Yashica YC-64

My Yashica YC-64 is the non-French version with composite output. The French version had RGB output instead.

This was my first computer, and it just turned 33 years old. It still works, but I figure it’s high time to replace the capacitors anyway.

Opening the case

Getting the case open requires the unscrewing of two screws on the bottom, towards the back.

Then there are three tabs where the red upper cover needs to be pried outward from the bottom part of the case: One tab in the middle of the back, and one tab in the middle of each of the sides.

Once these are loose, the back side of the red cover can be gently lifted up a few centimeters, and pulled towards the user or more precisely towards the front of the machine.

Note that the tab on the back, and the tabs on either side, are shallow, probably one millimeter or less. You should be able to pry them out with a flathead screwdriver without worrying too much about breaking anything.

The three tabs on the front are significantly longer, and the plastic is brittle. It is not necessary to use force on these, as they slide right out after you have loosened the three other tabs. If you do use force on them, they will probably snap right off.

After the cover has been lifted off, the two-pin connector for the power LED can be pulled from its socket on the main board before you can put the cover aside.

The keyboard has two flat-flex connectors that go to sockets on the motherboard. First unlock the sockets, then gently pull out the flat-flex. After that, the keyboard can be lifted right off.

Overview

The power supply PCB is secured with one screw, and two more screws hold down the (video?) daughterboard.

This is how it looks. Time to locate the caps.

Power supply

The YC-64 is rated at 20W power consumption, and has an integrated power supply that takes 220V at 50Hz. You plug a C7 (so-called figure-8) lead into the C8 socket, flick the switch on the side of the case, and off you go.

The power goes straight to the power switch, and to a transformer. The transformer has five output wires which connects to the power supply board at the right hand side of the case.

Among other things, we can see a discrete bridge rectifier. To the left is C4, the second biggest capacitor in the computer. At the lower right, C3. Top middle is the king of the capacitors, C2.

To the left of C2, you can see C8 and C9 with some shmoo on top of them.

A closer look at the other bridge rectifier, this one is integrated. I think maybe this package is called RC-2?

Again you can see the shmoo on top of C8 and C9. I wonder which one of them is guilty. (If any? More on that later.)

C6 hides behind the heat sink for U2, which is an AN7812 regulator.

You can also see the tops of two capacitors that sit on the main board; C35 and C5.

In the foreground, C4, and of course the lovely C8 and C9 with the leakage on them. (Or is it? More on that later).

The four outputs from the PSU are marked GND, -12V, +12V, and +5V.

Main board

I could see some date codes that indicated 1983 and 1984.

Some pictures of the caps on the main board.

Near the power header. From left to right: C35, C5, and C18.

A different angle.

 

Yet another angle.

Between the ROM cartridge slot and the back panel, from left to right: Composite video out, C87, mono audio out, C3, C79, C78 and C77.

To the right is RY1 the relay. I think it has something to do with controlling the tape drive.

This one is close to the Zilog Z8400A PS Z80A CPU (date code 8342).

I nearly missed C7, which is hiding behind one of the keyboard connectors, right between it, the tape drive plug, the printer port, and the relay.

The last two main board capacitors are pictured together with the daughter board.

Daughter board

I think this daughter board, marked EMC-PX3607, might be a video card. It has a 74LS 04 hex inverter, and a LVA510 IC.

No capacitor designations are silkscreened onto the PCB, which makes it a little bit more difficult to keep track of what goes where, but some pictures should help.

Here is the daugherboard from the opposite angle.

Notice the two caps on the main board on this picture, from left to right: C76 and C86.

The electrolytic capacitors

Every single one of the original electrolytic caps in the YC-64 are rated for 85°C.

A few of the capacitors in the machine are light blue, and branded Shoei. From what I could find on the web, Shoei is a Japanese manufacturer who makes caps of decent or even good quality.

Most are dark blue Panasonic/Matsushita caps with a triangular logo, CE, SU, and Japan. These seem to be regarded as top quality capacitors.

On my power board, C8 or C9 had leaked, I’m guessing C9. (Or not. See below.)

Curiously, all the caps on the power board seemed to measure more or less OK, but I’m going to replace them all anyway. 33 years is enough.

I did not open the RF modulator to look for caps yet, but I guess I should. I’ll update the page if/when I do. 

Here is a list of all the electrolytics I could spot. 

Power board

  • C2: 4700μF 16V
  • C3: 47μF 50V
  • C4: 3300μF 25V
  • C6: 1μF 50V
  • C8: 220μF 35V
  • C9: 1μF 50V

Main board

  • C3: 10μF 25V
  • C5: 100μF 25V
  • C6: 10μF 25V
  • C7: 10μF 25V
  • C18: 10μF 25V
  • C35: 470μF 16V
  • C36: 22μF 16V
  • C76: 10μF 25V
  • C77: 10μF 25V
  • C78: 10μF 25V
  • C79: 10μF 25V
  • C86: 33μF 16V
  • C87:100μF 16V

Daughterboard

  • 1 x 470μF 6.3V
  • 3 x 100μF 10V
  • 2 x 33μF 10V
  • 3 x 0.47μF 50V

Decapping the PSU

Since there was leakage, I decided to pull the old caps from the PSU right away, even if I don’t have any replacements yet.

