Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) - UK Vintage Radio Repair and Restoration Discussion Forum

Nickthedentist · 3rd Jun 2009, 3:24 pm

Hello everyone,

I recently bought this smart 4V+R Bakelite set from a forum member. It's AM only but I'd like to fit a tiny FM tuner PCB inside so I can receive my favourite stations. I have an ideal board salvaged from a 1980s Panasonic radio cassette. It could be powered from the dial bulbs' AC supply, replacing them with 0.15A lamps if need be to prevent overloading the mains transformer. I intend to feed the output into the set's PU input.

N.B. I am able to do this in such a way that the set could easily be restored to original condition one day leaving no trace of my modification. So don't worry about the possibility of a nice radio being spoiled.

I tried it yesterday, and it works well, but is a little quiet, with the volume control needing to be advanced to maximum for a reasonable listening level.

My question is this: Can someone explain how can I increase the gain of the set's amplifier a little?

Due to forum rules, I don't think I can publish an excerpt from the service sheet here, but it's readily available on Paul's CDs/DVD as "Baird 210". The PU signal goes via the volume control to the EBC41 and then to the EL84.

All suggestions gratefully received.

Thanks,

Nick.

KeithsTV · 3rd Jun 2009, 3:31 pm

As an alternative to increasing the gain of the sets amplifier could you add an amplifier to increase the output of the FM tuner.

Increasing the gain of the amplifier would probably affect the output on AM.

Keith

Steve_P · 3rd Jun 2009, 3:38 pm

Cut out R14 from the circuit on Pick Up.

Take it out, put that place straight to the Switch, and put R14 between C31 and R13 and the wavechange switch.

Put a .01uF cap in the positive side of the Audio of the tuner to protect it. Then try it.

Cheers,

Steve P.

Nickthedentist · 3rd Jun 2009, 3:38 pm

Thanks Keith.

You're right of course, but I thought I could make it switchable. I'm not really bothered about AM and FM being at different levels, I'd just like to make the sound on FM a little less feeble.

Nick.

paulsherwin · 3rd Jun 2009, 3:43 pm

You can post a circuit fragment showing the area being discussed.

I would build a one transistor preamp - you just need 4k7 in the collector lead and 1M between the collector and base, plus coupling caps. This will probably give too much gain so you will need to attenuate the input.

A higher quality preamp could be made with an op-amp but you don't really need it for an application like this.

Paul

Darren-UK · 3rd Jun 2009, 3:44 pm

Quote:

Originally Posted by Nickthedentist

Due to forum rules, I don't think I can publish an excerpt from the service sheet here, but it's readily available on Paul's CDs/DVD as "Baird 210". The PU signal goes via the volume control to the EBC41 and then to the EL84.

All suggestions gratefully received.

Thanks,

Nick.

Quote from the relevant Forum Rule....

Quote:

you may post a small section from published data (such as a section from a circuit diagram or other diagram) where appropriate to illustrate or help with a discussion.

Nickthedentist · 3rd Jun 2009, 4:30 pm

Thanks for the suggestion Paul, and the reminder re. the rules, Darren. Sorry, I obviously hadn't read that section for a very long time.

So, here's the relevant bit...

Nick.

kalee20 · 3rd Jun 2009, 5:36 pm

I'd use a simple transistor pre-amp, more or less as paulsherwin says. Resistor values that he suggests will be fine. Use a BC109 transistor. Attenuation at the input would be best achieved with just a series input resistor - then the feedback 1M bias resistor will act to give lower distortion.

You can get the power for this from the cathode of V4. Increase C39 though, or you may get motor-boating (or you could add a dcoupling filter using, say, 470 ohms and 1000µF). An extra milliamp or so will not hurt the EL84 - if you are worried you could increase R27 from 180 to 200 ohms.

paulsherwin · 3rd Jun 2009, 5:51 pm

Quote:

Originally Posted by kalee20

I'd use a simple transistor pre-amp, more or less as paulsherwin says. Resistor values that he suggests will be fine. Use a BC109 transistor.

A BC109 is ideal, but just about any general purpose Si transistor will be fine. You can use whatever you have to hand. Even a noisy transistor shouldn't be a problem given the signal levels involved.

Paul

PJL · 3rd Jun 2009, 5:56 pm

Check that the bypass caps are good C39 & C35. Only way of increasing gain would be to increase R18 but will require an increase in R17 or the delayed AGC bias will change. Probably not a good idea unless you want to play around.

timohaveri · 5th Jun 2009, 9:46 pm

Quote:

Originally Posted by Nickthedentist

It could be powered from the dial bulbs' AC supply, replacing them with 0.15A lamps if need be to prevent overloading the mains transformer. I intend to feed the output into the set's PU input.

