Help with 1960's Era Amplifier Problem (Wurlitzer Model 145 Electric Piano)

[Curts Dad]

I have reviewed this forum and I am hopeful that someone may be able to help us out with a technical problem. My son has acquired a Wurlitzer Model 145 Electric Piano. The piano was manufactured in the early 60’s and has a tube-type amplifier that is likely very similar in technology to the guitar amplifiers of the day.

I am helping my son troubleshoot via FaceTime as I am in Canada and he is in NYC. I have posted a similar question on “The Electric Piano Forum”, and members have provided some useful information but the issue has not been fully resolved.

The piano plays fine but has a persistent buzzing sound coming through the amplifier. The amplitude of the buzz can be controlled with the volume potentiometer, and the frequency remains constant. The buzz doesn't sound to be 60 Hz.
We have checked all the internal grounding/bonding in the piano. A couple of things were suspect but there was no change after minor repairs. From the other forum we learned that the piano originally came with a two wire input cable, and we observed that someone in the past had replaced it with a three wire cable. We removed the ground wire from the amplifier and the buzz was attenuated significantly (ground loop?). Now if any of the metal portions of the piano are touched with a finger the buzz is again significantly attenuated. If a ground wire from the electrical service is touched anywhere on the metal portions of the piano the buzz returns to the original levels. This is all demonstrated on a YouTube video at this link:

https://youtu.be/7LVkY8PV12M

The schematic for the piano is available on page 26 of the service manual at this link:

https://cdn.shopify.com/s/files/1/06...ice_Manual.pdf

BTW, I have been educated on the “death capacitor”, and this piano does indeed have one. Our next step will be to remove it.

Many thanks for any thoughts or ideas you might have.

[nickb]

Firstly, deal with the death capacitor and fit a three wire power cord.

I notice that as you move your hand closer to the keys, the buzz increases. Touch the chassis and it reduces. Part of what is going on here is that you ( well your son ) is like an antenna picking up noise. Move your hand close to the sensitive areas i.e the keys and you get more noise. If you ground yourself my touching the chassis the the noise current flows to safely to ground. This begs the question, does the cover, not shown, have screening material and does that make a good connection to the chassis? If it does, fit it and see how it is.

The second issue, the safety earth to chassis, may be a side effect of the death capacitor. Deal with the cap.

[olddawg]

For test... the first thing I would do it use a ground lift adapter to see if changing ac mains orientation polarity and floating the chassis ground makes a difference. Then like the guys say, remove the death cap and install the grounded cord properly. Be aware that many retrofitted 3 prong ac outlets have no ground in old construction . You might want to check that to for whatever/where ever you are plugging into. Does it have the original filter caps? Also... if you unplug the input to the amplifier and instal a shorting plug, do you still have the noise? You may have something open, resistive, or unshielded in the piano section. Also run a signal source from an iPod or anything really into the amp input. Notice it has a volume control.