HMV 1121 Receiver

 This wooden-cased receiver dating from 1950 arrived at the workshop in late May 2016. I've seen this particular model a few times before so was quite familiar with its construction. See one and another. This one has the serial number A15-120410 with the Royalty Paid license plate AF10034.

 

 

The design minimises the size of front of the set by placing the dial directly in front of the loudspeaker and arranging that sound can escape by tilting the glass sections at an angle. Thus the overall size of the dial is maximised and a large speaker can be fitted within the same space. There's a drawback, in that the glass needs to be thicker than normal because it may be used as a handle when lifting the set and the larger than average glass area means more risk of breakage. This view is supported by a missing glass strip in this example.

 
 

I decided to deal with the missing glass first. To fit new glass means the assembly shown above needs to be removed from the case. The glass pieces are held in place by brown felt strips and a small screw through the end of the left slot. I found a piece of glass and scored it with a glass cutter. Unfortunately the glass refused to break along the scored line as does virtually any glass that's not freshly made. After a couple of attempts, the results of the latter were quite spectacular. A piece of glass of correct thickness taken from the door of an old oven shattered into thousands of fragments instead of breaking along a scored line. Clearly safety glass is not suitable and I decided on an alternative.. a piece of plastic. After wandering around for ten minutes I spotted a stack of scrap laptops. One of these had a plastic front to the display (others had glass). I removed the display panel and dismantled it into its components, one of which was a plastic sheet which varied in thickness along its height but was exactly the right width for the HMV receiver dial. I scored the plastic, then decided to use a hacksaw and file and after fifteen minutes had a new plastic replacement for the broken dial section. I'll see about marking it later, but I won't be using exactly the same method as the manufacturer. Trotting around the School of Signals radio museum I noticed lots of exibits had little familiar looking labels stuck on them. Something about them reminded me of CND before I realised they were radiation hazard warnings. Like lots of WW2 radio equipment whose knobs are marked with luminous paint, this series of HMV radio uses a radioactive paint to make the dial lettering fluoresce to improve readability. In fact the glass strips are used as light guides by fitting a pair of incandescent bulbs at their ends making the markings glow when one of the four wavebands is selected.

 Below is the circuit diagram. The valves are a mixture of types reflecting three generations of design. V1 is an X78 (B7G), V2 is a W77 (B7G), V3 is a DH77 (B7G), V4 is a KT61 (IO) and V5 is a U10 (B4). Tone of reproduction is catered for by switching in different values of condenser in the audio section (S25-S28), rather than fitting a tone control potentiometer. To this set of switches is added S23 and S24 which broaden the IF bandwidth to allow more of the incoming signal to reach the loudspeaker (something I've not seen before). S29-S31 is a rear mounted switch for using a record player connected at P.U. (a feature usually carried as an extra position at the wavechange switch).

The four wavebands which unusually include the trawler band (SW1) are 16-50m (SW2), 50-187m (SW1), 187-582.5m (MW) and 719-2026m (LW). I imagine that SW1 listeners would be mainly interested in the HF end of the band which would carry African broadcasts together with a few European stations.

 

 
 The first thing I usually do when carrying out a repair of an old radio is to check there's nothing really serious that would make a repair impossible or uneconomic and powering it up should help indicate this. As with nearly all receivers of this vintage using a mains cable made from rubber I replace it with an alternative. Lately, instead of fitting a plastic covered cable, and where the set is not being restored to its original condition, I fit an IEC connector allowing use of a kettle lead (the type used with desktop PCs). There are three options. Adding a metal plate to the side of the chassis and cutting a hole in the rear fibre-board cover; cutting a hole in the chassis itself; or using an existing chassis hole. In this case I decided to detach the external speaker socket and fit the new IEC mains socket in its place. After a small amount of filing the new socket fitted easily and a short length of cable finished the job.