I once fitted a transistor FM radio in a valve radio because the owner wanted it (the radio was a kit made by Kemo). I tried to power it from the heater supply by rectifying and filtering the AC. It didn't work. It seems that there was a some sort of short circuit or more or less direct path from the heaters to the PU ground (?). The dial lights dimmed when I connected the PU leads because of the loading. Also it hummed quite badly. I ended up fitting a very small transformer taken from a small charger and powered it directly from the mains (after the on-off switch). Maybe there is a simple solution to make it work using the heater supply?

Timo

Nickthedentist · 5th Jun 2009, 10:26 pm

My set has a separate winding for the dial lights, with one side grounded. I'd planned on using a single diode for half-wave rectification of the AC.

Willl let you know how it goes.

N.

3rd Jun 2009, 3:24 pm	#1
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,864	Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) Hello everyone, I recently bought this smart 4V+R Bakelite set from a forum member. It's AM only but I'd like to fit a tiny FM tuner PCB inside so I can receive my favourite stations. I have an ideal board salvaged from a 1980s Panasonic radio cassette. It could be powered from the dial bulbs' AC supply, replacing them with 0.15A lamps if need be to prevent overloading the mains transformer. I intend to feed the output into the set's PU input. N.B. I am able to do this in such a way that the set could easily be restored to original condition one day leaving no trace of my modification. So don't worry about the possibility of a nice radio being spoiled. I tried it yesterday, and it works well, but is a little quiet, with the volume control needing to be advanced to maximum for a reasonable listening level. My question is this: Can someone explain how can I increase the gain of the set's amplifier a little? Due to forum rules, I don't think I can publish an excerpt from the service sheet here, but it's readily available on Paul's CDs/DVD as "Baird 210". The PU signal goes via the volume control to the EBC41 and then to the EL84. All suggestions gratefully received. Thanks, Nick.

3rd Jun 2009, 3:31 pm	#2
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,573	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) As an alternative to increasing the gain of the sets amplifier could you add an amplifier to increase the output of the FM tuner. Increasing the gain of the amplifier would probably affect the output on AM. Keith

3rd Jun 2009, 3:38 pm	#3
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) Cut out R14 from the circuit on Pick Up. Take it out, put that place straight to the Switch, and put R14 between C31 and R13 and the wavechange switch. Put a .01uF cap in the positive side of the Audio of the tuner to protect it. Then try it. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

3rd Jun 2009, 3:38 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,864	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) Thanks Keith. You're right of course, but I thought I could make it switchable. I'm not really bothered about AM and FM being at different levels, I'd just like to make the sound on FM a little less feeble. Nick.

3rd Jun 2009, 3:43 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) You can post a circuit fragment showing the area being discussed. I would build a one transistor preamp - you just need 4k7 in the collector lead and 1M between the collector and base, plus coupling caps. This will probably give too much gain so you will need to attenuate the input. A higher quality preamp could be made with an op-amp but you don't really need it for an application like this. Paul

3rd Jun 2009, 4:30 pm	#7
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,864	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) Thanks for the suggestion Paul, and the reminder re. the rules, Darren. Sorry, I obviously hadn't read that section for a very long time. So, here's the relevant bit... Nick. Attached Thumbnails

3rd Jun 2009, 5:36 pm	#8
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) I'd use a simple transistor pre-amp, more or less as paulsherwin says. Resistor values that he suggests will be fine. Use a BC109 transistor. Attenuation at the input would be best achieved with just a series input resistor - then the feedback 1M bias resistor will act to give lower distortion. You can get the power for this from the cathode of V4. Increase C39 though, or you may get motor-boating (or you could add a dcoupling filter using, say, 470 ohms and 1000µF). An extra milliamp or so will not hurt the EL84 - if you are worried you could increase R27 from 180 to 200 ohms.

3rd Jun 2009, 5:56 pm	#10
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) Check that the bypass caps are good C39 & C35. Only way of increasing gain would be to increase R18 but will require an increase in R17 or the delayed AGC bias will change. Probably not a good idea unless you want to play around.

5th Jun 2009, 10:26 pm	#12
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,864	Re: Increasing the gain of PU input on a 1950s radio (Radio Rentals M210) My set has a separate winding for the dial lights, with one side grounded. I'd planned on using a single diode for half-wave rectification of the AC. Willl let you know how it goes. N.