Servicing a Grundig Marlborough 3028 Radio

 

 I was contacted about this circa 1958 German radio with the request to sort out an annoying hum affecting an otherwise working set. The obvious culprit was the smoothing condenser as the customer said that the hum was present even with the volume control at its lowest setting. I arranged for its delivery and parked it in the workshop.

Later, after I'd looked at the Radio & TV Servicing volume that covered this radio, I upended the case and removed its inspection panel. Lots of sets have this feature but in this instance the area covered was limited to the central area of the underside of the chassis meaning that chassis removal would be required. I identified the HT line and the output valve audio input coupling condenser, as this latter can be leaky. Another cause of hum could be a heater-cathode short or perhaps too much current being consumed by the output valve (due to a faulty audio coupling condenser driving the grid positive). Anyway, I connected a new 33uF 350 volt capacitor across the HT line whilst monitoring the ripple on an oscilloscope. Sure enough the hum lessened appreciably dropping from a volt or so to a tenth of this. Checking the grid of the output valve showed this was only a few tens of millivolts. The HT line measured around 180 volts but the schematic indicated it should be nearer 230 volts. The connections to the reservoir condenser were hidden so chassis removal was needed to check this.

 

 Chassis removal is straightforward except it entails detaching speaker connections. There are four large screws holding the chassis in place that require upending the case and once removed the chassis needed unsticking from rubber washers before it was free to move. I found I had to detach a length of black tape along the top of the dial glass to make things easier. This might be used to prevent light from the dial lamps from escaping above the dial. Once the chassis has been pulled backwards the four speaker connections needed to be unsoldered at the transformer (shown above). I can add long wires later to check all is well before refitting the chassis. Note the reel of solder.. in many cases adding a bit of solder to an old joint helps unsoldering.
 
 

 The radio is awkward to work on. There's only a small inspection panel under the set so to access the HT power components means the chassis must be removed from the case. The wires to the three loudspeakers need to be unsoldered and temporary leads used to connect the output transaformer to the speakers.

Because of the wide glass dial and the ferrite rod assembly the chassis can't easily be turned over for access to the underside.

 

 I'd noticed, when first powering up the radio to check on the hum, several points of interest. Firstly, although the set worked well on FM, AM reception was weak. This mode uses a ferrite rod and rotating this proved it was working because there was a precise null as it turned. Secondly, dial illumination was poor with one side dim and the other absent. The reason can be seen here. The effect of AC on filament lamps is to deposit metal onto the inside surface of the glass resulting in lessening light output until the filament finally goes open circuit.

 

 
 

 AM/FM tuning is interesting.

To avoid losing a favourite AM or FM station when switching between AM and FM the two modes are separately tunable. This is done by a clever clutch arrangement which is still, after over 60 years, (almost) working perfectly.

Note the oddly shaped flywheel which isn't particularly effective because the whole tuning mechanism is stiff and needs lubricating.

 

 Above you can see the connections to the reservoir/smoothing condenser on the left and, on the right, connections to the selenium rectifier. Note the 2-core mains lead on the left. Below are other views.

 

 

 

 

 Nothing special about the condenser except one of the two (as expected) measured open circuit whilst the other (used as the reservoir) showed up as 57uF with an ESR of 0.45 ohms. The selenium full wave bridge rectifier is marked B250C75 indicating a rated voltage of 250 and a current of 75mA. This rating representing 600 volts max peak repetitive input with a maximum output current of only 75mA. I've not seen a selenium rectifier having this shape before.. usually they're housed in a flat aluminium-cased package.

I tested the rectifier and noted forward readings of about 1.2 volts for positive and negative connections with no anomalies apparent, however the HT was pretty low being 50 volts down so either the rectifier isn't doing its job or the reservoir condenser, although checking good with my ESR meter, isn't working well when HT is applied across it. The obvious option is to fit a pair of new capacitors plus a new silicon-based bridge rectifier with a ballast resistor (to be selected) to help mimick the poor efficiency (and lower voltage output) of the selenium component. I'll fit a 33uF reservoir and a 220uF smoothing capacitor. A tiny RB156 rectifier rated at 800 volts and 1.5A wired to a new tagstrip should fit the bill.
 

