This is the third Howard 474 to enter my
collection, purchased late 2015. It is the first of my three to have a
walnut coloured cabinet. The significance of the 474 is that it uses a
Fremodyne circuit for the FM section.
The Fremodyne is a single valve (using
a 12AT7 or 7F8/14F8) super-regenerative superhet FM receiver developed
by the Hazeltine Corporation. It appeared post war as a low cost option
for FM reception.
Fremodyne circuits appeared in a few mantel
radios around 1947-1949, and also stand alone FM converters, for use with
existing AM receivers or audio amplifiers. Many of these have been described
elsewhere on this site. The Fremodyne circuit has been extensively covered
elsewhere on this site. See here
for a detailed description of this unique method of FM reception.
My first Howard 474 was purchased almost
10 years ago, and it is described here.
A second one was purchased a few years later, but has not been restored.
This article describes my third acquisition.
It arrived in good cosmetic condition,
except for a crack on the top of the cabinet. It is hard to say if this
had been there for a long time, or was a result of the postage - it was
rather tight in the box.
This set had the aerial connection instructions
fairly well intact.
Aerial connection instructions on back of set.
Inside the set was something of a surprise. There was a radio shop repair ticket from 1949. That may well have been related to the interesting repair I found here:
The large dropping resistor at the side of the tuning gang is foreign
and had been installed to power the 6.3V dial lamp from the 120V mains.
The No. 47 dial lamp (6.3V 150mA) had been rewired to run off the 120V mains via an 850 ohm 20W resistor. A piece of asbestos had been glued to the side of the cabinet to reduce the effects of heat dissipation. An attempt had been made to mount the resistor by securing it in a spring clip which was soldered to the back of the dial. However, spring steel does not solder, and needless to say the whole lot had become detached and was flopping around. Interestingly, the resistor measured open circuit. The 474 is different from most U.S. made mantel radios in that instead of a valve rectifier with a heater tapping to run the dial lamp, a selenium rectifier is used to obtain the B+. This necessitates including the dial lamp directly in series with the other valve heaters. To prevent the surge current blowing the dial lamp when the set is first switched on, a thermistor is included. When I looked underneath, I could see the thermistor had been replaced with a 200 ohm 10W resistor.
Asbestos had been glued to the cabinet to protect it from the dial
lamp dropping resistor heat.
Evidently, the thermistor had failed, and
with a replacement not available, an ordinary resistor was used. Since
the dial lamp can no longer be included in the heater string, it was powered
from its own dropping resistor. It's certainly a way out of the problem,
but causes the power consumption of the set to be almost doubled, and adds
18W of heat to the inside of the c
The next strange thing I noticed appeared
to be a manufacturing error. The earth connection isolating condenser had
actually been connected to the FM aerial terminal.
The tubular condenser next to the 12BE6 is for chassis isolation.
Above it, with the red band, is the 2.2uuF FM aerial coupling condenser.
Wired thus, the FM input is shorted to earth, and there is no earth return
for the AM aerial primary coil.
In this situation with the FM input effectively
shorted to earth, reception would be limited to very strong local stations,
and any attempt to use the external AM aerial terminal would have
worked poorly.
At this point, it is relevant to examine
the circuit diagram.
Circuit of the Howard 474. it is a conventional AM receiver with
a Fremodyne FM receiver added on.
The AM receiver.
The AM section is completely conventional,
using a loop aerial built into the cabinet back to feed the input of a
12BE6 converter. The local oscillator section of the tuning condenser has
specially shaped plates which avoids the use of a padder condenser, and
should provide superior tracking. The loop aerial has a primary winding
of a few turns which allows the connection of an external long wire aerial
in weak signal areas. The external earth connection is via the previously
mentioned .02uF condenser (17) to the chassis. This is required because
of the live chassis design.
A remote cut off valve, type 12BA6, performs
as a 455Kc/s IF amplifier in the usual way. The second IF transformer then
feeds the diode plates of a 12AT6 to provide demodulated audio, and AGC
voltage.
The 12AT6 is an earlier version of the
12AV6, and is plug in compatible with similar characteristics. The
audio is filtered by means of a 47K and two 100uuF condensers before proceeding
to the volume control. The demodulated output is also filtered by the 2.2M
(43) and .05uF (16) to remove the audio signal. This provides a negative
going DC voltage which is proportional to signal strength, and thus becomes
the AGC voltage. This is applied to both the 12BE6 and 12BA6 grids. There
is no AGC delay, which means the set can be desensitised with noise, and
that the gain starts to reduce even with weak signals. Nevertheless, this
system of simple AGC works well enough and was standard in many sets.
