Denon AVR1912 problems

[Decee1]

Hey fellow EEVblog Ppl.
I've been working on my first big repair, an Denon AVR1912 i picked up for free which was dead. I have been looking at it in over 20 hours and still don't see anything that could make it shut down.
It gets the error code: Thermal A on the errors checking thing while the main power for the transistors is removed. I have been trying in a long time. Been staring at the whole HDMI board to see if any parts was missing or look faulty but without results. Also contacted Denon to get them to explain the error code but it takes so long. I hope somebody can guide me to the working state of my lovely Denon AVR-1912. I shoud have all the required equipment to measure things so don't hold back your thoughts

Thanks,
Stefan
 -17y old newbie

Manual: http://assets.denon.com/documentmaster/us/owners%20manual_avr1912.pdf

[kripton2035]

if you disconnect the power for the transistors, sure it won't start.
these denon avr all have lots of security circuits that shuts off if any voltage on many points on the circuit is not within range.

check the output transistors for any shorts, then check the small resistors beside the transistors. one ( or many) of these are shorted
change them and re-check.
also the power supply section check all output voltages. I remember of a faulty 7915 once there

if the hdmi board is fried, you get bad sounds clics (very bad...) but it does not prevent the unit to start.

[kripton2035]

and you will need that : http://www.manualslib.com/manual/986184/Denon-Avr-1912e3.html

[Decee1]

I've already checked all the output transistors, they are fine,
And I said that it would start if you disconnected the power to the transistors. I know it has so much security but it has to be something simple.
I believe the HDMI board works fine as I can start it without powering the transistors. So it has to be something around the Amplifier board but thanks for the ideas.

[Decee1]

And I also did share the manual in my first post

[kripton2035]

Quote from: Decee1 on Today at 16:20:32

And I also did share the manual in my first post

the manual in your first post is the user manual
the manual in my post is the service manual with all schematics... far more useful in your case ?

for example at page 23 : error check mode, the avr tells you where the failure might be...

[SoundTech-LG]

yes...  thanks for that post. will surely help in troubleshooting similar problems in possible purchase.

[SoundTech-LG]

and you will need that : http://www.manualslib.com/manual/986184/Denon-Avr-1912e3.html

That has to be one of the most beautifully done manuals I have laid eyes on...    a true work of art.
The full color shots, board layouts, and fully traced out schematics in highlighted colors. Wow. A real pleasure to see.

Worth a look for sure!!! Enjoy!

[SoundTech-LG]

Turns out it did help. No video out from this AVR-1912. The Hot Plug detect was stuck low at the processor Analog Devices ADV7623 pin 41 (of 144). Found the new part on ebay. 144 pins later, it works great!

[Decee1]

Hello folks,been a while since i've checked my EEvBlog acc.
I realised i had posted this post a while ago so i thought i would post an update.

I found out one channel was dead and i don't need 7,1 only 5,1 surround so i removed the transistor and if in the future i will need it, i can just repair it fully and if i end up doing so i would have much more experience then.
The other channels which was dead just got a new set of power transistors and it worked, i ended up putting it on the shelve for a long time because i was empty of ideas.
I here this week took it down from the shelve and began troubleshooting and after a few hours made it sound, then i wired the whole system up only to realize the front left channel was weird - DAMMIT! It has a high gain output and shuts off at around -39dB output. Today i began measuring resistance only to realise that the resistance between the two power transistors is 15Kohm where others it is 4.7 or around so.

Now only left to do is find out why!

[mzacharias]

Do you mean you are measuring from the emitter of one output transistor to the emitter of it's mate? If so, it should read quite low, and if out of spec will trigger the protection circuit prematurely. The emitter resistors on these Denon's commonly go bad if the output transistors have shorted.

[SoundTech-LG]

I'm into a 3312 here. Typical flashing red as if it had a protect problem. But I went so far as removing one of the 71V, 12,000 uF caps as it seemed one was weak (I measured way less DC on that one). Apparently not the case. Anyway if I disconnect CN701(main power in/Spkr out) it powers up no problem. That connector disconnected is where I measured the +/- rails, and found them not equal. Strange. Rectifier bridges appear ok. Could still be the Caps out of balance I suppose.

[kripton2035]

voltage regulator circuit problem ? I had that once (on an avr1912)

[SoundTech-LG]

Yes, VRs are a constant issue, but this is the Big Rails +/- 60 Volts unregulated to the Output Amps.
I think the Nichicon 71 Volt 12,000uF caps must be an issue. I see them being sold on ebay, and Amazon. Not sure if those are safe to use. Could be counterfeit.

[kripton2035]

measure each cap ESR, but with such a big capacity value, not sure what device can measure this !

[Shock]

If you can keep it running without going into shutdown you can put a signal source through and use multimeter or scope and compare channels and troubleshoot in that manner.

I would exhaust any service manual troubleshooting and reference voltages marked on the schematic (if any).

Typical troubleshooting, check supply voltage, ripple, the proceeding stage, if the unit runs hot or is a few years old check caps. Test transistors out of circuit, double check your work and where you sourced any replacements for fakes.

[SoundTech-LG]

I have both 12,000 uF caps out of circuit now. Charged both to the same 25 volts. Watching self discharge. One seems to be losing it's charge a bit faster, also seems to charge faster, which leads one to think it's value is less than the other. When I pulled the 2nd cap out of the board, one lug seemed to be loose enough to pop right out...  possibly a broken solder ring around that lug... should not pull out of the board that easy!!!