Vacuum-fluorescent display gone dim. - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 27th Sep 2013, 11:48 am

I've just unearthed a small "Micronta" digital alarm-clock I bought from Tandy something like thirty years ago and haven't used for around fifteen.

Plugged it in - the clock bit works BUT the green vacuum-fluorescent display is extremely dim.

In the dark, I can see the cathode-wires glowing dimly red so there's clearly adequate heater-power getting through.

I recall reading somewhere else that these VFDs can suffer with disuse and that they sometimes recover if left on for a few days, which is what I'm trying. If this doesn't work is it possible to rejuvenate the emission in the same way as with a tired CRT?

--G6Tanuki

turretslug · 27th Sep 2013, 12:14 pm

G6T, I'd certainly be inclined to leave it powered for a few days and see if it improves- twice recently, I've had items that started off very dull and steadily got better (Sony tuner, Pioneer clock) over a few dozen hours, presumably they'd been laid-up for ages. Reading around, they seem anecdotally a bit like CRTs, ageing-wise- sometimes they're just flat, flat, flat and that's all there is to it, sometimes (as above) they benefit from a few hours back in service, I've also heard of mild filament boosting working- obviously, as with any fine DH device, I'd be inclined to do this very gently!

If it does "come back", I'd look over the circuit and see if the grid bias could be throttled back with a rear-panel pot- a common tactic of mine with VFDs. They often seem to be set to bling bright, sometimes with grid connected to segment (anode) supply, which seems to be asking for short-lived phosphor brilliance. After all, they're only a consumer product, intended to shine brightly for a short time, it's awkward

like us who keep things going for decades...,

Colin.

G6Tanuki · 27th Sep 2013, 12:50 pm

Thanks Turretslug: I'll leave it powered up for a few days and see if things get better.

Looking at the wiring inside, it has an automatic brightness control (LDR behind a little translucent window on the front panel) - however putting a finger over the LDR doesn't dim it further and shining a LED torch directly into the LDR doesn't make it brighter. Perhaps the dimness is because the auto-brightness-control LDR or transistor's failed and it's defaulting to minimum brightness?

I think this area of the circuit needs further investigation.

Nickthedentist · 27th Sep 2013, 12:59 pm

Before condemning the VFD itself, I wonder whether the other supply lines are in order?

There was a range of Akai videos in the early 1990s which suffered from this problem, and the main cause was failing electrolytic capacitors. Worth checking, I suppose.

Nick.

G6Tanuki · 27th Sep 2013, 1:13 pm

I had thought about the smoothing: there's one big blue radial-leaded 470uF 16v e-lytic that looks a bit tired. Easy enough to change.

vidjoman · 27th Sep 2013, 1:38 pm

Quote:

Originally Posted by Nickthedentist

Before condemning the VFD itself, I wonder whether the other supply lines are in order?

There was a range of Akai videos in the early 1990s which suffered from this problem, and the main cause was failing electrolytic capacitors. Worth checking, I suppose.

Nick.

I remember those Akai VCR's. Vague feeling there were caps that were part of a voltage converter. Made the display very dim until you changed them.

double-triode · 27th Sep 2013, 4:37 pm

Hi G6Tanuki,

VFD's do not normally go dim due to lack of use and the problem is most probably dried-out or de-polarized electrolytics in the power supply or inverter powering the VFD.

Indeed, I have some first generation VFD tubes here which sat in a drawer for 20 or more years until I put them back into service in a an "eccentric" radio project and they still glow very brightly.

Good luck with the restoration.

double-triode

Nickthedentist · 27th Sep 2013, 5:38 pm

Quote:

Originally Posted by double-triode

VFD's do not normally go dim due to lack of use

Yes, as collectors of 1980s videos will tell you. When plugged in after years of disuse, they generally glow as brightly (or dimly) as they did when they were last used.

N,

G6Tanuki · 27th Sep 2013, 6:48 pm

Well, I did a bit of poking around, and discovered that - though there was some serious ripple on one of the supply-lines this wasn't due to the smoothing capacitor having gone low-capacity/high-ESR.

Replacing the capacitor with a known-good "Rubycon" Electrolytic from my stockpile made no difference. Then I noticed the scope showed ripple was rather more 50Hz than 100Hz.