From the AM/FM switch, the audio then feeds
the grid of the 12AT6 triode, functioning as the first audio amplifier.
Contact bias, by means of the 10M resistor, simplifies the circuit by dispensing
with the cathode resistor and bypass condenser that would otherwise be
used. With high gain triodes, the grid will charge up to a negative voltage
because of the electrons emitted from the cathode.
The grid resistor must be kept high for
this scheme to work, otherwise the charge is drained away too rapidly,
and the voltage is too low. Hence, grid resistors in this situation are
usually 4.7M to 20M.
Extra hum filtering for the 12AT6 plate
circuit is provided by the 47K (46) and .1uF (13).
The output valve is a 50L6 and operates
under completely conventional conditions. There is no negative feedback,
with the circuit simply relying on the 300uuF RF bypass condenser (21),
and the 50L6 plate bypass of .002uF (11) to provide acceptable frequency
response.
Power Supply.
Up to this point, the circuit is much
the same as hundreds of thousands of what are these days called "All American
Five" receivers. And the performance is the same. Like the AA5 circuits,
a series heater live chassis power supply is used, which allows AC or DC
operation from 105-125V AC or DC. One has to wonder just how often such
sets were used on DC, but the real reason for using a transformerless power
supply is for a much lower cost. Indeed, some such radios were really penny
pinching minimalist affairs, with short cuts taken wherever possible.
The chassis in the Howard is not directly
connected to the mains, but instead has a separate earth bus bar to which
all the earthy connections are made. This is in turn connected to the chassis
via a .22uF (18) isolating condenser. This is required for stability and
hum reduction. However, the reactance of .22uF at 60 c/s is 12K. At 120V,
this means a maximum current of 10mA can flow to earth. This is enough
to give an unpleasant shock, although probably not lethal in most situations.
The chassis and its securing screws are easily touched from behind the
cabinet, so there is a risk of shock unless the set is run from an isolating
transformer.
For this reason, the external earth connection
is made via a much lower value condenser, of .02uF (17). This gives a maximum
current to earth of closer to 1mA which is quite safe.
Unlike the usual AA5 circuit, there is no 35W4 or 35Z5 rectifier. With an extra valve added to the heater chain (the 12AT7 used for the FM section), there are not quite enough volts available to heat a valve rectifier, so a selenium rectifier is used here. The B+ power supply is a conventional half wave design, but actually does use a filter choke. In this regard, the set was not a penny pinching design.
All the valve heaters are connected in
series, along with the 6.3V 150mA dial lamp, and a thermistor, across the
mains. The thermistor is required because as the valves have a low cold
resistance (about 100 ohms for the whole string in this set), a large surge
current flows (a little over 1A) when the set is first switched on. Because
the filament in the dial lamp runs white hot, it is much thinner (i.e.
fragile) than the valve heaters which run at red heat. The effect is that
the valves warm up much more slowly than the dial lamp. Unless a thermistor
is included, the dial lamp would be subjected to this high surge current
and immediately burn out. The thermistor starts off with a cold resistance
of 791 ohms and falls to 100 ohms once fully warmed up with 150mA flowing
through it.
In most U.S. made series heater radios
made post war, a rectifier valve is used with a tapped heater across which
the dial lamp is connected. This means the dial lamp voltage rises slowly
at the same rate of the valve heaters. However, the scheme has some disadvantages.
I discuss it further in the Perco FM Converter article here.
FM Section - The Fremodyne.
The most important aspect of the Howard
474 is of course the Fremodyne FM receiver. Essentially, one section of
the 12AT7 is a conventional local oscillator, operating at 23.75Mc/s
above the carrier of the received station. The other 12AT7 triode operates
as a mixer with the VHF signal fed into its grid, and as a self quenched
super regenerative detector based around the plate and cathode circuits.
It operates at the IF of 23.75Mc/s. The aerial coupling condenser is of
low value (2.2uuF) to reduce aerial loading effects, and it also provides
mains isolation. The input is unbalanced, although the instructions imply
that a balanced transmission line can be directly connected. Of course,
the noise reducing characteristic of a balanced line is negated when connected
this way. A mains aerial is also included. This consists of chokes (63)
in both mains supply leads to isolate the mains cable (at VHF) from the
chassis. This allows the mains cable to function as a simple aerial in
good reception areas. A 100uuF condenser (68) isolates the mains supply
from the user and couples the signal into the FM aerial connection.
The detector has automatic stabilising
circuits which eliminate the need for a regeneration control. Audio output
is taken from the detector cathode circuit, with quench filtering and de-emphasis
provided by the 100K (38) and the 1000uuF (25).
As well as switching the B+ to either the AM section, or the Fremodyne circuit, the AM/FM switch also selects the audio source fed into the volume control. A problem that can occur here is that if one section is not used for long periods, the relevant cathodes become poisoned because of no cathode current flowing while they are heated. Indeed, a ruined 12AT7 has turned out to be not uncommon with these AM/FM mantel radios using Fremodyne FM receivers, when the set was only ever used on AM. One way around the problem is to supply a low current to the section not in use, as I did with this receiver.
The unrestored chassis. The replacement 200R heater dropper can
be seen. The grey and red wires leading to the bottom corner are not original
and supplied the dial lamp.
With a few resistors then replaced, that completed the B+ power supply, audio, and AM sections. Next was to deal with the Fremodyne. Here, service access is poor because the circuitry is assembled onto a sub chassis with the tuning gang. This is mounted on top of the chassis, and it is awkward getting into the depths. Furthermore, some of the components are mounted inside the FM IF transformer and this would have to be opened up.
Inside the Fremodyne's IF transformer. The actual 23.75Mc coil is
the one closer to the adjustment slug. Nearer the resistors is the 12AT7
cathode choke winding. Although mounted on the same former, any inductive
coupling is merely coincidental.
By desoldering a few connections, and a
couple of earth straps, the sub chassis is removable. Most of the resistors
were found to be high, including the 150K grid stabilising resistor. In
this set it is mounted inside the IF transformer. With wires wrapped around
tags it was a pain to extract, and I broke a tag as a result, but was easily
fixed. The 10uF electrolytic (9
Upon reinstalling the tuner sub chassis,
I could now turn to the heater and dial lamp wiring.
Restoring the heater and dial light
circuits.
I thought about the options here; ideally
using a thermistor to restore the original circuit. The only types I had
were CZ9A Brimistors, made by STC. These looked suitable at first with
a cold resistance of about 1000 ohms, but alas when I tried one the dial
lamp promptly blew. The trouble was the warm up time was way too fast.
Once I actually looked at the data I could see why. The CZ9A is totally
unsuited to valve heater strings, as it has a very rapid warm up time.
It's intended function is for things like reducing transformer inrush current
and incandescent lamp surge protection. The Brimistor data did show types
which would be ideal, such as the CZ3, but alas there's no way I would
find any in the modern day. In Australia, series heater circuits are few
and far between, so the demand was always minimal.
Dial lamp shunt resistor method unsuitable.
So, that meant using a normal dropping
resistor for the valve heaters. The dial lamp was then the remaining problem.
I considered (and tried) the shunt resistor method as I used in the Perco
and Emerson CF255
receivers. However, the shunt resistor had to be such that when the valves
had actually warmed up, there was only about 3V across the dial lamp. This
was just too dim.
In the Howard, the valve heaters add up
to 100V. The dropping resistor should then be 130R. As the valves have
a cold resistance of 100 ohms in total, when the set is first switched
on, the surge current is about 500mA. With a dial lamp of 150mA, it's obvious
it has to be shunted by quite a low value resistor to keep its voltage
below about 7V when the set is cold. Thus, once the valve heater string
current has reduced down to the normal 150mA, the dial lamp is now grossly
underpowered.
The reason for this method's acceptability
in the Perco and Emerson receivers is that the valve heater dropping resistors
are of much higher value, because of the lower heater string voltages (483
ohms and 250 ohm respectively). This in turn means the surge current at
switch on is much reduced, and the dial lamp shunt resistor doesn't have
to be as low.
(A variation of this method is to also
have the B+ current flow through the dial lamp and shunt resistor. As before
the lamp will run dim after the initial cold surge, but once the valves
warm up, an extra 60mA or thereabouts, flows through the dial lamp and
shunt resistor, restoring some of the brightness. When I restored this
474, I was not aware of this method so did not try it).
Next option was to operate the dial lamp
separately again, as our previous repairer had done, but with a bit more
efficiency. 18W just to run a dial light was not acceptable, especially
if the set was to operate off the 12V house supply via an inverter. A capacitive
dropper was considered, but it would be frequency dependent and cause a
low power factor, both of which can be problematic with inverters.
And then there's the switch on surge possible
with capacitive droppers. A diode dropper with a resistor is another option,
but the light would produce a visible flicker operating at half the supply
frequency.
A miniature transformer perhaps? Yes,
that too was considered, and would be the best option so far, although
it meant no DC operation, not that I'd ever need that.
130V dial lamp.
But then, I remembered 130V dial light
bulbs. These have the same miniature bayonet BA9s base as the No. 47 dial
light, have the same size glass envelope, and I had a small quantity in
my collection. They're used as indicator lamps in industrial control applications.
Current consumption is only 20mA. Trying one out proved the point - this
was the solution I was looking for! The lamp socket was connected across
the 120V supply to the heater string, and the 130V 20mA bulb inserted.
Only someone noticing the longer filament inside the bulb would realise
it wasn't original. The light output is the same colour and only slightly
brighter than the No. 47 bulb. The bulb socket is far enough away from
the celluloid dial glass for the slight extra heat not to be a problem.
All in all, this was a most elegant way out of the problem.
The data on these bulbs shows a life of
1000-2000 hours, depending on whose data you're looking at. This is not
a lot better than a household light bulb. However, remember that's at 130V.
On 120V or lower, the bulb life is extended considerably - it should be
at least doubled. While not quite the same life as the No.47 bulb, 4000
hours is still a lot of listening time. One disadvantage with these bulbs
is the longer, more fragile, filament. For a portable radio that's bumped
around in use I wouldn't recommend them, but for a radio that just sits
on a shelf this would not be a problem. I would recommend anyone looking
to purchase these bulbs to shop around because the prices vary from reasonable
to quite ridiculous.
The 130V 20mA dial lamp looks like it was always there. It was the
most elegant solution with no thermistor available.
With the dial light sorted out, next was the valve heater string. Now that the dial lamp was not to be included, I could calculate a suitable dropper resistor. The 200 ohm resistor that the previous serviceman had installed was a little on the high side. I calculated a value of 130R for 120V mains. In practice, I found 150R to be better than 120R. (130R is not a preferred value). Power rating was 5W.
Not long after the radio had been powered
up, I could smell something burning, and then the smoke came out. The culprit
was the .05uF mains filter (10). The original looked like it was mica so
I had left it in situ. Alas, it turned out to be paper, hence its failure.
The receiver worked in that stations could be received. However, it was
now time to check the alignment.
This all seemed to peak up OK and the
AM reception seemed very good. On FM things weren't quite right. Pressing
on the tuner sub chassis would shift the stations and cause reception at
the high end of the band to disappear.
Restoration complete. The new silicon rectifier can be see just
to the bottom right of the filter choke.
Aside from this, the B+ was a bit low.
Remembering my first 474, and selenium rectifiers in general, I decided
to replace the one in this set too. I removed the rectifier and installed
a four lug tagstrip in its place. Mounted on this was a 1N4007 diode and
a
22R 1W surge resistor, chosen to produce 140V B+ at the 50L6 plate. I became
aware of a hum on the upper end of the FM band which hadn't been there
before. Placing a .0047uF across the 1N4007 fixed that completely. Obviously,
the sharper switching of the 1N4007 had introduced modulation hum.
I also took advantage of the 22R resistor
with regards to the dial light. With the 22R resistor connected to the
120V AC supply before the diode, I connected the dial light to the other
side of the 22R.
There are two reasons for this. First,
the 22R will act as a fuse if the lamp socket insulation fails, and secondly
my theory is the surge current will be lessened through the filament when
first switched on. This is because with the filter condensers discharged,
there will be a high voltage drop across the 22R for a very short period
until they charge. Thus the lamp voltage is less until the condensers have
charged.
FM problems.
Finally, it was time to deal with the
erratic FM behaviour. First I found the metal shield on the side of the
tuning condenser was only just touching the side of the tuning condenser.
