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Old 4th Jan 2022, 7:29 am   #1
Radio1950
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Default Toshiba Clock Radio RC-K1, VFD Display Problem

Toshiba Clock Radio RC-K1.

This my bedside clock radio and has been on every day since 1982ca.
It has a VFD display, and it is getting dim, like its owner.
The display also has some number segments with bright “spots”.
I really would like to fix this radio.

I had the covers off today and checked most obvious things.

It has had a relatively recent “all electro capacitor replacement”.
PSU and Display volts OK.
VFD heater is nominally 3.0 V AC, measured 3.15 V AC at VFD pins.

I cannot remove the actual display from the PCB due to risk of damage, so at this stage, I don’t really know what the display type is, but it is certainly a special for Toshiba, so no chance of a new display per se.

I have no experience of VFDs.
I have read that I can partially restore the display brightness by raising the heater volts by 20 %?

With this particular radio, the only way I can see how to do this is by installing an extra tiny mains transformer.

Before I do this or other very intrusive surgery, can I please ask for the experience and wisdom of others.

The Toshiba original manual is mine, and offered freely here.
It is a bit big in a single PDF. Please PM me if you want the entire manual.

Attached is the circuit, derived from a Photoshop “PhotoMerge” of two pages, and is unchecked for minor merge errors.

Some photos of the display attached, but it is quite difficult to catch the “dotty” segments on a photo.
The red “alarm” section shows it better than the blue “time” section.
.
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Last edited by Radio1950; 4th Jan 2022 at 7:51 am.
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Old 4th Jan 2022, 3:42 pm   #2
G6Tanuki
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

In decades-past I had a "Micronta" green-display VFD clock bought from Tandy when they were still a thing; it started developing 'stripiness' on some of the segments.

Investigation showed that there was an open-circuit diode in the bi-phase half-wave part of the PSU which provided the segment-voltage, along with a smoothing-capacitor which had lost most of its capacitance.

A new 1N4007 (I don't bother stocking lower-rated 1N400x diodes since they're all the same price) and a new 1000uf 35VW electrolytic solved the problem.


Whether the diode-failure brought on the electrolytic's problems or they were coincident failures I don't know.

I'd be casting a suspicious eye over the power-supply diodes and their neighbouring electrolytics as a first attempt at fixing it.

*edit* I notice you've already replaced the electrolytics - could stuill be worth checking the diodes though; you could have half-wave rectification when there should be full-wave rectification.
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Old 4th Jan 2022, 6:43 pm   #3
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

VFDs do die eventually, depending on how many hours they've done (which in the case of a 1982 clock display is a lot.) You could try increasing the heater voltage a bit to brighten them up, but when they go dim they're pretty much on the way out.
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Old 4th Jan 2022, 7:25 pm   #4
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

I agree, a tired VFD may show uneven light/dark parts within segments. Getting common now with old cooker clocks and video recorders
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Old 4th Jan 2022, 11:32 pm   #5
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

I made a clock to show GMT/UTC time from some of the bits of one of those Motorola kits that came out in the 70's. I didn't have the original 7-segment LED display or the transformer but I did have a small 12volt transformer and a VFD calculator display. The VFD of course needs a heater supply which I did by winding sufficient turns of wire-wrap wire around the transformer core until you could just see the filament glow in the dark. The display I attached to a multi tag strip by it's leads and then spent hours sorting out which pins lit each segment or display. Now nearly 50 years later it still works although the display is just like yours - 'segmented' shall we say. Now that Paul and other have suggested upping the heater volts I may add an extra turn to the existing winding and see what happens. The pic below shows the "2" and the "3" merged due to the shutter timing.
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Old 6th Jan 2022, 6:29 pm   #6
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

I've got a Realistic clock radio, with a VFD, it has an automatic brightness control, controlled by a LDR. When it was new it lit up the bedroom in the dark, so I changed some resistors to improve the control. Now the display has dimmed with age, I keep meaning to redo the mod., to try and improve it, but I can't find the schematic.
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Old 13th Jan 2022, 11:36 pm   #7
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

Follow up.
Resolution.

. Thank you for your comments.

More investigation.

Recheck diodes, resistors, capacitors, in power supplies.
All DC lines measure OK.
Added trial extra filter capacitance to PS; no improvement.
Check wire wrap connections in AC heater circuits.
Measure normal 3.1 V AC at VFD pins, and current at 100 mA AC.

With power on, and using a small rare earth magnet over the VFD glass front, I was able to manipulate the internal electron beam so as to shift the beam over the length of segments of a number display.
A bright "trace" could be shifted through parts of the segments.
As this trace went through segments which previously had bright and dark spots, the spotting effects could again be observed.
This made me think that the phosphor had permanently aged segments.


With radio powered off, and using a small UV torch, I observed that the VFD whole display was uniformly phosphorescent and with reasonable brightness.
No segment spotting was observed.
An unexpected result.


Nevertheless I decided to trial slightly elevated VFD heater voltage.
Break VFD heater supply and isolate from original transformer.
Run VFD heaters from external adjustable AC source.

Set to 3.1 V AC and observe normal operation.
Adjust up to 4.0 V AC to observe some improvement in overall brightness, but segment spotting still observed in original places.
Heater current 110 mA.
No improvement after 30 minutes of 4.0 V and resetting to 3.1V.

Adjust up to 4.4 V AC to observe heaters just starting to glow, and with no real improvement to display.

I considered using an external adjustable HV supply (20V DC) to note any possible improvement, but as the segments were spotting, and a test arrangement carried complexity and risk, this lost any appeal.


Some web investigation and correlation of noted effects.
Concluded that the VFD display has aged and effective "restoration" is probably not possible.

The radio has operated well for 350,000 hours.

As I use my radio in a darkened room at night anyway, and it has sufficient brightness for now, I decided to keep it as original, and contemporaneously decline in sympathy with it.
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Old 14th Jan 2022, 7:43 am   #8
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Default Re: Toshiba Clock Radio RC-K1, VFD Display Problem

Wonderful research and conclusion!
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