DAC90A weird voltage readings.

[ms660]

Originally Posted by Nukeybrown

Ok resistor on v4 has gone high, which has caused all the voltage issues . Result the radio is now working as it should. Not sure why 6v on pin 6 v4 is strange?

Resistor had gone off the scale for some reason on the v4 cathode.

Good result, it shows how important voltage measurements and their interpretation is as an aid to logical fault finding.

Lawrence.

[Nukeybrown]

Thanks for all the help everyone , as I said I'm new to this and learning as I go so all you help has been invaluable. And with respect sideband thanks for pointing out my learning curve.

[ms660]

Yep, learn the basics, such as how resistors, capacitors, transformers, inductors respond to AC and DC voltages and current, those are generally termed passive components, active components are things like valves, transistors etc.

Learn the basics of valve operation, it's not too complicated, you don't have to bother with such things as space charge, transit time, contact potential at the early stage, that can come later as and when if desired, the basics of valves is all about anode current, grid bias voltage and the amplitude of the input signal, plenty of tutorials on the web about it, look for one that shows a typical valve characteristic curve, that should show Vg (control grid bias voltage) and Ia (anode current) and the input and output signal (usually represented by a sine wave) in graphical representation, a picture says a thousand words and all that.

Good luck and keep at it.

Lawrence.

[Nukeybrown]

Thanks again Lawrence I appreciate you highlighting some valve points that I'm not too knowledgeable and point me in the right direction of what info etc I should be looking at to learn rather than just pointing out that I'm a beginner in all this. I will be building a rocket ship some time soon I'm sure. Some people sometimes forget that we all start somewhere.

[Nuvistor]

There are many publications on the theory, one good one you may find in the local library is the ARRL Handbook. Does not really matter which version, the first few chapters cover a lot of ground, it's a well respected book.
Frank

[peter_sol]

I am pleased we have been able to help you get the set going again and hope we did not seem overly harsh with our comments.
It is quite difficult to put over in a few words, exactly what we are trying to suggest to a beginner, but maybe soon you will helping someone else in a similar position with your first hand knowledge.

[Sideband]

Yes wise words. You don't need to get bogged down with heavy theory but try and get to grips with simple Ohms Law...it'll give you a good idea of how current, voltage and resistance work together. The big problem beginners have these days is the lack of personal tuition....the one-to-one that most of us had. Don't be put off asking questions here. It's easy for those of us who've been in the trade for 50 odd years and it's up to us to pass on as much as we can to younger enthusiasts.

Enjoy your radio now that it's working well again.

[Nukeybrown]

Been busy all day restoring a 1950s Formica drop leaf table. My dad has the radio still so haven't changed anything or one anything etc but i did ask him to check the 6v voltage again on v4 pin 6 turns out it's a rising voltage , when first powered on reading 4v , after 10 mins it's reading 6v and prob rising higher after that . Told him to switch it off and leave it off for now . Strange thing is c18 has been changed on the originally none working radio but that also is reading the same voltages on v4 pin 6 as the working radio so even though the none working radio is now working , that also seems to have the same voltage issue on v4

[crackle]

Just to help to clarify the voltages around the cathode and control grid of the UL41.
The 5 volts listed in the voltage table in the Trader service sheet is measured with respect to chassis. In other words there is a 5 volt drop across R14 when the valve is working, making the cathode more positive than ground.
Because the voltage on the cathode is higher than the chassis this causes the control grid (which is more or less at the same voltage as chassis) to be in the region of -5 volts WITH RESPECT TO THE CATHODE. This can be confirmed if you put the -ve meter probe on the cathode pin and test with the +ve meter probe on the control grid, you should see a negative 5 volts, this is known as the "bias" voltage. This is something which confused me at first.

The output valves in radios almost always rely on a negative bias voltage on the control grid to help to limit the conduction of the valve. If the bias voltage gets too close to 0v (cathode resistor going low on value with age, R14 in your case) or even worse to a +ve voltage (DC voltage from the preceding valves anode, leakage through the audio coupling capacitor, C18 in your case) then the valve will over conduct, and the excess current will burn out the very fine wire in the output transformer primary. *
This is a common problem with the DAC90A, which is why in earlier posts you were being advised to forget the rectifier for now and to check the voltage on the anode of the UL41.
As it turned out the main cause of the problem was a cathode resistor which had gone high, or open circuit, causing the valve to not conduct and resulting in higher than expected voltages on the HT.

* The audio coupling capacitor is one of the most critical things to check/replace in every valve radio.

Mike

[peter_sol]

The UL41 valves (V4) sometimes cause that and may lead to distorted sound as the set warms up. The only solution is to replace them but they are scarce and because of that expensive.
I would see how the repaired one goes but do change C18 on the other set if nothing else.

[peter_sol]

Pin 7 is the cathode but pin 3 is also internally connected and should be left unconnected but Bush used pin 3 as an anchor tag for some components.
Cut pin 3 off the base but leaving the components connected to it.
Let us know if any change.

[Sideband]

Originally Posted by peter_sol

Cut pin 3 off the base but leaving the components connected to it.
Let us know if any change.

Yes...I did that on mine and it made a huge improvement to the leakage voltage on the control grid.

One of the problems with the UL41 is internal leakage of the valve. After many years use, deposits build up between the pins inside the valve and cause high resistance leakage.

[Ed_Dinning]

Hi NB, if you PM me when you get round to it again I may be able to loan you a UL41 for testing as I'm not so far away.

Ed