Retro radio alarm clock with LED display - interior, construction, principle of operation

p.kaczmarek2 04 Jul 2025 08:50 16 1851 Cool? (+7)

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TL;DR

The Crown Japan CR-S9028 DIGITAL LED-Uhrenradio combines an FM/AM radio, LED clock display, alarm, snooze, and 9V battery backup.
Inside, the clock uses the LM8560 and the radio uses the TA7613AP; the whole unit is built on a single-sided, single-layer PCB with through-hole assembly.
The LM8560 counts mains pulses from the transformer, dividing 50/60 Hz down to 1 Hz, and drives the display directly.
The LED display multiplexing is unusual: the LM8560 controls segments, while the common cathodes are driven from the transformer’s sine wave halves.
A battery-only supply keeps the clock running but the display stops, and the mains-frequency timekeeping raises questions about long-term accuracy.

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I would like to invite you to a presentation of the interior of an older Crown Japan radio alarm clock model CR-S9028, designated DIGITAL LED-Uhrenradio. The unit offers FM reception in the 88-108 MHz range and AM reception in the 540-1600 kHz range. It is equipped with an LED display showing the current time, an alarm clock function with snooze option, and battery back-up of the clock in case of mains power failure.

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The inscriptions on the housing are in German. Uhrenradio, as you might guess, is a radio alarm clock. Schlummer is a snooze (postponement of the alarm). Zeit is the time, Stunde is the hour, Schlaf is sleep (this is probably the radio automatically switching off after a set time).
At the bottom is a space for a 9V battery, just for backup:

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Let's take a look inside. The laminate inside is single sided and single layer. The assembly is fully threaded. The radio relies on a ferrite antenna and an adjustable capacitor and on coils protected with paraffin so as to prevent the coils from physically moving relative to each other.

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The first thing we see is the display and alarm clock controller - the LM8560. About it in a moment. First, let's turn our attention to the power supply section - there is no switching power supply here, just a simple transformer. Heavier, but simpler to build.

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Now the underside of the PCB. It's been a long time since I've seen paths routed like this, is it handmade? Modern CAD programs don't run tracks like this. Here you can also see that the LM8560 directly controls the display.

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The radio itself is based on the TA7613AP:

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Basically that's it, the button board has no more electronics.

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All that remains is the speaker:

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Now it is time to look at the principle of operation. Here we have two circuits:
- LM8560 - a digital clock controller with built-in alarm, display, pushbutton operation and clocking from the mains via 50 or 60Hz
- TA7613AP - a single-chip AM/FM radio controller, including AM amplifier, oscillator, mixer, etc.

Don't confuse the LM8560 with a microcontroller - this chip was designed directly for alarm clocks:

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Pinouts:

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The internal design shows the pulse counting route well. We have two inputs - CR (oscillator) and 50/60Hz (from the transformer). These go to a gate which decides which pulses are counted. Then there is a divider for 50 or 60Hz (1/25 or 1/30) and then a divider in half so that we get 1Hz. This is then processed by the blocks from counting time, alarm and snooze...:

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The LED multiplexing is solved in an interesting way. The display is with a common cathode, but the LM8560 only controls the segments. The common cathodes are controlled from... transformer, from the respective halves of the sine wave. The obvious consequence of this is that the display does not work when the whole thing is powered by battery only. Moreover, these common cathodes are two - and 14 segments each.

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193650bb6 .
The schematics illustrate well how the clock is clocked from the mains, this is done by connecting from the transformer before the rectifying diode to the 50/60 Hz Input pin. Right next to it you can see the battery backup and the oscillator with resistor and capacitor (CR Input pin). The alarm output can switch the radio or buzzer.

This leaves the TA7613AP. As I've written before, it's basically a single chip AM/FM radio controller:

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You don't even need an LM386 - there's already a Class B audio amplifier inside.

Summing up , there were essentially two separate sections inside - one was the simplest single-chip radio, all integrated into the TA7613AP, and the other was an IC designed for an alarm clock with a mains frequency-based countdown along with optional sustain on an RC resonator. I think I was most interested in this display multiplexed by that 50/60Hz from the mains.
Simple and functional, although I wonder what the stability of the mains frequency was like in the heyday of such products and whether the deviation of the time display grew rapidly.
Have you used this type of alarm clock, or do you still use one? My alarm clock in my primary school days was already a bit more technological, as it had an LCD....

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p.kaczmarek2 wrote 14318 posts with rating 12213 , helped 648 times. Been with us since 2014 year.

