Transformer update.
The K1801 power supply card has two transformers. I ordered them from Tektronix and they arrived in under a week. They are not cheap.
These are both Ferrite POT style transformers in 18 mm x 11 mm packages. The bobbin foot print is given in the attached pdf.
TR-285B ($193), core material 3E2A.
Primary, wire dia 0.0315" with insulation, this may be 20 ga. DC resistance = 0.0033 ohms. This is 3.9 inches of 20 ga wire.
Secondary, wire diameter 0.004" over insulation (difficult to measure), this may be 38 ga wire. The secondary winding is center tapped so there are two identical windings. DC resistance for one of the two matched secondary windings is 10 ohms. This is 15.4 feet of 38 ga wire.
When 0.1V RMS at 4.8 kHz is applied to the primary, half of the secondary generates 11.2V RMS.
TR-286B ($393), core material 3E27.
Primary wire dia 0.004" over the insulation (difficult to measure), this may be 38 ga wire. The primary is center tapped. The DC resistance of each half of the primary is 1.55 ohms.
This would be 28.7 inches of 38 ga wire.
Secondary #1 winding. This wire is smaller than the primary. I measured 0.003", again, this is very difficult to measure. I broke one of the wires trying to measure it. It may be 40 ga. This output is center tapped. The DC resistance of half of the winding is 12.6 ohms. This would be 11.7 ft of 40 ga wire.
Secondary #2 winding. This stranded wire is 0.028" in diameter (22 ga?). The wire is insulated as it goes into the transformer. The insulated wire is 0.047" in diameter. It has a DC resistance of 0.0061 ohms. This would be 4.5 inches of 22 ga wire.
With 2 V RMS applied to half of the primary, half of Secondary #1 developed 3.4 V RMS. Secondary #2 developed 0.0715 V RMS.
The TR-286 Secondary #2 feeds the primary of the TR-285 transformer. This looks like a technique to reduce leakage currents to the preamp. Insulated wire is used to reduce capacitive coupling inside the transformer and a low voltage excitation AC of less than 200mv is used to reduce capacitive coupling some more.
Before anybody asks, no I do not plan to take the cores apart and actually count the turns of wire. I can't count that high... The core material was the big unknown for me. The transformer winding details can be determined through experimentation.
All resistance and voltage reading have been double checked but I may still have made a mistake. All wire gauges are my best guess.
I got distracted this week when my K2002 showed up. It was calibrated in Aug of this year!. All the resistances measurements above were performed with the K2002 using 4 wire leads, offset comp was turned off. The inductance of the transformers may have disturbed the offset comp operation. I checked the K2002 against my 1 ohm Thomson standard before testing. The meter has the A10 firmware and I will upload that as soon as my ROM reader arrives. All the chips have 2009 dates codes so it's a pretty new meter. I will be doing the MEM2 mod as soon as my next package shows up from Digikey.