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Time for another repair adventure...
I have a Panasonic DVD recorder/player with a hard disk drive, Model DMR-EH57, purchased around 2008. The recorder isn’t used that much and normally sits plugged in but turned off under our second television in the master bedroom.
Figure 1. Panasonic Model DMR-EH57 DVD Recorder Player Front Panel
When we finally had cause to use the unit to play a DVD the display was lifeless (no clock) and the unit wouldn’t turn on. This was somewhat inconvenient but not a total disaster. We have a second DVD player (also a Panasonic but a more recent dual tuner model) in the lounge.
Servicing should start with the obvious. The power supply outlet is good, and so is the main cord and plug. The power switch on the recorder (front panel lower left hand side) makes an audible click when pressed so this is at least mechanically functional.
The dead recorder isn’t all that old (manufactured in 2007 according to the rear label), has been subjected to intermittent use and no abuse or harsh treatment. Maybe it can be repaired for less than the cost of a replacement?
Figure 2. Panasonic Model DMR-EH57 DVD Recorder Player Rear Panel
With the power disconnected I removed the five cover screws and gently slid the cover back and up. Note that you should take care to photograph every screw that you remove because there are a whole bunch of different screws, most with similar threads and lengths but with different heads, and placing the correct screws is the right hole will be difficult on reassembly, even with a service manual on hand.
Figure 3. Cover Removed with Functional Blocks Labelled
Inspecting the top of the power supply and main board I can see three aluminium electrolytic capacitors that have bulged tops. These components will be buggered. There were no other signs of obvious damage or over-heating, other than slight heat darkening on the upper surface of the power supply and main phenolic boards. These marks are typical of long term typical heating from Integrated Circuits (ICs) and did not raise any alarm bells.
Figure 4. Bulged Aluminium Electrolytic Capacitors on PSU Board
Figure 5. Bulged Aluminium Electrolytic Capacitor on Main Board
There are a whole bunch of board-to-board and board-to-wire connectors (some are difficult to see) but they all appeared to be securely engaged.
There was minimal dust on the boards or rear mounted cooling fan, consistent with minimal use in a relatively clean environment.
The 2 Amp slow blow cartridge fuse on the power supply board is securely connected and looks to be intact (confirmed with an Ohm meter). So we are getting power to the recorder and it hasn’t suffered a catastrophic fault resulting in fuse failure.
The power supply is an isolating switch mode design. I can see the high voltage designated area separation on the board silk screen, the step-down transformer, and the switching module (this has too many leads to be just a MOSFET or BJT). When the board was eventually removed I could also identify an opto-isolator IC on the underside providing feedback to the switching module from the low voltage side of the supply.
The unit has a two pin non-polarized mains plug so the case cannot be connected to earth. The case is therefore maintained at mains Neutral (within a diode drop) by the mains rectifier which is typical of devices that connect to other earthed equipment through shielded cables as this configuration reduces the potential for earth loops and associated mains hum.
Figure 6. Two Pin Mains Plug (non-polarized at DVD player)
There is nothing obviously wrong other than the three bulged electrolytic capacitors. Were these caused by some other failure or were they causal?
Non-solid aluminium electrolytic capacitors are based on wet cell chemistry and they have a limited service life subject to temperature and duty. Don’t expect them to last for ever. Some service agents will automatically replace all electrolytic capacitors by default when servicing old equipment but there are a large number of these on the main board, the majority of which appear to be completely serviceable, and replacing them all will make the repair uneconomical.
You may also have heard of capacitor plague. This is an issue with the formulation of electrolytes by some capacitor manufacturers circa 2000 that has been linked to premature capacitor failure. The three failed capacitors were all manufactured by ELNA with a 105°C temperature rating. So the capacitor failures may have occurred all by themselves.
The next question is that, given the capacitors have failed, could these have caused the no power problem?
In order to answer this question we must first consider how the capacitors are being used and how they have failed.
The rated electrical value of these components (100’s of micro Farads at up to 16 Volts) and their locations on the boards suggests that their primary function is to reduce ripple in DC power supply rails as part of an RC or LC filter.
They clearly haven’t failed catastrophically as a dead short. If they had then the mains fuse may have blown, the capacitor bodies might show signs of destructive damage such as case splitting perhaps with plates and insulation blown all over the place, and perhaps evidence of severe local heating ranging from blackening to a horrible ‘that’s been burnt’ smell.
