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advance-in: Servicing

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » advance-in: Servicing
           
Michael Watterson
 
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21.Sep.18 21:54

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The four screws are captive to the front panel, so undo each in turn only a couple of turns.
Originally the case was not earthed. A 3A fused plug and three core flex with Earth wire to chassis makes it safer.

The Oscillator trimmers should not need adjustment, even after 70 years! The HT point at C11 should be connected to a 30V to 60V DC current limited PSU (10mA) via a meter to measure milliamps. Replace C10 or C11 if the current doesn't drop to less than 1mA. C11 had a slight encrustation. like leaked electrolyte on the positive end, so not surprising the current didn't drop below 7mA. Cutting it verified that C10 had dropped to less than 400uA (0.4mA) at 65V supply. So C11 was replaced with 4 x 1uF met-poly 400V parts as I had no 4uF part. The 6X5G rectifier can develop a heater cathode short thus burning out the mains transformer if there is excessive current. The 3 K Ohms, R24 will protect it if C10 later fails.

C9 was high leakage, being a paper dielectric part. All the other capacitors are Mica (silver rot rare outside USA) or ceramic and thus likely fine. The generator works fine on all bands and on 200KHz was within 1% stated tolerance.

All the connectors are the old UK style car radio type.

The stitching had failed on the handle which has a steel band inset, so the edges of the leather were glued.

The mains tap should perhaps be set one more than your nominal voltage, which you should measure as the EU 220V is allowed to be as much as 245 in the UK and 235V in Ireland. it's accessible via the rear plate. Unplug mains first as the wander plug could be live. The power switch, S3 on the audio out level, is only single pole.

Michael Watterson
 
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Pict.: 2497
25.Sep.18 09:56

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Almost every old signal generator I've seen has dire warnings about touching the preset trimmers or coils. This because almost no-one had a frequency counter, or indeed one with an oven or TXCO. Today you can get a TXCO referenced counter for as little as $20. At school we had both cheap older counters using decatrons as counting element and display as well as a more expensive unit using digits (Numitrons / Nixies). I've no idea what internal reference they used.

Unmarked algnment holes looking from rear:

(F) (A)

(E) (B)

(D) (C)

You can see it matches the layout of the band switch.

Use an insulated trimming tool with a metal blade tip as there is a voltage on the screw head. There is no indication of what frequency to align at. Make sure the two end marks of the scale line up. Allow at least 30 minutes warm-up time. Likely adjustment near the higher frequency end of each band is best. The original specification is 1%, so 198 KHz at 200 KHz or 29 MHz at 30MHz wasn't bad. Drift overnight on 30MHz on Band "A" was less than 12Hz. The frequency seems stable a few minutes after warm up. Changing the mains tap from 250V to 230V raises the frequency slightly (Our mains is just below 235V). On some bands you may want a compromise between the middle of the band and the band edge, but much better than 1% across the band is possible.

Connecting anything to the "Full RF" out can pull the frequency on Band B and especially on Band A as well as changing the output level, so I used a J310 n-JFET buffer between the full out and the socket as even 30cm of coax affects Band "A". Supply was rectified and smoothed 6.3VAC to Drain. Gate 220K Ohms and 33pF input. Source load 390 Ohms with 10nF coupling. Open, short, coax unterminated or 50 Ohm terminated at counter then had no effect. The circuit was on stripboard and is easly removed and socket reassmbled if originality is desired.

It would be rare to connect the generator directly to a radio aerial socket. Use about 10 turns of wire at about 10cm diameter as loop to couple to loop aerial or ferrite rod. The E Model 2 has a connection pad suitable for isolated or earthed chassis. If the chassis live you need isolation. A small ferrite (maybe from a scrap pocket radio) with 5 turns for SW and 10 turns for MW/LW of PVC insulated wire over the same number of turns of PVC insulated wire makes a 1:1 isoiation transformer. You can use 40 turns on a 1.5" plastic pipe if you have no ferrite. No pad is needed. The Pad circuit looks like a dummy aerial, I've not tried the pad, I'd try 470uH for the L12 for Broadcast band. Possibly a short will do for higher SW bands.

Michael Watterson
 
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Pict.: 2497
26.Sep.18 13:06

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I've uploaded details of a simple RF buffer that can be fitted temporarily for alignment, or left in place if originality isn't needed. It's easy for anyone to subsequently remove.

It is simply inserted between the full RF out and the front panel connector with the approximately 5mA power drawn from the 6.3 V AC heater supply.

  
rmXorg