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Tube tester: Sencore MU-150

Yes there is not much left perhaps the only one left in the city..
Sadly they did not stock everything I needed.
Will try to buy some online and update the status.
I got the Neon bulb glowing again at the copper side of the circuit board, but
still getting too high GM readings. Since the passives components are all new
I really don't know what to do next.. Perhaps a new PCB + rebuilding the
whole thing again. :)
I hope a new signal pot
will bring back the tester to life.
 
So the calibration of the static emission is good. This is the basis of all.
And you are getting too high readings in GM emission tests with the test plug you've build?
 
Brice I can successfully calibrate to 100% when setting high signal Gm. No problem at all. Very steady.. (with the 1.6k resistor)

The problem occurs when I test my 12ax7s..actual tubes in place.

Maybe worth trying other type of tubes?

What is the correct value of the cr12?
I remember when I had a single 1n4007 the signals did not max out.. but this is not the proper remedy is it?
 
Oh I see. So if I summarize:
- So the static emission calibration is good.
- The GM calibration is also good, 100%.
- Now when you try test an actual tube, the reading is way too high.

Hum, that is odd. I am thinking something is off with the gm signal level.
Have you managed to calibrate the GM low setting successfully ?

On mine that was the problem I had. It is possible that the gm signal is too high at the tube's setting level, therefore the reading is way too high.
This tester is so sensitive (which is good) so with the signal dial a small variation in degree generate large reading variation.

That is why I am very happy with the decision to use a rotary 24 stepped switch to be able to make absolutely identical repeatable tests.

On my board, I replaced CR12 with a UF4007 and 1N5817 serial.
 
is that the signal pot you replaced with the attenuator Brice? Looks like a dual 50k/5k pot that is meant to track together. If those sections are off and/or track poorly I can see where that would cause measurement errors. Carbon pots tend to age poorly unfortunately.
 
Hi Gadget,

Yes it is. I had issues with them, exactly as your described. The stepped solution eliminate that issue.
And I don't know why Sencore used huge one like this when there is really no need.
 
Hi Brice,
I joined this forum because of this post and I wanted to say thank you for sharing your updated PCBA design and attenuator replacement idea! I just ordered one of these testers and will be swapping out the factory board for yours! Also, double bonus on the suggestion to use https://jlcpcb.com/, wow they are much cheaper than the proto board houses I've been using! Thanks Again! If anyone needs the service manual or tube charts let me know and I can try scanning them.

-Adrian
 
Hello Adrian,

Congratulations on your purchase. The Sencore MU150 is IMO the most sensitive non-military analog tester out there.
I wanted to help the community with the notorious issue with the PCB on this model. And it worked ! :D

Good point: I do have PDF file of the tube chart and the service manual, but it is too big to be posted here unfortunately.

Good luck in your restoration, it will be a fun project is it was for me.
 
I've been lurking on this thread for a bit. I bought an MU-150 several years ago and I've always wanted to find someone to restore it for me. Currently it works good enough to test whether a tube is good or bad on shorts, emissions, and grid leakage. But, I've never been able to get it to behave on Gm tests. I thought it was the switch and just lived with it. Now, after following this thread I think I might open her up again and check the PCB for cracks.

I thought any kind of work on this unit was beyond my skill set. I mean, I can find my way around a Dynaco tube amp without killing myself, but every time I look inside the Sencore I get overwhelmed. Now after this thread I think I might be able to tackle it.

Thanks @Brice for starting this. I sent you a PM about the service manual.
 
Thanks Guys!!!...I happened upon this tester mostly by luck at Ham radio site. After researching a bit it had everything I wanted and way more, then finding this forum and the work @Brice had done was icing on the cake. I've been doing solid state repairs for years but have just recently been diving into tubes so I am super excited to have a good tester. If i get really into it I guess i'll have to go for the uTracer next!

Hey @dogwan you can find the service manual and tube charts here http://bama.edebris.com/manuals/sencore/mu150/
I believe I saw there were 2 slightly different schematics for this tester, not sure which is in this manual.
 
Hello I would like to thank Brice and others who have helped me out restoring/calibrating the MU150 tester.
In the last stage I had a problem with the tester reading too high on GM tests, all tubes tested too high, even the weak ones.
Problem was solved by making a new signal switch as Brice here once did on his and throwing out the old signal pot.
Now the tester reads what it should.. I really think the signal switch replacement is a must on all MU140/150s.. wow what a change!
Remove the connection between the 2 wipers of the pot, check for jumpy/open spots one by one.
I also tried disassembling and thoroughly cleaning the old pot but it was a waste of time. Just change it if there is any
doubt about the old signal pot.

It took a while to get all the resistors and the rotary switch delivered due to the recent outbreak
and stupid me, I ordered 250R and 25R instead of 250R and 2.5K.. :(Ordered again!
I hate it when
this happens.

I really enjoyed bringing the tester back to life
and will enjoy even more testing my tubes!

Thankyou again Brice :banana:
God bless you!


DSC_0178.JPG DSC_0180.JPG
 
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Thanks.

I am glad you got it working perfectly.

I am still wondering why Sencore used a pot there, for such important parameter, and to such sensitive tester.
Also the min max signal circuit is wonky which makes the calibration not that easy.
But once all that is working, this is a very good tester super easy to use and reliable.
I used mine all the time, despite that I also have UTracer 3+.

Congrats and enjoy. :beerchug:
 
MOQ on the PCBAs was 5pcs so if anyone wants a Brice board for cost plus shipping PM me. Total should be around $15 shipped via ups.
 
Attached is my updated BOM based on Brice's initial BOM and notes in this topic, updated for instock parts at Mouser.

Notes:
1. It includes all parts except resistors under 1W, the Tube socket, the 5/50 pot mod
2. I removed the 62uF 160v caps as I didn't see where they were supposed to go.
3. If there is an Asterisk (*) then the part is not on the PCBA
4. I took my best guess on C18, C19, and C28 as I have an MU140 and these are not present
5. I've built the board but not have tested it yet...can't guarantee i am right on everything =)
6. Does not include any parts for the 5/50 Attenuator replacement
 

Attachments

Hey hey! Checking back in. Getting closer to pulling the trigger to get going on this project.

I have the PCB thanks to @The Ruined , I've got the BOM all loaded into Mouser. Still need to add the resistors to do the stepped pot replacement and have another project I need to open up and build a BOM for (SAE 2800 parametric equalizer).

SO, what did I just realize yesterday? Well, I have an MU140, not the MU150. From what I've read there are only a few differences....

"There’s some speculation about the difference between the MU140 and MU150 models, apart from some relocated front panel controls. On the schematic, the only differences are that R6 is split from a 10M resistor into a 4.7M and 5.6M in series, with a 220n cap from the junction to ground; and there are ferrite beads on many of the socket wires. Protection diode CR12 in the later models appears to be a 1N816, which has a higher Vf so that the meter can more easily reach full scale without the diode conducting. Anything with at least .66V Vf @ 1mA should work fine here."
https://atomiumamps.tumblr.com/post/19846211560/sencore-mu150-rehab

Does anyone on this thread have any input on whether I am good to go to proceed with this project?
 
Yes you are good. These 2 models are essentially identical, just a few "upgrades" added to the 150.
So in fact you are going to upgrade your 140 to a 150. Congrats :).
 
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