Given that the capacitors haven’t catastrophically failed as a dead short (and they haven’t – confirmed after removal) then the likely failure is a reduction in capacitance value and an increase in Equivalent Series Resistance (ESR). In a ripple filter design these two effects will result in increased power supply ripple and increased power supply droop under transient load, both of which may be sufficient to prevent or interrupt the operation of down stream circuitry.
While switch mode power supplies can be stand-alone it is not uncommon to incorporate feedback from the load to the supply to shut the supply down under fault conditions or in sleep modes. So the failed capacitors may be causing the power supply fault.
Okay, on with the repair. With the power still removed we need to get at the underside of the main and power supply boards to replace the failed capacitors. This means taking anti-static precautions because the undersides of the boards have all manner of Surface Mounted Devices (SMDs) including large integrated circuits on them. Ground the unit case, yourself and your test bench. Any mains powered tools such as soldering irons should also be grounded.
With the top case already removed disassembly continues with removing the plastic front panel. This is clipped in place by tangs in the chassis at the sides and along the bottom. Gently ease the clips away from the case and press the panel straight forward.
Remove the power supply board (three screws through the board into the chassis and one by mains plug on the back panel). Remember to photograph each screw before removing it. Unplug the ribbed power supply connector (front right of the board) and the board should lift clear.
Unfortunately we also need to remove the main board to effect the repair and this means pretty much removing everything from in the chassis. This starts with the hard disk drive (HDD) (four screws, and two connectors at the front of the drive). Lift clear. Do not attempt to open or adjust the hard drive or the optical drive and leave them on their metal cradles. These are sealed units and opening them up, even just for interest, is inviting disaster.
Now we can remove the RAM/Digital board immediately between the HDD and optical drive. There are four corner screws. The board has three board to board connectors on the underside, three flexible filament cables to the optical drive unit, and a flexible cable to the HDD. Gently lift the board upward and fold it gently, along with the HDD connectors on top of the optical drive unit.
Now remove the optical drive assembly (three screws). Note that there is a metal lug that sits over the right hand front chassis mount. With the optical drive removed it will come free and it you don’t know where it came from then you will have a spare bit rattling around in the case with no obvious home.
We can now remove the HDMI board (centre rear). It is fixed with a screw through the rear panel by the HDMI connector and supported by a metal bracket. The board has a board-to-board connector underneath to the main board. Lift it gently straight upwards from the connector and bracket.
There are three further sub-module boards and a metal bracket that we need to remove at the front of the unit, and the HDMI support bracket near the middle of the left hand edge of the board.
One of the front panel boards is hard-wired by a ribbon cable to the main board but it still needs to be removed from its bracket in order to access all of the main board chassis mounting screws.
With all of the boards removed we can now get at all of the mounting screws for the main board, including one at the front left hand corner which was previously concealed by other boards and a metal bracket above. Remove the rear panel connector mounting screws and all of the board to chassis screws. Gently move the main board forward and up, free of the chassis. If there is any resistance then you have missed a rear panel or chassis mounting screw.
I completed a number of Ohm meter checks on the PSU board of inductors, ceramic capacitors, diodes and the main rectifier. Nothing was amiss. Be aware that the main rectification capacitor (the largest one rated at 450 V) may have an appreciable residual charge and this can be a significant shock hazard. It does not appear to have a parallel bleed resistor across it so make sure it is discharged through a moderate value resistor before working on the board.
There are any awful lot of SMD components on the underside of the main board. I strongly recommend that you don’t go probing around with your multimeter willy-nilly because you are likely to induce a fault through inadvertent current injection or reverse bias. Handle the board with care from the edges.
Find some suitable replacement components. These should have at least the original voltage specification and at least the original capacitance. We can increase these marginally without any undue effect but note that increased working voltage and/or smaller physical size are likely to increase ESR. Electrolytic capacitors will generally have a tolerance of +/-20% and in my experience the actual value tends to be low.
The three capacitors that I needed to replace were:
PSU: C1401: 1,200uF, 6.3 V. Replaced with 2,200uF, 16 V
C1402 : 330 uF, 6.3 V. Replaced with 470 uF, 16 V
Main Board: C7401: 470 uF, 16 V. Identical replacement.