Comments

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zgierzman 04 Jul 2025 14:47

They can, if one wishes - either by default, or with an additional plug-in.... https://github.com/mitxela/kicad-round-tracks [Read more]

p.kaczmarek2 04 Jul 2025 14:57

Interesting plugin, somewhat along the lines of 'the exception proves the rule'. I don't normally see this type of track on modern projects. And you've reminded me that I was supposed to learn Kicad, but... [Read more]

Ktoś_tam 04 Jul 2025 16:25

That's how fast the deviation grew. I don't remember anymore, I associate something 20s but whether it was per day or per week.... My memory is not that good anymore. I remember that it had to be corrected... [Read more]

stachu_l 04 Jul 2025 16:57

This stability probably varied according to use. In the 70s/80s it was rather poor and indeed such watches but also such "flip-flop" watches driven by a synchronous motor had to be corrected. By this... [Read more]

perkins1 04 Jul 2025 20:38

Today it is very good , the network holds stably 50Hz I have such a clock controlled by the frequency of the network in the microwave and practically does not need correction except for the time chang... [Read more]

Citizen75 04 Jul 2025 22:31

In the case of the battery, there is no 50 Hz clocking and it is replaced by an internal 900 Hz oscillator with RC elements attached to pin 27 of the LM8560 chip. Without this oscillator the clock would... [Read more]

CHCl3 05 Jul 2025 02:57

I have a radio alarm clock on the same chip, or perhaps on its twin brother (TMS3450) I'm not sure now. It's been going strong for a couple of years now; a few minutes a day* and I've been collecting to... [Read more]

Citizen75 05 Jul 2025 09:35

By using SMD ICs and instead of a TTL 7490 chip a CMOS 4013 chip, such a generator can be significantly miniaturised. In the process, the LM78L05 stabiliser is eliminated. Pin 26 of the LM8560 chip is... [Read more]

misiek1111 05 Jul 2025 10:31

. I had a similar radio alarm clock during communism in the '80s - from abroad I got it :] The deviations were b. large and you had to set it often, at least once a week. But I was catching on it even... [Read more]

kris8888 05 Jul 2025 11:08

. Unlikely, but if even that, just filter out with a suitably sized RC circuit these potential interferences on the 50Hz clocking input of the TMS3450 chip. So as not to significantly attenuate the 50Hz... [Read more]

Citizen75 05 Jul 2025 17:48

. We have two 120Ω resistors in the cathode circuit of this display and the value can be increased slightly if required. [Read more]

kris8888 05 Jul 2025 19:38

. That's right, and I do this in any of my older devices (designed for 220V) that I care about that have an LED display or NIXIE powered by a non-stabilised voltage. I also add a small resistor in series... [Read more]

CHCl3 06 Jul 2025 03:56

. SMD soldering doesn't really work well for me; making boards with such tiny tracks even more so. I used whatever circuits I had at hand. I've already checked the electrolytic capacitors... [Read more]

cranky 09 Jul 2025 13:49

I had one. Timing accuracy on mains bearable. On battery backup time accuracy of 20%! It worked great for years when testing remote controls for a car. [Read more]

CHCl3 15 Jul 2025 23:13

So I added two ceramic capacitors; 100 nF in parallel to the electrolyte in the power supply filter and 4.7 nF between the 50 Hz input (pin 25) and circuit ground (pin 15). And the alarm clock stopped... [Read more]

kris8888 16 Jul 2025 00:12

This happens because the 50Hz signal, apart from being a time pattern, is also used to multiplex the display. It is this Blank& Flash block in the internal block diagram of the IC. When this 50Hz... [Read more]

FAQ

TL;DR: With FM 88–108 MHz support and a 9 V backup, this FAQ explains why retro LM8560 clock radios drift, why the display uses mains multiplexing, and how to repair or quartz-mod them. As one expert noted, "the grid is currently very stable." It helps restorers, collectors, and repairers diagnose timing and display faults fast. [#21598061]

Why it matters: These clocks mix radio, timekeeping, and LED drive in unusual ways, so one small change on the 50 Hz path can fix drift or create new display faults.

