Two repairs

So I recently did two more repairs although neither was terribly interesting and I didn't take enough photos. Still worth a quick entry though.

Sharp JH1600E Solar Inverter

I am a member of a small sailing club and a few years ago the government were offering what was probably a way to generous incentive for people to install solar panels. They would pay 66c per kW.h for the electricity you produce where here in Australia we pay around 25c when buying the same power off the grid. I signed up the sailing club and the unit paid itself of in around 18 months and then continued to pay healthy dividends for the next few years.

The scheme ended last December and now they only pay 6c per kW.h for electricity produce but we now have the option of changing to a nett metering system so we can consume the power we produce and save 25c per kW.h consumed.

The inverter also went out of warranty at about the same time and then within a few weeks of this it failed. We got quotes of between $350 and $450 for repairs but when we earn so little from the system this becomes a major outlay. Interestingly these units seem to fail often and there are a lot of broken ones on ebay. The company that repairs them seems to be doing a good trade.

I decide to have a shot at it. The concern is if I get it wrong the unit could catch fire and damage the sailing club (which is empty most of the week so nobody would notice until it was too late). Trusting that the circuit protection will save the club I had a go anyway.

There is a service manual for the unit here but it only contains a block diagram and no schematics. As you'd the service manual shows a DC boost converter (to level the DC voltage coming in off the solar array) feeding a full-bridge converter. There is loads of EMI protection (both DC and AC) plus transient protection. The system can monitor the DC voltage, AC voltage as well as the power being pushed back onto the grid. There are loads of temperature sensors so it presumeably can shut itself off in case of trouble.

The unit failed after a thunderstorm. The AC breaker had opened and the system reported an F01 error indicating it didn't have a grid connection. Restoring the breaker didn't fix it so I dug deeper and found there is a 20A fast blow HRC fuse between the breaker and the AC output of the unit and this had opened. I ordered some of these, replaced the fuse and the unit then detected the grid and began its sequence to connect to the grid. As it got towards the end it made a loud pop, opened the AC breaker and destroyed the fuse again.

My theory was that a MOSFET in the full bridge converter had failed short either due to heat (it has been *really* hot here last summer with days as high as 47C) or line transient.

Taking the unit apart wasn't much fun. You have to unscrew the end caps, undo the bolts holding the lower section on, undo the screws holding the outer extrusion on and lift it off. Then you are looking at the bottom of the board so you have to unscrew the heatsinks from the back panel and lift the board out (which is insanely heavy).

There is a digital board on top that interfaces with button, generates the warning lights and has the LED numerical display. The main board has a small daughter board that contains the controller for the main inverter but this is hard to access and soldered in. The main board is conformally coated also which made just getting continuity readings difficult.

After some poking around with a DMM I found one MOSFET that was a dead-short. Even though it was screwed to a heatsink I had to desolder the four MOSFETs on that heatsink and unscrew the heatsink from the board to get it out as there was no clearance to get a screwdriver onto the transistor.

Here you can see the heatsink desoldered and unscrewed. In the forground are the heatsinks for the AC EMI, the DC boost converter and .. actually I'm not sure what the other one is. Lots of fist-sized transformers for filtering. There are some massive capacitors for the DC coming in (left side) and loads of MOVs inside little sleeves complete with thermal fuses.

The transistor was a SPW47N65C3 from Infineon. Unfortunately it wasn't carried by RS or Element14 so I had to get one from Xon (via ebay) for an eye-watering $28.

There wasn't really a safe way to test this and in fact it was going to be difficult trouble shooting the unit unless I partially re-assembled it. I couldn't find anything else that looked suspicious so I took a chance and put it back together with the new transistor. Sure enough it worked fine (with a new fuse) so the repair was a success.

The only annoying thing was I somehow lost a space for the selection button and haven't been able to get that working again. Given how much effort it takes to disassmble and reassemble the device I decided I would fix that another day.