 Above is an under-chassis view. Connections are made by very thin wires in thinner than usual insulation. Because of the poor AM performance I'll need to check the various resistors and condensers, although it could just imply bad alignment.

 The first change I made was to replace the selenium rectifier with a silicon bridge rectifier and fit a 470 ohm ballast resistor. The old rectifier has a significant voltage drop even when new and to compensate for this I added the extra resistor. The original circuit uses part of the audio output transformer as a low frequency choke to help set and smooth the HT voltage. This is left in place but a new reservoir capacitor and a significantly larger smoothing capacitor in combination with the ballast resistor produce a much reduced hum level (even over the original circuit). See drawing below.

The two-wire mains cable without a ground wire has been modified by a previous owner to include a ground connection via a lead fitted with a wander plug to fit into the receiver earth socket. Without this inserted results in some modulation hum.

 I measured the output from the rectifier as 292V and at the ballast resistor feed 259V. The receiver consumption is therefore about 70mA. The feed (a gold coloured wire) from R42 goes to a new 220uF the smoothing capacitor and measured 215V.

A pair of new 6.3V 120mA lamps were fitted. Audio for FM and AM was clean and hum free but the AM band needs some work to improve overall performance. A slight puzzle was poking in a long wire aerial produced nothing extra by way of reception. I looked at the schematic and it showed a switch so having checked the rear socket for a built in switch and finding nothing I rotated the ferrite rod beyond its furthest anti-clockwise position and sure enough there was a loud click and Radio Somerset boomed through... in fact I had to back off the volume control as I guess the AGC isn't coping too well.

A quick check with a signal generator showed the IF response was fairly clean at 466KHz, a bit lower than its factory setting of 468KHz. Below is an alignment aid. Full tracking is only available for the medium waveband with LF settings adjustable for long and short wavebands. See the Radio Servicing document for more details.

 I aligned the IF amplifier to 468KHz after noticing the transformer coils were reversed to that shown in the servicing details. I also aligned the medium and shortwave bands.

I then decided to change a couple of capacitors.. these being a 47nF AGC decoupler and a 4.7nF audio coupling capacitor, both of which need to be leak free. I noticed that the main tuning condenser crackles at near full mesh although no plates are visibly bent, and whilst checking this I discovered complete SW deafness must be due to a discontinuity between the aerial winding and the aerial scket or perhaps an open circuit RF coil because touching the aerial directly onto the tuning condenser resulted in SW reception. Tuning Radio 4 Longwave produces about 2.5 to 3 volts of AGC voltage so this is probably satisfactory bearing in mind this is a domestic set and not a communications receiver. The magic eye is driven from the AGC line and works OK when strong stations are tuned however I'm not too sure looking at the schematic whether its used in FM mode.
 
 

 I noticed some mechanical noise when intially switching from VHF FM to AM and found it was due to a stiff mechanism which prevented the FM clutch from immediately disengaging. It took about 10 seconds for the brass part to slide back against the spring until I lubricated it.

I also found virtually no flywheel tuning effect until I'd lubricated the numerous small pulleys carrying the wires and cords.
 

 The new power supply components in place with the improved mains lead which had consisted of a three core cable terminated at an insulated terminal block which connected to the old two wire mains lead and a flying lead carrying the safety earth.

To complete this I secured the new capacitors to the chassis with tie wraps.
  
 
 

 A view of the tuning condenser which carries, on the right the oscillator tuner of 433pF and on the left the 512pF RF tuner.

At something like three-quarters mesh there's a loud crackling noise which is probably due to something shorting the plates.

I can't see how to fix this but as there isn't much to hear on the far right of the dial by way of broadcasts it shouldn't be a real problem for a listener.

 

 The receiver back in its case. It appears the shortwave aerial connection might be getting lost in the push button switching so I soldered a 68pF capacitor to the tuning condenser and wired this to a terminal fitted in the rear panel. This enables shortwave broadcasts to be received. Because the connection isn't switched this terminal can also be used for enhancing medium and long wave reception. The drawback is it might introduce locally generated noise into the set as it bypasses the ferrite rod switching. Ideally the proper aerial socket for long and medium waves is preferred because this only connects when the ferrite rod knob is turned the its off setting. For short waves this isn't an option.
 

  

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