Wariant	Baza czasu	Wyświetlacz LED	Typowa dokładność z wątku	Główne ryzyko
Praca z sieci	50/60 Hz z transformatora	Działa normalnie	od „znośnej” do stabilnej	zakłócenia na wejściu 50 Hz
Podtrzymanie bateryjne	oscylator RC ok. 900 Hz	Nie działa	nawet ok. 20% błędu	duży dryft czasu
Mod kwarcowy	generator kwarcowy + dzielniki	Zależnie od wykonania	lepsza długoterminowo	brak miejsca i przeróbki połączeń

Key insight: W LM8560 sygnał 50 Hz nie służy tylko do odmierzania czasu. Ten sam przebieg współuczestniczy też w multipleksowaniu LED, więc zbyt mocne filtrowanie pinu 25 psuje wskazania. [#21608093]

Quick Facts

Crown Japan CR-S9028 odbiera FM 88–108 MHz i AM 540–1600 kHz, a zegar podtrzymuje bateria 9 V po zaniku zasilania sieciowego. [#21597161]
Tor zegara LM8560 używa wejścia 50/60 Hz z transformatora albo wejścia CR, a z dzielników 1/25 lub 1/30 oraz dalszego podziału powstaje sygnał 1 Hz. [#21597161]
Na baterii wyświetlacz LED nie pracuje, bo wspólne katody są sterowane połówkami sinusoidy z transformatora, a nie w pełni z samego układu scalonego. [#21597161]
W praktycznej naprawie pomogły kondensatory 100 nF równolegle do elektrolitu zasilacza i 4.7 nF między pinem 25 a masą; 10 nF i 100 nF na pinie 25 powodowały już świecenie nadmiarowych segmentów. [#21608064]
Starsze wersje projektowano na 220 V AC, a przy dzisiejszych 230–245 V warto lekko zwiększyć dwa rezystory katodowe 120 Ω, aby zmniejszyć stres prądowy LED. [#21598409]

1. What is the LM8560 chip and how does it work in a retro LED radio alarm clock like the Crown Japan CR-S9028?

LM8560 is a dedicated digital clock IC for alarm clocks, not a microcontroller. It handles time counting, alarm, snooze, pushbuttons, and direct LED segment control in one chip. In the CR-S9028, it takes timing from mains 50/60 Hz or a backup oscillator path, then drives the display and alarm output that can switch either the radio or a buzzer. The thread also notes battery-backed operation and mains-derived display multiplexing as key design features. [#21597161]

2. How does the TA7613AP integrate the AM/FM radio stages, oscillator, mixer, and audio amplifier in this type of clock radio?

TA7613AP integrates most of the radio into one IC. It combines the AM/FM radio stages, including amplifier, oscillator, and mixer, and it even includes a Class B audio amplifier, so this design does not need an LM386. In the shown clock radio, the radio section is largely built around this single chip plus the tuning components such as the adjustable capacitor, ferrite antenna, and coils fixed with paraffin. [#21597161]

3. Why is the LED display in an LM8560 alarm clock multiplexed from the transformer’s 50/60 Hz mains waveform instead of being driven entirely by the IC?

Because this design offloads the common-cathode switching to the transformer waveform. The LM8560 drives the segments, but the display’s common cathodes are switched by the respective halves of the 50/60 Hz sine wave from the transformer. That reduces what the IC must drive directly, but it creates one hard limit: on battery only, the LED display does not work. The thread also notes there are two common cathodes serving 14 segments each. [#21597161]

4. How do you trace the timekeeping path inside an LM8560, from the 50/60 Hz input or CR oscillator input down to the 1 Hz clock signal?

You trace it through the LM8560 divider chain. The IC accepts pulses from either the CR oscillator input or the 50/60 Hz mains input, then a gate selects which pulses are counted. After that, the chip divides by 1/25 or 1/30, then divides by two to produce a 1 Hz signal for time counting, alarm, and snooze logic. This path is shown directly in the internal block diagram discussed in the thread. [#21597161]

5. What happens to time accuracy in LM8560-based alarm clocks when they run on battery backup instead of mains frequency?

Time accuracy gets much worse on battery backup. One user reported about 20% accuracy on battery, while mains timing was still bearable. Another post explains why: without mains clocking, LM8560 uses an internal 900 Hz oscillator with RC parts connected to pin 27. That keeps the clock running, but RC timing drifts far more than stable mains-derived counting. The display also stays off in battery mode because its multiplexing depends on the transformer waveform. [#21601745]

6. Why does adding too much capacitance on pin 25, the 50 Hz input of the LM8560, make extra LED segments glow?

Too much capacitance distorts the 50 Hz signal that also multiplexes the display. One repair test showed 100 nF on pin 25 to ground caused clearly wrong segments to light, and even 10 nF still produced faint extra glow. A later explanation states that the 50 Hz waveform is used both as the time pattern and by the internal Blank & Flash display logic. If you heavily distort or weaken that waveform, multiplexing fails. [#21608093]

7. How can you filter interference on the 50 Hz clock input of an LM8560 or TMS3450 alarm clock without breaking display multiplexing?

