taylor: 65B; All-Wave Signal-Generator: Repair and Restoration

Initial condition

It seemed untouched inside. The old rubber & fabric mains cable was disintegrating and dial glass cracked.

I found an Instruction manual (Search for "Thevalvepage"). The manual has a basic schematic but without any values.

Repair

Two obvious capacitors were short circuit: A TCC cardboard & waxed paper 30nF (0.03MFD) 350V part for audio oscillator (C7 on my annotation) and a TCC "metalmite" 50nF (0.05 MFD) for grid coupling. I replaced the 30nF with a new 27nF 400V (some kind of plastic foil kind) and the 50nF with an ex-equipment 47nF 400V part.

I prised open the soldered can (easier than melting the solder) over the attenuator and RF out socket and  the 10nF DC isolation capacitor had no DC isolation. I replaced it with a 10nF ceramic.

The mains cable replaced with PVC 3 core and Earth to case. Isolation to case was OK and across mains too.

Plugged in and power tripped! I opened the cylindrical mains filter (C1 ... C4 & L1, L2 on my annotation) and found a loose coil with a centre tap. The coil had presumably "fallen over" and shorted to the can (earth). I could see two other coils inside, but schematic only has 2 not 3. I unsoldered the four Hunts 1nF parts that might be Silver Mica (OK) or paper (bad idea as "Y" capacitors). The lower coil was burnt out, at some stage insulation had failed and it had shorted to the common of C1 ... C4. Someone had added (badly) a 3rd coil to replace it. I mounted 4 x 470pF 1kV "Y" rated ceramic on an ex-SMPSU common mode choke (two coils wound on a ferrite ring), fitted insulation and tie wrapped this to the original mounting bolt. 

The HT was only about 100V. I checked Transformer drive to 6X5 and it is 110-0-110 AC

The unit was then operational. But after a short while there was no Audio or Modulation. After checking voltages I and swapping 6J5G tubes and windings of modulation transformer I concluded one or other  of the two capacitors I labelled  C7 & C8 must be open circuit or near open circuit. The 47nF had dropped to under 800pF! I replaced it with two of the new 27nF parts  in parallel (54nF). Audio out/modulation was now working.

I can get a new glass cut for under €5, but for now I superglued the two parts to keep out dust. What looked like corroded Aluminium polished up well with brasso (the dial bezel)

I checked the Attenuator and output on all ranges with Frequency counter and HP141T spectrum analyser with 8553B 11MHz/110MHz plug in. The calibration/stability very reasonable for a 1945 instrument. The 400Hz wasn't measured but viewed on 'scope. Quite distorted, by no means a nice sinewave (see the Operation article), but usable as "tone" to trace Radio set Signal path.

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