New Project Marantz 2250b

[pjf610]

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

 

[Mooly]

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.

 

[pjf610]

Hi Mooly, as you can see in the pictures not very good, I tried to set the scope to 1v but had to turn it down to 20mv to get the signal, turning up the volume to get what you see

 

[Mooly]

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 ?

 

[pjf610]

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

 

[Mooly]

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.

 

[pjf610]

Yes no bass sounds like the treble turn up all the way, i did recap the board when i had it out

 

[Mooly]

Check the values are correct. If you recapped the front end (preamp/radio) then the issue could be there.

 

[pjf610]

I only recapped the P700(left channel only) and the PE01 Pre Tone Amp board

 

[pjf610]

Hi Molly, can I remove the 1K resistor that i tagged across R718

 

[Mooly]

Yes, but removing it will increase the bias current so be sure to turn it back down to zero first.

 

[pjf610]

Done no problem

 

[pjf610]

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

 

[Mooly]

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.

 

[pjf610]

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

 

[Mooly]

No problem although its all a zillion posts back in time for me. The scope and some careful signal tracing should narrow things down.

 

[jdubble79]

Gosh, this was such a wonderful dialogue! And then the it stopped with no closure! How does the story end? (i ask because i just got a 2250b that has no sound, and i'm about to start working on, and i found this thread to be absolutely thrilling)