help setting up idle current

[AndrewT]

I don't remember reading that. I will go and enlighten.

Thanks for the link.

 

[lensmanMk2]

well, set up the idle current a few days back at 10mv per channel, both were miles out,left one was 35mv and the right was 14mv. worked great for a few days, then a fuse popped, so replaced that one then both went :/

ive found a trashed resistor that was in series with one of the trannys on the left channel i replaced (R336), with some serious burn marks on it.but getting to the point now where im thinking "is it worth it?" if it is as simple as fixing that resistor and all being well then that would be great, but im thinking if its blowing fuses there must be something a bit more nasty going on,and i dont have the equipment to do any proper fault finding.

so what do you think guys? replace the resistor and check the rectifier bridge (caps are ok)

..or just give up and bin it?

thanks for all your help everyone, i learned quite a bit even though the amps dead!

 

[nigelwright7557]

well, set up the idle current a few days back at 10mv per channel, both were miles out,left one was 35mv and the right was 14mv. worked great for a few days, then a fuse popped, so replaced that one then both went :/

ive found a trashed resistor that was in series with one of the trannys on the left channel i replaced (R336), with some serious burn marks on it.but getting to the point now where im thinking "is it worth it?" if it is as simple as fixing that resistor and all being well then that would be great, but im thinking if its blowing fuses there must be something a bit more nasty going on,and i dont have the equipment to do any proper fault finding.

so what do you think guys? replace the resistor and check the rectifier bridge (caps are ok)

..or just give up and bin it?

thanks for all your help everyone, i learned quite a bit even though the amps dead!

Its possible just replacing the resistor will result in the new one burning out.

I suspect an output transistor or more gone wrong.

 

[lensmanMk2]

i should have taken up fishing..cheaper and less hassle

to top it off,tonight my backup pioneer amp has died and part of the plastics on my headphones have broken for no apparent reason

anyone want to place a bet on me getting struck by lightning by tomorrow morning?

 

[godfrey]

What transistors did you replace to start with? Was it blown output transistors or something else?

Sometimes there's collateral damage when output devices fail, with other components getting hurt too.

Replacing with different parts to the original could cause problems too e.g. instability.

 

[lensmanMk2]

i replaced Q310 309 311 as these had actual burn marks on them, all on the power amp board. they were drop in replacements,luckily they still seem to be manufactured

just to go off at a complete tangent here,looking at the schematic, could i bypass the power amp board all together and put some buffer amplifiers on the pre out and use it just for headphones?

 

[lensmanMk2]

thanks to everyones help so far, i "think" ive found the problem..

one of the NPN power transistors is giving funny diode readings, and its out of circuit so that rules out any interactions with anything else on the board this is the chip:
2SC2706 pdf, 2SC2706 description, 2SC2706 datasheets, 2SC2706 view ::: ALLDATASHEET :::

completely open circuit E>B but C>B is fine , on the right channel the corresponding chip shows normal values for an NPN, so im hoping this is the problem

plus ive found one of the trannies i replaced (Q331) is tested dead, the rest i replaced seem to be fine as per the rest on the board,theres 2 opamps on there but i dont know if its possible to test them with just a multimeter

heres a pic to show you guys, the burnt resistor is hidden underneath the transistor

An externally hosted image should be here but it was not working when we last tested it.

now given i think i know whats failed, what should i check for as well? failed rectifier bridge, transformer etc? given that the power amp board is seperate from the rest what are the chances of it damaging anything else like the preamp section?

thanks for all the help so far

*edit*

just found all the chips on the heatsink for the left channel are dead so thats q501 -2 -3

 

[richie00boy]

Usually, if one any output transistor has failed it will have at least damaged its driver transistor. I would replace all drivers and outputs to be on the safe side.

