Ok this is what I got, 18v at the Zener diode, 0v DC offset at the speaker terminals at the R732 0mv R733 2.1mv
Those readings all sound good.
Connect the scope across the speaker output and set the attenuator for around 1 volt/div (or 0.1/div if using a divider probe) and see if any oscillation is present as you tweak the preset and the bulb suddenly jumps from dim to bright.
Connect the scope across the speaker output and set the attenuator for around 1 volt/div (or 0.1/div if using a divider probe) and see if any oscillation is present as you tweak the preset and the bulb suddenly jumps from dim to bright.
I wouldn't like to say on that... it could be just RF noise such as stray radio pickup. It is not 'oscillation' in the way I was thinking.
Does that noise disappear when you turn the bias back down ?
Does this channel now work OK with headphones ?
Does that noise disappear when you turn the bias back down ?
Does this channel now work OK with headphones ?
Hi Mooly, well I had to replace that trimmer resistor it as just too sensitive and now at least it is very stable I can adjust the bias with no problem. Now when listening to the left channel there is no more hum but the sound is very tinny
That sounds more promising.
What do you mean by 'tinny'. If you mean no bass then that can only be caused by a problem with a capacitor in the audio path. The only two in the power amp that could cause that are C701 and C705.
What do you mean by 'tinny'. If you mean no bass then that can only be caused by a problem with a capacitor in the audio path. The only two in the power amp that could cause that are C701 and C705.
Check the values are correct. If you recapped the front end (preamp/radio) then the issue could be there.
Yes, but removing it will increase the bias current so be sure to turn it back down to zero first.
I put a 1khz tone through the right channel( with my signal gen.) all is well but when i hooked it up to the left channel nothing. I get 0v on the DC offset 10mv on the bias yet can not get a tone
So you need to apply the same signal to both channels and use your scope to look at the signal directly on the base of H702.
You should see identical signal levels for both channels. Remember there will be a slight DC offset on the base but the signal will be superimposed onto that.
You should see identical signal levels for both channels. Remember there will be a slight DC offset on the base but the signal will be superimposed onto that.
Hi Mooly, It has been a while, Have not done much with my audio gear we sold our home at the end of Feb. and had to find a new place to call home, well low and behold we are in our new place and getting settled in. I plan on returning to my 2250b, I did go through to familiarize myself were i left off. I remember having that tinny sound on the left channel, I believed i narrowed that down to the tone board, after recapping the left main amp i fed my phone through it to play music through the main in (inputs) and clear as a bell, so i thinking it is the tone board. I do not have my work station at 100% at this time still need some time but i am itching to get back to this unit
Pat
Pat
No problem 🙂 although its all a zillion posts back in time for me. The scope and some careful signal tracing should narrow things down.