Old zm100 photodarlington wavelength response - Page 1
Forum section: electronics / beginners

author: Dataforensics
author_rank: Unknown
timestamp: December 23, 2011, 06:41:03 pm
content: Hi I am repairing a thirty year old racal ra1772 hf radio receiver. One of the faults I have diagnosed and repaired concerned the shaft encoder used to change frequency. It uses two 14v wire ended filament bulbs that shine on two zm100 photodarlingtons. I changed a bulb and it now works.Question is that considering this set does have LED displays, why did they not use them for the encoder?I assume as only red ones were available then, that the darlingtons response would be bad.Second question if that is the case is are shorter wavelength LED,s such as green, blue or white likely to work?The zm100 datasheets I have found so far only state light intensity reponse.I would have thought that if they did I could get longer life than the filament bulbs have.Mind you the filament ones are very under run to give long life.Tony

author: Fraser
author_rank: Super Contributor
timestamp: December 23, 2011, 09:26:57 pm
content: Logic suggests that if filiment lamps work, a good broad visual spectrum light source like the common white LED should also work well. You could do a test by temporarily fitting a small white high brightness LED with appropriate series resistor and see what happens. As you say, the LEDs will be more reliable and long lived than filiment lamps.Good luck

author: Dataforensics
author_rank: Unknown
timestamp: December 25, 2011, 08:27:20 am
content: Thanks for reply, I need to get near encoder to fix another fault, so will try a couple of white leds. just for fun. This set has a few separate loops in its early frequency synthesiser and is proving a bummer to fix.

author: jahonen
author_rank: Super Contributor
timestamp: December 25, 2011, 08:36:44 am
content: My experience has been that red (or IR) leds work best with phototransistors, so you might want to try them out too. Most power of the white leds is radiated at blue wavelengths (usually huge peak there). Silicon phototransistors seems to be most sensitive at near-IR wavelengths.This might make sense also in the way that incandescent bulbs radiate most of their power also at IR, too.Regards,Janne

author: rf-loop
author_rank: Super Contributor
timestamp: December 25, 2011, 09:47:56 am
content: Quote from: Dataforensics on December 23, 2011, 06:41:03 pmI have repaired/refurbished many 1772 receivers.Some times I think same.But also these lamps are easy and robust. Also easy and fast repair in field situation. No need any mechanical adjustments etc. Bulb in and it worrks. Also in these years when it (encoder) was designed originally, who really know how reliable leds are. These years also some use mechanical display. Also there have been lot of Nixie tube displays.This encoder construction is more old than this receiver model?If there is original lamp and driven with low voltage they are really robust.Also there was lot of leds available and Hewlett-Packard use incandescent lamps inside encoders. (these very small lamps around 2-3mm dia) And some of these equipments was really really expensive and made for long time reliable use. So why they use lamps? (I do not know)Tektronix use incandescent lamps in many oscilloscope display baclight and also time where LED are very common.Aviation have used incandescent lamps long time after LED have been possible. Why?One nice thing is that filament radiates to all directions. Led may need littlebit more mechanical adjustments.Racal 1772 where this lamp filament have burned to end must have extremely high counts of power on hours. One year is only 8760hour and 10 year is only 87600 hour Maybe it have been 20 years power on as 24/7.In what meaning LED is more advanced. 1. is price today. If someone make leds they are so cheap that paper works for selling is more expensive than 1000 or 10000 leds. (if littlebit carricated as humour) or extremely robust incandescent lamps example for aviation or equipments, price is maybe 100 - 1000 times more? 50000 hour or 200000 hour is easy bisquit.If take 12V lamp. and then start drop voltage. I have seen this curve about lifetime.. it is amazing (not just fast find it)One of most common need for service is this MHz selecting switch (and also filter and mode swithc). Swithc construction is extremely good and clever (and originally really expensive) If this do not work reliable it need special work for "renew" it. It need fully dismantle and adjust mechanically perfect agen, only if it is poerfectly adjusted for contact force and so on.. it can last long time agen. It can also adjust poor and then it fails (permanently) after some turnings. Many times it may also need cleaning (but not how ever). But swithch is one of best made serviceable swithch what I have seen. (but this model is nod full golden, it is some silver+? alloy) (switch have really made serviceable)One more rare problem may sometimes be hidden inside IF filters. (yes hermetically enclosed filters) It depends so many things that it can not predict. If with experience look this metal can metal surface "outlook" it may give some signs that it can suspect IF same time receiver suffer some special problem. (IF filter BW may be totally chaotic or receiver may suffer bang, wheeze, or continuously or randomly very different or step variable volume with a USB, or LSB, CW or AM.(some of these filter may have tin whiskers inside case surface. Most long tin whiskers I have find was over 5mm. One can was full of these making many small random short circuits there. Opening and repair these filters need good tools and lot of experience and skill to do it. If have not and sure supect this problem, it is best to send someone who can do it.(I know there have been many these to waste station just becouse not any knowledge about this problem. Only thinkkin that IF filter is bad, need change. But today... where from to find these. Nowhere.This receiver is also nice for service. Whole front plate turn 90degree down to service position, it takes 20 second.In military field,if have spare partsthis receiver can nearly always repair around <15 minutes exept if heavy mechanical damage.It is one of best receiver what ever have made (but it is not with tens of knobs and hongkong lights toy for "dx" hobbyboys). If it is with tunable front end it is even more good. This receiver front end is extremely good. No interferences... no chost frequencies..Racal 17 and 117 was good but 1772 is of course better, and it is even better receiver than some later LCD display models. IF filters have lot lot of variations (with BW shape but also in quality). I have seen so many different manufacturer filters inside that if someone talk about one 1772 maybe other 1772 is just different. (This receiver have lot of variations - nearly so that buyer have designed what he pay and then they make)Also these receivers are high-end history... so please do not modify! (of course everyone do with his own unit what ever he want but... )But as fact: there is not any reason for change these encoder lamps to leds but of course it can do easy and I have seen models where it have done by professional user own repair lab.Electrolytic capacitors is so self-evident and this need no explanation.For better hobby use (DX etc) it is maybe nice if littlebit change ALC timing / curve but one like apples one like oranges.

