Can I build better myself? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Ted Kendall · 11th Apr 2022, 10:51 pm

Quote:

Originally Posted by Bufo Bill

Hi all, I've built my little Frank Jones Mod circuit, and tested it on the bench. It seems the metal shell is fouling the pins and shorting out, so the mod works outside the case only! I was wondering about painting on enamel paint in an attempt to stop the shell conducting, as I have some on hand.
Is there something else that would work better as insulation?
Very enjoyable project all the same! Let me know if you have any ideas.
Thanks chaps, Bill.

Insulating tape would be quick and effective.

Bufo Bill · 12th Apr 2022, 9:50 pm

Hi Ted, I cut a thin strip of tape and wound it around the pins.
Didn't have much spare time today to look at the preamp. I found a couple of resistors had drifted badly, but that's as far as I got. Will hopefully start tomorrow checking voltages. Suppose I should check the tone arm cable, it's pretty old now.
Cheers from Bill.

Bufo Bill · 17th Apr 2022, 12:14 am

Hi guys, I need some help with the schematic shown in post #16.
It says there should be a 270k resistor in series with the HT wire; should this be 27k instead? The HT should be 180V, 0.0018A before the resistor I think.
Also the 680k resistor seems too large, or am I wrong there?
Many thanks from Bill.

Ted Kendall · 17th Apr 2022, 8:45 am

What is the thing not doing? I built this years ago as shown and it worked fine, albeit on a stereo control unit. What voltage are you getting across the zener diode? If it's a steady 18v the dropper is about right, as is the HT. I shouldn't worry about the 680K - it's merely biasing the transistor.

Radio Wrangler · 17th Apr 2022, 9:15 am

The "From HT on other channel" label on that schematic is decidedly misleading!

He's used "HT" for the name of both the incoming (180-ish volt) supply AND the outgoing 18v supply to the duplicate transistor circuit for the other channel.

Inevitably, someone is going to put it to 180v from the other Quad channel.....

The BC109 has quite a lot of current gain, so the base current is small campared to the collector-emitter currents, so the base bias network can be quite high impedance, and it isn't necessary to waste a lot of current down its potential divider.

Using a big resistor from a high voltage, and a shunt zener to make that 18v power supply for the transistors is quick, cheap, and works, but it is monumentally inefficient. You can live with that. The designer has saved a bit of current where he can, but he's relying on the gain of the BC109. Use a much lower gain transistor and the bias voltage will sag so the circuit won't behave properly.

David

Bufo Bill · 24th Apr 2022, 3:53 pm

Hi, sorry not to update you sooner, I've had no time to myself this week. I found I got a faint output from the module, and either through bad luck or mismanagement on my part the transistor I was using was found not good when removed and tested. I will have to buy some more, and that got me thinking . . .
Some of the original carbon resistors have wandered out of spec in the last few years, so I was thinking of replacing them with metal film substitutes. That should be an improvement, noise floor wise, do you think? Shares in RS will be through the roof at the news!
Cheers from Bill.

Bufo Bill · 24th Apr 2022, 11:11 pm

Decided to totally rebuild the module, with insulation as far as the eye can see . . . I'm using more black rubber than the average S&M Dungeon! I am presuming that the original little Quad units weren't potted back in the day? Not going to open an original one to find out, I have nightmares about the amount of these that must have been thrown out or trodden on.
Bill.

Bufo Bill · 12th May 2022, 2:05 pm

Update: built one on veroboard, with an umbilical connection. Works well. Hurrah!
Thanks to all from Bill.

Ted Kendall · 12th May 2022, 2:33 pm

Good show! Pleased it's worked out.

Bufo Bill · 12th May 2022, 2:51 pm

Thanks Ted, you've been a great help.
Bill.

12th Apr 2022, 9:50 pm	#22
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Can I build better myself? Hi Ted, I cut a thin strip of tape and wound it around the pins. Didn't have much spare time today to look at the preamp. I found a couple of resistors had drifted badly, but that's as far as I got. Will hopefully start tomorrow checking voltages. Suppose I should check the tone arm cable, it's pretty old now. Cheers from Bill.

17th Apr 2022, 12:14 am	#23
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Can I build better myself? Hi guys, I need some help with the schematic shown in post #16. It says there should be a 270k resistor in series with the HT wire; should this be 27k instead? The HT should be 180V, 0.0018A before the resistor I think. Also the 680k resistor seems too large, or am I wrong there? Many thanks from Bill.

17th Apr 2022, 8:45 am	#24
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Can I build better myself? What is the thing not doing? I built this years ago as shown and it worked fine, albeit on a stereo control unit. What voltage are you getting across the zener diode? If it's a steady 18v the dropper is about right, as is the HT. I shouldn't worry about the 680K - it's merely biasing the transistor.

17th Apr 2022, 9:15 am	#25
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: Can I build better myself? The "From HT on other channel" label on that schematic is decidedly misleading! He's used "HT" for the name of both the incoming (180-ish volt) supply AND the outgoing 18v supply to the duplicate transistor circuit for the other channel. Inevitably, someone is going to put it to 180v from the other Quad channel..... The BC109 has quite a lot of current gain, so the base current is small campared to the collector-emitter currents, so the base bias network can be quite high impedance, and it isn't necessary to waste a lot of current down its potential divider. Using a big resistor from a high voltage, and a shunt zener to make that 18v power supply for the transistors is quick, cheap, and works, but it is monumentally inefficient. You can live with that. The designer has saved a bit of current where he can, but he's relying on the gain of the BC109. Use a much lower gain transistor and the bias voltage will sag so the circuit won't behave properly. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Apr 2022, 3:53 pm	#26
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Can I build better myself? Hi, sorry not to update you sooner, I've had no time to myself this week. I found I got a faint output from the module, and either through bad luck or mismanagement on my part the transistor I was using was found not good when removed and tested. I will have to buy some more, and that got me thinking . . . Some of the original carbon resistors have wandered out of spec in the last few years, so I was thinking of replacing them with metal film substitutes. That should be an improvement, noise floor wise, do you think? Shares in RS will be through the roof at the news! Cheers from Bill.

24th Apr 2022, 11:11 pm	#27
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Can I build better myself? Decided to totally rebuild the module, with insulation as far as the eye can see . . . I'm using more black rubber than the average S&M Dungeon! I am presuming that the original little Quad units weren't potted back in the day? Not going to open an original one to find out, I have nightmares about the amount of these that must have been thrown out or trodden on. Bill.

12th May 2022, 2:05 pm	#28
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Can I build better myself? Update: built one on veroboard, with an umbilical connection. Works well. Hurrah! Thanks to all from Bill.

12th May 2022, 2:33 pm	#29
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Can I build better myself? Good show! Pleased it's worked out.

12th May 2022, 2:51 pm	#30
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Can I build better myself? Thanks Ted, you've been a great help. Bill.