on Long Wave and Short Wave. I’ve been using this circuit to check values: https://www.************************...onic-ra640.pdf

This should be easy to sort but I’m stumped. The radio just goes very quiet when MW is selected. My first thought was that the local oscillator was dying on MW but it’s running and tuning perfectly.
My next issue is that the circuit doesn’t seem to match what’s built. For example R17 (150k) brings the AGC line back to the mixer oscillator V1 but on the circuit it’s only connected via the cold end of the MW coil whereas in the radio (which doesn’t appear to have been tinkered with) it’s connected to the band change switch as well.
The voltages are similar under no signal conditions on V1 except the screen grid voltage drops a little when switching to MW.
My next thought was a failure in L4 but the resistance of the various coils seems reasonable and I haven’t got the data sheet fit a QGA532 to know exactly what they should be. I’ve checked for no shorts having developed across any of the trimmers and all seems fine. There's so little to go wrong I'm stumped as to why MW is dead.

Sorry if this is long winded but has anyone got any ideas where I should look next.
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Old 23rd Feb 2021, 3:02 pm   #2
john_morris_uk
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Default Re: Not working on one waveband!

Sorry for the strange typos. They have been introduced every time I use an apostrophe for some reason.
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Old 23rd Feb 2021, 3:31 pm   #3
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

I’ve checked the voltages again and the voltage on the screen of V1 is 120 volts when receiving R4 LW and drops to 62 volts when I turn to MW and rises slightly to 75 volts on SW. I think this merely reflects the fault and the change in the screen current rather than the fault itself.
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Old 23rd Feb 2021, 4:01 pm   #4
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Default Re: Etronic RA640. Not working on one waveband!

On the schematic as drawn, the circuit wouldn't work properly on MW because there's no RF grounding from the bottom of L4, it looks like they've missed out a connection dot on the schematic, also in the schematic on SW, L3 is connected to ground which would appear to suggest no AGC at least for the mixer on SW, I think it should have been drawn with at least the bottom of L4, L5 and R17 connected to C5 as well as the switch contact that shorts out the LW coil (L5) when MW is selected...so far as I can make out.

Lawrence.

Last edited by ms660; 23rd Feb 2021 at 4:13 pm. Reason: correction
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Old 23rd Feb 2021, 5:04 pm   #5
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

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I’m starting to wonder if it’s got a coil missing. The MW tuning trimmer that parallels up the main tuning capacitor is open circuit the whole time and there’s a hole next to the LW coil (that gets shorted out correctly on MW). If I could find a chassis layout diagram with which trimmer and which coil is which I could compare and contrast.
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Old 23rd Feb 2021, 5:28 pm   #6
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Default Re: Etronic RA640. Not working on one waveband!

I had one of these a while ago but passed it on. Just checked and sorry I don't have a photo of the underside of the chassis.
Mike
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Old 23rd Feb 2021, 5:43 pm   #7
Restoration73
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Default Re: Etronic RA640. Not working on one waveband!

You do indeed have a missing coil, smaller than the adjacent LW coil. On my chassis
it measures 230uH, dc resistance 4.0 ohms.
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Old 23rd Feb 2021, 5:49 pm   #8
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

Quote:
Originally Posted by Restoration73 View Post
You do indeed have a missing coil, smaller than the adjacent LW coil. On my chassis
it measures 230uH, dc resistance 4.0 ohms.
That will explain the problem!
All I’ve got to do is find a 230uH coil ( I don’t expect the resistance is that critical). Easier said than done.

I could do with a new glass panel for the front as well if anyone had one In good condition kicking around. (The lettering has come off a large patch of the one I’ve got. )

Last edited by john_morris_uk; 23rd Feb 2021 at 6:08 pm.
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Old 23rd Feb 2021, 5:50 pm   #9
vinrads
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Default Re: Etronic RA640. Not working on one waveband!

