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Sansui G-901 Transformer Buzz – Quiet Under Dummy Load, Plus 72 VAC Between Chassis and Earth Ground.

I'm currently troubleshooting a transformer buzzing issue with my Sansui G-901 and was preparing to measure AC ripple using my oscilloscope.

Before connecting the scope, I decided to check for any voltage potential between the receiver's chassis and the earth ground terminal of the AC wall outlet supplying power to the unit.

To my surprise, I measured approximately 72 VAC between the chassis and earth ground while the receiver was powered on.

I'm trying to determine whether this is normal behavior for this receiver, a high-impedance phantom voltage reading, or an indication of excessive leakage or an insulation problem.

I'm also hesitant to connect my oscilloscope's ground clip to the chassis until I understand what's causing this voltage, since the scope is earth-grounded.
A few questions for those familiar with vintage Sansui receivers:
  1. Is approximately 72 VAC between chassis and earth ground typical for a G-9000/G-901 with a two-prong power cord?
  2. Could this voltage be related to the mechanical transformer buzzing I've been investigating?
  3. What's the safest way to determine whether this is simply capacitive coupling or an actual leakage problem?
  4. Is it safe to connect an earth-grounded oscilloscope to the receiver's chassis under these conditions?
I'd appreciate any insight from those who have encountered similar measurements on vintage Sansui equipment.
Thanks in advance!
 
Thread title changed as requested - to
Sansui G-901 Transformer Buzz – Quiet Under Dummy Load, Plus 72 VAC Between Chassis and Earth Ground.
 
Mecii said:
A few questions for those familiar with vintage Sansui receivers:
  1. Is approximately 72 VAC between chassis and earth ground typical for a G-9000/G-901 with a two-prong power cord?
  2. Could this voltage be related to the mechanical transformer buzzing I've been investigating?
  3. What's the safest way to determine whether this is simply capacitive coupling or an actual leakage problem?
  4. Is it safe to connect an earth-grounded oscilloscope to the receiver's chassis under these conditions?
I'd appreciate any insight from those who have encountered similar measurements on vintage Sansui equipment.
Thanks in advance!
Click to expand...

Definitely not a Sansui exclusive situation.

1. Varies.
2. Not likely, IMO, if the buzzing is actually mechanical from the trafo.
3. Perform the leakage test that's often included early on in the service/repair manuals.
4. On the presumption it's just coupling it's probably not going to blow up anything. But, I think a lot of folks use isolation transformers.
 
I reviewed the Sansui G-9000 service manual but couldn't find any reference to a chassis leakage current test or the recommended procedure for performing one. What type of test equipment would I need, and what would be the proper procedure?

While investigating the transformer buzzing issue with my G-901, I took several AC voltage measurements between the amplifier chassis and the earth ground contact of the AC outlet.
All measurements were taken with the G-901 plugged into the AC outlet and the power switch in the OFF position.

I also reversed the orientation of the two-prong AC plug to see how it affected the readings.

1. Transformer Secondary Windings Connected​

With all transformer secondary connections in their normal configuration, I recorded the following:

AC Plug OrientationChassis-to-Earth Voltage
White-striped conductor connected to HOT (narrow slot)106.4 VAC
White-striped conductor connected to NEUTRAL (wide slot)2.22 VAC

2. Transformer Secondary Windings Disconnected​

Next, I disconnected and isolated all transformer secondary connections going to the F-2807 and F-2808 boards.
The amplifier remained plugged into the AC outlet with the power switch OFF.

AC Plug OrientationChassis-to-Earth Voltage
White-striped conductor connected to HOT (narrow slot)69.3 VAC
White-striped conductor connected to NEUTRAL (wide slot)0.91 VAC

​

Disconnecting the transformer secondary windings reduced the measured voltage from 106.4 VAC to 69.3 VAC when the white-striped conductor was connected to HOT.However, when the plug was reversed, the measured voltage was considerably lower in both configurations.

At this point, I'm trying to determine whether these readings are the result of normal capacitive coupling, leakage through an electrical component, or possibly an insulation issue within the power transformer.

My questions are:
  1. What equipment would you recommend for performing a proper chassis-to-earth leakage current test?
  2. Is there a recommended leakage current limit for vintage, two-prong audio equipment such as the G-9000/G-901?
  3. Could these measurements indicate a problem with the power transformer, or should I be investigating other components in the AC primary circuit, although there is not much there?
1792809691814.png
Any thoughts or suggestions on how to proceed would be greatly appreciated.

Thank you.
 
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