2226B Low Volume & Distortion Both Channels

[grey_beard]

I have a 2226B that's new to me. It has low volume in both channels (and headphones) and distortion when the volume knob is turned up high (volume is still low through speakers and headphones).

I've cleaned the pots and switched best I could without taking the board out and there's no scratching or intermittency from any of the pots or switches with the exception of the Loudness button which I can't hear any difference with it engaged.

Routing input through Main In yields strong, clean audio. Output from Pre Out is quiet and distorts.

I've been able to do the bias and power supply adjustments on the Main Amp board with no issue so I'm lead to believe the issue is with the Tone/Pre board.

I have the service manual but I could use some help figuring out where I should begin tracing. I also have a decent meter and an oscilloscope.

Anyone with experience working on the tone/amp board have any tips to removing the board for service?

Thanks in advance.

 

[runningdog]

Good work ruling out the main amps and everything after them. The preamp/tone board on the 2226B may be the hardest to get to of all the 22xx models. It is sandwiched to the front chassis panel, and is accessed by removing a lot of screws. But...since the issue is in both channels then it could be the power supply, which would affect both channels equally. For example if the preamp/tone board is not getting the required voltage the result could be the distortion you are hearing. The 2226B power supply is on the same board as the main amplifiers and the protection circuit and relay.

A good next test may be to see if the preamp/tone board is getting power. It looks like J721 is the 35V, and J722 is the 14.7V.

 

[grey_beard]

Measured 14.3V at J722 and 35.2V at J721 so those are both quite close.

It seems like the next step would be to trace a sine wave. I've hooked up a wave to AUX and now need to trace it through the PHONO/SELECTOR board but I'm not sure which jumpers are the outputs on the Phono Amp / Selector board.

I'm looking at the schematic but I can't figure what's the output from the Selector. Can anyone point that out for me and possible explain how to read this Selector schematic?

 

[runningdog]

Test to see if J414 and J415 are the Selector output when set to Phono.

The left/right outputs from the phono board go to S401-2F-12 and S401-3F-12. Here's how to read that:

Switch #1 on the P400 board, 2nd wafer, on the front of the wafer, Location 12

Switch #1 on the P400 board, 3rd wafer, on the front of the wafer, Location 12

When the switch is rotated clockwise two notches to Phono:
2F-12 is connected to 2F-9, which goes to J415
3F-12 is connected to 3F-8, which goes to J414

This is my best guess at the L/R phono signal path through the switch.

 

[grey_beard]

Test to see if J414 and J415 are the Selector output when set to Phono.

The left/right outputs from the phono board go to S401-2F-12 and S401-3F-12. Here's how to read that:

Switch #1 on the P400 board, 2nd wafer, on the front of the wafer, Location 12

Switch #1 on the P400 board, 3rd wafer, on the front of the wafer, Location 12

When the switch is rotated clockwise two notches to Phono:
2F-12 is connected to 2F-9, which goes to J415
3F-12 is connected to 3F-8, which goes to J414

This is my best guess at the L/R phono signal path through the switch.

You nailed it. I went one selection past phono and used the AUX input which exits at J118 and J119. The wave is still pretty at this point.

Now onto the Tone / Preamp Board. It looks like the AUX signal comes onto the board at JE01 and JE02. By tracing the wires I found a clean wave made it to the input jumpers on the TONE AMP board. From there I need to find the output from the board and work my way backwards I think.

I did check the wave at the speaker terminals and it goes flat on the top of the curve in case that helps anyone.

 

[runningdog]

I'm getting nowhere with the tone/pre board schematic and where the signals come off it. But you can test what is reaching the main amplifiers at J701 and J702 (J703 is ground). And see if you can work backwards from it to the tone/pre board.

 

[grey_beard]

I traced the signal to the Preamp board and it comes to the board clean and continues clean through the tone section. I don't think there's any more I can do without pulling the board.

 

[grey_beard]

I freed the tone amp board by removing the interior faceplate under the exterior faceplate. This allows the pots the space they required to gain better access to the board.

Now to figure out what needs to be replaced on here.

Troubleshooting advice appreciated.

 

[grey_beard]

Here are a couple clues that may help someone more knowledgeable than myself.

1. It is the top of the sine wave that distorts.
2. It is both channels.
3. Sine looks clean coming of of the volume pot and into CE102.
4. 35V is present at JE25

 

[runningdog]

@grey_beard , you have gotten further on that 2226B than I have ever have. Thanks for the information. It's challenging to follow the signal and compare channels on the circuit board with the physical layout having little relationship to the logical flow of the circuits.

There is that "both channels" thing again. I don't think there are many components shared by both channels. RE75, and CE35? The latter being a 220uF/35v capacitor. I'd replace CE35 right off and then check the signal at the base of QE01 and QE02 to see what it looks like there. Assuming it can be powered up to do so?