Use light filtering, not heavy filtering. A working example used three steps: 1. add 100 nF in parallel with the power-supply electrolytic, 2. add only 4.7 nF from pin 25 to ground, 3. verify that no false LED segments appear. The thread warns that 10 nF to 100 nF on the 50 Hz input can already disturb multiplexing. As one expert put it, you must not significantly attenuate 50 Hz while cutting higher harmonics. [#21608064]

8. What causes an old mains-synchronized radio alarm clock to gain a few minutes per day even when the power grid frequency is supposed to be stable?

A gain of a few minutes per day usually points to the clock, not the grid. The thread says the current grid holds the 50 Hz fundamental stably enough that large daily drift more likely indicates damage, waveform contamination at the clock input, or poor filtering. One user improved a rushing clock significantly by adding 4.7 nF on the 50 Hz input and 100 nF in the power filter, which suggests the problem can come from the local signal quality seen by the IC. [#21598061]

9. How do you retrofit a quartz time base into an LM8560 or TMS3450 clock radio using parts like CD4060, CD4013, 7490, or MM5369?

You replace the mains-derived timing with an external frequency generator and feed the clock IC the expected reference. The thread gives a concrete path: build a quartz generator with CD4060 and 7490 around a 4.096 MHz crystal and 78L05, or miniaturize it with CD4013 or a single MM5369. You then change the clock IC connection for the required output, because one user’s generator produced 60 Hz, not 50 Hz. Pin 26 selects 50/60 Hz on LM8560. [#21597984]

10. LM8560 with mains-frequency timing vs a quartz generator mod — which gives better long-term accuracy in a vintage radio alarm clock?

A quartz generator mod gives better long-term accuracy. Mains timing can be stable today, but the thread records older clocks needing correction weekly, and battery RC timing can be far worse. A quartz mod replaces that dependency with a dedicated crystal-based source, which one user pursued specifically because his TMS3450-class clock still gained a few minutes per day. The tradeoff is practical, not theoretical: extra parts, tight internal space, and the need to adapt for 50 Hz or 60 Hz operation. [#21597903]

11. What is an Uhrenradio and what do German panel labels like Schlummer, Zeit, Stunde, and Schlaf mean on old clock radios?

"Uhrenradio" is a clock radio that combines radio reception, time display, and alarm functions in one appliance, usually with labeled front-panel controls for sleep or snooze behavior. In this thread, the German labels are explained directly: Uhrenradio means radio alarm clock, Schlummer means snooze, Zeit means time, Stunde means hour, and Schlaf refers to sleep, likely the timed radio shutoff function. These labels appear on the Crown Japan housing. [#21597161]

12. How can you reduce LED display stress in older 220 V clock radios now that mains voltage is often 230–245 V?

Reduce current in the display path. The thread notes these alarm clocks were designed for 220 V AC, while modern mains can sit at 230 V or even 245 V. That higher voltage can overdrive LED segments when the display uses a non-stabilized supply tied to the transformer waveform. A practical fix is to slightly increase the display’s series cathode resistors so the segments see less current. One contributor says he does this routinely in older LED, NIXIE, and VFD equipment. [#21598409]

13. What role do the two 120 ohm cathode resistors play in the LED display circuit, and how should their values be adjusted safely?

The two 120 Ω resistors limit current in the display cathode paths. Because the common cathodes are driven from transformer halves, these resistors directly affect LED stress and brightness. The safe adjustment is modest: increase their value slightly if present-day mains voltage is higher than the original 220 V design target. The thread presents that as the practical remedy when older clocks now run from 230–245 V sockets and segment burnout becomes a concern. [#21598325]

14. How does a single-sided, single-layer, fully through-hole PCB with rounded tracks compare to a modern KiCad or Eagle PCB layout?

It is simpler, more manual-looking, and less typical of current auto-routed boards. The opened CR-S9028 uses a single-sided, single-layer, fully through-hole PCB, and the author specifically remarks that modern CAD tools do not usually route tracks in that rounded vintage style. The comparison in the thread is visual and practical: old layouts often prioritized hand-friendly geometry, while modern KiCad or Eagle projects usually show straighter, more standardized routing unless a designer forces a retro style. [#21597161]

15. What is the KiCad round-tracks plugin and when would someone use it to recreate the vintage PCB routing style seen in old electronics?

It is a KiCad add-on that generates rounded PCB traces to mimic vintage routing. In this discussion, it is presented precisely as a way to recreate the curved track style seen on the old alarm clock PCB, even though that look is uncommon in modern projects by default. The follow-up also notes the plugin has limitations, so you would use it mainly for aesthetic recreation, restoration, or retro-inspired layouts rather than as a standard routing method for every board. [#21597482]

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