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Tube tester: Sencore MU-150

Brice said:
Jay,
This is from my notes.

Here is my 10 steps GM calibration procedure.

CALIBRATION (After the Emission Calibration 60 is properly done).

1) Set a sensitive voltmeter to AC.

2) Set the FUNCTION switch to SHORT, this power on the tester and warm up the 12AU7.

3) Add the 1.6k resistance between the wiper of SIGNAL F pot and the Cal point on the board.

4) Set the B LOAD pot to position A.

5) Set the SIGNAL F to maximum 100.

6) Adjust R24 to reach 100%. Sometimes you need to adjust R22 for fine tuning so you're
not at the edge of R24 (if made adjustable during the PCB build).

7) Measure the AC voltage at the SIGNAL wiper and ground on the PCB: I read 0.722VAC.

8) Set the SIGNAL F to minimum (0) position.

9) Disconnect the 1.6K resistor.

10) Measure the AC voltage at the SIGNAL wiper and ground on the PCB and adjust R21
1/100 of the previous voltage, in my case 7.2mV.

That's it. If you have a scope you can do it like this too: you just have to have 1/00 of the signal amplitude at the end. Same principle but just visual.
Click to expand...
So I try to do Gm and I went through this and I am unable to get ANY movement out of the meter while it is in shorts test. If I switch it over to GM-emissions it instantly flies to the 100% where I can further adjust it using R24. Is this correct?
At 100% when measuring with the 1.6k resistor in place I only read around .175vac on the wiper. Pot moves good from 0-5kohms

I set it this way then attempted my best at the minimum 1/100th but my meters not the best so it will need to be redone. But when I went to test a tube and press the Gm button, the meter only moves maybe 1/8th of the way up. Which doesnt seem right.
 
Brice said:
Hi Guys,

I am not sure this belongs to the Tube Audio forum but I didn't find a more suitable category.

I worked of this tester, a Sencore MU-150, that I found very nice so I am sharing this with you.

The good points:

- This tester is one of the last tube tester been produced, therefore is more modern than the classic oldies.

- It has a very sensitive as it uses a 12AU7 tube as voltage differential. The meter could pick up variation that my fluke couldn't.

- It does the classic tests for static emission, shorts, grid leakage and tube life.

- And does a superb job as trans-conductance test, by generating a 5 Khz signal to the tube and measure it that way. It has 3 transistors for that function.

- The voltage reference is also made with a tube too, an OB2 voltage regulator.

- It is a very robust and heavy piece of equipment. It has 2 transformers, one for the heaters and one for the tester itself. The case is a made of solid metal protecting the tester very well. The layout is clean and well design.

The weak points:

- The main problem is the PCB. Very brittle and prone to cracks. It attached directly to back the meter.
- The calibration is unnecessary complicated.

Here are some pictures.
View attachment 1477235

Inside:
View attachment 1477236

Here is the schematic:
View attachment 1477239

Most of the components on the board were out of specs and had to be changed.
All external caps were bad.
But all the rest was in excellent condition.

So I first tried to change all components on the board and give it a try.
Here it is:
View attachment 1477241

The tester was working. I used a SS voltage regulator too.
I was happy, but the board was still fragile, and the calibration wasn't easy as some resistor needed adjustment and without a trimmer, it the painful de-soldering/re-soldering process, try, retry.

I liked this tester so I decided to make a new board, and improve a few things.

- First I wanted all the caps to be mounted on the board and not externally mounted to the chassis.
- Increase within reason the PS filtration.
- I wanted to only use 1 tube, and replace the OB2 with a solid state solution.
- I wanted to shield the 12AU7 and keep it tied to the socket.
- Add trimmers were I needed to make the calibration easy.
- I added a current sensing resistor in the meter path so I could connect a VDM to it.

After 2 iterations, one without layout change (V1.0) and all these changes (V1.1), I got a perfectly working solution.

Here it is:
View attachment 1477251

Only one tube mounted on a plate, shielded. All caps in the board, more trimmer, neon bulb on top, etc..

Here is it finished.
View attachment 1477279

The calibration was easier.

1) The static emission needs a plug to be build with 2 resistors and a diode, as per the manual. Then you insert it and adjust the meter mid range with 1 trimmer. That calibrates the tube voltage differential for all measurement.

2) The GM emission calibration is a 2 step process. You need a 1600 ohm resistor to the arm of the signal pot, to the 0.47uf cap of the voltage amplifier (marked CAL in to board), as the user manual's instruction.

- Max signal first, you adjust the meter at 100% with 2 trimmers. Then forget about the scope things in the manual. Get a DVM and measure the voltage at the arm of the signal pot

- Min signal, remove the 1.6 resistor and adjust the voltage to 1/10 of what you measured before. That part is tricky.

