• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Trouble with preamp on 2250.

PoDuck said:
No, but it wasn't present after the recap either until I went to adjust the phono board. Everything had been working perfectly for months except the phono amp. Now all inputs have trouble except when I connect directly to main.
Click to expand...

That's odd. The only adjustments on the phono board is an ASO adjustment for symmetrical clipping. These rarely need adjusting.

What kind of test gear did you use and how was it hooked up?
 
I don't think it is the phono board, since it happens on all inputs except the main input. It even happens on AM and FM, although it doesn't seem to cut out completely on those like it does with the other inputs.

As for what kind of test gear I used, I have a Rigol DS1054Z oscilloscope, and a cheap DDS signal generator, that goes down to 1mV amplitude, but I cut that resolution in half by connecting a 50 Ohm load to it so I can get finer resolution. I hooked up the signal generator to the phono input, with a separate 1K signal to each channel. I hooked up the oscilloscope to the Tape 1 output, and connected both left and right channels to separate inputs on the scope.
 
Strangely, it doesn't seem like my edit took. I forgot to mention that the problem persists on Tape 1 and 2, AUX, and while AM and FM seem to have some immunity, as I can still hear sound when the bass is all the way up, it still gets quite distorted.
 
Okay, I've been doing some probing. Somewhere between the the output of the BA312, and the input of HE03, I am getting some distortion. I'm not sure what is between those two points that could be causing this distortion. You can see that whatever is happening is clipping the bottom of the waveform. The higher I turn the bass knob, the more it clips, and it does it linearly, without any jitter as I move the knob. In all these images, the cyan trace is the signal I am getting at Tape 1 output. The yellow trace is the specific item being tested.

This first image is the input on JE01 pin 3 of the PE01 board.
DS1Z_QuickPrint4.png

This next image is the output of pin 6 of HE01, which is the BA312.
DS1Z_QuickPrint5.png

This is the image of the base of HE03. It already appears distorted here, but gets worse.
DS1Z_QuickPrint7.png

This is the collector of HE03
DS1Z_QuickPrint6.png

This is the emitter of HE03.
DS1Z_QuickPrint8.png

Does anyone have a clue what would be doing this? It's only happening on one channel right now.
 
I am assuming you want to know if both BA312s read the same on all pins? They are all identical, other than a 0.5 V difference on pin 1.
 
i see some 0 ohm resistors . maybe fuse resistors .. easy to rule out as voltage across them should be 0v .
saying that it shouldn't affect the signal before the op amp . :idea:
 
All 0 ohm resistors read 0 ohms.

I'm trying to figure out why you are saying that the problem likely resides before the board when the signal coming into the board is fine. That's the first image I posted. Also, the output of the op amp is fine, which is the 2nd image I posted. The signal goes from the BA312 through several things and winds up bad at HE03, from what it appears to me. I'm just unsure what would be clipping the bottom of the signal between those two spots. As I said earlier though, I am suspecting a capacitor, since it builds up distortion over time, and even faster if I turn the bass knob all the way up, and I have tested a bunch of the capacitors I think are inline, and they seem to test okay.

The distortion in those images above is with the bass control about half way up. If I go all the way up, I lose signal completely.
 
What would I be checking for? That would remove the signal coming in.

Also, yes, pin 2 is the input.
 
its clipping at the negative side of things . disconnecting the tone amp will rule it out as it might be pulling it down somehow ... signal should look good there from what i see . .. this manual is a nightmare to navigate .
 
Okay, I removed CE05, and as I thought, there are no signals to test. The signals at the transistors are flat too, in case you were wanting to see if there was some errant signal causing the clipping.
 
By output, I am talking about the output that is giving me a bad signal, which is the left channel. I have considered disconnecting the wire the comes from pin 6 and seeing if something is bringing things down from that end. I can't find an easy way, so I have been avoiding it.
 
Here's a snip of the schematic. Looks like the output of the ba312 goes directly through the tone mode switch. Is CE13 a polarized cap? Is it in backwards?

Screenshot_20180810-194310_Adobe Acrobat.jpg
 
You must log in or register to reply here.
Back
Top Bottom