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Michael Kukat

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Planned obsolescence? Let's check

A friend gave me this nice kitchen scale yesterday and asked me if i can find out if there is some counter or so that renders it dead after warranty expired, because this was nearly exactly the time it failed. Completely dead, no display.

I had a quick look into it this morning, found out that something still switches power to some circuitry on and off as intended when operating the capacitive power button. The main CPU is unreachable because it's bonded to the PCB, but there is a TM1620 next to it. Checked the datasheet, it's an 6x8 LED driver with serial input. Hooked up the scope and the input looks plausible, the data fields change when pushing on the sensors, but the outputs of the chip stay off, not driving the LED.
Okay, doesn't look like planned obsolescence, but still leaves the possibility, maybe the thing is tricky enough to simply not send a chip enable command when it's time has come.

But i just had another idea - hot air. So i grabbed my hot air pencil and heated up the TM1620 while the scale was turned on and i suddenly noticed it drawing more power (had it upside down to reach the chip, so i couldn't see the display, in such situations, the amperemeter of the lab PSU is very handy). Turned it around - voila, it works!

Okay, no permanent solution, as soon as the chip cools down again, the LEDs go off. Operating it with that 80W hot air pencil would not be very energy efficient, but i told the owner that he can get some TM1620 (you get them in 10-packs for $3 from China) and i'll replace it, if he wants to.

But at least it's proven - not planned obsolescence here, at least none that's not forced by the consumer asking for very cheap electronics and getting exactly the quality he asks for. While 5 years for a 36€ scale isn't even that bad.

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Selenium rectifier inside

Why throw away broken stuff when you can take it apart and learn something?

This is the selenium rectifier i replaced in a tube radio today. You can easily see 8 small stacks of selenium diodes and some pieces of metal to connect them. Many diodes for a single rectifier, but selenium diodes can't handle large voltages, so this is necessary. If you ever cracked open an old HV rectifier diode from a TV (i did decades ago), you find many hundreds small chips in it.

Interesting. And i think i found the problem of this rectifier. The second cell in the upper row and the corresponding chip doesn't look so healthy.

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Selenium goes silicon

Well, it didn't work to put this Nordmende Carmen into the corner to complete the repair a different day. After the problem wasn't an open resistor, i checked the anode voltage (well, i always tell people to check supply voltages first, so why should i do it myself? :). 160V instead of 260V. That's a bit out of tolerances, maybe.

As my isolation transformer showed just a small current consumption of 160-180mA (the smaller Blaupunkt radio needed 220mA), it likely isn't overloaded, but the rectifier is broken.

It's an old selenium rectifier and in Germany, they are also called "gleich riecht er" (will smell in a moment) for a good reason - at end of life, their voltage drop increases, the power loss across it thus also increases, it gets hot and finally gives up with a very evil smell. Good that i didn't reach this point, i read it's hard to get that smell out of the room again...

As selenium rectifiers are less than optimal when it comes to voltage drop (back then, it didn't hurt to lose 20V at the rectifier), you can't simply throw in a silicon rectifier, the voltages would be too high due to the lower voltage drop. Besides this, silicon diodes have much harder transients when switching, which can result in enough distortion by overtones that the low frequency AM bands get unusable.

So a common trick is to use 1N4007, add some 4.7-22nF capacitors across them and add some resistors to simulate the higher resistance of a selenium rectifier. I never spent so much effort for just a simple rectifier, but it works great. I had some 15nF Y2 capacitors around, i had 150Ω 2W resistors, which sum up to 600Ω, which should be a useable value for this radio (other sources tell around 470Ω-1KΩ for similar types). And it looks nice. You could easily sell it as an audiophile bridge rectifier :)

After carefully putting this thing together and installing it in the radio, i first measured the anode voltage. 250V instead of 260V. Perfectly okay, i have set the radio to 240V and run it off our official 230V power grid.

And the most important thing - it works now. No fluctuations any more, clean FM reception, great sound and the EM84 is bright enough that i decided to keep it.

