# Replace/refresh 7 segment display of vintage 80's clock radio

Past two weeks working on this clock radio because I love the looks of it and it is also pretty rare (it must be restored in original condition or better). Fixed almost every non-working feature such as (touch-) knobs, backlight (replaced bulbs with warm-white LED's - awesome result), potentiometers, speaker, ground noise, dirt and dust, casing etc. Arrived in bad shape however after all the effort it turns out to be very promising.

Still one problem left to be perfect, most important part of a clock, the numeric 7 segment display. Very dim, checked the voltages and such, it is just old, aged, the reason why it is dim because it needs to be replaced.

However, unable to find a replacement and searching for another route. Display is a red LED Toshiba TLR4202 (noticed in the service manual of device). Don't know if it is possible to take the display apart and replace the LEDs with some new smd LEDs. Searched on this topic however cannot find anything/anyone done this before. To be sure don't screw up the display before trying because current one is only one I have. Cannot find either a display with same characteristics (same layout) and same size.

Another problem, each LED must fit the old school environment requirements, a LED can only consume 5mA (noticed in the limited datasheet of the display), the MK50366N (no datasheet found) display driver is a MOS IC (needs careful threatment experimenting with this).

Any suggestions what to do, to find a solution to this?

Model

Service manual (TLR4202)

Display layout (TLR4202)

Display Datasheet

• If I read correctly, all you want to do is to replace the 7-segment displays and the LED colons. You do NOT want to replace (or damage) the clock chip: MK50366N. We don't have any information on the bezel (polarized?) But what have you excluded as unsuitable? I imagine that with some added effort, you should be able to find suitable 7-segment individual displays and can source individual LEDs for the colon dots. You may have to construct something to properly fix them with relationship to each other so that the display height and width is what you want. Is there a reason that can't be? – jonk Sep 10 '19 at 5:10
• @jonk Hi, thanks for the comment. The window on the case is transparent. The LED display has a dark red window. Because the window on the case is transparent, everything you do behind this window is clearly visible. Maybe I have to include a picture of it, so I will add this after this comment. Just a moment please ;-) – Codebeat Sep 10 '19 at 5:20
• Preserve that dark red window. At their simplest, they hinder reflections. But they were often also polarized so that reflections were completely killed and the contrast was maintained as high as possible. Do not damage it. Having it will likely allow you to use modern devices, appropriately mounted and positioned. Have you looked for high efficiency red 7 seg displays? – jonk Sep 10 '19 at 5:28
• Just posted a picture, added it to the question. Just read your comment, just a moment. – Codebeat Sep 10 '19 at 5:36
• Are you sure the problem isn't with the Automatic-Brightness-Control? (see the CDS photocell next to the LED module) – amI Sep 10 '19 at 7:51

You can buy individual common-cathode displays and use discrete LEDs for the four additional LED colon and anunciator positions. There will be a bit of wiring to do, which could be done with a bit of perf board and some solder-through magnet wire of ~AWG30. Should not take very long.

You'll have to evaluate if there is enough room, you'll need something like 11 or 12 mm thickness, which is probably more than the original display.

For example, Rohm LA-601EL. Use indicator LEDs with similar nm wavelength to the LED displays to reasonably match the colors. You might need to fiddle with the discrete leds by adding tubes, flattening the ends or doing something like that to get a good appearance, but fortunately they're cheap so experimentation is no problem.

A modern LED display such as the Rohm will be much brighter than the old displays at the same current, possible way too bright. Since you have a way to dim it, that should not be a problem.

• Hi, thanks for the great answer. Nice idea however the 'glass' of the case is transparent (it isn't a dark window) so any change made is visible. There is a dark red diffuser on top of the original display that cannot be taken off without damage because it is heavily glued (cannot reused). Have done this before with a sony clock display with same problem and destroyed the film/diffuser (by carefully peeling off). When it is possible to find a diffuser, your solution could be an awesome solution because there is enough space left to implement this. – Codebeat Sep 12 '19 at 17:25
• Just for info: The sony display I am talking about have a black frame with segments cutouts instead of a white frame. To make the display brighter, I removed the green diffusor and replaced with a transparent diffuser (of backlight) of an old LCD screen. This was possible because the frame was a black one, result isn't that bad/ugly. Replacing the colored diffuser with a transparent one makes the display less dim, not a real solution. Because your solution is much brighter, I have to find such material to 'hide' the implementation behind a colored transparent window. Any ideas? – Codebeat Sep 12 '19 at 17:53
• Build it up with a 3-d printed or X-acto knife cut (from black-painted foamcore) frame and add a thin red filter in front. – Spehro Pefhany Sep 12 '19 at 18:43
• @Sphero: Great idea, black foam. But what is the material of the thin red filter? Can I buy this somewhere? Searched on the web and cannot find anything. – Codebeat Sep 14 '19 at 4:50
• Look for filters “gels” used in stage and similar lighting. You can even find assortments on Amazon but theatrical supply outfits carry a wide range. There are sources of other materials that are more suitable for mass production situations, but as you just want one... – Spehro Pefhany Sep 14 '19 at 14:44

Inspect what controls the Intensity input on pin 5 of the MK IC and record the range of voltages.

This should control the common cathode voltage on pins 1 or 24 of the LED display, where, closer to 0V is a brighter display . Each segment is rated for 15mA absolute max, 10mA typical but is probably operating at <100uA from the brightness indicated.

• Hi, thanks for the answer. When I connect the ground pin (pin 1) of the display directly to ground, it is at full brightness, the same as the dots (dots are not affected by light intensity) however isn't very bright. – Codebeat Sep 10 '19 at 15:00
• What about V pin 5? And is Vcc=5V? – Tony Stewart EE75 Sep 10 '19 at 15:05
• Vdd = 5.8v which is plausible because datasheet specify 6v. Take a look at my answer, I disabled the ABC-feature. – Codebeat Sep 12 '19 at 1:26

This is not a real solution however it is some better now by shorting the pins of the ABC-LDR (Auto Brightness Control) at the front. Especially at night (dark room) it is an improvement. Don't want to spend more time on it and don't want to ruin something, leave it this way for now. At daylight it is still not very bright. I made it easily reversible, when I find better solution it is easy to reverse this fix.

• Increasing Vdd will increase current slightly. Measuring Vf on Segment may reveal current. If >2V – Tony Stewart EE75 Sep 12 '19 at 13:54