DENON DCM-270 Five Disk CD Player

This was my sister's CD player from the 90s that started to skip and have problems playing CDs. It got worse progressively until it wouldn't play CDs at all anymore. When I tested it the unit would spin up the CD, click the focus a few times and then stop and move on. It couldn't even read the table of contents.

I found a service manual for the unit complete with schematics and exploded diagrams. The schematic was truly awful - lines everywhere and the lines were so illogically laid out it made the whole thing really hard to follow. The device uses a bunch of special purpose ICs for controlling the motors, processing the signals from the CD mechanism and decoding the audio. Thankfully the manual showed internal block diagrams for these so you have some clue what is going on.

First off I checked the power supply and inspected the board. There was a big hots spot around the -26V supply and signs of corrosion near a capacitor. The capacitor measure ok in circuit and the voltages looked ok so I moved on (more on this later).

From some googling I discovered that the usual failure mode of these devices is the laser sled. They use a pretty common Sony KSS213C mechanism and these are available all over ebay. I ordered a replacement and parked the troubleshooting.

When the replacement mechanism failed I found that it worked worse than the original! The replacement mechanism didn't even spin the disk where the original at least tried to read.  I checked the motors and they were fine (I could make them go with my bench supply).

I started looking at the Sony chipset used to drive the CD mechanism and decode the signals. Meanwhile I was trying to understand how the mechanism worked and understand what was going on.

A Quick CD Player Primer

So the CD mechanism had connections for:

• The disc motor
• The sled motor (to move the laser)
• Laser focus
• Laser power

Then it had a bunch of other connections labeled A-E. It turns out these are opto-diodes and the way they work is quite clever. The opto-diodes are arranged as follows (see below - this was stolen from here ).

                     |<--- ----="" array="" photodiode="">|
                               +---+---+
   ---------_________ +---+  +-| A | B |-+  +---+
   Track--->          | E |- | +---+---+ | -| F | ________
                      +---+  | | C | D | |  +---+         ---------
                        |    | +---+---+ |    |           Track--->
                  /|    |    |   |   |   |    |
    Focus       / +|----|----+---|---+   |    |    
    Error o---<    |    |    |   |   *   |    |    |\
    (A+D)-(B+C) \ -|----|----|---+-------+    +----|+ \       Tracking
                  \|    |    |   *       |         |    >---o Error
              FE Amp    +--------------------------|- /       (E-F)
                             |           |         |/ TE Amp
    * Since the photodiodes  |           |           
      are current sources,   |           |         |\
      the simple junctions   |           +---------|+ \       Data Out
      implement a sum.       |                     |    >---o RF Test Point
                             +---------------------|- /       (A+B+C+D)
    All Amps: current mode inputs.                 |/ DO Amp

So there are 6 opto-diodes in total (A-F). The four centre diodes are used both the read the disk and to keep the laser focused on the disk. The laser optics cause the laser spot to become elliptical along one or other diagonal depending on if the focus is to close or too far. By comparing the power at A and D with the power and B and C you can tell if one diagonal has more brightness than the other. This tells you if you need to focus in or out.

The focus mechanism has to be accurate to within 1um (!!) and this is while the disk is moving at 500RPM and could even be a little warped (spin unevenly).

The E and F sensors are placed such that one is inward of the current track and one outward. By comparing the power detected by E an F the mechanism keeps the track centred. The track is a continuous spiral over the whole CD. The data on the disk is read by summing the four centre detector outputs (A-D).  

The encoding of the disk data is such that by tracking the signal edges you can detect the speed of rotation. The system uses this to manage the rotation of the CD and keep it at a fixed velocity (using a PLL).

CD Chipset and Debugging

The CD player uses a Sony CXA1782BQ for the RF signal processing and CD mechanism servo control and a Sony CXD2500BQ to handle the signal processing of the CD data.

