BC-348-Q Repairs

 
 

"On the Bench"

 

2nd October 2003.

This set was rescued of the local waste dump for the princely sum of 5. When I got it, it seemed to be reasonably complete, with the addition of a few modifications to make it more useable as a general purpose communications receiver.

The BC-348 series of receivers were designed as HF receivers for use in heavy aircraft (B17, B24, B29 etc.), and were generaly paired with an ART-13 transmitter. They were also used in some ground installations.

The BC-348-Q is an 8 valve set, covering 200KHz to 500KHz, and 1.5MHz to 18MHz in 6 bands. It has a switchable crystal filter in the IF stages, BFO, and can be used with either manual or automatic gain control. The output is designed to feed a load of 4000 Ohms, either through one of the front panel headphone sockets, or through the main power connector.

The BC-348 is designed to run from a 28V DC power source, with an internal dynamotor to provide the HT supply. There is also a 14V version, known as the BC-224 series.

I found an online service manual here you will need to apply for a pasword to view the manual (free).

In the picture, you can see the scruffy appearence of the set, but also evident are some of the modifications. There are two extra controls mounted on the RF section cover (above the makers plate), marked "AF Gain" and "RF Gain", and an additional toggle switch near the top left corner of the dial, marked "Mains ON/OFF". At least all the knobs are present!

Taking the covers off, you can see that a homemade mains power supply has replaced the original Dynamotor in the top left corner - this means the Heaters have been rewired for 6V operation. In the middle of the set, an extra chassis has been added containing two B7G valves. The original dial lamp dimmer has been replaced with another control of unknown purpose.

After a thorough visual inspection, and replacement of the mains cable, the set was powered up slowly via a Variac, with no ill effects. IT WORKS - well after a fashion. The bandswitch is intermittent, as is the Original and added gain controls, and the output seems distorted.

 

3rd October 2003.

The Bandswitch has been thoroughly cleaned and lubricated with Servisol, which has improved matters greatly! The other switches were treated in the same way, as they looked in need of a little care.

The Original gain control (bottom left control) was alittle more troublesome. This is a dual gang potentiometer, which acts as a normal Volume control when the set is set to AVC, but as an RF gain control, with limited AF control when the set is used with MVC. To clean and lubricate the control, it had to be removed from the set and dismantled, as it was designed to be moisture proof for tropical operation. It can be dimantled by carefully bending the tags which hold it together, then rebuilt in the same way.

The set is now much improved, though the action of the various gain controls is very coarse, and the set appears to be distorted. It is possible to receive a number of commercial stations on HF using only about 10 feet of wire, and even to resolve a SSB transmission on the 40m amateur band.

The small added in chassis is an extra audio stage and limiter, connected to the additional AF gain control, while the added Rf gain control varies the screen voltage of the second RF stage - it would have made more sense to control the first, in order to avoid overload! The third added control (in the dial lamp dimmer position), seems to be some form of tone control!

 

7th October 2003.

In the interests of originality (and performance!), I have removed the additional AF stages, and the additional tone and gain controls, returning the circuit back to the original - most of the distortion and overload have now gone! The set performs nicely, with the original gain control giving smooth operation in both the AVC and MVC position.

(picture shows the additional chassis with two B7G valves - an add in AF amplifier and limiter)

The internal noise level of the set has dropped significantly, though this has shown that the AF output stage is microphonic - I have a spare valve (VT152A or 6K6GT) somewhere, though changeing it is not critical at the moment.

The output valve seems to run a little hot, so I must check the grid coupling capacitor for leakage. I also think that the set would benefit from re-alignment.

 

8th October 2003.

The cause of the overheating output valve has been found! I referred to the manual, and found that there should be a negative bias of about 8V on the grid of the output valve, when it was measured with a high impedance meter (Fluke DVM), there was only -0.7V. My first thought was to check the AF coupling capacitor for leakage, but this proved to be OK.

After careful study of the circuit diagram, I noticed that the HT -ve rail is not connected directly to the chassis, but is used to provide the negative bias for various stages, with the chassis being connected to HT -ve via a choke and a 50 Ohm resistor. A close inspection of the home made PSU revealed that the -ve line from the power supply was connected directly to chassis - rewiring this corrected the fault and got rid of a lot of distortion. The bias voltage is now -20V measured with the DVM.

(Picture shows the added in PSU components under the chassis, the large choke at the left, the two red capacitors, and the small sqaure tagboard.)

I suspect the higher (measured) level of bias is correct, as the voltage chart in the manual recommends a standard meter for the measurements, which will have a much lower resistance than the grid resistors, and so will load the circuit. My Brimar valve data book recommends a grid bias of -18V for the 6K6 (VT152) with 250V on the anode, which is about what we have here!

Now that the gain control works correctly, the set operates well, but it does need re-alignment, as the crystal filter peak does not match the IF peak - you need to retune slightly when you switch the filter in. Interestingly, early manuals tolerate this, so long as the peaks are within 1KHz of each other, but later manuals recommend aligning the IF to match the crystal filter.

13th October 2003.

Work on the BC-348 has been suspended! I came to use my Marconi TF2002 signal generator to align the set (I wanted to use th TF2002 as it is easy to generate a swept signal for aligning the IF symmetrically about the crystal resonance), and it wouldn't work. I also have the dilema of what to do with the power supply, ideally Iwould like to find a dynamotor unit, but these seem to be very scarce. The existing internal power supply needs to be re-built, so I may make this an external unit, to reduce the heat within the case. I also have to decide whether to leave the heater chain in its current 6V configuration, or re-wire it to the original 28V arrangement - this will mean an extra transformer, but the dial light dimmer will work correctly. I will update this section when I get to do some more work on the set.

 

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Copyright J.Beacon & M.Wroe-Parker 2003