CurtPalme.com Home Theater Forum

[mdc]

Hey everyone I recently picked up an AVR1906 (fulfilling a desire to replace the AVR1905 I lost during a move about 6 years ago ) with the dreaded protect mode problem.

First thing I checked was the IC reset (A+B+Power), but no luck. I quickly stripped the unit down and discovered that C182 was blown (bulging with the top cracked open, in fact), and on the underside of the mainboard, the ZD + D combo on the SL channel were literally fried, so desoldered those.

I then removed C182 and R184 (which was also dead), then checked all of the Darlingtons and found one of them (on the SL channel) was shorting, so removed the 3 heatsinked transistors for that channel as well.

Unfortunately, it STILL goes into protect mode. I'm guessing that C182 and R184 NEED to be replaced before there's any chance of it powering up, as it doesn't look like they're confined to the blown SL channel (they appear to come off one of the Darlington transistors for FL, and then feed back to the -B line (not sure what that is, tbh :-\).

Given the incredible work you guys seem to have done on the 2307 which seemed to be exhibiting the same symptoms, I figured it was worth posting on here just in case anyone has any ideas Any advice would be greatly appreciated

[mdc]

This is the schematic for the main board, btw:

[macgyver655]

You seem to have some knowledge of electronics. Are you a repair shop? Or is this your receiver?

[mdc]

'tis mine Grabbed off eBay at a relatively bargain price with the knowledge it had probably blown a channel or needed the voltage regulators changing, but unfortunately I seem to be stuck now

I'm getting a 41V spike on powerup on each of the Darlington's emitters with respect to the chassis ground, so I'm guessing that one of the components that are currently removed provide a ground path or reference voltage to the Darlingtons for the protect circuit, the likely culprits being C182 and R184 as they seem to be "shared" components rather than channel-specific, but unfortunately the complexity of the Denon is stumping me Most of my previous jiggery-pokery is limited to MOSFET-driven Class AB stuff, never worked on something with a DC protect circuit like this Denon before.

[macgyver655]

I need all the location id's of the parts you removed and that ZD you say was fried.

I also need you to test that cap with your DMM and tell me if it is blow short.

[mdc]

They're both in 4B on the schematic; I just tested the Cap in Diode test mode and it's blown open.

The ZD and diode pair actually aren't ON the schematic for some reason, they're located underneath the board on every channel.

EDIT: Just done some more testing; the emitter AND base are at +42V with respect to chassis ground, the collector is at +53V. PD across the base and emitter is 0.

[macgyver655]

I'm sure they are on the schematic, but I'm not going to guess. I want the id numbers off the board itself.

[mdc]

Ah apologies; on the board itself there's no additional information that I can see beyond the names of each component.

The ones that have been removed thusfar are:

D102SLT (fried)
ZD102SLT (fried)
Q101SL (shorted)
Q102SL
Q103SL
R110SL (blown open) 0.22ohm/1W
R111SL (blown open) 0.22ohm/1W

C182 (blown open) 10uF/100V
R184 (blown open) 10ohm/1W

I've taken some photos of the affected areas so you can see their position on the board itself if this is any help

http://i.imgur.com/fZJ680P.jpg
C182 and R184

http://i.imgur.com/WTMhZqr.jpg
ZD102SLT & D102SLT

http://i.imgur.com/BMdjpWk.jpg
R110SL & R111SL

http://i.imgur.com/fjpz46j.jpg
http://i.imgur.com/gsKXb5r.jpg
General layout shots

EDIT: The cap had actually blown its top and two of the three removed resistors had blown their casings. The two diodes underneath had actually exploded and taken the PCB laquer off as well in the process as you can see.

[macgyver655]

That's what I was looking for. Give me a few hours to looks things over. I have other stuff going on today also.

[mdc]

Thank you very much Much appreciated!

And no rush, will take a couple of days for the replacement parts to arrive anyway... I miss the days where you could walk down to your local Tandy or RadioShack where they actually had component drawers

[macgyver655]

I hate when service manuals leave parts out. I'm trying to locate the D102slt and ZD102slt. Nothing in schematic section, nothing in parts list and in the section of board layout the foil side of the main board is not there. Every other board layout shows both sides except this board. WTF is wrong with these people?

How do you plan on ordering replacements for these 2 diodes? Are there any markings on the same diodes in adjoining circuits?

Also, in your image it appears as though 1 end of ZD102slt is on a trace that appears to be ground. Can you verify?

It then appears as though the 2 diodes then go end to end. Is that correct?

And then can you verify where the other end of D102slt goes?

[mdc]

Tell me about it haha

I'm out at work at the moment, but I'll confirm those details when I get home. You're correct in saying both diodes are connected in series; they were soldered together in the middle (in an L-shape) with endpoints either side of the exposed piece of PCB.

I read somewhere online during my googling (although I can't for the life of me find the source of the information) that they comprise part of an additional protection circuit designed as a clamper which comes into play at high volumes, and that simply leaving them off may be an option.

With that said, I don't actually mind having S/L out of play entirely, as my main use for the amp will be as a stereo unit, POSSIBLY with added rears at a later date... planning on putting together a complete Mordaunt Short setup eventually, but sourcing the components of it is proving to be difficult due to the relative rarity of specific models on the second hand market. (MS25is for the fronts, and MS10is for the rears, possibly with a 1000 for a center).

Since the MAXIMUM I'll be running is a 5.0 setup, losing that channel isn't the end of the world, although if that ends up being the case I need to find a way of disconnecting it safely in such a way as it won't bias the other channels, which is where my knowledge is failing me currently. I'm guessing that the channel-specific R117s are what link each channel to the protection circuit, so disconnecting that should allow me to power the amp on without that channel intact?

[the big E]

[mdc]

[mdc]

Right, this diode pair is a bloody nightmare. It appears to be connected between the trace that C101SL is on (on the side that J102 is also on), and then down to the trace between R118SL and inductor L101SL. It appears to be the only component DIRECTLY connecting those two traces.

Ideally I'd like to check the polarity and values of the components but unfortunately they're in a bloody awkward position which means actually getting probes on them with the unit powered up would mean balancing things in precarious and possibly electroutey-dangerous ways lol.

[macgyver655]

Set your DMM on continuity and test between that D102slt pad and pins 1 and 2 at connector CP501 and see if you have continuity.

Then check between the ZD102slt pad and the pad of TP104 and see if you have continuity.

[mdc]

[macgyver655]

[mdc]

I'm guessing the unit went through a couple of revisions, and they never updated the service manual. With photography gear (flash units, cameras etc) they typically add an addendum to service manuals, usually a little 1 or 2 page insert, which isn't always included. Can be a complete pain in the backside when repairing stuff.

Serial number is 5103311235.

Q103SL is a 2SC947F, although says D947F on the chip itself.

R101SL is a Red Purple Red (bloody difficult to see that one as it's right up against the heatsink so difficult to get in with my loupe lol).

[macgyver655]

Alright, I have most of your answers now but I don't have time to post them as I'm on my way out. I'll write it up later when I get back. But I have something for you to try while I'm out.

On the back of the board where those 2 diodes are gone I want you to remove the other 2 diodes, D101slt and ZD101slt. The see if it will power up without going into protect.