Note that the boards in this unit are all PBF (lead free). This means that the solder will have a much higher melting temperature than lead based solder and will not flow to form a bright shiny surface. Use an appropriate flux and lead free solder for removing and replacing the defective capacitors. The boards are single sided which makes removing and replacing leaded components relatively straight forward using solder wick. Remember to observe electrolytic capacitor polarity during replacement.
Now reassemble everything in reverse order to disassembly. About this time I discovered a lug floating around loose in the chassis with no idea where it had come from (mentioned above). After too much time spent considering unsuccessful options I decided to lash out $15 for an on-line service manual. The errant lug sits on the chassis under the front right hand mounting screw of the optical drive assembly.
With the capacitors out of circuit we can now do some tests on them to determine the extent of failure. This requires a signal generator and an oscilloscope, or more advanced instruments such as an LCR bridge.
Set up a square wave signal generator with a 50 Ohm output impedance with a 1 Volt ppk square wave at about 100 kHz. Connect the capacitor under test across the signal generator output and attach an oscilloscope across the capacitor. The simplified model of what we are testing is shown in Figure 7. This circuit is simple enough for hand calculation of the transient response in the quasi steady-state (after a long time to allow C1 to approach its final average state of charge).
Figure 7. Test Setup
Note that Resr and Rleakage are internal to the capacitor.
We can’t actually attach a test probe to Node 3.
Figure 8. Input V1
100 kHz 1 Vppk ground referenced square wave without test capacitor in circuit.
Figure 9. Output V2 with 1,200 uF Capacitor
This should be almost a straight line at 0.5 V (ignoring inductive spikes).
We can estimate the leakage resistance (Rleakage), the ESR (Resr) and the capacitance (C1) from the output waveform at Figure 9, at least to first order on the basis of simple circuit analysis.
We can calculate the leakage resistance from the average DC voltage. With no leakage this should be exactly half the peak square wave value, or 0.5 V. The DC average voltage was 0.492 V. So the leakage resistance was about 3K Ohms. This is quite low.
Rleakage = 2 x Vaverage x Rout / (Vpeak - 2 x Vout)
If we ignore the inductive spike at the switching transients (all wiring has series inductance and the test circuit does not incorporate matching), we will see a step of 3.5 mV between the end of the discharge and the start of the charge. This gives us a direct measurement of the ESR which is about 0.17 Ohms. This is about 20 times larger than anticipated.
Resr = Vstep x Ro / (Vpeak - Vstep)
= 0.0035 V x 50 Ohms / (1 V - 0.0035 V)
= 0.175 Ohms
Finally we can calculate the capacitance from the amplitude of charge (or discharge) curve and the charge (or discharge) time. The capacitance has dropped from 1,200 uF to about 8 uF.
C1 = - t / [Rout * LN(1-2 x Ccharge / Vpeak)]
= 0.5E-6 / [50 Ohms * LN(1 - 2 x 0.006 V / 1 Volt)]
= 8.2 uF
The other bulged capacitors showed similar degradation. Yep, these capacitors are stuffed. To confirm our simple calculations we can perform a circuit simulation of the circuit shown in Figure 7.
Figure 10. Circuit Simulation
Compare with Figure 9. We can conclude that our first order hand calculations
provide a reasonable estimate of actual circuit performance.
With the unit reassembled (less the font plastic panel and top cover) it was time to tempt fate and apply power. Whoot! The player is working again. The display is showing sensible stuff like No CD Inserted, Please Wait, AV Channel and a flashing clock (the clock does not have a battery back-up and flashes to indicate that it needs to be reset). I fitted the plastic front panel.
This repair took under three hours with the only actual expense being the service manual which, with a little more attention during disassembly, would not have been required. Replacement capacitors would have a cost a couple of dollars but I used near equivalent stock items on hand. Had I lashed out and purchased the service manual before commencing the repair the service time might have been halved.
The only thing left to do is to test the unit connected to a television with some media.
Test Time. After about a minute from power on the recorder is switching off all by itself, but otherwise it appears to be fully functional. Using the service manual I brought up the last error code, H19. This corresponds with a fan failure. Oops! I don’t recall reconnecting the fan to the main board at all during reassembly. I powered down, removed the case cover and sure enough the fan was disconnected (my dumb). With everything back together the recorder is operating perfectly.
Job done.
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