A Dynakit Stereo-70 I acquired recently has me concerned about the output transformers. I have made some measurements using the equipment I have on hand which consists of a Knight KG-625 VTVM and a Heathkit IB 2A Impedance Bridge.
DC Resistance Measurements:
Test Conditions:
Center tap disconnected from power supply electrolytic
Allied KG-625 VTVM
Output tubes removed from sockets
Left Channel:
Center Tap to front plate lead = 92 ohms
Center Tap to rear plate lead = 103 ohms
Center Tap to front screen lead = 32 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 192 ohms
Right channel:
Center Tap to front plate lead = 180 ohms
Center Tap to rear plate lead = 178 ohms
Center Tap to front screen lead = 58 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 390 ohms.
I am concerned that the transformers do not measure close to the same in the plate circuit measurements.
My inductrance measurements of the two transformers with their center taps disconnected from the power supply electrolytic were:
I set up the Heathkit bridge to operate using its internal 1kHz generator frequency.
Following the Heathkit instructions for the bridge the setup of the bridge that gave the lowest left hand 100uA reading of the AC level when the range switch set to the 10H range gave the lowest set-up reading after the AC set the meter to 100uA at left end of scale. Then the other dials are adjusted for best null reading as the generator level is gradually increased to maximum. The controls are basically adjusted for the best deflection back toward the left end of the scale on the meter.
DQ dial adjusted to read 1.0
Final readings of the CRL Dial were:
Left channel transformer 1.57 X Range dial reading of 10, so 15.7 Henries
Right channel transformer 1.42 X Range dial reading of 10, so 14.2 Henries.
A post on Antique Radio Forum said that he tested a known good transformer measured 192 ohms plate to plate and 300 Henries at 20Hz. My inductance readings are so far off from those posted on the forum that I am not sure which inductance readings are to be trusted. What are your thoughts? What is a normal inductance reading plate to plate of the A470 transformers?
Do you think the right channel transformer has a partially open winding that is being bridged somewhat by a carbon trace? I suspect it is bad.
Joe
KC5LY