BC348 valve screen voltage

[Cliffgeorge39]

I have recently acquired a nice looking bc348 model O that someone else has previously worked on. I have lowered the ht from about 320 to 230 volts wih two dropper resistors.
When I measure the screen volts on the rf, mixer and IF 1 and 2 I get 200 volts.
A copy of the manual says on page 22 and 30 that it should be around 100 volts.
The screen voltage supply comes from a series of resistors off the ht, namely r42.6 4.7k, r56 10k, r62 47k and the 20k manual gain pot. They check out ok resistance wise, but with those values I dont see how you get less than about 200 volts at the screen take off junction between the 10k and the 47k.
Has anyone else come across this problem?

[ms660]

Is that 200 volt screen volts in AVC mode or MVC mode? or both?

Lawrence.

[Cliffgeorge39]

Thank you for your reply Laurence.

Firstly to answer your question about screen volts AVC or MVC. It goes from 200 down to 170 on AVC. This is measured with my trusty Radio Shack 50uA analogue meter.

I have some other voltages for us to consider. If we concentrate on the 1st tf stage it has a 10k screen resistor and a 380 ohm cathode resistor accessed via a removable panel on the front. I am making the measurements in MVC with gain at max because the manual says so. With my meter on the supply side of the screen 10k and the gain on max, I measure 200V. If I check the other side of the 10k I measure 190 volts. So I get 10 volt drop across 10k which is 1 mA.

When I do the same with the gain on min I get 260/250V. Still 1 mA screen current which I find strange because turning the MVC gain to min puts positive volts on the cathodes which should reduce th G2 current, but it doesn't even though the volts rise from 190 to 250.

I think that's enough for now and welcome your feedback.

Regards, Cliffgeorge.

[Cliffgeorge39]

Hello its cliffgeorge again.
I have double checked the volts agin around the screen of the first rf amp and come up with some more sensible results.
On pin 4 of the base which is g2 i get 170 volts on max gain and 250 on min.
The supply side of the 10 k i now get 200 on max and 250 on min.
I now make this a 30 volt drop on max (3mA screen current) and no volts drop on min suggesting the valve is more or less off.
This makes more sense. Sorry for the mistake.

[ms660]

The schematic and the rest of the manual for the BC348 can be a bit of a brain teaser sometimes.

So far as I can make out with regards to the main screen rail supply to the 2nd RF, Mixer and 1st IF:

In both MVC and AVC modes the screen supply rail is via a potential divider that's connected between the main HT rail and chassis.

In MVC mode the potential divider (starting at the HT end) consists of....4.7k>>10k>>47k>>20k(volume pot)>>chassis. When in that mode the wiper of the volume pot is connected via a 3.7k rheostat and a 100 ohm resistor to the junction of the 47k & 10k resistors, the junction of the 47k & 10k resistors supply the screen rail concerned in both MVC and AVC modes.

When in AVC mode the wiper of the volume control isn't in circuit, the end of the rheostat that was connected to the volume pot's wiper in MVC mode is now connected via a 68 ohm resistor to chassis.

If you draw that out schematic wise you should be able to get an idea of the voltage divider ratio, and, taking into account the screen grids quoted current (if it is quoted, I've not checked) be able to get a reasonable idea as to what the screen rail voltage should be in either MVC mode (say volume at min and max) or AVC mode.

If you have trouble figuring out the schematic then let me know and I'll try and post up a simple schematic for the divider in both modes.

I hope I've got the above description correct, maybe someone else could check that out also, just to be sure.

Lawrence.

[ms660]

Ignore post#5, the description/values etc were taken from an annotated schematic I have and just realized it was for the P version not the O version.....having a bad day...

Lawrence.

[Cliffgeorge39]

Thank you for your time and effort Lawrence. Yes there are some differences between the different models. The O model is as you say with the chain of resistors and no extra ones around the gain pot from the diagram I have.

I would like to think that with 5 valves under control of the MVC screen circuits and they each pass about, say, 3mA screen current on max gain, then 5x3 equals 15mA, and this current must come from the ht via the 4.7 and 10k.
It would cause a voltage drop of at least IxR equals 15mA times 15k comes to about 175 volts. In theory this would drop the 230 volt ht rail to 230 minus 175 which is round about the screen volts I expect but it doesnt happen.
I know now that the first rf amp 6k7 draws 3 mA when the gain is max. I need to check the other 4 valves tomorrow and will report. Thanks for your help. Its good to have contact with other knowledgable radio people.

[ms660]

With 230 volts HT, the screen feed take off point on the divider should be approx 189 volts unloaded from what I can make out, might be worth checking that?

Lawrence.

[ms660]

Just had a look at the manual, so far as I can make out the HT with respect to chassis is given as 210 volts, so maybe your 230 volts is too high?

Lawrence.

[ms660]

Another look at the manual suggests that a Weston 665 meter was used for the voltage measurements, from what I can gather it was a 1,000 ohms per volt job, so far I haven't found anything in the manual that says what voltage range the meter was set to for the readings?

Allowance needs to be made for that if using a high impedance meter for the voltage measurements as the shunting effect of the 1,000 ohms per volt meter would have had a noticeable effect on the actual voltages present at that time.

Lawrence.

[Cliffgeorge39]

Thanks for your feedback Lawrence.

I didn't get round to the radio yesterday but hope to do some measurements today.

Rgds, cliffgeorge.

[Cliffgeorge39]

I have managed to measure five screen volts. Three of them i have measured each side of the screen resistor. The mvc gain was set to max.

1RF. 170/200. 3mA?
2RF. 150/175. 2.5mA?
Mix. 175/175. 0mA?
1IF. 195
2IF. 195

Seems to be a problem with the mixer circuits/valve.
I will investigate. The radio works quite well!

I would have liked to spend a little more time and found the feed volts for the IF but I am having some negative effects from the steroids I have been taking and feel very tired the last few days.
Rgds cliffgeorge