So i can finally archive that topic :)
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Nordmende Carmen restoration

A bit more to do on this one, but i got it very cheap, 10€ or so. After working on this radio, i know that restoring tube radios will not get a new hobby here. Sure, it's interesting, the result is extremely satisfying, but there are also some very annoying moments around this. Big respect for all out there who frequently spend all the time and effort to keep those old radios alive.

But let's start. First problem was a FM tuning capacitor that was stuck, you could hardly rotate it at all. I tried some Kontakt 701 externally, but it wasn't enough, so i had to remove the cover of the FM tuner to get to the innards of the capacitor. There, a very small dose of 701 fixed the problem. Not sure if it's a permanent solution or if the grease re-hardens with the time, but at the moment, it works fine.

After this, i could test it a bit more. Fluctuations. Everywhere. The reception quality on FM was coming and going, the whole magic band flashing, but in general, it wasn't that dead. I already checked the main filter caps some days ago, they were good enough. But after seeing all those infamous ERO 100 capacitors, some of them in a very questionable condition, i decided for a bigger recap job.
I ordered replacements for the capacitors, most are Mallory 150, just the ones i didn't get the required values are TAD Mustard, Orange Drops 715p and one silver mica capacitor. Add another nearly 30€ for those parts...

Yesterday evening, i could finally start to replace all those capacitors. It was no fun on the PCB because reaching all the necessary spots on the solder side without removing the whole PCB is a bit difficult, especially if you need to take care all the time to not touch the wiring and burn it.
Replacing the capacitors underneath the chasiss also was no fun, because this radio is built to last, not to be repaired. If those capacitors would live forever, the whole radio likely would also do so, but well. Capacitors. So it took a while to replace them all, being confused here and there if i connected it to the correct place, constantly verifying with the schematics, but finally, all the capacitors in the radio were replaced.
There was one shielded capacitor (with 3 wires), those are unobtainium today, so i used a standard unshielded one for now. If i would need to shield it, i could just wrap it in some aluminum foil and connect this to gound. But i don't notice audible problems at the moment.
And while working on this, i destroyed two resisotrs, they easily fall apart.

Another careful check of all the places i worked on - you don't want to make mistakes in a device running off several 100V.

Let's continue with the visual appearance. Just some dirt here and there, i have a nice set of small nozzles for the vacuum cleaner, so removing the dust was easy. The plastic parts were cleaned with glass cleaner, in some places an old toothbrush helped a bit.

The dial glass backlight diffuser is just a sheet of paper wrapped around a cord. The lower end was getting loose, so i glued it back in place, didn't more on this it's still good enough.

Old foam strips that totally dissolved over the decades were replaced by some self-adhesive felt strips.

The dial glass was a bit of a problem. Nordmende attached several self-adhesive things to the painted back and the paint suffered under them. I hope the chemical process meanwhile ended, but i left those stickers in place, i fear removing them also removes the paint. It's not nice, but a new paint job would be really too much, i have to live with this. The painted side was just wiped with a damp cloth, the front side got the usual treatment with glass cleaner (finally it's used for what it's intended for :).

After putting everything together, it just worked on the first try. Sort of.

On FM, i still have fluctuations, and it gets even more crazy. When it's connected directly to the wall outlet, FM nearly doesn't work at all. On the isolation transformer, it mostly works. And it also depends on the orientation of the radio, which is not that surprising with the type of the antenna, but the effect is very dramatic.
I assume the gain control doesn't work, maybe an open resistor. The magic band gives plausible readings, but maybe the control voltage doesn't reach it's destination. The AM bands all work fine.
But that should be a solvable problem. I think i'll give it a new EM84, it doesn't look so healthy, but i'll dive a bit deeper into the schematics first. Not today. I have to move forward nearly 6 decades in technology now :)

But the sound. Wow. This radio has 3 speakers and with this really large box around them, the sound is really awesome. This is how i recall the old tube radios from my childhood.
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Blaupunkt Verona ECL86/PCL86 tube conversion

During repair of this nice tube radio, i found a faulty ECL86. As those are hard to obtain those days and i have plenty of PCL86 around, i decided to consider a conversion.

