Marantz 2235b - Tone Board Not Responding

[GnarlyBus]

I recapped this unit and after doing so noticed that the tone board doesn't work anymore. The bass, mid and treble pots don't do anything. The audio signal is just passing right through and ignoring them. the loudness and Hi filter still work, but I can see in the manual that they aren't really connected to the PreAmp board.

I've disassembled the unit again, thinking that maybe I had flipped a cap around but they look good.

Any ideas? I appreciate the help!

 

[Jonstigator]

Put up a pic of the back of the board. Check all solder joints. Maybe a wiring issue. Did you clean and lube the pots? Does balance work?

Did you do this one board at a time checking results after each board?

That tone board is used in other models. My 2220b and 2225 has that board. They can be touchy but I have never had this issue.

From the top the work looks good.

 

[GnarlyBus]

Thank you. Here's a pic of the back. I checked results one board at a time but didn't notice the tone board wasn't functioning because the pots were centered. Audio came through sounding good, so I thought it was fine. I've since noticed that the mix sounds much better when loud than when at a lower volume.

The pots were DeOxit'd and are clean. Balance works and was cleaned with FaderLube.

 

[ocdecio]

I find it unusual that the tone board stopped working on both channels - can you confirm with a square wave and scope?

 

[GnarlyBus]

I don't have a scope, yet. No audible change in either channel when moving any of the tone knobs. Any way to do tests with my DMM?

 

[ocdecio]

If all you have is a DMM you could try to carefully measure the voltages and compare to the schematic in the service manual. If you are using a probe put some tape around it and leave only the tip to do this test or use grabbers turning it off and on for every measurement. I know it can be tedious but you won't want to risk shorting something.

 

[AE Bob]

You have installed C19 and C20 incorrectly. You have one capacitor installed between the + leads of C19 and C20, and the other capacitor installed between the - leads of the two capacitors.

 

[ocdecio]

It looks as though you have installed C19 and C20 incorrectly. You have one capacitor installed between the + leads of C19 and C20, and the other capacitor installed between the - leads of the two capacitors.

god almighty @AE Bob I can only wish having such a sharp eye. It sure looks they are wrongly installed.

 

[AE Bob]

Well it is virtually impossible for the tone controls to have NO effect, unless something is disconnected, missing, or improperly installed. I was looking to see if possibly one pair of coupling capacitors had been left out, when I saw that the C19 and C20 pair of coupling capacitors was incorrectly installed. I suspect that the amplitude from the tone board is wrong with those capacitors installed that way, and that varying the tone controls would tend to strangely alter the stereo separation as well.

 

[GnarlyBus]

Thanks Bob! That was it! Thank you for spotting that screwup. The controls work again!

 

[Steven Tate]

While you're at it, check these two solder joints. When you stretch the photo, they sure look like they could stand to be reflowed. Could just be the photo.

 

[Jonstigator]

You have installed C19 and C20 incorrectly. You have one capacitor installed between the + leads of C19 and C20, and the other capacitor installed between the - leads of the two capacitors.

I did at least ten of those tone boards and that flew right by me. Good eye!