It just keeps blowing...

[madan1]

Re: It just keeps blowing...

So just checked what main transistors I got last time from the local shop - they are a pair of SanKen 2SC3856 - 2SA1492

Are you doing the adjustment on page 12? So your problem is happening on the surround sound R channel? This is happening when no speaker are connected?

No, I adjust the bias once I have a fully working channel.
The maiden power-up is always without speakers attached.
Yes, the SurrRight is the blown channel.

Using a variac would spare you lot of excessive costs . It let you also localize the gradually heated area ..

Yes, variac would be perfect, but the electronics repair is still just a hobby and I'm trying to keep my hobby running costs low .

A variac and/or a series "bulb tester"... Not to mention the thought of NOT just swapping a fuse if the previous one blew.

can you provide more info about the "bulb tester" ( good link would be just fine ).
Yes, just swaping fuses is not a good idea, but back then, when I got the AVR I needed to see if there is any life left in it ( the seller did not provide any information about the unit's faults ) and the thing was so cheap I really didn't care if it will live or die on me .

remove outputs and tack in a 1k resistor between base and emitter connections for each output .
check dc voltage on output ..

By "outputs" you mean the main transistors, right? What power rate should be the 1k resistors?
What should I check on the dc?



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and here are some good screenshots from the last bonfire
It looks like this time one of the emitter resistors blew.

[petehall347]

Re: It just keeps blowing...

remove outputs and tack in a 1k resistor between base and emitter connections for each output .
check dc voltage on output ..

[Khron]

Re: It just keeps blowing...

A variac and/or a series "bulb tester"... Not to mention the thought of NOT just swapping a fuse if the previous one blew.

[jiroy]

Re: It just keeps blowing...

Howdy everyone.

Are there any procedures you do before powering up the unit?
From the damage, do you see an element which might be faulty?
Any advices are welcome!

Using a variac would spare you lot of excessive costs . It let you also localize the gradually heated area ..

[keeney123]

Re: It just keeps blowing...

Are you doing the adjustment on page 12? So your problem is happening on the surround sound R channel? This is happening when no speaker are connected?

[madan1]

Re: It just keeps blowing...

Well... here is the service manual and it says tr532 - 2sd1763a and tr534 - 2sb1186a. Those at least are easy to find, so no replacements here.

p.s. Oh, you mean the main power transistors.
The original are TR536 - 2sc3855, TR538 - 2SA1491.. let me see if I can find here ( at the office ) what I got last time as a replacement





ithe manual is too big to upload here, so use these links

http://www.filedropper.com/hfedenonavr-2801981serviceen

[keeney123]

Re: It just keeps blowing...

Are you sure TR536 is a NPN and TR538 is a PNP ? It would seem both transistors are turning on at the same time

[madan1]

Howdy everyone.
I got a non-working denon avr2801 from ebay. Since it was (almost) for free I just changed the fuse, powered it up and I got a pretty good bonfire from one of the "non-burning" resistors . OK, I have another blown amplifier.. cool, change the blown parts and I'm good.. at least that's what I thought back then .
While I was waiting for the parts, I removed the blown transistors, assembled it and hooked it to my home audio setup... maaan this denon sounds great!
I got the parts ( the power transistors, the diodes and the zener diodes were all substitutes ). Soldered everything on the pcb, checked it for any visual damage ( after the fire ), crosschecked the resistance of every component with the same on a working channel.. everything was good to go. Powered it up and.... bang! another bonfire from a brand new "non-burning" resistor.
Removed the new set of blown transistors and just left it for a resident amplifier.
A week ago I was out of things to repair and decided to finally fix that receiver ( I also wanted to try a new music setup for which I needed one more working channel ).
Alrighty.. got new transistors, new resistors, new diodes.. well pretty much the standard thing. This time I also thoroughly checked the pre-amp board and the output circuitry for faulty elements. I also tested out of the board TR540 and TR528 together with a set from a working channel.
This time I was totally sure that everything should be fine... and while I was brushing out the flux residue I noticed that there was something wrong with the path between R614 and ZD506. The path is less than 1mm wide / ~10mm long and was interrupted right in the middle of its length. So I guess this explains the enormous amount of damage the channel had ( indeed this is the first time I'm changing that many elements on a blown channel ). Made a bridge, checked again all paths for interruptions and assembled everything back.
Power.. bang!... only smoke and a flash from the fuse this time .
At first glance, this time, it blew only the power transistors and few resistors ( R622, R620, R624 and most likely the emitter resistors ). The pre-stage ones ( tr534/532 ) seem to be fine and currently the receiver is working with them on the board ( but with removed R620 ).
So I already purchased the emitter resistors ( turned out those are hard to find ) and in the next days I will also get the power transistors, but... damn, I do not want another fire.. I almost have paid for transistors what I paid for the whole receiver.

How do you proceed when fixing blown channels?
Are there any procedures you do before powering up the unit?
From the damage, do you see an element which might be faulty?
Any advices are welcome!

Also, with what standard parts would you substitute ZD506/508 and D534/536?


The first picture is with the damage after the first repair attempt ( before I discover the broken path b/n r614 and zd506 ) and the second one - after the second ( with fixed path ).

specs:
https://www.hifiengine.com/manual_li...avr-2801.shtml