The ECL86 and PCL86 are nearly the same tubes, just the heater differs. E tubes are commonly used in radios and heated by 6.3V, all heaters are connected in parallel, while P tubes are commonly used in TVs and heated by 300mA, all heaters are connected in series and their heater voltages added to determine the heater winding on the transformer (resulting in 300mA through the whole string).
There are some special cases like the ECC83/12AX7, which provide several possibilities to heat them, like 6.3V, 12.6V and 300mA.

Not the PCL86, this one wants 300mA and to make this current flow, it needs 13-14.5V.

But first, i wanted to try if my diagnosis is correct at all. The PCB layout in this radio allows to easily disconnect the heaters of the up to two (for stereo) ECL86s at least on one side. After some soldering (and using heat shrink tube to connect the unsoldered heater wires for the original 6.3V circuitry), i had a separate heater input for the output tubes. With the lab PSU, i could check if everything works - yes, the problem was gone, so it's really a faulty ECL86.

Now it's time to decide about a way to fix this. There are several options.
- Delon voltage doubler, this would make cutting PCB traces necessary
- Separate transformer just for the PCL86, this would remove 700mA load from the original heater winding, possibly increasing the heater voltage for the other tubes
- Additional transformer in series with the existing heater winding, this would remove just 400mA from the original heater winding, a good compromise

And after digging through my transformers, i found a nice small 2x3V/300mA transformer that looks good for the purpose. With a provisoric setup, i determined 13V heater voltage for the PCL86 at 220V mains voltage, that's the lower end of the range, but enough. And even if it's just rated for 300mA, it doesn't get too warm, so this transformer is usable for the purpose.

While thinking about how to install it in the radio, i had some more complex setup with a PCB and 2 mounting brackets in mind, but then i found this single transformer mounting bracket that perfectly fits the transformer used in the radio. Double-sided adhesive tape and a robust cable tie to fix the small transformer to it, problem solved. I usually don't like such unstable solutions, but a tube radio isn't something you shake too much, so it's robust enough this way.

Wired up everything with the original transformer (it's important to take care about the winding directions of the transformer to add the voltages, not subtract them), i had this new output for the PCL86 heater. Now i just need to bring this to the PCB.
There are 2 free pins on the connector, but they are gaps between primary and secondaries/capacitors of the transformer, so i kept them unconnected. Two pins are connected together at the radio end (one end of the heater and GND), i also left it this way to not introduce hum when i move this connection away from the electronics. So i needed another single connection for the new heater. Not as elegant as the connector for the power supply, but as you don't take this apart frequently, a luster terminal is good enough here.

After putting it together, everything works as expected. The PCL86 needs a bit longer to heat up, a bit more power would have been nice, but for now, it's fine, the radio works nice and i will never again have a problem with the ECL86 getting unobtainable with the time.
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Blaupunkt Verona model 23200 cleanup/repair

From the 3 tube radios i got yesterday, this one was a candidate that i wanted to sell after minor service. But i started to like it and might keep it also.

The main "defect" was tons of dust and traces of usage. Not surprising after more than 50 years. But for this age, the enclosure is in great condition. If you look at this amount of dust, you might get an idea why those radios have this special smell when operated :)

A secondary problem was a significant distortion of audio output after several minutes after power up. Usually, the problem is a leaking coupling capacitor somewhere and the WIMA Durolit used for output pentode decoupling was a very hot candidate for this. Measurement has proven a bias shift when the distortion starts.
But after replacement, the problem wasn't gone, so i measured the current into/out of the output pentode's grid. And to my surprise, the current was flowing out of the tube, so the ECL86 is the problem.

As i didn't have one around and they are a bit hard to obtain those days, i used one of my plenty PCL86 to verify my diagnosis. This required some changes of the heating circuitry and proved to work in a test. So i'm looking for a permanent solution and adding a small 2x3V transformer in series with the normal heating voltage (still used for all the other tubes and the lamp) to feed the PCL86 looks good enough. This way, the normal heater winding is still loaded a bit, otherwise, heater voltage might increase too much for the other tubes, and the PCL86 gets 13V, which is the lower tolerance limit and thus okay. I don't want to add the complexity of constant current DC heating.

Up to now, the additional transformer (which is rated for 300mA, no headroom available) doesn't get too warm, but it will have to stand this for a while until i really build a permanent solution from it.

