Help with BC-348R

[Grix]

Hi All
This is my first post to the group. At the moment i an trying to get my BC-348R in working order and need some help. How do i replace the R.C.A 991 voltage regulator with zenner diodes ?.as you can sea i am new to this electrickary thing and am just learning. second point how do i put a 8 ohms speaker onto the output witch is set to 4000 ohms. regards Rick

[Sean Williams]

Hi Rick,

Why change the regulator? - Is it broken, or missing?

You will need to use a transformer to get the set to drive a speaker - and probably modify the circuitry a little.

First step will be to get the manual - BAMA have them on line.

If you have no experience with electronics this is going to be one steep learning curve!

Have a look at the manual, then come back to us.......

Cheers
Sean

[Grix]

Hi Sean
Thanks for your reply. I found this radio in a skip it looked good so i am having a go with it and learning all the tine, I have built a power supply for it(220 V DC and 6.3V AC), i have got the manual from BAMA and followed it as best i can.I have replaced all resister that are over 25% out of speck and have replaced some caps that are dead short. The regulator is broken and from looking on this forum there is referance to putting in zenner diodes in place of the valve and would like to know how to do this. What sort of transformer di i need for the speaker as i would like to keep the set as ariginal as posible. regards Rick
P.S my next set to get working is a R1155N

[Sean Williams]

Hi Rick,

Well, the 348 should work without the regulator tube - the VFO will be a little flaky, but it was only a neon tube, so it wasnt really regulated.......

A standard output transformer, nicked from any old valve radio will do the job - Just wire it into the anode circuit of the output valve, connect a speaker to the secondary, and enjoy......

Lets have a picture of the set - both outside and inside eh?

Cheers
Sean

[Grix]

Hi Sean
I have powered up the set and it works well sort of a bit more work to do on it like build the power unit into the set and get an output transformer but i am pleased to get this far. thanks for all your help with my first victory.
If i can get them in working order it will not cost so much to get them tuned up by someone who knows what they are doing. regards Rick
P.S how do isend a pic of the set.??

[Kat Manton]

Originally Posted by Grix

P.S how do isend a pic of the set.??

This thread may be of use.

[PJL]

I fixed one of these recently and had the same shorted capacitor problem. The neon regulator runs at about 60V and I used a couple of 1.3W types from Maplin in series to make up the 60V soldered underneath the socket.

For a speaker why not use a powered PC or portable CD type that way you won't need to mess about with transformers?

Other problems I had:
The crystal filter switch needed cleaning. The filter attenuates the signal badly but dramatically improves SSB reception on my set but commonly fails altogether.
The potentiometer marked 78 on the circuit drawing was open circuit. This sits on the end of the tuning capacitor and the set works fine if you short it out!

[Sean Williams]

Potentiometer 78 is to vary the gain of the RF amplifier - it works to improve the gain at the high frequency end of the band - As a consequence you may suffer overloading at the LF end of the band if this control is bypassed.

Cheers
Sean

[PJL]

Sean, somewhere I read this was to keep the gain consistent so background noise doesn't vary across the scale. I thought the AVC (or volume control in MVC mode) would prevent overloading. The pot is an odd value and fitting and difficult to replace so I tried shorting it out and it has behaved OK but should I be trying to find a replacement? I can live with the change in background noise and anyway I should be getting a bit more sensitivity.

[Sean Williams]

Depends entirely on how well the AGC works! - And of course how good the antenna system is, for the most part this repair should be fine - i am somewhat of a pedant when it comes to repairing Military kit - I like to see the repairs done as the designer intended.........

What you have read could well be a true statement as well - the circuit will of course do that too, however, seeing as the valves used in this circuit present a lower gain at the higher frequencies, I would consider this to be the primary reason for the odd gain control...

Yes, a bit of a problem to replace this control, as you say, it is not exactly a standard part!

Cheers
Sean

[Biggles]

When switched to SSB mode won't the AGC be disabled anyway and go to manual RF gain control?

Biggles.

[Grix]

Hi All
Again i want to thank you for all your help with my set.Thanks to Kat on how to send picks,and PJL for the tip re the zenners, i to have the problem with R78 mine is just open circuit to but has not been shorted out, when i turn on the crystal filter the audio decreases by 75% and it sounds as if it is under ''under water'' but you can still get ssb on the set.
i have to build the power set into the radio and clean it up and then get it tuned up as that part is over my head. have put in some picks the 1155 is the next one to do (got it on ebay for £45.00)
regards Rick

[PJL]

Rick, mine has been stripped of more of the 24V supply components and I see you replaced all the paper capacitors which I couldn't face doing! You seem to be building up quite a pile there - all the best Peter