Repairing a Panasonic DMR-EX77 PVR

After nearly 10 years of continuous operation, my Panasonic DMR-EX77 PVR died. The display went out and it didn't respond to the remote control. Sometimes this type of failure is just a fuse which has been stressed too many times by the inrush current produced when charging large capacitors in the power supply, but not this time.

Initial Failures

It was immediately obvious that the electrolytic capacitors had failed. I could see their bulging tops which are designed to blow out in the event of failure. All electrolytic capacitors have an expected lifetime which is mainly due to the wet electrolyte drying out at which point the capacitance drops and the equivalent series resistance goes up. It seems to be a common problem with these PVRs.

On this product, the capacitors which needed replacing were identified as C1401, C1402, C4056 & C1557. The service manual explains the disassembly procedure and has the schematic and bill of materials which makes finding replacements easy.

I ordered spares from RS Components and disassembled the PVR carefully. Since many different types of screws are used, it is a good idea to stick them down to a sheet of paper with a note of which screw was used in which location so that reassembly is easy

The PCBs are well designed with labelling on both sides and marking of the negative side of the capacitors, which makes replacement easy. These are through-hole parts so a hot air station isn't necessary, just a decent soldering iron and some desolder braid or a solder sucker to get the hole clear again after removal.

More Capacitor Failures

That repair gave me three more years of use before another problem became apparent. The DVD drive would not read any disks, neither ones that this machine had burned nor pre-recorded ones, and lit the "disk present" indicator even when empty. I found that some more capacitors had bulged and needed replacement: C1271, C1607, C7401. And while I was replacing C1271 it seemed prudent to replace C1270 since it is the same type. In the meantime I had acquired a scope and signal generator which allowed me to test the removed caps for ESR using Alan Wolke's method. I set the signal generator to wave type CMOS which makes the trough level 0V, and the frequency 10kHz. The difference between a good and bad capacitor can clearly be seen in how the square wave collapses when applied to the capacitor. These GIFs show the square wave on the scope before and after the capacitor is applied in parallel.

New capacitor - waveform completely collapses apart from inductive spikes.

Bad capacitor, RC charging curve apparent.

ESR Meters

The signal generator and scope method only makes sense if that's what you already have. If not, and you're going to be doing a lot of equipment servicing, it makes sense to get a dedicated instrument. SDG and MJLorton say that the Peak ESR70 meter is highly recommended, and eevblog says that the DER EE DE-5000 meter is good, whilst Defpom likes the East Tester ET4401 bench LCR meter.

When you've measured the ESR, you need to compare it with the expected value for that capacitance and voltage rating to see if it's out of spec. The meter itself cannot produce a good/bad indicator because it doesn't know what component you're testing.

The ESR should be below the value in this table for it's voltage rating and capacitance value.

DVD Drive Replacement

Although this improved the PVR in that the frequent disk parking noise has disappeared, unfortunately the DVD drive still did not work. I tried to find the mechanical disk detection switch but this is either very well hidden or done by the firmware when the laser head attempts to locate the disk so I could not repair my unit this way. Instead, I took a risk of purchasing a known faulty unit off ebay in the hope that the DVD drive from that would still work.

When the ebay unit arrived, I found that it too suffered from the same electrolytic capacitor failures, so I replaced those because I wanted to see if the DVD drive worked before transferring it into my machine. Some of the failures of those capacitors are really obvious and others don't take a lot of experience to detect.

Replacing a handful of capacitors was all that was needed to bring that old machine back to life. I could then see that the DVD drive worked correctly in terms of reading and even writing disks; there were some old TV programmes on the HDD that could be copied as a test. The dates on those showed that this machine had been sitting around for some years unused.

The DVD drive is paired with it's control board so the two have to be moved together into a new machine - the manual is clear on this. That means that the 4 ribbon cables between the two are left connected, and the only ribbon cable that needs to be detached is the one into the HDD.

I wanted to keep my HDD because I had a lot of programmes on it that I wanted to keep, which was a big part of the motivation behind doing this repair instead of buying a new one. In any case, there doesn't seem to be a current version of this on the market, Panasonic's last similar model to upgrade to is from 6 years ago; newer models don't record to DVD.

Transplanting the DVD drive and control board across is actually very simple and can be done with only a screwdriver. The front and back panels can remain fitted and the PCBs stay in place. For safety, you need to disconnect the mains cable first before removing the top cover, but then the DVD drive and control board is easily accessible.

Testing

The PVR retained all the old settings apart from the timers which were easily re-entered. I tested the DVD drive read and write functionality and apart from a slight oversensitivity to the tray position it is as good as new. Sometimes it will only pop the draw out for a second before pulling it back in again but I can live with that.

Circuit Design Lessons

Let's look at the schematic to see what those capacitors are doing.

They are on the output side of switched-mode PSU circuits which need to be low ESR capacitors, and the highest temperature rating you can find for the longest life.

I replaced them with Panasonic brand through a reputable retailer. It is a shame that for a small fraction of the cost of the unit, Panasonic had compromised the reliability of these machines and shows the importance of using low ESR high quality capacitors in these circuit positions.

Summary of Replacement Capacitors

If you've got a machine with the same problems, you might also find repair kits of capacitors on ebay.

Note: This post has been languishing in draft since 2021.