This 1956 radio had been got at. The valves were all switched around in their sockets, only one being in its correct position and with a surplus UF41 but a missing UY41! Testing revealed; UCH42 50% triode/35% hexode; UF41s 15% and 28%; UBC41 75%; UL41 50%. Both the UL41 and the UBC41 had heater/cathode leaks making them unusuable in the AC/DC circuit. The heater ballast resistor was open circuit and one of the two dial lamps was blown. Because of budget constraints and the difficulty of getting new valves I decided to modify the circuit and proceed as follows:-
(a) Convert the heater chain to DC operation, fit a 1N4007 and a 47uF 400v reservoir and a 100uF 200v smoothing capacitor
(b) Fit a 1N4007 with a surge limiter in place of the missing UY41
(c) Fit a heater ballast resistor paralleling a 2.2kohm and 3.3kohm 11watt to cover the loss of the UY41
(d) Fit a pair of 24 volt dial lamps with a surge limiter into the heater chain
(e) Fit a modern mains lead and a pair of 1.25amp fuses
(f) Realign the RF stages
|To complete the work it was necessary to shunt the output valve grid leak with a 27kohm resistor to reduce the effect of its heater/cathode leak, fit a new coupling capacitor between the AF and output valves and to add a decoupling capacitor across the output valves bias resistor to improve gain. To improve the life of the dial lamps I ran them at 16volts instead of their rated 24. The set worked very well and looked good after the case had been cleaned and polished.|
Work necessary to bring the set back to life was as follows:-
(a) Replace the 10nF audio coupling capacitor
(b) Fit an alternative mains lead
(c) Realign the tuned circuits
(d) Refit the tuning capacitor because its mounting rubbers had perished
(e) generally tidy up the dial glass and cabinet
The valves had tested OK apart from the triode section of the UCH42 which had poor emission but worked well enough in prcatice. I left the old chassis-mounted mains plug in position (but unwired!) and fitted a new captive mains lead. This early model DAC90A uses an IF frequency of 465kHz rather than the 470kHz used in later models and marker idents on the rear of the dial backplate facilitate realignment of the RF stages. Slipping tuning is often caused by the tuning capacitor mounting bushes going brittle and falling out leaving the capacitor leaning over so always check these before fiddling with the dial cord.
The bakelite cabinet finish was restored by cleaning off half an inch of nicotine by first using isopropyl alcohol then rubbing with a light oil then finishing off by buffing with a proprietry dark liquid polish. This restores shine without leaving it prone to fingerprinting.
I always warn prospective users of these old AC/DC sets about the dangers of live chassis. Never use the set with a knob missing...always connect the blue wire to neutral and the brown wire to live if the plug is removed for any reason...don't poke things in the back...don't use out-of-doors..and in the case of this model don't connect an external aerial and certainly don't earth the chassis.
|This old set, first introduced in 1947 for £25:14s, was made in 1948 and is an AC/DC model. The set purports to have "Bush Bifocal Tone" which is basically the use of negative feedback to produce a more linear audio response. As with any AC/DC set there is a live chassis. In this example a previous owner probably unaware of this had decided to carry ouit a modification (now unknown) perhaps to add an extension speaker. The hole in the rear cover to facilitate the mod has been patched over, proably after the culprit, or member of the culprits family, had received a shock.|
The tuning knob had been damaged some time ago but the repair is sound and the knob should remain safely in place.
An odd feature of the radio is the "gram" facility. An external record player can be plugged intio the back but the only way the radio can be silenced is by unplugging the aerial and tuning the dial to a quiet piece of ether.
Valve tests produced the following results:- CCH35:triode 40%, hexode 28%; EF39 65%; EBC33 65%; CL33 50%; CY31 80%, and all were found to be serviceable.
Restoration work undertaken was:-
(a) Clean case and chassis and superglue cap of one of the valves. Renew speaker wires which had perished. Clean speaker cloth which was stained. Apply scratch remover to cabinet and polish.
(b) Clean off old grease & lubricate tuning mechanism. Fit new rubber bushes because the old ones were so bad the dial cord slipped off its drum.
(c) Apply switch cleaner to controls (volume, tone and wavechange switch)
(d) Test/renew capacitors; new AF coupling capacitor ( C32 is shown on the circuit diagram as 0.05uF; a 0.001uF was fitted and I decided to fit a 0.01uF 1200 volt component)
(e) Test/renew resistors. All were OK.
(f) Realign tuning. The IF was set to 465kHz and had drifted badly as had the long, medium and short wave settings of all the coils and trimmers. The set was very lively and its audio tone very acceptable once realignment had been completed and the set reassembled.
(g) Fit new mains lead with a 13Amp plug fitted with a 3Amp fuse.