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Old 09-28-2020, 03:40 PM   #8
momaka
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Default Re: Denon AVR-X2000 mainboard caps bulging

Quote:
Originally Posted by re-atari View Post
The X2 cap is indeed placed across the AC input of the standby PSU. I have attached page 121 of the service manual, showing its schematic. I wanted to encircle the culprit cap (c4143) in the schematic, but sadly after the last Nitro Pro update I cannot edit pdf's anymore...
No worries, I found it easily.

That cap is for EMI/RFI filtering/compliance only, so it's not that important to the operation of the amplifier.

For this cap to loose capacitance / go nearly open-circuit like that, one of the two must have happened: either there was indeed a voltage surge on the line/phase or the cap is very old. After all, it is normal for metal film caps to loose capacitance over time. However, this amplifier isn't that old (2013-2014 "vintage", according to online sources), so I think this rules out age-related failure.

Quote:
Originally Posted by re-atari View Post
I'm in 230V territory (EU), our wall sockets carry 3 wires, brown (phase), blue (neutral) and yellow/green (GND). One strange thing is that the receiver's AC input socket does not have a GND pin, only phase and neutral are connected.
Right.
A lot of audio equipment is not grounded and uses only a 2-prong cable for phase/line and neutral so that way there is less likely chance of creating a ground-loop, which can sometimes result in a lot of hum created from the amp/speakers due to 50/60 Hz line noise getting picked up on the ground look and getting amplified.

That aside, the real question is whether the house in which this amp failed had only a single-phase or 3-phase power. If it's 3-phase, there is a possibility to have a high line voltage from a failed neutral connection (depending if the building uses Delta or WYE connection.)

Quote:
Originally Posted by re-atari View Post
The internals of the receiver don't appear to have been excessively and prolongedly overheated. There's no visible massive discoloration on the PCB's or on the cooling heatsinks. Besides that, I would expect the cooling paste on the output transistors to be dried out then as well, and that's not the case. The regulators are bolted to the steel bottom halve of the receiver housing for cooling, and the steel isn't discolored there either.
Exactly, I don't think heat was the reason why those Elna caps bulged.

Quote:
Originally Posted by re-atari View Post
There is a tiny spot on the mainboard PCB that looks slightly discolored. It's the area where r4025, r4026, r4027 are situated (see service manual pag. 127). They are three 1,1K resistors that are connected in series immediately after the positive lead of ELNA cap c4052. The resistors all measure in spec, as do r4029 (10K), d4007 (1SS133) and zd4002 (ZJ6.8B, a 6,8V zener) that come after them. These components produce the Mute+7V signal. The discoloration may very well have been caused by a possible power surge, which is what you suspect has happened.
Yes, it could be that those components got a little overheated from the power surge.

Of course, it could also be due to poor design. In particular, I see this sort of cost-cutting technique done on a lot of amplifiers. Instead of adding separate transformer taps from the main transformer to get those lower voltages needed for the pre-amp and logic circuits, they just use several resistors and a linear regulator to drop down the voltage from the main amplifier output stage voltages, since the pre-amp and logic circuits usually don't draw a lot of current. Of course, over a large voltage drop, the power dissipation can get up to several Watts high. So it's actually not too uncommon to see this section in a lot of amplifiers run a little "toasty" if they used a cheap design like this.

Since your components (the resistors and Zener diodes) are in spec, I don't imagine anything was damaged in that section. Of course, it wouldn't hurt to measure the voltage with the amp plugged in to check.

Quote:
Originally Posted by re-atari View Post
Curiosity already got the better of me, so I disassembled my working AVR-X2000 to swap boards. This showed that only the mainboard of the failing receiver is at fault causing it to not start up, all other boards are OK.
So you got the broken receiver to work with the main board from the working AVR-X2000?

Quote:
Originally Posted by re-atari View Post
I took my investigation one step further and desoldered the two bulging large ELNA caps. This wasn't easy, as the bottom of the cans was stuck to the mainboard PCB with some kind of white silicone glue, but in the end I got them out undamaged. I was astonished to find both caps measured completely in spec: 11 mF (or 11.000 uF) on my M4070 LCR meter and an ESR of 0,018Ohm on my MESR-100!
Ah, I've seen that a few times, but it's rare. Usually only happens to good caps, because they are built better.

I suspect the surge was high enough to over-volt the caps way past their surge rating, but perhaps it didn't last long enough to destroy the caps, so only a little pressure built up inside them from Hydrogen production, as the caps were over-volted.

Quote:
Originally Posted by re-atari View Post
I had been thinking of measuring the voltages that the five 78XX and 79XX regulators put out beforehand, just to exclude obvious and easy to fix faults there. Decided doing these measurements wouldn't be easy, though, as the regulators are quite hard to reach.
Well, if you got the receiver to work with the main board from the good one and the regulators are not located on that board, then I don't think the issue would be with the regulators.

That said, if you do want to check voltages in a tight spot where it's trecheros to stick your hand in with the power On, then what you can do is solder some extension wires to the input and output of those regulators and measure that way.

Quote:
Originally Posted by re-atari View Post
I agree with you, that the material Denon used for making the PCB's (Phenol) sucks big time. I mean, pertinax has already been around for more than 40 years, why would a serious manufacturer even think of using Phenol.
Denon did use pertinax for the CPU board, probably because it's a double sided PCB, whereas all other boards are single sided. Must probably have something to do with cutting cost corners...
Yeah, Phenol board are cheaper, so it is done for cost cutting. What I was referring to, however, was the black/brown die they used to coat the top of the PCB, instead of leaving it uncoated or using some lighter color.

Quote:
Originally Posted by re-atari View Post
I really wonder if and in what way the ELNA caps could be failing. Both measured completely in spec, 11.000 uF on my M4070 LCR meter and an ESR of 0,018 Ohm on my MESR-100. Could it be they fail when under load? The receiver shuts down 2 sec. after being switched on, no loudspeakers attached or input signal fed into it. So no real load present. I'm stumped, but then again don't have an education in electronics.
If they measure OK for capacitance and ESR both in spec, they likely are OK.
As mentioned earlier, probably the power surge didn't last long enough to damage them, so they only got a little "gassy" inside from the over-voltage, but the vent and other seals didn't get compromised.

Over time, though, that pressure will find its way out and may even force electrolyte leakage from the bung on the bottom. So you can probably continue to use the caps for testing. But once the main issue is found and fixed, then it would be best to replace those caps for better long term reliability.

Quote:
Originally Posted by re-atari View Post
I now realise there's one thing (well two, to be more precise) I didn't measure yet, and that's the inductance and voltage output by the large linear transformer. Maybe it took a hit by a power surge and has some windings shorted. I will do those measurements with my LCR meter and multimeter asap and post the results here. It's very convenient that I can compare the readings with measurements on the transformer in the working receiver.
Yes, if you can, get readings with both the working and non-working board of the voltages from the main transformer (both AC and rectified DC would be good to see.) This might give us a better clue to what's going on.

Quote:
Originally Posted by re-atari View Post
Hi Momaka, many thanx for your insights, it's always a pleasure to read your posts on Badcaps!
Thanks, I appreciate the compliment.
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