Denon AVR X2000 amp problems.

[ajlelectronics]

As detailed on another thread, I am working on a surround amplifier that has been got at in the past. It has been taking out its power transistors on the front left and front right channels, seemingly at random although all Voltages are in sensible figures. I have a new set of expensive transistors in there now and am trying to set the idling current. This is done by adjustment of the trimpot to achieve a Voltage of 6.5mV, waiting ten minutes and then setting 8mV. This is specified with no connections anywhere, to anything. The interesting point is that there is no Voltage being developed at all, which tells me that the output transistors are not pulling any current. I have tried two other Voltmeters plus my 'scope. The circuit diagram is particularly unhelpful in terms of Voltage readings so I am scratching my head a bit.

Before the last transistors died, I did have an amount of wooly audio from the front channels with the volume set flat out, but nothing like what would be expected. Circuit of one channel below. The test voltage is across the emitter resistors, via a 10K to TP401. I have plus and minus 60V DC being supplied to the PA board.

Audio Section
Adjusting Idling Current
Required measurement equipment: DC Voltmeter
1. Preparation
(1) Temperature should be at avoid direct blow from an air conditioner or an electric fan and humidity should be moderate,
and place the set at normal usage environment.
15 °C ~ 30 °C (59 °F ~ 86 °F)
(2) Presetting
• POWER (Power source switch) STANDBY
• SPEAKER (Speaker terminal) No load
(Do not connect speaker, dummy resistor, etc.)
2. Adjustment
(1) Remove the top cover and set VR401, VR402, VR403, VR404, VR405, VR406, VR407 on at fully 7CH AMP UNIT at
fully counterclockwise (c) position.
(2) Connect DC Voltmeter to test points (FRONT-Lch: TP401, FRONT-Rch: TP402, CENTER ch: TP403, SURROUNDLch:
TP404, SURROUND-Rch: TP405, SURROUND-BACK Lch: TP406, SURROUND-BACK Rch: TP407.
(3) Connect the power cord to AC Line, and set the power switch to "ON".
(4) Presetting.
MASTER VOLUME : "---" counterclockwise (c min.)
SPEAKER (Speaker terminal) : No load
(Do not connect speaker, dummy resistor, etc.)
MODE : MCH STEREO
FUNCTION : DVD
(5) Within 2 minutes after the power on, turn VR401 clockwise (x) to adjust the TEST POINT voltage to
6.5mV ± 0.5mV DC.
(6) After 10 minutes from the preset above, turn VR401 to set the voltage to 8.0mV ± 0.5mV DC.
(7) Adjust the Variable Resistors of each channel in the same way.

 

[ajlelectronics]

The other 5 channels show around 10mV at their respective test points so there is something going on with the two front channels on the amplifier board.

 

[Seadog]

I'm assuming that the Darlingtons are your output drivers. Is the circuitry to the right a protection circuit? I had a lot of problems with a NAD PA because of that.

 

[ajlelectronics]

I'm assuming that the Darlingtons are your output drivers. Is the circuitry to the right a protection circuit? I had a lot of problems with a NAD PA because of that.

Yes on both counts.

 

[ajlelectronics]

Emitter resistors are O/c on both channels one side. They appear to be in perfect order so made the daft assumption that they were OK. 21 transistors to desolder again to get at them and a special order as I don't have anything that low value in stock. Surely these aren't fusible resistors?

 

[ajlelectronics]

 

[ajlelectronics]

I found some fusible 0.47r 1 watt at Switch Electronics. Not many suppliers it would seem. They have to be fusible because they would have burnt else, surely?

 

[Misterg]

They have to be fusible because they would have burnt else, surely?

Does the (2) after the value on the schematic relate to any sort of foot-note?

Not convinced that them being open would give you zero volts on the test point...?

Does the bias adjustment pot (VR401) look OK? I think that if the wiper lost contact (e.g. due to vibration) then the bias current would increase, possibly killing the output transistors in due course.