author: Dataforensics
author_rank: Unknown
timestamp: December 27, 2011, 01:05:12 am
content: Thanks rf-loop for taking the time for such a comprehensive reply. My 1772 is now working, thanks probably to you for the pointer. It had very low 34MHz drive, so I gave the MHz switch a clean with switch cleaner and cotton buds. Low and behold it came to life. Followed that up with a clean of filter and mode switch. Seems almost ok now. Been listening to amateur ssb traffic. As you say though either the agc needs tweeking or there is still something wrong as the set seems to work best with agc turned off.I have almost a full set of Racal receivers, a RA17, RA1792, RA1795 (vhf-uhf) and now the 1772. So now I can compare performance, although need to get an antenna switch and 19inch rack.RegardsTony

author: ejeffrey
author_rank: Super Contributor
timestamp: December 27, 2011, 04:03:34 pm
content: All silicon Phototransistors and photodiodes intrinsically work anywhere from around 400 nm (blue) to 1000 nm (IR -- past the 'reddest' commonly available LEDs). In actual use they are frequently fitted with plastic filters to block out unwanted light. This is normally done by using IR LEDs and a filter that blocks visible light. The purpose here is just to reduce noise that would otherwise be picked up. Especially displays like CRTs generate a lot of flickering light in the visible spectrum but almost no IR.

author: carlos
author_rank: Newbie
timestamp: January 09, 2012, 06:21:11 pm
content: Hello,I have also got a nice Racal 1772 receiver. I am quite glad with it but I find it a little noisy.I have replaced the 8Khz filter for a McCoy 6.8Khz I would like to get a 6Khz for AM Dxing.It has developped a problem, the OUT OF LOCK does not work, I have checked the red led diode and it is blown. I replaced it but now is always on.I have all diagrams but I have not been able to find the damage. Maybe I would need an oscilloscope but I have only a multimeter, signal generator and millivoltmeter.What would I check firstly? Could it be a bad capacitor or IC?Thank you very muchKind regards.

author: Dataforensics
author_rank: Unknown
timestamp: January 10, 2012, 03:58:03 pm
content: Hi CarlosI spoke too soon about my ra1772 working, after a few hours on my out of lock light was also always on. That is except when actually spinning the knob.After a few days with the service flowcharts traced it to the pm588 board in synthesiser.Thought initially that it was a failed 7474, put a socket in and new one, it worked.However it also worked with original put back in socket, so most likely was a dry joint.The set has quite a few logic lines feeding the lock circuitry, at least one for each synth board.There are also 7410 three input NAND gates on the pm588 combining signals from vco's etc.Trouble with my set is that each fault cured, shows up another.The VFC on mine does not work, but this was masked by synth fault.So you could use multimeter to check each boards lock lines.Although I used a scope as seemed quicker to differentiate between logic one and zero.And do not forget to check each end of the cables as they flex a lot when moving board to service position.Good luck with your repair.Tony

author: carlos
author_rank: Newbie
timestamp: January 11, 2012, 06:01:29 pm
content: Hello Tony,Thanks a lot for your advices.Maybe the cause of your continuous faults it is the mains voltage setting. If your local mains voltage is 220 I would use 220-110v voltage setting on the rear panel, with the arrow pointing to 220v it is possible that if the mains voltage drops for some reason the receiver will go below its normal voltage because it has been set, for example, to 250-125v which is too high for local conditions.I say this because I set mine to 250-125v and one day the receiver went slowly off, like a time fuse, and suddenly it went on I think that this blew up the OUT OF LOCK diode. I t does not matter if I spin the tuning knob or not, the led is always on.I have read that RA177x do not like low input mains voltage.Recently, I aligned all the phase loops, so there must be an incorrect logic level being generated between the loop output and the LED driver logic.The lock alarm indications are 'summed' together so if only one is incorrect it will cause the LED to light.I have also a Lowe HF-150 receiver, sometimes this small receiver outperforms the Racal on AM it has lower noise floor and I can better understand the weak signals.My next step is getting an AOR 7030+ it has excellent dinamyc range and has much better filters options, Racal lacks of it.Thank you !

author: Renaud F5ZR
author_rank: Contributor
timestamp: March 16, 2024, 11:06:50 am
content: Hello,I was wondering if I could tell about my "new" (for me!) RA1772 here?Just got it and it is fine but in need of some TLC, bad contacts also probably.If inconvenient I'll open another topic but there's interesting experience here.Best,Renaud