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
I 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 parts this 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.