1176 weirdness..

ok finished wiring my dual 1176 box and decided to try it out.
no smoke, checked PSU voltages, all looking good! plugged the input and output in and then houston we have a..
PROBLEM..

Audio is VERY weak and very distorted. started poking around with the multimeter and most of the voltages are looking good.

next I pulled the FET, no change in the audio.
third I ran a lead from the wiper of the input pot to the input of the output pot to bypass everything and i get audio loud and clear(however the bass seems rolled off around 200hz or so..) so my problem is in the area after the preamp and before the output driver section. I have doublechecked just about every cap and resistor for value and they are all good.

so.. I decide to try the other board. SAME PROBLEM. hmm so i start looking again and see one thing that I am unsure of.

the PCB has BC107B written on it. the parts i am using are BC107. the schematic on Gyraf's site shows BC107 and so does the parts list. I've been through the datasheet and see nothing that would cause the BC107's not to work so I'm sure it's not that but what else could it be?

so here's the facts in a nutshell:

the audio seems to be lost after the FET but before the output stage
It doesn't matter if the FET is installed or not leading me to believe that the FET isn't bad nor is the GR section at fault.

any ideas?


I'm headed back down into the studio to test some more.

:thumb:

have you double checked that the transistors are in the right way?

dave

well.. i took them out completely tested them, compared to the datasheet(st micro parts..) and put them back in. same deal. I took another look at the datasheet and realized that ST micro has a PNP transistor shown on the datasheet for a BC107, an NPN part.

however i checked them with a transistor checker and they show ok. I know that doesn't mean much, I've checked transistors before and they showed ok but didn't work.

I just looked up another NPN transistor in TO18 format and the datasheet actually shows an NPN this time. it shows the exact same footprint.

looking at a bc107 from philips, the numbering is different but the pinout is the same.

I also found that the voltages of all 3 BC107's in the audio path are way too low. where the 30v goes through the 22k to the collector of q3 i'm only getting something like 2v. same for the emitter output of q4, only around 1.5v.

strange..

If all 3 voltages are low look for something that is common to all of them.

when i take the transistors out, the voltages on the transistor side of the resistors(like the 22k i mentioned above) rise to 30v.. guess i'll replace them one by one and see what I get.

ok still nothing. i've swapped out the transistors with some more and still nothing. for the hell of it I stuck them in backwards and only succeeded in killing a few of them. now the traces are coming up from the board i've been fucking with this so much. my meter also died so reasons unknown. I'm getting the feeling that someone(something) just doesn't want me to have these stupid things.

guess I gotta buy more BC107's now too.

what a bitch of a day it's been.. :sad:

on the schematic it has two part numbers, one that Gyraf put in, the BC107 and another handwritten one, 2n3391A. is that the original part? i see it has a 25v max though...

another thing..

I've read in a number of threads that NTE replacements have a different pinout.. however the crossreference to the Bc107 is the NTE123 which has an identical pinout according to the datasheet. a local store has these in stock, I guess i'll give them a try.

from memory, the 2n3391a I think is an original part but if Im not mistaken, these were available in the US from either mouser or digikey when I built my 1176. This is from memory so beware...

dave

I thought the BC107`s should be the BC107B for this. The B variety have a gauranteed slightly higher HFE.

whew, ok I played with this forever trying to track down the reasons that this doesn't work.. still nothing. the only other clue is that if i shunt the 27k resistor before q2 i start to get louder audio through the 3 transistors, not as distorted too, but the voltages are still very wrong. other than that I'm still at a loss.

:roll:

If you shunt the 27k resistor you would get louder audio because you`ve turned the signal up in level, I don`t think this is much of "a clue".

I`m sure that in a thread somewhere it states that you need the BC107B & not the plain low gain BC107

If you look at these 3 transistors Q2-Q4 they are directly coupled. Therefore if the bias is out on the first one Q2 it will have a knock on effect cascading throught Q3 & Q4. If the collector of Q2 is high then Q3 will be switched on,which will make its collector low, turning Q4 off.

If all the voltages are low it is likely to be something that is common to all 3 transistors, like their all not getting any juice.

If you can`t find the problem maybe you could post voltage readings for c b & e for all 3 transistors.

i'm going to try some substitues, if that doesn't work then I'll post the voltages.

thanks!

Svart,

My stereo G1176 has plain BC107's and works fine.

Dean

yeah that's what i figured. I fried a few messing with it so I have to get more anyway... might as well get the better ones.

ok here's an update.

i took some idle voltages..

q2: b=1.8v, e=1.5, c=2.6
q3: b=2.6, e=2.0, c=2.0
q4: b=2.0, e=1.5, c=30.3


i replaced the transistors with bc107Bs, and unhooked the section at the coupling caps. i've tried grounding the inputs too with no change.

i still don't understand why these aren't biasing correctly so i started probing with a 10k pulldown resistor. biasing the base of q2 does nothing. pullling down the base of q3 with 10k to ground immediately brings the rest of the voltages on q3 and q4 to the correct levels. this means that the issue must be with q2 and it's biasing. this happens with the coupling cap before/after the stage are inplace or not.

so I continued to look at q2 and realized that the bias network on the emitter must not be working correctly. I probed at the two 180R and it hit me like a ton of bricks..

brown, grey, RED, ORANGE, brown???????????????????????????????????????????????????????

182KKKKKKKKKKKKK

AHHHHHHHHHHHHH
fuck.

replaced with 180R and all voltage is normal. will have listening tests done this weekend!!!!

:thumb:

i must have overlooked that Resistor 4 freaking times!!

live and learn.

Glad you found it.

herehere!

now geeet squishin! :thumb: