Calibration of AVO tube testers
This question comes so often by Email, I want to answer it here.
"Hello Jac, I have bought this AVO tester from somebody, and the appearance is nice. Only I have a small problem with it. I have a box with tubes, but when I test them, the tester seems to reads too low. I have been looking the calibration setting at the inside, but I am afraid to do something wrong. What should I do? Can you help me with this"
So the expectation is to turn a calibration screw, and then the job is done. Unfortunately it does not work this way.
First of all, it does not work this way, because we not a church. My lawyer charges 295 Euro per hour for advise only. Even the man who paints my house, wants 80 Euro per hour, and all he will do is move a brush up and down, if he comes at all.
The second reason why it doesn't work like that, there is no such screw inside, saying "Turn right to increase precision". If there was, I would tell you here.
The only way to get good result, is use the SERVICE manual for this. If I have those, they are linked elsewhere on this website, you find it automatically when reading the right part. The procedure in there assumes it is the year 1960, and the tester is may be 10...20 years old, and ready for a service. Only, now we live 65 years later, and best way the tester was never touched. Meaning there may be coffee drops in some tube sockets, causing leakage, bad capacitors or bad resistors, cracking pot meters, and a long list of other issues, cause by old age, not anticipated in the service manual, but at least no fool opened the tester.
The service manual is a good document to start with. Though it can of course not really help, when somebody messed the tester up. Like, as a matter of principle, the low reading is suspected to be a panel meter fault, so they open up the meter, see the adjustment screws, and then guess what they do... Then, to their great surprise this does not help, but they meter is now worse then before. 99% of the meters I get send for repair are like this. So when they find out, the meter was not the main problem, they look deeper inside, and it doesn't take long to find the calibration settings. But.. there is the factory calibration, which will not change itself, it should not be touched, and there is no procedure published for this. And there is the user calibration, which must be done sometimes. Well, I am sure you understand this, but 65 years is a long time, and often, some fool tried to improved the meter and the factory calibration.
What is needed do?
For calibration of an AVO tube tester you need some good equipment.
- The SERVICE manual. This is not the user manual.
- Mk3, Mk4: 30uA 0.2% current meter to check the meter sensitivity. (The Mk3, Mk4 panel meter must be within 0,5%). Test Meter must be of averaging type, not true RMS or simulated RMS.
- 100uA Ohms meter to measure meter impedance. These are very rare. 99% of all ohms meters, also digital, and very expensive ones, use 1mA. That is possible with Mk1, and Mk2, but it may l damage the Mk3, and Mk4 meter. As these cannot take an overload of a factor 33.
- A good working AVO 8 multi meter, to check the voltages inside. As they were specified by AVO with only this meter. This is a 50kOhms/Volt averaging type meter. A meter with another impedance will not show the right voltage, as circuit load is another. Also you may get another result because the way an AVO8 averages the wave shape may be another with any other" meter. As there is a lot of distortion in the signal, due to the tube under test being a non-linear device, you cannot work with a true RMS meter, or RMS simulating meter. We do need to measure average value here. Moreover, many old AVO8 on Ebay are imprecise, and not suited anymore to calibrate other instruments with.
- A tube tester to check the rectifier tubes of the Mk3.
- Not bad is an oscilloscope to check the waveforms for abnormalities, so a defect of the Mk3 Selenium cell becomes visible, or any other leakage or issues. Also using a scope is better to do the factory calibration of the grid voltage.
- Three exactly known tubes, with a full set of tube curves. After calibration the tester must be vitrified with a tube that has a very low grid voltage, then with a tube that has medium grid voltage, and finally with a power tube. It has to be tested at various plate voltage and plate current, if the grid voltage corresponds to those tube curves.
- A Variac to check the result at various mains voltages, all in the end.
The AVO has some verification, and two calibrations to do.
- First follow the checklist completely. It includes transformer verification. If the transformer is not within specs, calibration is impossible. Inside are three mains transformers, these are often good, but not so often with testers that are sold..
- Meter verification. If the meter is not within specs, calibration is impossible. The panel meter is very often out of specs.
- Replace the capacitor across the meter with Mk4, it is bad always. It is hidden deep inside a switch. Just cut it out. Replace with Wet Tantalum type. Value is not extremely critical. 8...15uF. Put it directly across the meter, so you do not damage the switch. Check the polarity with a working tester. ADD this capacitor with a Mk3. Do not replace the meter protection diodes by "better" ones. There are no better ones.
- Verify leakage current function. If totally outside specs, calibration is impossible. This is often good, but coffee drops or Coca Cola drops inside the sockets cause trouble.
- Factory calibration. This is the position of the dial wheels. VERIFICATION ONLY.
- Verify the protection relay, and adjust if needed. You can use an old power pentode for this.
- User calibration. These are the internal pot meters for the mains voltage, and grid voltage. This absolutely requires as a minimum a set of calibration tubes of type 6SN7 and 7SL7, to verify your work. Best is the use the complete calibration package we offer. This includes a calibration tube 6SN7, 6SL7, 6L6, a leakage test tool, a 60 page manual about repairing tube testers in general, and email support about how to use the calibration tubes. A guided enmail session about hoe to calibrate a specific tester is not included, but I help when it stays fair, and when I can. For Mk3 and Mk4, it also includes a table to check the grid voltage setting of the grid wheel.
- But most of all, the above is not the official procedure. You need to use only the SERVICE MANUAL for this.