Yamaha CA-1000 hum and hiss from tone board

[spaghetti]

I am trying to figure out how to fix my new Yamaha CA-1000 amplifier. It works fine except for an audible hum, accentuated by tone control operations... and setting the tone highs to max introduces some hiss too.

After some inconclusive hours trying to fix the issue I noticed that the noise completely disappears if I move away the tone board from its lodge. Please see this picture, if you're not familiar with this model:

The tone board is placed just above the power switch and lamp, and the class A switch operator. There's a metal shield that should protect the board from interferences, but it seems it's not effective. In the picture the tone board has been moved aside, where it produces no hum.

Is there something I can do to fix the noise issue or is this a design flaw of this Yamaha model?

Thanks,
Andrea

 

[mbz]

First up, hums are difficult to sort out, many possible causes.
From the symptoms I'd suspect that when correctly installed, there
is a new ground path from the tone board, that or by moving the
board you've disturbed some of the earth/ground wires.

Suggest the following,
- reflow the earth wire solder connections on the tone board. These
are the middle black wires, I think there are 5 or 6
Also reflow solder on earth wires on volume board, 4 maybe?
- reinstall the tone board, amp powered off/unplugged.
With black multimeter probe connected to phono ground, measure the
resistance at each of these black wires in turn, repeat for each of
the black middle wires on the volume board. Should be less than 2ohms.

To complete the resistance measurements you can also measure at,
- black speaker post
- EM tab/wire on electrlytic board
- 4 black wires on electrolytic board.

 

[avionic]

There are 4 audio cables Red,brown white and orange. Each of these cables has a soldered connection marked "E". "E" is a ground connection.

 

[spaghetti]

Thank you for the suggestion. I've just checked all the ground wirings on the tone board and they're all good.

One thing that puzzles me is that the closer the tone board to its lodging, the more audible the noise, just like it was picking some sort of static from the air.

 

[avionic]

Reflow your solder connections on the base and treble controls.Center pins are all "E" grounds.

 

[avionic]

I've just checked all the ground wirings on the tone board and they're all good.

How did you test that ? Did you put a multimeter on them and measure there resistance to chassis ground ?

 

[avionic]

I'd still reflow them with a little new solder, even if they look OK.

 

[spaghetti]

How did you test that ? Did you put a multimeter on them and measure there resistance to chassis ground ?

Yes, that's what I did. I am reflowing the tone controls now, I'll reflow the wires as well and update soon

 

[avionic]

Super..

 

[spaghetti]

Unfortunately there's no good news yet, the noise is still present.

This is so annoying because the power amp section, when separated using the back panel switch, is so crazy silent...

 

[spaghetti]

I've just noticed something weird: on the power board (the one with the 2 big caps) the +B and -B voltages read +10V and -97V... according to the SM they should be +50 -50. How is it possible that the amp works totally fine except for the tone control issue?

 

[avionic]

How is it possible that the amp works totally fine except for the tone control issue?

How are you measuring that raw B+/- voltage ?

 

[spaghetti]

How are you measuring that raw B+/- voltage ?

With a DMM, black on the chassis, red on the B+/B- solder spots.

But meanwhile, I think I found the faulty board: it's the "function circuit board", the one under the removable metal cover. If I remove the B+ voltage cable from that board, then things go back to normal: +50V and -50V.

At first sight I can't see any evidence of damaged parts, the fuse resistor is spot on 220ohm, but the board is full of tantalum caps, which I've learned to look suspiciously. I'll start checking transistors, diodes, and probably my next move will be checking those pesky tantalum caps...

 

[blhagstrom]

There are a few fuse resistors in that model.
Some on the PS IIRC.
Hunt them all down and verify or replace every one is a good first step.

FWIW, the last one I rebuilt had bad solder joints on the RCA input jack board inside the rear panel.
Years of connecting and un-connecting seem to have cracked some joints.
Just noting that as an area that may be a problem on these, not that it is the problem you are experiencing.

 

[restorer-john]

...One thing that puzzles me is that the closer the tone board to its lodging, the more audible the noise, just like it was picking some sort of static from the air...

The high impedance tone stage is likely picking up radiated AC from the exposed two core cable directly below it, on the power switch wiring. Is there more hum in Class A as compared to AB mode with the tone switched in? More current is drawn and I would expect the hum to increase if that were the case.

I have come across problems like this before, especially with poorly routed or untwisted AC power switch wiring, running near high impedance tone stages. Sometimes, even a foil suppression cap over switch contacts can radiate enough to sensitive circuitry immediately adjacent.

 

[mbz]

Since you've mentioned the function board as possibly being involved, you might want to check for a burnt track that often goes, it does result in hum and
odd voltage reasons, this did not match the original symptoms of alls well when tone board relocated.

 

[spaghetti]

The track was damaged indeed! I jumpered it with some tiny wire. I can't test the board right now because I had it washed with water and soap it to remove all dust and crud, so it needs to dry. I think I will wait until tomorrow for a test.

 

[spaghetti]

Today I put everything back in place and now the amp works fine: the weird voltages are gone, as well as the hum. I suppose that the hum sensitivity was caused by the insufficient B+ voltage (about 10V).

Many thanks to everybody, you taught me so much in this thread, and special thanks to @mbz who pointed me in the right direction.