Turns out there was an open-circuit diode in one leg of the centre-tapped-full-wave rectifier.

Lob in a spare 1N4007 and green vacuum-fluorescent happiness was restored!

Thanks to everyone for all the suggestions on what to look for: sometimes the most-obvious fix is right under your nose but you don't see it.

Nickthedentist · 27th Sep 2013, 7:33 pm

Well done!

I do love these old VFD clocks. Being locked to the mains frequency, their timekeeping potential is extremely good, and it's nice not having to fumble to press a button to check the time at night.

Nick.

G6Tanuki · 27th Sep 2013, 7:42 pm

Quote:

Originally Posted by Nickthedentist

Well done!
I do love these old VFD clocks. Being locked to the mains frequency, their timekeeping potential is extremely good, and it's nice not having to fumble to press a button to check the time at night.
Nick.

Thanks!

This one even has a little R-C oscillator powered by a PP3 battery to maintain a semblance of timekeeping if the 50Hz mains sometimes goes away. And a circuit to check that the battery's not dead - that's the "Battery Sentinel" function which lights a dot on the display when it's time for a new PP3.

turretslug · 28th Sep 2013, 10:36 am

Good stuff, a cheap and conclusive fix.

Quote:

Originally Posted by Nickthedentist

I do love these old VFD clocks.

Nick.

Me too- perhaps my favorite display type, crisp and clear and a good temperature range. It's genuinely a "valve", too! Perhaps one of the last outposts of thermionics with the cliff-edge demise of the CRT

VFDs are still clinging on in the form of diagnostic edge-of-card displays in some of the kit I work with. Just a shame they are prone to fade. Far nicer than LCDs but one can understand the manufacturer appeal of the LCD- cheap mass production of complex arrays and easy to power and drive. Too bad if they lose segments over the years as contact deteriorates... VFDs are worth nurturing, I think the most prone to permanent fade are the small, narrow segment bright types that are on all the time, i.e VCRs. A clock like G6T's with bigger segments is probably less fierce on the phosphor.

Must try some gentle filament boosting on a sacrificial item at some point

double-triode · 28th Sep 2013, 11:20 am

Well done G6Tanuki,

Glad you restored the VFD clock to full working order. I love those VFD's particularly for clock displays. Much easier on the eye than LED displays and a much better/wider viewing angle.

turretslug wrote:
"It's genuinely a "valve", too! Perhaps one of the last outposts of thermionics"
*** End-quote ***

I took this point to heart a few years back while looking for some tubes which could simulate the low-mu pre-WW2 triodes and built this eccentric project for fun....

http://www.qsl.net/m0ayf/VFD-Regen.html

It works really well and my only regret is that I did not make the construction more "open" to show the green glow of the VFD's when in operation

I used the VFD's in order to preserve my pre-war valve collection for future "retro" projects.

73,s

double-triode

Quote:

Originally Posted by G6Tanuki

Well, I did a bit of poking around, and discovered that - though there was some serious ripple on one of the supply-lines this wasn't due to the smoothing capacitor having gone low-capacity/high-ESR.

Replacing the capacitor with a known-good "Rubycon" Electrolytic from my stockpile made no difference. Then I noticed the scope showed ripple was rather more 50Hz than 100Hz.

Turns out there was an open-circuit diode in one leg of the centre-tapped-full-wave rectifier.

Lob in a spare 1N4007 and green vacuum-fluorescent happiness was restored!

Thanks to everyone for all the suggestions on what to look for: sometimes the most-obvious fix is right under your nose but you don't see it.

28th Sep 2013, 12:07 pm

I wonder if one could be used as a triode in a one (or more) valve radio? Also how come they last well but 'magic eyes' don't.

AC/HL · 28th Sep 2013, 12:12 pm

I believe it's the (metal) target that fails in these, not the cathode. The later one's on the glass seem to be OK.

