northenElectric
New member
- Joined
- Jun 13, 2010
- Messages
- 1
Amp: Northern Electric (Canadian Hammond) organ preamp circa 1937, no internal B+ supply. Newly rebuilt, original power and output transformers. Please refer to schematic: http://farm5.static.flickr.com/4053/4694201969_4eb60a021b_b.jpg
Symptoms: somewhat quiet output and a prominent 60 cycle hum.
Here's what i've got so far after poking around with the scope and multimeter. Any ideas are appreciated!
- Power transformer measures 6.3v with centre tap to ground.
- Measured 6 to 8 volts 60hz ripple between chassis ground and either side of the output transformer secondary.
- No ripple on the primary.
- Only 0.1V ripple between two ends of secondary. Same 0.1V ripple is present on the B+ feeding this amp from the Leslie cabinet (320VDC).
- Checked for transformer leaks to ground, got infinity resistance between all transformer wires and ground. Tried disconnecting the centre tap ground, same results - I don't know the purpose of this ground.
- Checked output transformer with 12V AC adapter (actually reading 16V). Measured 1.25V on secondary, giving 12.8:1 ratio. Spec is 4000:200 which would be 20:1. Bad transformer?
Thank you,
Adam
Symptoms: somewhat quiet output and a prominent 60 cycle hum.
Here's what i've got so far after poking around with the scope and multimeter. Any ideas are appreciated!
- Power transformer measures 6.3v with centre tap to ground.
- Measured 6 to 8 volts 60hz ripple between chassis ground and either side of the output transformer secondary.
- No ripple on the primary.
- Only 0.1V ripple between two ends of secondary. Same 0.1V ripple is present on the B+ feeding this amp from the Leslie cabinet (320VDC).
- Checked for transformer leaks to ground, got infinity resistance between all transformer wires and ground. Tried disconnecting the centre tap ground, same results - I don't know the purpose of this ground.
- Checked output transformer with 12V AC adapter (actually reading 16V). Measured 1.25V on secondary, giving 12.8:1 ratio. Spec is 4000:200 which would be 20:1. Bad transformer?
Thank you,
Adam