 

[5th element]

thanks to everyones help so far, i "think" ive found the problem..

one of the NPN power transistors is giving funny diode readings, and its out of circuit so that rules out any interactions with anything else on the board this is the chip:
2SC2706 pdf, 2SC2706 description, 2SC2706 datasheets, 2SC2706 view ::: ALLDATASHEET :::

completely open circuit E>B but C>B is fine , on the right channel the corresponding chip shows normal values for an NPN, so im hoping this is the problem

plus ive found one of the trannies i replaced (Q331) is tested dead, the rest i replaced seem to be fine as per the rest on the board,theres 2 opamps on there but i dont know if its possible to test them with just a multimeter

heres a pic to show you guys, the burnt resistor is hidden underneath the transistor

An externally hosted image should be here but it was not working when we last tested it.

now given i think i know whats failed, what should i check for as well? failed rectifier bridge, transformer etc? given that the power amp board is seperate from the rest what are the chances of it damaging anything else like the preamp section?

thanks for all the help so far

*edit*

just found all the chips on the heatsink for the left channel are dead so thats q501 -2 -3

First, the images you posted don't enlarge in anyway when you click on them. At least not for me.

If the input to the amplifier is capacitor coupled it's unlikely that the stage before it would have failed.

Also I doubt anything in the amp PSU would have been injured either, unless the components aren't particularly robust.

What puzzles me a little is how the amplifier was working and with a correctly set bias. It has been my experience that if any transistors are dead, the amp will not function at all, let alone allow you to listen to music or set up the bias.

I did have one problem once that caused spontaneous combustion of the output transistors though. This was a defective trace on a PCB that caused the tracking of the bias to go right off resulting in thermal runaway.

I would check all of the resistors also for any troubles. Sometimes they can fail without looking damaged, this is rare, but I have had a couple of cases like this.

There are a couple of other possibilities that I can think of. The first is that there is parasitic oscillation occurring somewhere and may not be present all the time, hence the amp seemed to work, but then exploded later on. It is possible perhaps that a capacitor has changed it's value over time and while once was stable, this drift in value is now causing intermittent instability (I do doubt this however).

The other is the protection circuitry, if this has become damaged in some way it is possible that you managed to overdrive the amp. The second time round, did the amp fail when doing anything specific?

 

[lensmanMk2]

The other is the protection circuitry, if this has become damaged in some way it is possible that you managed to overdrive the amp. The second time round, did the amp fail when doing anything specific?

nope was just listening at normal volume levels when it just stopped dead, with no power, im wondering if the burnt resistor was damaged in the amps inital death, and as the amp got warm with use has topped itself and somehow destroyed the outputs

good news is the output transistors are still available, kind of, the other 2 are still manufactured but the 2sc2706 are now retired with these being touted as a replacement :

2SC3280 datasheet pdf datenblatt - Wing Shing Computer Components - NPN PLANAR SILICON TRANSISTOR(AUDIO POWER AMPLIFIER DC TO DC CONVERTER) ::: ALLDATASHEET :::

heres the 2706's:

2SC2706 pdf, 2SC2706 description, 2SC2706 datasheets, 2SC2706 view ::: ALLDATASHEET :::

i dont know nearly enough about Ics to judge if theyre a drop in replacement, as was mentioned earlier using a non original part can result in some problems, there is a redesigned circuit for the ouput on the last page of the schematic using the 3280s, but that means more cost and more work with the potential for more to go wrong, so it would be good if i could just drop the 3280s in place of the originals.. think this is possible given their specs?

 

[lensmanMk2]

lets see if these will work this time, i tried the bb code from imagebam last time, but it didnt want to link, so i'll try html

http://www.imagebam.com/image/20f74371480051

http://www.imagebam.com/image/e445dc71480046

 

[5th element]

The pictures worked.

That looks like it's R337, which in the schematic ties the emitters of the pre drivers together, what Self calls Rd. I can easily see it blowing if transistors start failing.

Simulating an EF output stage you can omit Rd and it will still work, you can even still set the bias. What implications with regards to stability and bias stability, omitting R337 might have I don't know. You'll have to let someone more knowledgeable chime in on that.