Customer repairs: R.A.P. receiver


R.A.P. Console Model, S/No 10533; Repair No 20

 The customer declared the set to be "not working" and there was little chance of it working as there was no mains lead
As the set is very unusual I have gone to more trouble to record information on it.

The radio is an expensive model made in the latter half of 1945 or early 1946 and uses many good quality military type components no doubt surplus to requirements of the day. The chassis is, unusually, chrome plated and is made in three sections which allowed different models to be fabricated from common sections. The better models in the series used a larger centre section and utilised additional valves (noted by the blanking panels fitted to this chassis). The cabinet rear panel is made of glass instead of the usual material so that the interior can be viewed and this explains why the chassis is finished in chrome plate and is mounted at an angle allowing one to see the construction.


 The valve line-up is standard being 6K8 frequency changer, 6K7 IF amplifier, 6Q7 detector, AGC and audio amplifier, 6V6 audio output and 5Z4 rectifier.
The IF was found to be around 360-380 KHz and is unusual, normally being 465 KHz in contemporary sets.

The overall design probably dates back to 1938.
The volume control is most unusual and may have its origin in the designs of the early 30's. The audio output is set at a switched potentiometer, comprising a combination of resistors, via a multi-way rotary switch of the make before break type giving the impression of smooth adjustment.
There is also a switched tone control and a two position selectivity switch.
The type of arrangement used for volume control is believed to be unique and may have been used because of a shortage of reliable, noise-free, rotary potentiometers.

Interestingly, certain safety aspects are considered pretty horrific by today's standards, notably:-
(a) there is an extension mains supply provided on the same panel as the extension speaker. Both use similar style sockets.
(b) extension speaker connections carry 300 volts, being designed for a mains energised type with a high voltage anti-hum circuit carried by the coil. (c) mains tapping holes are carried on the same panel as the speaker leads thus combining in one small area 240 volts AC, 300 volts DC and chassis.

The design of the antenna/earth connectors are useful in not requiring plugs although care must be taken not to short the end of the aerial wire to chassis.

The tuning control is not functioning

In summary, I reported that the radio is considered to be quite rare and despite the problem with the tuning should command a good price if offered to a collector. If restoration is carried out on the cabinet this should be done sympathetically to preserve as much as possible of the old finish.

Before connecting power to the set the following tests were made:-
(a) all passive components across the internal supplies were checked for shorts (see note 1)
(b) all resistors were measured (see note 2) and critical items fixed.
(c) critical capacitors were checked for leakage and value (see note 3) and critical items fixed.
(d) a safety check was carried out and results shown in note 4.

 Mains was then connected to the set. The rectifier valve was observed to be sparking and glowing purple, a sign of internal gas. No sound was heard from the speaker. The primary winding of the audio output transformer was found to be open circuit. One dial lamp was found to be u/s and two others were of the wrong type. The mains transformer was observed to be very noisy due to loose laminations.

As the dial pointer was very loose and inoperative it was tightened but when the tuning mechanism was operated the pointer was found to hit the ends of its travel when the tuning capacitor was still part meshed. Because of this the frequency coverage was considerably restricted. In addition because of the mechanical arrangement the pointer became loose after hitting the end of the range and fell to one end of the dial.
Examination of the mechanism indicated that in all probability a vital mechanical component had been replaced some time ago when a geared assembly connecting the dial pointer to the tuning capacitor had been replaced with a straight-through flexible bellows coupling. This had the effect of either reducing the tuning range of the receiver or, if tuning was attempted beyond the natural limiting effect of the pointer, forcing the pointer to follow its spindle and hence be placed up to 60 degrees out of true. If tuning was then attempted to the other end of the scale, the same thing happened and eventually the pointer would work free and no longer function. The tuning scale, if the pointer were to be re-fixed would be completely wrong, being up to some 60 degrees out.

 

The plastic tuning scale is distorted and slightly damaged through ageing.

See notes 1-4 for other faulty items needing replacement.

Repair:

 The major item requiring repair, the dial mechanism, was carefully examined. The solution adopted, because of the non-availability of a new component, was to revise the coupling between the tuning capacitor spindle and the pointer. A new mechanical assembly having a mechanical advantage of about 3:4 was made which links the two. Because the coupling is extremely critical the new part is adjustable (see note 5). The design of the new assembly requires a tensioning spring to ensure its correct operation and this is adjusted by trial and error to achieve proper operation (see note 5). The new coupling unfortunately results in a slight mis-calibration of the dial originally calibrated using a linearising device which had to be discarded in the new design. By altering the tracking of the tuned circuits the scale misalignment has been minimised.

The loudspeaker output transformer has been replaced with a pre-war universal type and the correct tapping found by trial and error.