The two largest caps, C2 and C4, were attached to the board with some glue that had turned brittle and crusty over the years. I pried the caps from the glue, and they snapped loose pretty easily. After desoldering the caps, I scraped the glue away with a pointy metal tool, and cleaned up the rest with some isopropyl.

After removing all the caps, I cleaned up the pads with some solder wick.

This is how the PSU board looks right now.

I don’t want to wait for ages for capacitors of questionable quality from Ebay, so I ordered a full set of replacement caps from Digikey. The capacitors themselves cost less than the postage, all in all a fair deal.

I’ll update this post when the replacements arrive.

Stage 2 – hey, wait a minute

I had already desoldered the PSU capacitors from the Yashica YC-64 and measured them with my cheapo LCR-T4 tester. To my surprise, they all measured decently. While waiting for the replacements, I started to think.

The gunk on top of C8 and C9 was brittle and dark. It didn’t look like other leakage I’d seen. Maybe it wasn’t leakage after all, but just more of the same dried out glue stuff that held C2 and C4 to the PCB. Unfortunately, I had already cleaned up the glue, so I had nothing to compare it to.

When the replacements came, I measured them as well, and to be honest I could not see any significant difference between either of the old caps and their corresponding replacement.

Conclusion (all of a sudden)

I ended up soldering the new caps into the PSU board anyway, but I no longer feel like replacing all the capacitors in the machine just for the sake of it.

If any of the capacitors had measured bad, I would have looked at it differently. Same if there had been any actual leaks, but now I’ve begun to doubt that.

So now I think that a full recap would be too drastic. If it ain’t broke don’t fix it.

The capacitor plague of the noughties gave electrolytic capacitors a reputation for being unreliable, and it conditioned many of us to replace caps as a knee-jerk response to anything really, but there’s no way a machine manufactured in the 1980s could have been affected by that.

I think I’m just going to clean the keyboard, play around with the machine a bit, and try to check on the capacitors every decade or so from now.

Disassembling the keyboard

Most keys have two springs under it. There is one wide spring that makes the key bounce back after you release it. The spacebar is the only exception; it has two of the wide springs.

Then there is a thinner spring that sits in the middle of the wide one. The thin spring is the actuator that makes a keypress register when you depress a key.

It is possible to remove most keycaps by pulling them straight out, but it’s not something I can recommend. Each keycap has two plastic tabs that holds the cap down to the board, and if the plastic has become brittle, they might conceivably snap.

Besides, the only way to remove the left shift, space, and enter key without damaging them, is to unscrew the back plate first. These keys are rather long, and they are attached to metal stabilizer bars that ensure that the keys depress evenly.

On the Yashica, these stabilizer bars are really rugged, and they need to be removed before pulling the keycaps. If you don’t do this, you will damage the keycap.

Unscrew all the little screws that held the back plate in place, and everything will come apart nicely. Slide the three stabilizer bars for the left shift, space and enter keys sideways, and pull them out.

Now get the thin springs out and put them in a container.

Use a small screwdriver or other pointy object to carefully push the keycaps out of the black plastic frame. If you have long or pointy nails, you might even do without any tools for this job.

Use some soap and lukewarm water to clean up the keys. A soft brush will also help get the grime off. Afterwards, rinse with clean water and dry all the parts thoroughly.

You might want to give all the metal springs a thin coating with WD-40 or some other oil after cleaning, to protect them from corrosion.

Reassembling the keyboard

Putting the keyboard back together was pretty easy once I figured out the right way to do it. Since this requires everything to be done in exactly the right order, I made a list. Here’s how to do it:

  1. Insert all the keycaps into the black plastic frame, with one of the wide springs under each of the keys. Except for the spacebar, which takes two wide springs.
  2. Insert the stabilizer bars for the left shift, space and enter keys.
  3. Double check that all of the keys are in the correct position: It is not easy to change them around later. A single error brings a risk of having to disassemble everything and start from scratch.
  4. Lay the frame on a flat surface, with the keycaps facing down.
  5. Put one thin spring into each keycap. The pointy end of the spring should point up towards you. The flat end of the spring goes down into the keycap. Warning: If you get one of the springs the wrong way around, that key will start to autofire the moment you switch the computer on, and you might have to disassemble everything and start from scratch to get it working properly.
  6. Lay the metal metal backplate with the membranes on top of the frame. The frame has some plastic studs that should fit with holes in the backplate.
  7. Use a precision screwdriver to fasten the backplate to the frame.

Now that the keyboard is reassembled, you can open the two keyboard connectors on the motherboard, and insert the flat flex cables. Insertion requires some force, and there is little space, so you will need some patience and nimble fingers for this.

Lock the keyboard connectors. If all went well, the computer should be ready for testing now. If all the keys are working and none are autofiring, you can put the top cover back on. Rremember to plug in the power LED when you put the top cover on.

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