Ground Hum with different transformer tap

[bjoneson]

Working on a Scully 280B deck from around 1977. The main power supply has multiple taps on the primary for different mains voltages.

It was set up for 115V, but has taps for 105, 125, 220, 240 as well.

I've been troubleshooting another issue with the machine with an unregulated supply running a bit higher than spec.

My mains voltage sits consistently around 122-123V, so I decided to move to the 125V tap on the transformer.

Now, any time I engage the transport, I get a very noticeable 60Hz hum from the outputs.

There are certain transport functions that do not cause the hum. Such as EDIT, and with the ATL defeated on fast wind mode.

I'm perplexed that simply changing the tap (essentially dropping the supply by 10V, would have this affect.

Returning to the 115V position removes the hum.

Any thoughts on what is creating this hum?

(PSU Schematic Attached)

 

[JohnRoberts]

without looking at the schemo if the power supply sags to lower unregulated voltage, any regulated PS may run out of headroom and drop out of regulation at the bottom of ripple troughs and pass that noise through on regulated voltage rails.

JR

 

[bjoneson]

without looking at the schemo if the power supply sags to lower unregulated voltage, any regulated PS may run out of headroom and drop out of regulation at the bottom of ripple troughs and pass that noise through on regulated voltage rails.

JR

John... thank you. I was just about to send a reply. There's a pair of big 2900uF computer grade electrolytics that filter the main unregulated supply. I'm 90% sure they're 40 year old originals.

It makes sense that at a lower supply voltage they may have trouble keeping up. Just noticed all the lamps (running off of a 24V regulated supply fed by this PSU) dim slightly when the transport is engaged.

I do think you're on to something,  and I'll check out the ripple to confirm.

Many thanks!

 

[bjoneson]

Sure enough... with the transformer tapped at 125V, the +35V unregulated supply measures 34.2VDC, but 30.6VDC under load. The bottom of the trough in the ripple is under 30V:

https://www.youtube.com/watch?v=sbeWua59jzc

With the transformer tapped at 115V, the +35V measures 37.3VDC (33.6VDC under load).

I've avoided changing put those filter caps, since they're a bit on the expensive side, and seemed to be functioning to this point. Though it appears their age has been masked by running the machine at a higher supply voltage.