The rectifier valve has been replaced with a GZ34 which is superior, drawing less heater current and having a better HT rating.

A burned out resistor in the RF gain circuitry has been replaced with one having a value surmised to be appropriate.

A resistor disconnected from the RF gain switch has been re-connected.

Two replacement decoupling electrolytics have been substituted for the faulty items.

New dial lamps have been fitted.

The coil pack has been re-aligned and tracked as closely as possible to the scale (see note 6).

A replacement mains on/off switch has been fitted. The original was missing and would have been unobtainable. The replacement is fitted to a contemporary potentiometer which has been coupled to the tone control via a brass bush. The coupling is fairly critical and may require resetting in the future.

A new mains lead has been fitted together with a blanking plate fitted over the hole for the original (missing) connector.

The tuning scale was reset by gluing transparent plastic pieces between its edges and the backing plate. This results in a reasonable degree of flatness and allows the illuminating bulbs to backlight the scale.

Parts fitted
Output transformer
Rectifier valve
Three dial lamps
Two 25uF 25VW electrolytic capacitors
240 Kohm resistor
Mechanical parts
Mains cable
Mains switch

Notes

 

1 Reservoir electrolytic, 8uF measured at 8.5uF and minimal leakage
Smoothing electrolytic, 8uF also measured at 8.5uF
Additional smoothing electrolytic, 4uF measured OK

2 Resistors (1st value is "indicated" ohms, 2nd value "measured" ohms)
Associated with 6K8
47K 48K
220 220
56K 59K
240 251
Associated with 6K7
330 271
100K 143K
68K 93.5K
220K 282K
Associated with 6Q7
100K 110K
4.7K 5.3K
470K 651K
Associated with 6V6
470K 621K
270 224
Associated with switched potentiometer
410 540
120 116 one end was found to be disconnected
1000 1139
2.2K 2.4K
8.2K 9.3K
15K 19.8K
33K 36.3K
68K 91.9K
120K 145.7K
3.9K 4.36K
240K found to be burned out and open circuit

3 The only leaky capacitors found were 2 x 25uF 25 vw decoupling the audio output valve and the AGC loop

4 The original mains switch had been removed and the wires twisted together uninsulated- the wires were soldered to the new switch
The mains tap panel carries the loudspeaker extension socket but as the speaker is a mains energised type and carries a potentially lethal voltage this facility must not be used.
There is a mains outlet socket provided on the panel. The safety aspects of this fall far short of current practice and must not be used.
A back panel should be fitted to the set as high voltages are present at the loudspeaker output transformer terminals.

5 New coupling:
The original coupling I believe geared up the tuning capacitor by a factor of about 4:3 and applied a frequency linearising motion to the pointer via a system of levers. The combination also ensured that the pointer was limited in its travel to 180 degrees. Without the step up gearing and the straight coupling used as a replacement, the pointer swept 240 degrees fouling the scale end metalwork and eventually becoming loose.
The new coupling gangs the tuning capacitor directly to the pointer in a mechanically linear fashion (i.e. not a linear frequency movement).
This results in some variation from true tuning and that marked on the scale.
A sketch is supplied of the new coupling which incorporates an adjuster to ensure smooth operation. As the mechanism is inherently not structurally determinate at its vertical position, being able to cross over and fold up, a spring is fastened to the higher of the bearings to constrain motion to that desired (i.e. a trapezoid shape). The spring is tensioned to overcome friction in the bearings and could conceivably require adjustment if the mechanism stiffened up. Some hesitancy in the movement of the pointer may be observed as it passes the vertical position- this is a consequence of the design and is normal.

6 Long waves
Dial reading followed by true frequency in KHz
160 163
180 173
200 189
220 204
240 226
260 248
280 272

Medium waves
Dial reading followed by true frequency in KHz
600 575
700 640
800 720
900 800
1000 890
1100 990
1200 1094
1300 1212
1400 1360
1500 1500

Short waves
Dial reading followed by true frequency in MHz; the third reading is the image frequency which will also be received (see note below)

6 5.9, 6.7
7 6.5, 7.3
8 7.2, 8.0
9 8.0, 8.8
10 8.6, 9.4
11 9.3, 10.1
12 10.5, 11.3
13 11.5, 12.3
14 12.5, 13.3
15 13.4, 14.2
16 14.8, 15.6
17 16.2, 17.0
18 17.6, 18.4

As the IF is only about 380 KHz there is significant "image" reception. This means that stations some 760 KHz higher in frequency than that desired will also appear at the same point on the dial. This is generally normal, but at such a low IF, the rejection capability of the receiver is only around 10dB, which on short waves is insignificant.

Retuning was carried out to optimise gain, image rejection and tracking (i.e. optimising gain across the whole wave band) and to attempt to minimise scale error.

 

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