Another really nice looking vintage radio :)

Don't do this wiring setup without a good isolation transformer! (You should have one anyway before opening any device connected to the main power grid). In addition, i use it to power the radio with 220V because i don't like 230V conversions. And running a 220V tube radio off 230V without some modifications isn't the best idea.
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Roland TR-909 brainswap

This time, it's the legendary TR-909 having a visit in my lab due to a problem. It's from a friend, who usually fixes her stuff on her own, but the problem on this one was too difficult. She replaced all the capacitors already, rebuilt the power supply, did many things that might lead to success, but didn't. So it was time for some real debugging.

The symptoms in a very simple form: crashes after some minutes and during this crash also destroys the whole memory contents. But sometimes, works fine for weeks. The problem started with crashes every 4-5 weeks around a year ago and got worse during the last months.

I was happy that it crashed right after i unpacked it, so i could start taking some measurements. After checking the usual suspects - power supply, CPU clock, rough visual inspection of address/data/control signals, i couldn't find something strange yet. But i noticed those not so optimal connectors the owner added because with all her disassembly/reassembly while trying to fix the problem, the cables fell off, so she made this disconnectable also. Nice, but the choice of connectors isn't optimal for a bunch of critical signals like the CPU's data bus. I didn't want to revert her change, so i used some hot glue to add some amount of stability to all this.

And voila - the machine worked perfectly. Had it running for hours, no problems at all. I was a bit disappointed that it was that easy, but okay, you can't always have the big challenges.

I talked to the owner after this and she reminded me that it sometimes really works for hours without problems, but the problem comes back when it's powered off for a while. While "a while" can be anything between several hours and several weeks. Okay, so there still is a chance to get a challenge :)

I left it off over night, put it outside at 13°C for half an hour in the morning (to make the problem even worst if it's ot a thermal nature) and powered it up. Boom, 2 minutes later, crashed again. I took some measurements and some minutes later again reached the point where it doesn't crash any longer. So i put the switch board (with the CPU and all this digital stuff) into the fridge for half an hour and retried - no crash, works stable.

While analyzing, i also tried some other details, so it's time to summarize the complete problem now.

- If the machine is powered up, it first works great for 1-2 minutes, then crashes. If it crashes, a CPU reset (connecting the /RESET input to GND for a short time) doesn't work, but a power cycle does. After the power cycle, it again crashes some seconds later.
- If it's in "crash mode" and you power cycle it several times or just leave it on for 10 minutes (not recommended if the multiplexer leaves the last state of LEDs on) and power cycle it once again, then it works perfect. For hours. The problem is completely gone.
- After the problem magically disappeared, you can power it off for an hour, power it on again, still no problems, works great. During my analysis, i finally found that waiting 7 hours is a good time, so i can take 3 measurement sessions over a day.
- The problem is completely independent of temperature. And it's also nothing mechanical, because i bent the board while it was running, without any effects, and in addition, the problem is too "systematic" to look like a hairline crack in a solder joint or PCB trace. So even if some people wanted to convince me that just resoldering everything is the way to go (i had to delete this post because the discussion was getting incredibly stupid and was just wasting my time), i insisted on finding the real problem by structured troubleshooting.

So i knew the nature of the problem, i had a schedule when i could take some measurements and i made a plan what i want to measure. The thought behind this was that for some reason, data/address/control lines get disturbed somehow, making the CPU fetch wrong instructions, ending up in chaos. Like Windows.

First to check was the address decoder, a 74138 plus a 7400. I triggered the first channel on the output i wanted to check and had the 3 relevant address bits on the other 3 channels of the scope. Not perfect, but hopefully enough to spot some oddities. Checking all 4 relevant outputs this way revealed no problems.

Next were address, data and control lines, measured on at least RAM and EPROM, i have chosen multiple spots to also find a possible connection problem this way. If i resolder something, i want to know why :) But the signals also looked fine everywhere and sort of plausible up to the crash, sometimes even beyond.

The MODE0 or MODE1 line looked a bit strange, but i just noticed this and didn't think too much about it. Wasn't too suspicious and they seem to be inputs on reset only, then switching to open drain outputs.

