DM70/DM71-LED Bargraph Display Modification?

[a_strong]

I have recently taken delivery of an Eddystone 840C receiver which uses the above valve as a signal strength meter. I am no longer in the first flush of youth and my eyesight is none to good. Even though I have replaced the display, it is still difficult for me to make out any change.

I would be interested to hear from anyone who has successfully replaced a DM70/DM71 with an LED bargraph display.

Thanks in anticipation - Alan

[paulsherwin]

Have you seen this thread?

https://www.vintage-radio.net/forum/...d.php?t=118220

[David G4EBT]

Originally Posted by a_strong

Even though I have replaced the display, it is still difficult for me to make out any change.

Though the display is rather small, it may not be your eyesight, but that there isn't much change in the display when on tune Alan. Might be worth checking the swing in AGC Voltage when a station of good signal strength is on/off tune.

When off-tune the whole 'question mark' display will be illuminated, and it takes quite a strong signal resulting in several AGC Volts negative to get the display to diminish to just a dot. I've attached a pic of a DM70 in a Pye radio that I have, and when relying just on the internal frame aerial and telescopic whip, the display doesn't change a great deal. If I connect an external aerial it will dim on strong broadcast station signals, but really, it's more of a gimmick than a useful indicator.

I'm inclined to think that our ears are a better judge of peaks and nulls than a DM70!

The DM70 is also used as a null indicator in those dinky Nombrex C/R bridges, and I've attached a pic of mine.

The redeeming features on the DM70 are that they seem not to wear out, and they're not generally expensive - unlike for example, EM34s.

Good luck with the LED mod if you decide to go down that route.

[G6Tanuki]

Truth is, the AGC action of the Eddy 840 series never really made that much difference to the tuning-indicator display. I always thought of it as a gimmick rather than anything really useful.

And nowadays to resolve SSB you'll be operating with the switch set to "BFO" mode anyway - so the tuning-indicator will respond purely to the setting of the RF Gain control.

[a_strong]

Thank you everyone for your replies. I have ordered a 10 segment bargraph display in green to start playing with. Somewhere I have a driver I.C which should make very compact unit which can easily be changed back to a DM70 if its next owner wants the set to remain as it was on the day it was born, not that I am planning to sell for yet a while.

Alan.