And that's it. In fact they wanted the max signal to be 100% of the meter, and the min a tenth of it to have a linear range.

I added a fuse for safety, changed the neon bulbs (nice one from Radioshack), and all load resistors mounted on the rotary switch. Nothing difficult.

I am sure I am forgetting things but that's mostly it.

I like the ease of using of that tester. The little book has most of the tubes in it, you start by the shorts, then emission, then grid leakage, then gm emission then life test. Simple.

I attached my gerber file so anyone can have a PCB made for it.

And here is the rendered picture.

Thanks for reading.

View attachment 1477278
Click to expand...
Hello,

I am going to be building out the board to use in my MU-150. I was wondering if you have more info on how you mounted the tube and where you got the socket and shield etc. It looks very professional. I would like to follow what you did.

Thanks so much!

-Pete
 
Hi Pete,

Unfortunately, there is no off the shelve solution for this as I couldn't fine 90 degree socket anymore.

So I made a simple plate to mount the socket to it, and I mounted the plate with 90 degree brackets to the board.
There are 2 holes on the PCB for the brackets.

It's not fun to do, as I don't know for you, but for me the mechanical part is always where I mess up.

But at the end, it worked well, and I had zero issue with my tester for 5 years now already. And I use is quite often.

Also some other AK'er used it so it had some use, transportation and such.
Only a 8 pin Chinese socket saver broke, ha ha.

I hope it helps, and good luck with your board.

Brice.
Here is the picture for reference.

20190412_143534.jpg
 
Brice said:
Hi Pete,

Unfortunately, there is no off the shelve solution for this as I couldn't fine 90 degree socket anymore.

So I made a simple plate to mount the socket to it, and I mounted the plate with 90 degree brackets to the board.
There are 2 holes on the PCB for the brackets.

It's not fun to do, as I don't know for you, but for me the mechanical part is always where I mess up.

But at the end, it worked well, and I had zero issue with my tester for 5 years now already. And I use is quite often.

Also some other AK'er used it so it had some use, transportation and such.
Only a 8 pin Chinese socket saver broke, ha ha.

I hope it helps, and good luck with your board.

Brice.
Here is the picture for reference.

View attachment 3307399
Click to expand...
I have another question. The board I have I bought off of Ebay. V1.1. But I was going by your download files above which included an xls sheet of mouser parts. The spreadsheet seems to be different in some cases than the part values on the board. For example on the spreadsheet For C1, C2A, C2A-2 the board says 82uf 250v. But the spreadsheet says 100uf 250 volts. I assume that the spreadsheet parts list will definitively work for this project? There are also some item quantity issues for example it says 2 rectifiers are needed when only 1 is needed. And shows 10 36v 5W and 10 35v 3W. I just want to make sure I have everything correct before placing my order. Thank You! -Pete
 
Hi Pete,

V1.1 seems like my board, yes.

C1,C2A and C2A-2 are power supply caps, so here the bigger the better.

What's important is the caps lead spacing and diameter to fit on the board, here 7.5mm LS and 16mm D, but also the length of 20mm max else it may touch the enclosure.
100uF is good.

The spreadsheet is old and most likely not accurate anymore. I don't know if all the parts are still listed.

Anyway, the 10x is most likely my purchase order as I often buy parts by 10 as so cheap.

Check the board for exact quantity as at the end of the day, you need to populate that board.
 
Brice said:
Hi Pete,

V1.1 seems like my board, yes.

C1,C2A and C2A-2 are power supply caps, so here the bigger the better.

What's important is the caps lead spacing and diameter to fit on the board, here 7.5mm LS and 16mm D, but also the length of 20mm max else it may touch the enclosure.
100uF is good.

The spreadsheet is old and most likely not accurate anymore. I don't know if all the parts are still listed.

Anyway, the 10x is most likely my purchase order as I often buy parts by 10 as so cheap.

Check the board for exact quantity as at the end of the day, you need to populate that board.
Click to expand...
Got it! I am super paranoid so just wanted to ask. Thank you so much for the response!

-Pete
 
Brice said:
Hi Pete,

V1.1 seems like my board, yes.

C1,C2A and C2A-2 are power supply caps, so here the bigger the better.

What's important is the caps lead spacing and diameter to fit on the board, here 7.5mm LS and 16mm D, but also the length of 20mm max else it may touch the enclosure.
100uF is good.

The spreadsheet is old and most likely not accurate anymore. I don't know if all the parts are still listed.

Anyway, the 10x is most likely my purchase order as I often buy parts by 10 as so cheap.

