|
Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
|
Thread Tools |
18th Apr 2017, 10:03 am | #1 |
Hexode
Join Date: Oct 2007
Location: Bristol, UK.
Posts: 445
|
RA1772 optical encoder
Hi,
I've acquired an RA1772 receiver as spares or repair with the original idea to remove the mains transformer to repair my existing 1772. However, it might be a better bet for the moment to repair the new one. It is receiving signals but the frequency only changes in the upwards direction and the Fast Tune is not working. I can see that only one side of the optical encoder is sending out a signal. However, the encoder is different to the one shown in my manual. This one has a circular pcb on the rear of the assembly with an LM239 comparator and associated components. Does anyone have any info on this particular encoder? Don m5aky Just for info: RA1772/X/S3/R/b3/0/C Ser number 1849 |
18th Apr 2017, 2:43 pm | #2 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,213
|
Re: RA1772 optical encoder
Perhaps it's because I've been tracing out schematics for years, so I no longer worry about it, but that thing looks pretty simple to me.
The LM239 is closely related to the LM339 and has the same pinout. It's a common quad comparator chip. You can easily find a data sheet/pinout. Given that, it shouldn't take long to see how it's all connected. Looks to me as though the 2 comparators on the 'left side' (pins 1-7) of the IC take the signals from the phototransistors in the sensors, those on the 'right side' are inverters from the output of those. The thresholds of the former are set by the presets on the board, that of latter seems to be set by the drop across the sensor LEDs? Start tracing signals. As with a stereo amplifier, you have another 'channel' to compare against. OK, the voltages won't be indentical, but the signals will be much the same. As a starting point, look at the signals on pins 1 and 2 of the IC. Do they both toggle as you twiddle the knob? If so, the problem is in the inverter stages (probably the IC). If not, check the inputs to those comparators (pins 4,5,6,7) What do you see there? |
18th Apr 2017, 3:11 pm | #3 |
Hexode
Join Date: Oct 2007
Location: Bristol, UK.
Posts: 445
|
Re: RA1772 optical encoder
Thanks for the reply.
I haven't been idle. As you say it's not a complicated circuit. I've atached a rough diagram which I'll check and tidy up later. R4 should read 1.0K. I would have thought both presets would have been the same value. Voltage on pin 5 was 8.6V and on pin 7 it was 3.5v. Tweeked this up to about 6V and it all started working. Fast Tune as well. Tune into a broadcast station at around 6.0 Mc/s and it appears to be in the right place. More checks later. Cheers Don m5aky Last edited by vosperd; 18th Apr 2017 at 3:16 pm. |
18th Apr 2017, 4:04 pm | #4 |
Nonode
Join Date: Apr 2012
Location: Worcestershire, UK.
Posts: 2,052
|
Re: RA1772 optical encoder
Daft question ... are both of the filament bulbs working?
Guy
__________________
"What a depressingly stupid machine." [Marvin: HHGTTG] |
18th Apr 2017, 4:38 pm | #5 |
Hexode
Join Date: Oct 2007
Location: Bristol, UK.
Posts: 445
|
Re: RA1772 optical encoder
I don't think this version has filaments bulbs. Looks like it could be leds.
Don |
18th Apr 2017, 6:44 pm | #6 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,213
|
Re: RA1772 optical encoder
I don't quite agree with that circuit from looking at the PCB.
I see a lot more going straight to +12V (after L1, but on the input side of R1). The top ends of the presets, for example. Then R1 feeds pin 8 (I assume the power to the 2 LEDs in series) from the 12V line.That point also goes to pins 8 and 10 of the comparator chip. That would make sense, it would set a suitable voltage on said inputs of the comparator. |
18th Apr 2017, 8:13 pm | #7 |
Hexode
Join Date: Oct 2007
Location: Bristol, UK.
Posts: 445
|
Re: RA1772 optical encoder
I did say it was a rough sketch. All of the 12V connections are after L1 and I missed out the connection of R1 to pins 8 and 10.
I have got the encoder working in both directions now but just discovered that the frequency is 100Kc/s lower than the display. i.e I have to tune to 3.7megs to receive a signal on 3.6. So that's my next problem. Cheers Don |
18th Apr 2017, 8:24 pm | #8 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,213
|
Re: RA1772 optical encoder
What was the problem with the rotary encoder?
|
18th Apr 2017, 9:04 pm | #9 |
Hexode
Join Date: Oct 2007
Location: Bristol, UK.
Posts: 445
|
Re: RA1772 optical encoder
I just increased the voltage from R5 to pin 7 and it started working.
Don |
18th Apr 2017, 9:25 pm | #10 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,273
|
Re: RA1772 optical encoder
This sort of thing is used alot in industrial controls, my main day job. Sometimes the led itself loses emission and takes the detector below the threshold. Sometimes the tweak-up works, and other times it masks a slowly failing led. If such a thing needs tweaking after a further period of use, the led is usually to blame. A bit like tweaking the laser up on a CD player.
__________________
Kevin |
18th Apr 2017, 9:46 pm | #11 |
Hexode
Join Date: Oct 2007
Location: Bristol, UK.
Posts: 445
|
Re: RA1772 optical encoder
It would help if I knew the correct settings. This is obviously a later version of the original simpler arrangement that is not covered by the manual. I guess there must be an update around somewhere.
Don |