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Old 09-23-2020, 10:37 AM   #1
re-atari
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Join Date: Oct 2010
Posts: 102
Default Denon AVR-X2000 mainboard caps bulging

I have been using a Denon AVR-X2000 receiver in my living room for several years now, works perfectly, am very pleased with it. Recently I acquired another one, advertised as defective with flashing red led a few seconds after switching on. Picked it up to try repair and use in hobby room. After switching on, the on/off indicator led indeed initially flashes green for about 2 seconds, during which the display is visible, and then starts flashing red (at 2Hz) with a switched off display.

I'm afraid the service manual is too large to upload here (55Mb), it can be found at https://www.manualslib.com/manual/10...-Avr-E400.html

After opening the receiver up I straight away noticed the tops of both large electrolytic caps c4052 and c4054 (ELNA 12000uF 71V, marked 'For Audio' on the casing) are bulging. The breach lines are still intact, so no gunk oozing out, the two tops are just domed. I have never seen this phenomenon yet on any amp or receiver I have worked on, and don't see a reason why the two caps would be domed either. Looking at the service manual page 127, they filter the voltage for the power amps, get this voltage fed from a large linear transformer and bridge rectifier, so are not part of a SMPS. On my working AVR-X2000 the tops of these two caps are completely flat, no bulging or domed tops at all.
Could these two bulging caps be the single cause of the receiver shutting down? And I'm curious, what could have caused this bulging? Massive overheating, eg. if the receiver had been built into a tight cabinet without enough space for cooling or running it at max. volume for a long time at a party?
I'm a bit reluctant to order replacements just yet, as caps like these are quite expensive, and there might be other gremlins lurking beneath the surface.

I already did some mesasurements after dismantling the PCB's. On the power amp PCB all output transistors and resistors are OK, no shorts detected. I thought of running the receiver with the power connector to the power amp board disconnected, to see if it will do a normal start up then, but as I measured no shorts on it, I don't really see the point in doing so.

On the standby PSU board I did find a defective cap, a 100nF 275V AC X2 made by Carli (c4143), that is located directly after the 230V input socket (see service manual page 121). Its capacitance had dramatically decreased, my meter showed it now was down to only 15nF. I happened to have a proper replacement available, so this one is already replaced. As I expected, this didn't solve the problem, though. The standby PSU outputs a solid 5,2V standby voltage, so seems to work OK. It is built around a TOP258MG, which is known to fail easily, but it appears to be OK here.

I measured the bridge rectifier and all diodes on the various boards, no shorts detected there. The five voltage regulators (two 7805's, 7808, 7905 and 7908) showed no shorts either. Sadly they are just about impossible to reach to do measurements with the receiver switched on, so no way of telling if the regulators output the correct voltages. The internals of the receiver are quite cramped overall, which makes it hard to measure the voltages going to the various parts of the receiver.

Another thing I noticed, is that on the HDMI board the 5,2V standby voltage is filtered by two 470uF 6,3V SMD electrolytic caps in parallel (c3789 and c3802). Strange enough, they are not the same type. Where c3789 is a standard run off the mill one, with no manufacturer's marking visible, c3802 looks to be a special one made by ELNA. This is exactly the same on my working AVR-X2000. I don't see a reason for this, looking at the service manual page 142 both are just filter caps for the 5,2V standby voltage. Can one of the experts here shed some light on why Denon took this particular route?

I can upload photos if needed.

Thanx in advance for any info!
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