Any chance of a picture of the coil . Mick.
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Old 23rd Feb 2021, 5:54 pm   #10
G6ONEDave
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Default Re: Etronic RA640. Not working on one waveband!

The circuit is in the RTS book volume 1 part 1 on page 258.
Dave
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Old 23rd Feb 2021, 6:13 pm   #11
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

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This is a photo of the underchassis of the RF section atm. The large coil is the LW input tuning circuit coil and the hole next to it on the paxolin (sp?) strip is where I assume the MW coil that would look similar but smaller would sit.
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Old 23rd Feb 2021, 6:22 pm   #12
Restoration73
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Default Re: Etronic RA640. Not working on one waveband!

Here it is;

Note on the overall pic I have transposed volume/switch and tone controls
for safety.
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Old 23rd Feb 2021, 6:52 pm   #13
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Default Re: Etronic RA640. Not working on one waveband!

This doesn't really help, but the best shot I have underneath an RA640.

It almost looks like your coil has never been there!

Regards,

SR

(Sorry, I didn't see these better pictures arrive whilst I was rummaging for mine!)
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Old 23rd Feb 2021, 6:55 pm   #14
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

Quote:
Originally Posted by Restoration73 View Post
Here it is;

Note on the overall pic I have transposed volume/switch and tone controls
for safety.
Interesting as thatís certainly showing Iíve got a missing coil Also the other inductors on mine donít have the tuning slugs.
Just need to find someone with a scrap chassis thatís got a suitable inductor going spare.
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Old 23rd Feb 2021, 8:25 pm   #15
vinrads
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Default Re: Etronic RA640. Not working on one waveband!

I have a coil that should work it reads 0.22mh on my Chinese meter ,if you want to try it you are welcome to it ,it does have a tuning slug , you just need to find out how to connect it ,Mick.
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Old 23rd Feb 2021, 9:41 pm   #16
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

That would be really helpful. I can easily find a way to connect it. How much? Shall I send you a stamped self addressed Jiffy bag? I’ll pm you.
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Old 24th Feb 2021, 9:53 am   #17
vinrads
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Default Re: Etronic RA640. Not working on one waveband!

Quote:
Originally Posted by Restoration73 View Post
Here it is;

Note on the overall pic I have transposed volume/switch and tone controls
for safety.
Hi I am about to send John a coil , any chance of finding where the connections go , I take it this is an aerial coil , Mick.
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Old 24th Feb 2021, 10:51 am   #18
Restoration73
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Default Re: Etronic RA640. Not working on one waveband!

My set is not the same model but the circuits are all similar.
One connection (nearest to former) goes to the lower tag linked to the 100k resistor tag.
The other goes via a piece of sleeving to the trimmer capacitor tag directly underneath.

Oscillator coils normally have three or four connections.
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Old 24th Feb 2021, 10:59 am   #19
john_morris_uk
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Default Re: Etronic RA640. Not working on one waveband!

Quote:
Originally Posted by vinrads View Post
Quote:
Originally Posted by Restoration73 View Post
Here it is;

Note on the overall pic I have transposed volume/switch and tone controls
for safety.
Hi I am about to send John a coil , any chance of finding where the connections go , I take it this is an aerial coil , Mick.
An accurate circuit diagram would be very helpful certainly. The one Iíve referenced (and that I got free off the net) Iíve lost confidence in! As others have pointed out, it wouldnít work as shown (I did wonder?)
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Old 24th Feb 2021, 12:20 pm   #20
vinrads
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Default Re: Etronic RA640. Not working on one waveband!

Quote:
Originally Posted by Restoration73 View Post
My set is not the same model but the circuits are all similar.
One connection (nearest to former) goes to the lower tag linked to the 100k resistor tag.
The other goes via a piece of sleeving to the trimmer capacitor tag directly underneath.

Oscillator coils normally have three or four connections.
Of course silly me, it must be the aerial coil then , that's ok Mick.
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