I figured out that the CXA has a FOK (Focus Ok) output and with the original mechanism in the CD player this would go high while it tries to read the CD but with the new mechanism it didn't.

I assumed the mechanism was broken and worked with the original. I was trying to understand what was going wrong as clearly the motors were working and I could hear the laser focus operating.

I found there was an RF_O signal that is effectively the processed data signal coming off the CD. This is supposed to be very closely synchronized and you should be able to measure the jitter in the CD speed control by triggering the oscilloscope of either the positive or negative slope and viewing an 'eye' diagram of the signal. In the case of this CD player the signal was a total mess. You could see the waveform get shorter as it speed up the disk to try and lock onto the signal but it was like the waveform was moving up and down (i.e ripples at the top then bottom).

I looked at the FE (Focus Error) output but this too was pretty messy and it wasn't clear if it was failing to focus, failing to track or what. I wasn't sure if the problem was the mechanism or something else.

Mechanism

So I thought maybe the laser power was too low and this was why it couldn't get a good signal. I read that you can't really adjust the power without a power meter since if you even slightly over power the laser diode it will fail in a matter of hours. I took a risk and turned up the power on the new mechanism but it responded exactly as before (no focus).

Doing some more reading I found out that new mechanisms are sold with a solder bridge across a critical spot on the board. This is to prevent damage from electrostatic discharge through the board during transit. I found this guide explaining where the bridge was located and how to remove it. With some excitement I removed the bridge on the new unit and installed it in the CD player. Unfortunately while it now spins the disk it still didn't read the CD. Basically it was the same as the original mechanism.

There is a FE (Focus Error) bias adjustment pot on the board and I tried changing this. I noticed that if I adjusted this I could make the problem worse but not better. When it was worse the player wouldn't spin the disk or would give up reading the TOC sooner.

Back to the Power Supply

Removing and re-fitting the laser unit is quite difficult and requires the removal of a number of connectors from the main PCB and removal of the CD loading mechanism from the chassis (as you can only access the laser unit from the bottom). I had done it a few times now and at some point I must have left the power connector for the front panel off and when I re-connected it the VFD didn't come on.

I decided I needed to tackle this. The VFD supply is powered by a separate transformer tap but is referenced to a -28V supply generated from another transformer tap. After a bit of poking around I realized this was missing. The -28V supply is generated with a pretty dumb linear supply consisting of a PNP transistor and a 27V zener diode. It turned out the zener was a dead short in both directions so I replaced it. This brought the supply back and now the VFD worked again.

The CD reading still didn't work however. While looking at the RF_O signal I noticed that even when it wasn't reading a CD there was a bit of noise. The noise looked like a 2MHz signal but only at about 200mV. At some point it dawned on me that this is actually a pretty significant proportion of the RF signal amplitude and is probably why the signal is so messed up.

I then proceeded to go hunting for the source of this noise. I think in part I ignored this before as I assume it was just bad grounding but even with the ground point closer to the power supply section the noise is loud and clear.

The CD Player has +14V, -14V, 8V which is generated by a monolithic regulator and 5V which is generated from the 8V by the motor driver chip and the -28V I fixed above.

The corroded cap in the image above was part of the -28V circuit so I decided I would replace it anyway. I pulled it out of circuit and the ESR measurement was through the roof at over 10K plus the capacitance was way off what it should be. I replaced it but the noise was still there (maybe a little less).

I found the 8V signal was most noisy and the 5V signal was also pretty noisy. There is a transistor controlled by the motor driver that generates the 5V line and this had an output capacitor. This tested Ok in circuit but I replaced it anyway. When I pulled it out of circuit the capacitor measured way off spec and again a very high ESR.

Replacing this capacitor fixed it! The RF_O signal now looked as it should and the CD player played CDs! I stepped through tracks and left it running for about an hour and all was well. I replaced a few more capacitors that looked suspicious but overall this was the extent of the repair.

So at least I learned a few things about CD players. I now have a spare Sony CD mechanism too.