2285B No BIAS (Driving me nuts)
- Thread starter caesar56
- Start date
caesar56
Active Member
Follow up:
I changed all the 1345 transistors in the pre and it became a lot quieter (Way less hiss at the tweeters). Also discovered a cap installed backwards. CH16 in PE01 board. A small 10uf 25v lytic.
I ran the amp for a couple of hours and it worked well. Today in the morning turned it on and the right channel was dead.
Decided to grab the signal generator an the scope and take measures:
Dummy loaded the left channel works fine and goes til 26.7v AC wich equates (Grossly) to about 89 watts before clipping.
Right channel stays silent no signal coming out of the amp either feeding signal to 'main in' or 'aux in'.
.....Well, pulled out the iPod and ran some tones trough both of the inputs and it decides to work well. Switched to some Jazz and both channels operate as normal.
DC offset and Bias work as expected.
Signal is being properly Injected and can confirm using pin 721 and 701 @P700 board Wich correspond to 'Main in output'.
At this point I'm puzzled my bucks are at a incorrectly biased transistor somewhere in the P700 board.
Now, I'm gonna ask a pretty dumb question but how critical is to match the transistor gain specifications in the sm?
I'm wondering if this circuit was designed with pretty tight margins.....
I changed all the 1345 transistors in the pre and it became a lot quieter (Way less hiss at the tweeters). Also discovered a cap installed backwards. CH16 in PE01 board. A small 10uf 25v lytic.
I ran the amp for a couple of hours and it worked well. Today in the morning turned it on and the right channel was dead.
Decided to grab the signal generator an the scope and take measures:
Dummy loaded the left channel works fine and goes til 26.7v AC wich equates (Grossly) to about 89 watts before clipping.
Right channel stays silent no signal coming out of the amp either feeding signal to 'main in' or 'aux in'.
.....Well, pulled out the iPod and ran some tones trough both of the inputs and it decides to work well. Switched to some Jazz and both channels operate as normal.
DC offset and Bias work as expected.
Signal is being properly Injected and can confirm using pin 721 and 701 @P700 board Wich correspond to 'Main in output'.
At this point I'm puzzled my bucks are at a incorrectly biased transistor somewhere in the P700 board.
Now, I'm gonna ask a pretty dumb question but how critical is to match the transistor gain specifications in the sm?
I'm wondering if this circuit was designed with pretty tight margins.....
caesar56
Active Member
I've found the issue (Mostly).
I gather by your name that you're from Brazil, South America. Right? (I did read about your own issue with the 2285B)
This is important because of the following:
The replacements I used for pre drivers and drivers were sourced locally, more specifically MJE15032 and C3421.
It turns out these were FAKE. Chunk FAKE PIECE OF GARBAGE.
I discovered this after injecting/following a generated sinewave as far as I could and then, all of sudden the channel woke up but had 200mV of DC at the output.
I remebered immediately that I sourced these parts locally, so I took them out, gathered the drivers of my Frankestein 2270 and the amp was back as normal.
Oh I do have a small buzz in my dead channel tweeters but this is WAY better than having nothing at all.
Went to ask the electronics shop about these parts but all they were willing to say is: 'No warranty for electronics parts'.
This however is a cancer eating South America as a whole: we've got loads of Chinese garbage and owners don't care if they cause damage to you by selling this BS.