While taking all those measurements, i noticed another effect. Right before the crash, the display and LEDs were flashing in a strange way, displaying erratic stuff, getting back to normal for a short period of time, very strange. Because if the CPU crashes because it got some wrong code, it's highly unlikely that it comes back to normal code for a short moment, then going crazy again, getting back to normal... There is something very strange with the behavior of the CPU.

Sure, before unsoldering anything, i replaced the EPROM, it's in a socket, the owner wanted a spare anyway, so i quickly copied it (i even had the original type used by Roland around) and retried with this one - same effects. I also added some 100nF SMD capacitors in some strategic locations on the PCB because Roland uses just electrolytic caps for bypass, which is very uncommon. Also no success with the crashes, but some minor improvements elsewhere.

So at the moment, i don't know the source of the problem 100%. But the overall behavior made me at least 80% sure that the CPU has a problem. Not because it's not the first time i have a very obscure problem caused by a broken CPU, but because all measurements collected so far don't leave much room for a different source of the problem.

Luckily, i had an old PCB from an Epson printer around with a µPD7810HG on it. Even if the TR-909 uses a mask programmed µPD7811G, the internal ROM is not used, so i can use the ROMless version also. Only difference - it's a H version, which can work at a slightly higher clock speed. So no 100% guarantee that the problem is really found if it works with this chip because the behavior might be different in some very small details. Anyway, worth a try. So i unsoldered both brains, added some SIL connector strips as a socket and inserted the 7810. Works. Waiting over night, tried in the morning, works.

So i plugged the old chip back in, crashed. New chip, works. Well, that's a 99% confirmation that i fixed the problem. The owner had some spare NOS 7811s around, they arrived today, so i put one of them into it and put the whole machine back together. First test - works fine. This looks very good so far.

I'll keep this machine here for another week, playing with it over the weekend, not powering it up the whole week, having a final check running it a full day the next weekend. I don't expect the problem to return, but you never know.

So once again, i managed to analyze the problem down to one single component, replaced it and the problem is solved. This is so much more satisfying than "fixing" stuff by guesswork and trying around, blindly resoldering things and so. And all in all, even if it took 3 days to find the problem, it was just around 2-3 hours of real work due to the long pauses between the phases where i could measure something.
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Sony PS-T1 refresh

After i recently fixed the reel to reel tape from my neighbor, she asked me if i could have a look at her record player also, she would love to listen to some vinyl. Sure, so i did. Available budget: 100€.

It was a bit dirty, but thanks to being a direct drive player, no aged rubber parts that need to be replaced. The motor control uses standard parts, so even if something were wrong there, no problem to fix it. It just needed some adjustment for the speeds.

The only problems were the cover hinges being too weak to keep the cover open, the arm lift having no damping at all and the pickup having no cantilever any more.

Cleaning it was most of the work, but after this, it looks much better now.

The cover hinges were quite simple - i took them apart, bent the springs to give them some more force and put them back together. Cover stays open like new now.

The arm lift was a bit more work again, because removing it required disassembly of several parts of the arm mechanism. The fix itself also was simple - just added new 300000cSt silicone oil, there was nothing left in it. Works like new also, perfect drop speed.

And for the pickup, i bought an Ortofon OM 10 and used the rest of the budget for a record brush and a needle brush, so she has a complete vinyl listening survival kit now.

Today, the order arrived and i put everything back together. While this is a really simple semi-automatic player with no big tone arm voodoo (but at least it has an antiskating adjustment), it sounds really great with the new cartridge. And Sony had the great idea to include an adjustment helper on the other side of the pickup, so it was very easy to install and adjust the new cartridge.

Mission accomplished. I'm sure she will be very happy when she comes back from her vacation, it was the first and only record player she bought brand new in 1978 or so and now it's in nearly brand new condition, even her New York sticker is still intact after all the cleaning :)

Another retro device brought to the 21st century.
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Korg RADIAS visual improvement

In my last post, i described how i replaced the crappy LEDs of the Korg RADIAS, which are failing those days. But even after this, i still was not happy with the look. First, the Oscillator 1 sawtooth LED was much brighter and second, there was a lot of bleeding between the LEDs of a group and especially between LEDs and potentiometer backlights.

I first tried some black tape on the sides of the blocks for the potentiometer backlight, but that wasn't enough. It looked a bit better, but still not great. Next, i thought about just painting this whole white panel block black, but first, let's have a closer look for the real reason of the problem.

There are all those light guides in the panel block, transmitting the light of the SMD LEDs to the front panel indicators. They are not isolated against each other or their surroundings. But after a closer inspection, i have seen that the front panel block doesn't reach the PCB, so it's not surprising at all that everything leaks everywhere.

Now what to do? I tried a bit with heat shrink tube, but because they need to be longer than the light guides, i didn't shrink them. This looks very good already. So you can use any tube for this, as long as the wall is thin enough, they need to fit between the hole in the panel block and the light guide for the best effect. On the PCB side, they are long enough to cover the LED, completely isolating everything against each other.

Okay, everything is a bit darker now due to the missing reflection, i even thought a bit about flooding the tube with silver paint first, but maybe that would have gone too far :)

When i look at this now, it looks very nice when i sit in front of the machine, the brightness of the buttons and the LEDs seems to fit, and the bleeding is totally absent, everything has a great contrast now, very pleasing look.

While working on it, i also "upgraded" the board - there was a change in the service manual solving the problem with the Osc1 sawtooth LED being too bright - it got it's own resistor. My unit seems to be old enough (serial number in the mid 400s) to still have the uncorrected board.

Replacing the yellow LEDs still is in the queue for a different day, it's not important at the moment. Now, i really like my RADIAS.

You need to do everything yourself those days...
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Korg RADIAS with fresh LEDs

My new Korg RADIAS had one issue - an annoying difference in brightness between the LEDs. Some buttons were hard to detect as illuminated in direct sunlight from a distance, but mainly, i simply hated the overall look, it looked too broken. After checking the service manual, it didn't look like a problem in the multiplexer, because the very dim LEDs were spread randomly across the mux rows/columns. And the dim LEDs also weren't just those being most frequently used, so the "GLOBAL" button also was very dim and you usually don't need that very often. So also no burn-out of the chips, just aging.

Besides this, the potentiometer illumination in the upper left corner was completely dead on one LED.

I have never seen such a massive LED degradation in gear of the 1980s or so, but i'm aware that you have plenty of problems with modern low quality LEDs. Looks like Korg has chosen exactly such a sort of product here. Searching the web didn't reveal that this was a common problem with the RADIAS, just with the PA2x which seems to be known for LED failures, Korg even replaced the front panel board for free on some units and officially stated that they had a problem with a bunch of LEDs they got from a supplier. I'm a bit disappointed by Korg now. Looks like re-LEDing can get the new recapping...

Well, it doesn't help, i don't think they will replace the front panel board on this 11 year old synth, so i need to fix it myself. Ordered 110 red and 55 yellow Kingbright chips because they came closest to the specs of the original LEDs. I initially just wanted to replace the single broken yellow LED in the upper left corner and do the rest during winter or so. But while replacing the yellow LED, i found out that the resistor for this LED was shorted. After unsoldering it, it wasn't even a faulty resistor, the board was also okay, i soldered the resistor back in place and everything was fine. Maybe a solder whisker shorted it. Very strange... There was some more solder work done on this machine already, from what i've seen. Anyway, the new LED was brighter than the others, so i replaced the other 2 in this row also. So the two potentiometers in the upper left are a bit brighter now, but this problem is way less annoying than the brightness difference of all teh red LEDs. At the moment.

But while i was on it, i replaced all the 104 red LEDs and thought, i'll also replace the yellow ones if i still have some motivation left - which surprisingly didn't happen after 3 hours of replacing 0805 SMD LEDs :)

Anyway - everything looks very nice again now, uniform brightness, just the oscillator 1 sawtooth LED is much brighter. It has a separate resistor, but according to the schematics, it's also 330Ω like the rest, so maybe the previous owner also changed the resistor when he replaced the LED by a green one. Will check this next time i open the unit to replace the remaining 47 yellow LEDs.
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