Newb Stumped on a Derwent

[oldticktock]

Originally Posted by Station X

You have to measure heater voltages by connecting your meter on the AC range between pins 1 and 6. Don't do this with your VVM though.

42 readings are fine. I'm still thinking about the 75.

Ahhhh DOH! ok avo coming out

[oldticktock]

heaters read 6vac

[Station X]

Heater voltage is fine.

Bias on the 75 is a bit high. 3 VDC instead of 2 VDC. Did you check the resistance of the 1.8K? You can check it in circuit with the set off, but expect a delay as the 25uF charges.

Check the voltage pin 5 to top cap, but don't use the VVM. (both points relate to 75).

While you're at it check the voltage pin 4 to pin 5 of the 42. (expect 15V)

[oldticktock]

Originally Posted by Station X

Heater voltage is fine.

Bias on the 75 is a bit high. 3 VDC instead of 2 VDC. Did you check the resistance of the 1.8K? You can check it in circuit with the set off, but expect a delay as the 25uF charges.

Check the voltage pin 5 to top cap, but don't use the VVM. (both points relate to 75).

While you're at it check the voltage pin 4 to pin 5 of the 42. (expect 15V)

1.8 is 1.8
no reading on TC avo set to AC signal goes low avo set to DC hum introduced

when you say check voltage pin 4 to 5 do you mean the same way i tested heaters one probe on 4 and one on 5, if so 0 reading volume drops greatly

[Station X]

Try the 42 first. AVO on DCV. One lead to pin 5 (cathode), one lead to pin 4 (control grid) expect about 15V.

Then on 75. Set volume to maximum. AVO on DCV. One lead to pin 5 (cathode) one lead to TC (grid) expect 3V.

[Station X]

HANG ON!

From post 219.

Valve 42
pin1.....11vac
pin2.....260vdc
pin3.....270vdc
pin4.....2vdc
pin5.....16vdc
pin6.....0

You're showing 2V on pin 4 the control grid. Is that a positive reading? ie with negative lead to chassis and positive lead to pin 4 does the meter read forwards? If so you may have a problem with THAT capacitor. We went there yesterday.

[oldticktock]

Originally Posted by Station X

Try the 42 first. AVO on DCV. One lead to pin 5 (cathode), one lead to pin 4 (control grid) expect about 15V.

Then on 75. Set volume to maximum. AVO on DCV. One lead to pin 5 (cathode) one lead to TC (grid) expect 3V.

Ok Did all that and in both cases 0 reading and sound drops right off. needle seems to want to deflect backwards (too the left)

Meter reads dc fine checked on the rectifier

[oldticktock]

Originally Posted by Station X

HANG ON!

From post 219.

You're showing 2V on pin 4 the control grid. Is that a positive reading? ie with negative lead to chassis and positive lead to pin 4 does the meter read forwards? If so you may have a problem with THAT capacitor. We went there yesterday.

Using the VVM common to chassis and dc(red) to pin four reads 3vdc
using the AVO does not register

could this mean the AVO can read High DC but low DC no might be the problem I was having the other night perhaps the AVO is faulty.

THAT cap is brand new

[Station X]

I'm beginning to suspect the test gear. Check the voltage with the AVO. Then, if you've seen a positive reading, disconnect THAT capacitor from pin 4 and try again. You should see zero volts.

Then connect the meter between chassis and the disconected end of THAT capacitor. Again expect to see zero volts.

[jjl]

I've just noticed that OTT is measuring voltages with an AVO 7.
An AVO 7 is not an ideal instrument for measuring voltages in valve circuits as it is a 1000 or 500 ohms per Volt instrument depending on settings. It will provide low, inaccurate readings unless it is used on a high voltage range.

John

[oldticktock]

Originally Posted by jjl

I've just noticed that OTT is measuring voltages with an AVO 7.
An AVO 7 is not an ideal instrument for measuring voltages in valve circuits as it is a 1000 or 500 ohms per Volt instrument depending on settings. It will provide low, inaccurate readings unless it is used on a high voltage range.

John

I was starting to think the avo was poo, which is why i moved to the VVM which is very accurate.

Why can i not use the VVM to measure and forget the avo

[Station X]

We'd better stick with the VVM then, but obviously it can only be used for measurements to chassis because of the earthed probe.

[oldticktock]

Originally Posted by Station X

We'd better stick with the VVM then, but obviously it can only be used for measurements to chassis because of the earthed probe.

I have a heathkit VVM with just a 2 wire mains lead its a v-7au

only problem is its DC probe socket seems to be a 1/4 jack odd!!!!!

just found a 1/4 jack in my box of spares

[Station X]

Stick with the Cossor. I'd like to get to the bottom of the voltage on pin 4 (control grid) of the 42. Yesterday you measured 0 volts there which is correct.

From post 115.

Went to my test equip cupboard and pulled out my mains powered Cossor instrument reads AC DC-DC+ and OHMs

42 readings

pin 1 cannot get a reading
pin 2 250
pin 3 270
pin 4 0
pin 5 23
pin 6 cannot get a reading

[oldticktock]

Originally Posted by Station X

Stick with the Cossor. I'd like to get to the bottom of the voltage on pin 4 (control grid) of the 42. Yesterday you measured 0 volts there which is correct.

that's correct, so must be something we did today

[oldticktock]

Just remembered when I inherited the test gear, the Cossor VVM's mains lead was naff so i replaced it and I introduced the earth. Guess that was a no no....... damm my stupidity hurts its a 1956 model so guess they would not have earthed it

[oldticktock]

radio has been on all this time. 42 pin 4 now reads 5vdc

[Station X]

that's correct, so must be something we did today

Either that or one or both of the readings was incorrect. Try the tests in post #229 and let's see if we can get to the bottom of this.

Don't worry about the earth on the VVM, we''l stick with readings to chassis.

[oldticktock]

42
pin1..6
pin2..260
pin3.270
pin4..5
pin5..15
pin6..6

[Station X]

radio has been on all this time. 42 pin 4 now reads 5vdc

Perhaps the valve is drawing grid current (grid acting as an anode). Check the voltage across the 510K with the 42 removed from its socket.