27th Sep 2013, 11:48 am	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Vacuum-fluorescent display gone dim. I've just unearthed a small "Micronta" digital alarm-clock I bought from Tandy something like thirty years ago and haven't used for around fifteen. Plugged it in - the clock bit works BUT the green vacuum-fluorescent display is extremely dim. In the dark, I can see the cathode-wires glowing dimly red so there's clearly adequate heater-power getting through. I recall reading somewhere else that these VFDs can suffer with disuse and that they sometimes recover if left on for a few days, which is what I'm trying. If this doesn't work is it possible to rejuvenate the emission in the same way as with a tired CRT? --G6Tanuki

27th Sep 2013, 12:14 pm	#2
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Vacuum-fluorescent display gone dim. G6T, I'd certainly be inclined to leave it powered for a few days and see if it improves- twice recently, I've had items that started off very dull and steadily got better (Sony tuner, Pioneer clock) over a few dozen hours, presumably they'd been laid-up for ages. Reading around, they seem anecdotally a bit like CRTs, ageing-wise- sometimes they're just flat, flat, flat and that's all there is to it, sometimes (as above) they benefit from a few hours back in service, I've also heard of mild filament boosting working- obviously, as with any fine DH device, I'd be inclined to do this very gently! If it does "come back", I'd look over the circuit and see if the grid bias could be throttled back with a rear-panel pot- a common tactic of mine with VFDs. They often seem to be set to bling bright, sometimes with grid connected to segment (anode) supply, which seems to be asking for short-lived phosphor brilliance. After all, they're only a consumer product, intended to shine brightly for a short time, it's awkward like us who keep things going for decades..., Colin.

27th Sep 2013, 12:50 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Vacuum-fluorescent display gone dim. Thanks Turretslug: I'll leave it powered up for a few days and see if things get better. Looking at the wiring inside, it has an automatic brightness control (LDR behind a little translucent window on the front panel) - however putting a finger over the LDR doesn't dim it further and shining a LED torch directly into the LDR doesn't make it brighter. Perhaps the dimness is because the auto-brightness-control LDR or transistor's failed and it's defaulting to minimum brightness? I think this area of the circuit needs further investigation.

27th Sep 2013, 12:59 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,843	Re: Vacuum-fluorescent display gone dim. Before condemning the VFD itself, I wonder whether the other supply lines are in order? There was a range of Akai videos in the early 1990s which suffered from this problem, and the main cause was failing electrolytic capacitors. Worth checking, I suppose. Nick.

27th Sep 2013, 1:13 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Vacuum-fluorescent display gone dim. I had thought about the smoothing: there's one big blue radial-leaded 470uF 16v e-lytic that looks a bit tired. Easy enough to change.

27th Sep 2013, 4:37 pm	#7
double-triode Retired Dormant Member Join Date: Sep 2013 Location: Lincoln, Lincs. UK. Posts: 9	Re: Vacuum-fluorescent display gone dim. Hi G6Tanuki, VFD's do not normally go dim due to lack of use and the problem is most probably dried-out or de-polarized electrolytics in the power supply or inverter powering the VFD. Indeed, I have some first generation VFD tubes here which sat in a drawer for 20 or more years until I put them back into service in a an "eccentric" radio project and they still glow very brightly. Good luck with the restoration. double-triode

27th Sep 2013, 6:48 pm	#9
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Vacuum-fluorescent display gone dim. Well, I did a bit of poking around, and discovered that - though there was some serious ripple on one of the supply-lines this wasn't due to the smoothing capacitor having gone low-capacity/high-ESR. Replacing the capacitor with a known-good "Rubycon" Electrolytic from my stockpile made no difference. Then I noticed the scope showed ripple was rather more 50Hz than 100Hz. Turns out there was an open-circuit diode in one leg of the centre-tapped-full-wave rectifier. Lob in a spare 1N4007 and green vacuum-fluorescent happiness was restored! Thanks to everyone for all the suggestions on what to look for: sometimes the most-obvious fix is right under your nose but you don't see it. Attached Thumbnails

27th Sep 2013, 7:33 pm	#10
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,843	Re: Vacuum-fluorescent display gone dim. Well done! I do love these old VFD clocks. Being locked to the mains frequency, their timekeeping potential is extremely good, and it's nice not having to fumble to press a button to check the time at night. Nick.

28th Sep 2013, 12:07 pm	#14
Guest Guest Posts: n/a	Re: Vacuum-fluorescent display gone dim. I wonder if one could be used as a triode in a one (or more) valve radio? Also how come they last well but 'magic eyes' don't.

28th Sep 2013, 12:12 pm	#15
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Vacuum-fluorescent display gone dim. I believe it's the (metal) target that fails in these, not the cathode. The later one's on the glass seem to be OK.