 

 

 

 With a safe way of powering the set I connected a loudspeaker and applied power. Something was wrong. Although valves lit up and HT started to establish some wisps of smoke appeared so I turned it off. I decided to check the two electrolytic condensers C34/C35 by disconnecting their red and brown leads and applying HT to each condenser in turn via a 1.8K resistor. I'd already checked the condensers with an ESR meter and found one to be OK but the second showed a short. However, powering with an HT supply showed both to be OK with only a small and acceptable leakage. Presumably the short indicated by the ESR meter had been removed when HT was applied.

I removed the rectifier valve and turned on the set, but using an external variable HT supply. Connecting the HT to the red lead (from C34) and turning up the voltage showed a drain of 50mA at 200V but as the voltage was increased the current rose to over 70mA and wisps of smoke appeared again (from R19). Unplugging the output valve (an EL37 which was in place of the original KT61) reduced the current drain to a low value of circa 15 to 20mA. Checking the grid connection at the IO valve socket revealed about 1.5 volts. This fault is a stock problem with nearly all the valve radios I've looked at.

The problem was a leaky condenser C24. This couples the audio stage anode to the output stage and any leak in this (because of the high circuit impedances involved) places a small positive voltage on its grid which causes the valve to draw too much current. As with many leaky condensers the leak increases as leakage current raises the internal temperature of the condenser hence after a minute or so the HT current would rise to the point where R19 smoked. I located C24 and replaced it with a new, modern capacitor. If you look at the circuit diagram you'll see that C24 is buffered from V4 grid by C25 (47pF) and therefore the effect of leaky C24 will not be apparent unless C28 (part of the tone selection) is closed. As the EL37 cathode decoupling condenser (C31) was adjacent to C24 I and tested it with my ESR meter. This indicated that C31 was open circuit so I removed it and checked it out of circuit. It was indeed open circuit so I fitted a new 47uF capacitor. Retesting showed that the HT current was now stable although a bit on the high side for comfort, but by now the receiver was working and pulling in stations at good strength. I swapped the EL37 for a 6V6GT and the HT current dropped by 20mA or so. Removing the external HT supply and refitting the U10 rectifier proved the set was now working normally.

The next job is to align the IF strip and the four wavebands because over the past 65 years components will have drifted and no doubt at least one twiddler will have messed up the tuning. To assist with alignment the designers have incorporated a scale (in inches) printed on the inside of the large tuning pulley fitted on the tuning condenser spindle. There's a pointer which is used to make readings and this needs to be aligned to exactly 9 inches when the tuning condenser is fully meshed. You don't need to move the pulley, just bend the pointer.

It's important to take care of the dial cord and to check all is well because you don't want the cord to detach itself once the set has been reassembled and returned to its owner. Visually check that the various cords aim exactly at the centre of the grooves in the pulleys. In this set the cord detached itself during initial testing and was due to the position of the large pulley being a tiny amount out of alignment. Repositioning the pully by slackening its securing screws, moving it a millimetre along the tuning condenser shaft fixed the problem. There's nothing worse than a customer on the phone reporting the tuning knob has no effect.

 I tested the electrolytic condenser C34/C35 on the right of the chassis. Looking at the hole for the valveholder I suspect the designers must have intended the rectifier to be fitted adjacent to the mains transformer but moved it onto the bracket holding the fuses etc. I think this was done to make it easier to fit a new rectifier and as a benefit,to allow air movement to cool the HT smoothing resistor R19 (just visible through the hole in the chassis. Resistor R20, also used for smoothing the HT line has been fitted conveniently close to C34/C35. I like the wire clips used to hold the wires to C34/C35. I wonder if C34/C35 was added to reduce hum as there are already two other condensers C30 and C33 (reservoir). Clearly part of the redesign and no doubt influenced by HMVs QA Department. I wonder if the design change specification was coupled with "absence of smoke from R19". The replacement EL37 didn't help, smokewise, especially when it's grid was positive as it will draw more current than the original KT61. You often get lots of unused chassis holes in receivers because of variations in different models in the range. For example a more expensive set might have a pair of output valves to provide greater speaker volume.