Check the board for exact quantity as at the end of the day, you need to populate that board.
Click to expand...
Sorry I just went through every post and could have answered most of my questions. I am now putting together the parts list to order. I was looking at CR12 and see there are two in series 1N5817 and 1N4007. Does position matter for those? On the ebay listing for the board they show CR12B (1N5817) and CR12A (1N4007) but the board has both as CR12. Thank You!
 
I tried to do the emissions calibration and the meter does not move when adjusting R34. I built the test module correctly. I am able to zero the meter. I then set up the test per the instructions. Any change to R34 or R35 yields no change in the meter. I will dig in further after work, but was wondering if anyone has any ideas. (also I wasn't sure when to adjust R35. It originally was a fixed 470 ohm.

Also.. The Neon I put in the circuit was "Lamps 65VAC .6mA NE-2 Mouser #606-A1A". Is this just an indicator or is this a voltage regulator. I was seeing conflicting info on 60V or 90V? Could that be the problem?

Let me know and I will continue to troubleshoot after work today and this weekend.

Thanks for the help!!

So I did not realize to do the emission test you have to switch the function from SHORTS to Gm - EMISSION. The instructions did not mention this but luckily someone here mentioned this. I was able to calibrate to 60 with no problem.

I then did the gm test. So the instructions say set the meter to 100%. But it actually goes up to 120. So should I be calibrating it up to the number 100?

My first calibration I went to 100% and my initial voltage was 1.299 volts. I then remove the resistor and followed the instructions and adjusted voltage to 1/100th or .013 volts.

I tested a few tubes and it seems to be working. I did have a question about grid leakage. How can I verify that test is working correctly?

Also please let me know the gm meter setting is 100% or at the 100 mark and not 120 mark.

Thanks much!!!

-Pete
 
Last edited:
Got answer. For the Gm test we should calibrate the meter to the 100 mark not the 120 mark. (Thank you Brice)

Now I am all set!!!! I do have a completely up to date Mouser parts list for this project with notes if anyone is interested.

I am so happy to have completed this project and now I have a wonderful tube tester as I work on and repair old radios!!

Thanks to Brice again for creating the board and all of the great info!!!!

Awesome!!!
 
OK.. Let me work on cleaning it up this weekend. I want to make sure all of my notes are clear. Also I will note which components should be lifted ABOVE the board. When I started building the board I did not do that and ended up having to reorder some parts and properly space them above the circuit board. This is typically for the higher wattage resistors and some of the diodes etc. I should get it done this weekend. If you dont hear from me please ping me to remind me.
 
Fellows,

I am in need of some help.

I have a new to me Sencore MU150 that I purchased cheap with the understanding that it did not function.

Without testing I just purchased a Brice designed V1.1 PCB off of e-bay.

I completed populating it today and installed in the tube tester. Before power up I ohmed out all resistors to verify the installed values where correct, verified electrolytic cap polarities, verified that all diode bands matched the images on the PCB and that the tube socket was wired correctly.

I now have a problem.

Does the black wire from the 2000 pf ceramic capacitor circled in the photo below (not my PCB but an image from Brice's first post) go to solder tab 6? I labeled the wires as I removed them from the original PCB but my label maker did not underline the 6s and 9s and they look the same but upside down from each other. I am unable to find this ceramic capacitor on the schematic I have.

PCB with circle.jpg



Thank you,

Eric aka Vlad (a monkey with a soldering iron)
 

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  • PCB with circle.jpg
    PCB with circle.jpg
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Last edited:
Hi Vlad.

Yes the ceramic cap does go to point 6 on the PCB.

On the schematic, it's C18 0.002mf on the far right mid top.

Brice.
 
I believe I have my Sencore MU-150 tube tester repaired and calibrated properly.. It does seem to test tubes comparably to my Hickok 600A.

The repairs where not uneventful. I did replace the PCB with a Brice version 1.1 unpopulated board purchased from a vendor on ebay. With the board fully populated and installed I turned on the power to the tester and immediately had a resistor (R8 180K ohms) burst into flames. It turns out that different transistor manufacturers use different pinouts on their 2N2923 transistors. The transistors I used where unharmed and I did find a pinout that worked even though the manufactures data sheet did not provide this info as the transistors are no longer produced.

I also had a problem with the meter movement that I found to be a wire crushed under the PCB when I installed it and the red/yellow ground wire to PCB terminal 8 (solid wire) broke off from the PCB without my notice.

Those issues are in the past though.

I do have a question about the meter movement when I first power the tester on. The meter is static for about six seconds, climbs to just under the 30 mark and then returns to zero. Is this normal power on action for the MU150 tester?

Thank you gentlemen.

Eric aka Vlad (a monkey with a soldering iron [and the burns to prove it])
 
No worries. That's the tube warming up. Try another one, more balanced and it won't happen or much smaller amplitude.
 
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