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
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
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
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
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.