Mu-Tron Phasor II has power but no sound

[norseofcourse]

I have a sick Mu-Tron Phasor II where it receives power, the red LED turns on, there’s crackle from the amp when I touch the ¼" jacks, but no other sound is heard.

Checked both ¼” cables and the guitar for sound and passed. There are no obvious burnt out components on the board. De-oxed the three pots, two trim pots, and rocker power switch. Re-flowed a few joints that even mildly looked cold.

Checked continuity of the rocker switch which passed, but bypassed it anyway with a short wire. Fuse is good.

Can anyone verify the voltage and ohms I should see? Pots and transformer were made by Chicago Telephone Company in the 70's.

• The transformer receives 120v from the power cable and the diode bridge on the other side of the transformer read 16v.
• Removed the three pots and measured the Rate pot at 23.5k audio, and Depth and Feedback pots at 11.2k linear.

Thanks,

-Sean

 

[z2amiller]

What a beaut, I love the hand-drawn traces.

Has someone been in there already to replace the op-amps? Those look like they're date code 0821 (2021) but maybe
I'm mis-reading. Could someone have bungled a previous repair?

I guess one thing I might check would be the electrolytic caps - electrolytic caps Of A Certain Age can have their
electrolyte dry out and stop capacitating. None of them have the telltale signs of dead caps, but it might be worth
going through and measuring them if you can.

I found this service manual:

https://www.bruynooghe.fr/public/effets_vintage/Phaser/MutronMU-01PhasorIIServiceManual.pdf

It doesn't match your PCB layout exactly, maybe you have an earlier or later PCB revision.

Do you have an audio probe? Might be interesting to trace the audio through the circuit and see where
it disappears.

 

[norseofcourse]

dio probe? Might be interesting to trace the audio through the circuit and see where
it disappears.

Hi, thanks for that manual! That confirms my pots are pretty close so they're probably not an issue. No one has been in it since i bought in 1995. It was working one day a few years ago, i put it in a box, then a few weeks ago i took it out of the box and plugged it in.

The big caps read a voltage drop under power, i forget exactly what it was but seemed reasonable at the time. Unfortunately I don't have a scope, just an average multimeter.

 

[Cybercow]

If the fuse is good, I'd start with a full recap of all the electrolytics and any tantalums (didn't see any pants in the photos). As already mentioned, they tend dry out and drift in value over time. Then I'd check the power supply to see if it's getting proper DC values.If all that checks out, it's audio probe time.

 

[Scruffie]

Hi, thanks for that manual! That confirms my pots are pretty close so they're probably not an issue. No one has been in it since i bought in 1995. It was working one day a few years ago, i put it in a box, then a few weeks ago i took it out of the box and plugged it in.

The big caps read a voltage drop under power, i forget exactly what it was but seemed reasonable at the time. Unfortunately I don't have a scope, just an average multimeter.

Don't need a scope, you can just build an audio probe.

If you post full voltages may be possible to spot the fault.