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Tube tester: Sencore MU-150

Brice

Brice

AK Subscriber
Subscriber
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.
20190412_160406.jpg

Inside:
20181212_212916.jpg

Here is the schematic:
Sencore_MU150.jpg

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:
20181227_094610.jpg

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:
20190412_143534.jpg

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

Here is it finished.
20190412_143518.jpg

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.

Sencore-MU150-V1_1.JPG
 

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Brice said:
I forgot: here is the spreadsheet of the mouser's order. Can save you time.

Cheers,
Brice.
Click to expand...

Thanks for the useful posting here. :) I do need some advice as I messed up my 'Load Switch' when cleaning the pots and switches on my MU150 tester recently bought. The load switch consists of 4 separate switches as shown in the photo. I tried to take them apart and clean them individually. The problem is that I do not remember where the switch knob was positioned when I took out the 3 switches starting from the upper section. The last switch was not removed so that one is fine. the other 3 are in alignment but I am not sure how to align them with the last 4th switch. I turned the knob clockwise/counter clockwise many times without thinking :(

Can anyone show me the switch orientation of the 4th switch when the knob is position in A section.

It is a 50/50 chance but I do not want to risk damaging this good tester. IMG_20190708_124732.jpg

Thankyou :)
 
I truly appreciate your help towards solving my small but big problem :). I have just realized that I asked the wrong question to solve this problem. What I need is a photo of every switch in series as they are taken out one by one. :) I guess the knob position does not matter since the last switch on the bottom was never removed. So I just need to look at the D shape indentions on each rotary switch dial. When or if you have the chance that is. :). Thankyou very much again. Btw. Excellent workmanship!

TJ
 
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Nah. These boards can't be trusted due to micro cracks not visible with naked eyes when they flex.
That is why I made a new one.

TJ, you are asking me to disassemble the load switch and take picture for each pole to see the D position?
Even with this you could still be 180 degree off.
Maybe it is better to take measurement at a set position and you replicate it to align all the D shape correctly?
 
Nice work, Brice! I have the MU-140, and it had been great. I rebuilt my original board, and also all other components, including the load resistors, it works great. The calibration for your board sounds close to the MU-140, I do not remember needing a scope to calibrate mine.
Thanks for the files, they may come in handy sometime down the line.
 
IMG_20190711_113712.jpg Yes Brice

If I can have a look at the D shape cavities (not D switch position) on each switch things would be much easier as I am not an expert and incapable (i suppose :( ) of measuring the switch for certain values. I did not remove the 4th bottom switch so the switch knob position does not matter. If given some photos on each of the switches(in the same knob position) I can trace the D shape and get them back together. :)
 
Quick note on this tester, and an "improvement" done to it.

One thing I do not like in this great tube tester is the signal adjustment pot for the Gm test.

It is an odd double pot, 50K/5K, oversized for no reason.
Mine was at the end and ratling. I unbuilded it, clean / lube it, but no luck.

I've read it on other boards, and indeed those go bad.

So I wanted a reliable solution, with repeatability of tests, as adjusting by eye a pot is not very precise.

I then looked around for solutions but did not find anything suitable.

So I decided to make some kind of stepped attenuator as a replacement.

I used a 24 rotary switch, a 20 x 250R and 2.5K to recreate the pot, using 20 positions only to respect the graduation.
So I now step 5 unit at each step which enough, but also now the steps are defined and always identical.
It worked like a charmed. Installed, re-calibration which now takes very little time, and the tester is ready again.

I thought that could help others to find a solution to their dead potentiometer.

Here are some pics:

The old pot on top and the naked switch in position.
20190915_160933.jpg

Then I build it:
20190915_172136.jpg

Installed:
20190915_180448.jpg

A better view. Thanks for reading.

20190915_190643.jpg
 
Thanks Andy,
Not really. I only have 2: 1 Utracer digital, I could not happier with.
And this Sencore MU150 I using when I want to test a tube the analog way, rectifiers and voltage regulators.
I have to say that 90% of the time, the Utracer is what I use.
 
100% Agreed.
On small triodes, I can trace both sections at the same time. It's amazing from one tube to another, how the 2 traces differ.
Visually matching is so easy.
At the tracer price point, it's unbeatable.

Also I always try to buy tubes tested with an Amplitrex as my tracer and the Amplitrex always match.
TV7 and Co too, the results are good but both the UTracer and the Amplitex give a % result, which to me is much easier to assess then a Gm=2345 for which I have to get the official result to make my judgement.
And they both gives the % Ip static current, which is often omitted (often for good reason) and which IMO, is one of the most important qualifying result for a tube.

Did I tell you I like my tracer ? :)
 
Dear Brice

Thankyou
I have not been able to get a hold of the MU150 for months but finally had some time to work on it.
I just gambled (since it is just a 50/50 chance) on my load switch and took my chances and got it
to the right alignment again. :) Took me 2 tries on a emissions test.

Then went on changing some of the parts such as electrolytic caps, diodes (8.2 zener, 1n4007)
and found out the TR1 was shorted so changed all three TRs. (Before changing TR1 I did not get a flicker
on the Gm test)

Luckily, almost everything works :)

1. I am able to successfully calibrate emissions to the 60% scale. (using the diode + 1k resistor test module)

2. I barely get a 100% reading on my Gm Signal at full power. (but need to turn the R24 pot. to max, so I assume
I should be changing all carbon resistors on the pcb + load switch since some of them are drifted 10~20%)
The voltage on the signal pot arm reads 1.410V (measured AC, chassis ground to center of signal pot wiper)

Removed the 1.6K resistor after setting the high end of the signal,

3. However I failed to calibrate the 'low end' of the signal.. which should be a 1/100 of the max signal level.
To be honest, I don't know where the ground point should be when measuring voltage with my DVM.
All points show somewhat different values. The lowest I can measure is around 0.050V but seems inaccurate.
Is there something wrong with my unit?
I tried the chassis, the metal plate where the tube socket sits on..
Maybe my situation is causing the problem since I use a non grounded step down transformer (220v to 110v)..and this
causes AC 'noise'? that interferes with low level AC readings? I cannot get an accurate reading for low AC voltage. :(

**So my question would be where is a good ground point when measuring AC for the signal?
**Is there another way to measure the signal to calibrate the 1~100 scale? For example by measuring DC on the connection (pcb to bolt/hex nut)
to the MU150's meters? in this case do I still need the 1.6k resistor?
** Is it still necessary to change the carbon resistors or other components if I can successfully(but barely) get a 100 reading for the gm test?
I will do so anyways, but just out of curiosity. Since the purpose of calibration is to adjust/compromise to the components that have changed values and consequently get the
right output.


Since I am beginner in fixing things I do end up asking basic questions.
Thank you so much in advance~
 
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Great ! You've made great progress.

1) The ground is a PC8, where the red and yellow PT CT wire goes. You can grab it easily at the negative of C16, upper left on the board with a clip.

2) Yes, the lower signal can be hard for some DVM to grab maybe because the signal is not a sinus. My handheld Fluke could not. I used an old DVM from the 90's and that one registered, go figure. If you have a scope, you can do as described on the doc. It's not that easy but the scope will have enough sensibility for you to measure the 1/100 of the max signal.

3) Yes. the CC resistors most likely have all drifted. And on mine the transistors were measuring DC ok, but were not working. So SS do age too... :)
The PCB is fragile and micro cut happens very hard to see with naked eyes. Mine wasn't reliable enough at the end, that is why I made a new board with some improvements.

And I do not think your step down transformer is at play here.

I hope it helps.
20181213_133226.jpg
 
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