When it did so, the stations shifted. The cure was to bend it further back
so it didn't touch. There was still the disappearing of the stations at
the 108Mc/s end. Pressing on the sub chassis still affected this. The sub
chassis is supported by three rubber grommets. When it was pushed down
enough to make contact with the main chassis, all was well. But when isolated
by the three rubber grommets and connected only by the one piece of braid,
the signals disappeared. Out with the spectrum analyser, and sure enough
the local oscillator dropped out. Two extra pieces of braid to the chassis
fixed that. I am curious why most sets isolate the tuning gangs with rubber
grommets. They do not seem to serve any useful purpose.
These two pics show the extra copper braid added. The original tinned
braid has too much inductance at 108Mc/s.
This is obviously a design fault because
I had this exact same problem with the first 474. How many Howard 474's
are out there unable to receive stations at the 108Mc/s end of the band?
How many of the owners know what the cause
is? I suspect most simply blame the Fremodyne concept as cheap and nasty
instead. It's situations like this that a spectrum analyser is an extremely
useful instrument.
Having dealt with that little problem,
I could now align the FM section correctly.
Finally, the crack was glued, the cabinet
polished, and the dial glass re-glued.
Performance.
As I've mentioned elsewhere in other articles,
the Howard is the best of the AM/FM sets using the Fremodyne circuit, both
in sensitivity and sound quality. Indeed, the AM section of this set is
very sensitive.
Not only could I pick up 2ZB from Wellington
NZ, but also 2YC. This was just with the internal loop aerial.
Aerial and earth connections can be seen here. Upper Fahenstock
clip is for the FM input. The wire connected to it goes to the mains cable
via a 100uuF condenser. The lower Fahenstock clip is the earth connection
for both AM and FM. For the external AM aerial, connection is made to the
coiled up piece of wire (why not a Fahenstock clip for this too?). Notice
that the back of the chassis and its securing screws can easily be contacted
by the user.
The FM sound quality is very good with a decent signal, and is certainly up to mantel radio standards. The power line aerial works very well with the local station about 5km away, and acceptably enough with the major Sydney stations located about 80km away. Anything weaker than that requires a better aerial for entertainment quality reception.
Ad from September 1947 in "Radio & Television Retailing".
The other two sections of the filter electro were tested with the Electronics Australia ESR meter, developed especially for testing electrolytic capacitors. Both were excellent, so a 30uF was squeezed in and connected across the defective one. Perfect stability! Next, a test of the B+ voltage was done. Although the ESR was good, I wondered if there might be leakage dragging down the voltage. As it turned out, the volts were down to about 80 at the third filter, whereas it should be 100. However, it wasn't the capacitor at fault, but the 2k filter resistor having drifted to around 3.2k. Replacing this brought the voltage back up. Low value resistors drifting high is rare which is why I had not tested this resistor previously.
Finally, it had been noted the FM was not performing as it should be. Sensitivity was down, and tuning was more critical than normal. Something was not right with the Fremodyne circuit. Remembering the cathode poisoning problem in AM/FM sets, and that the 12AT7 in this one was original, I tried another and all was as it should be. I had been using this set for my evening entertainment for a few years, mostly on AM, so it was perhaps not surprising. As mentioned previously, if a valve is run with the heater powered up but with no cathode current flowing, the cathode eventually becomes poisoned. This incidentally was a problem with early valve computers, where valves might be in a cut-off state for long periods, and new special types were developed to overcome this.
To prevent this happening again, I connected
a 100k resistor between the AM and FM B+ rails. When switched to FM, the
AM valves receive about 16V, enough to prevent cathode poisoning, and similarly,
when switched to AM, the 12AT7 receives about 26V. These voltages are low
enough so the relevant stages are inoperative to the point where there
is no risk of audio breakthrough.
So far so good, but it was found that
when switched to FM, there was some kind of interference which wasn't there
before. Sure enough, connecting a CRO to the AM B+ showed it oscillating.
The cure was simply a .01uF across the B+.
Modification prevents cathode poisoning. Resistor was later changed
to 180k.
March 2024: It had been noticed the AM performance was not what it should be. About every 20kHz across the AM band there was a hetrodyne whistle. Suspicion fell upon the 12AT7 oscillating because of the 100k resistor I'd added to prevent cathode poisoning. Shorting out the FM B+ got rid of the whistles. Evidently, even at 26V, it seemed the super-regenerator was not only oscillating, but sufficiently so that it quenched - hence the interference being harmonics of the quench frequency. Increasing the resistor to 180k dropped the FM standby B+ to around 12V which fixed the problem.