 
 Above, the leftmost spindle is the volume control with mains on/off switch. This is a replacement Radiospares potentiometer and was fitted many years ago by a repairer who fitted labels (see below) to the wires so he could refit them easily. The coupling condenser C24 connects within the tag strip to the 33K resistor near the bunch of red wires. C31 is visible near the left end of the 33K resistor. On the right under the metal cover holding the trimmers and coil adjusters is the coilpack.

 

 

 

 

   

View of the RF alignment adjustment points (see above for their designations). These all look to be untwiddled and are set exactly as they were in 1950. 
 

 
 These tables give the manufacturers details for RF alignment of the receiver using the markings on the circular dial rather than those on the tuning scales. Before starting alignment it's important to check that the maximum mesh of the tuning capacitor corresponds to the marking of 9 inches (see above). Once alignment is complete the dial pointer assembly can be moved to correspond with markings on the glass strips. "ROCK" and "RETUNE" may reflect difficulties in RF alignment on the production line?

COIL

 ADJUST FOR MAX

 FREQ

IF

 L18

 465KHz

IF

 L19

 465KHz

IF

 L17

 465KHz

IF

 L16

 465KHz
 

 REF

 ADJUST FOR MAX

 FREQ

 NOTES

RF

 9

 TUNING

MIN 

SET DIAL 

SW2 

 8+7/16

 L12

 6MHz

 TUNE

SW2

 8+7/16

 L5

 6MHz

 ROCK

 SW2

 8+11/16

 L12

 6MHz

 RETUNE

 SW2

 13/16

 C41

 17.8MHz

 TUNE

 SW2

 13/16

 C36

 17.8MHz

 ROCK

 SW2

 13/16

 C41

 17.8MHz

 RETUNE

 SW1

 6+5/8

 L13

 2MHz

 TUNE

 SW1

 6+5/8

 L6

 2MHz

 ROCK

 SW1

 6+5/8

 L13

 2MHz

 RETUNE

 SW1
 27/32

 C42

 5.5MHz

 TUNE

 SW1

 27/32

 C37

 5.5MHz

 ROCK

 SW1

 27/32

 C42

 5.5MHz

 RETUNE
 

 REF

 ADJUST FOR MAX

 FREQ

 NOTES

  RF

 9

 TUNING

 MIN

 SET DIAL

 MW

 7+3/16

 L14

 588KHz

 TUNE

 MW

 7+3/16

 L7

 588KHz

 ROCK

 MW

 7+3/16

 L14

 588KHz

 RETUNE

 MW

 7/16

 C43

 1605KHz

 TUNE

 MW

 7/16

 C38

 1605KHz

 ROCK

 MW

 7/16

 C43

 1605KHz

 RETUNE

 LW

 7+1/2

 L15

 162KHz

 TUNE

 LW

 7+1/2

 L8

 162KHz

 ROCK

 LW

 7+1/2

 L15

 162KHz

 RETUNE

 LW

 1+1/2

 C44

 353KHz

 TUNE

 LW

 1+1/2

 C39

 353KHz

 ROCK

 LW

 1+1/2

 C44

 353KHz

 RETUNE

 A few comments on the alignment which can be a bit messy... the trimmer condensers are fixed by 4BA nuts (note the serrated washers) and these nuts should not be touched whilst the coil cores are secured by their 4BA nuts which are slackened for adjustment. The trimmers are not a problem using a slotted adjustment tool but the coil cores are little awkward to tighten once adjusted. As is usual, the higher short wave band needs to be aligned with care as the 465KHz IF means it's easy to incorrectly choose the image rather than the correct frequency.

I found that the rotary switch needed a few squirts of switch cleaner before intermittencies were cleared up.

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