 

[ajlelectronics]

No there are no footnotes. It may indicate 2 in parallel maybe. If they are O/C then there can be no Voltage dropped across them, so that all tallies.

 

[ajlelectronics]

I am getting closer, after many hours of head scratching...

The protection system does not trip now with the amplifier disconnected from the speaker board, which carries the sensing components. The emitters of all the amplifiers read around 30mV to earth apart from the Front Right channel which is showing -200mV and it doesn't change with adjustment of the idle current pot. The idle current sets fine at 8mV.

I have looked and looked at the circuit and it surely HAS to be leaky output transistors again doesn't it? I have already replaced them in that channel once but I can't see anywhere else that the emitters can be getting that Voltage from. All the emitter and base components are good and have been replaced.

Waiting for another batch of transistors before I can test the theory.

 

[Martin-Sy]

I looked at this last week .... Are the components marked 'open' not populated on the PCB?
The clamp made from a fwd diode and a zener in series (ZD402 D403 ZD403 D404) marked 'E3 Only' .... Again does this mean not fitted?
Does the dashed line around them indicate thermal coupling?
The power darlingtons have quite a high FT. Is it hooting? (Oscillating at RF)

 

[ajlelectronics]

I looked at this last week .... Are the components marked 'open' not populated on the PCB?

Yes that's right. It seems to be different versions for other territories.

The clamp made from a fwd diode and a zener in series (ZD402 D403 ZD403 D404) marked 'E3 Only' .... Again does this mean not fitted?

Yes, it is a different version to the X-2000.

Does the dashed line around them indicate thermal coupling?
The power darlingtons have quite a high FT. Is it hooting? (Oscillating at RF)

A) I will have a look

B) Likewise, but the reading is too stable for it to be taking off I think.

 

[Seadog]

Waiting for another batch of transistors before I can test the theory.

Why not swap the transistors from a known working channel? That way you prove at a stroke whether it's the channel or the transistors.

 

[ajlelectronics]

Why not swap the transistors from a known working channel? That way you prove at a stroke whether it's the channel or the transistors.

1) Because they have been in and out so often, the legs are liable to get metal fatigue. In order to get to the amplifier board, all of the power transistors need to be desoldered and the legs bent back and forth.

2) There are two FFC ribbon connectors in the way that are starting to look worse for the wear.

3) Perhaps the actual reason is, that I'm sick to the back teeth of the damn thing and need a break. This should have been an easy and quick job, but I may well do that if I can muster the required enthusiasm.

 

[Seadog]

Point taken Andy.

Some designs are definitely not made to be technician friendly.

 

[ajlelectronics]

I still haven't cracked this. I have replaced the 220uF electrolytic between the bases and checked for matching Voltages in many places to the other channels, yet I am still seeing 200mV at the emitters on FR with respect to earth. All the other channels are around 20-30mV. All the idling current settings have been made as per the book and are all at 8mV or close.

I took the PA board out to put it on the bench and in probing managed to short out the - supply. That melted two pins on the connector, so had to butcher a JST socket for pins to mend that!

 

[Misterg]

I am still seeing 200mV at the emitters on FR with respect to earth.

What voltages do you have on C, B & E of the transistors in this channel compared to the others?

 

[ajlelectronics]

What voltages do you have on C, B & E of the transistors in this channel compared to the others?

That is the weird thing, they are all similar. When I calm down a bit I will do a chart of readings.

 

[ajlelectronics]

Centre good channel:
With respect to chassis and no output load or input signal...

Q434 Base 1.2V, Collector -58V, Emitter 47mV } Fairly typical across all but FR channel.
Q428 Base 1.1V, Collector +58V, Emitter 27mV }

All idle currents set in allowed range.


FR channel...

Q422 Base 1.2V, Collector -58V, Emitter 200mV
Q416 Base 1.05V, Collector +58V, Emitter 188mV

Heatsink is warming up but not getting excessively hot.

 

[ajlelectronics]

It's honking!

Good channel is flat line at the same settings and reconfigured gives

So now it is a question of why it is honking.