Tube tester: Sencore MU-150

[dogwan]

There is one 10K resistor too.
Here us a better shot.
View attachment 2159670

Ok, now I see they are virtually the same except for type of switch.

PM sent.

I am guessing I can easily make a bracket to hold that type of switch. I also like the idea of a latching switch to hold the reading.

If this doesn't work I was thinking I could find a 3pdt toggle switch that I could make work too. Just need to figure out the voltages on the switch.

 

[dogwan]

Well, I'm at a loss. After completely replacing the switch thanks to @Brice I still get no real reading on the Gm test.

It's exhibiting the same behavior as before I started this rebuild process. When the Gm test button is pushed the meter stays at zero. It does want to move if I push the switch slowly. And now if I mess with the B, D, or E switches while the Gm test is engaged I can get the needle to move (almost like bad contacts on switches).

So, I did disassemble the B Load switch (after very carefully marking the positions of each deck with it's wipers' positions) and clean each deck. They were pretty sad looking and I managed to get them nice and shiny, dried out, and lightly lubed again before reassembly. I also cleaned the exposed side of C Grid switch since it's exposed. I also sprayed contact cleaner on the Screen and Plate switches as best I could while rotating them through their range and let them really dry out. Still getting the same results.

When I go through the alignment procedure with the 1.6k resistor I have no problem getting the meter to read 100%.

Any ideas?

At least it seems to really be reading the rest of the tests very well, so I am happy with that.

Here's some pics of modifying it to use the latching push button switch...

Had to drill out the rivets holding the old slide switch to the brass colored backing plate.



After grinding the opening with a Dremmel to alter the switch opening from a rectangle to a round hole it was easy to insert the switch and wire everything up. I literally transferred the wires over one by one.



And this is what it looks like from the topside. The switch has a much better feel and action. It is built much more substantially than the old fiber/phnelolic slide switch.

 

[Brice]

As you get the 1.6K reading ok, your volt meter is working correctly.

I would start to check for the 5k sin generated by the oscillator and follow it to a tube under test grid, to see if you indeed have the oscillating signal to the grid of tube under test.
If you do then follow the amplified signal by the tube under test downstream, as it should reach to the VTM for the integrated measurement. I don't think you get it as the meter would register, as it is very sensitive.

 

[dogwan]

Just picked up a scope this last weekend. Hopefully that will be helpful to follow @Brice 's directions. Now, to learn how to use this thing.....Bwahahahaha.

No seriously, I do need to learn the basics of a scope. But, I think this will be the next evolution of my electronics repair abilities.

 

[dogwan]

Well, looks like I lost a post from a couple days ago. Must have left it in "draft" mode without posting.

Anyway, here's what happened.

I decided to soak the D and E controls in Deoxit since those were the last two that I couldn't easily disassemble and clean. I know it's ill advised, but I did it anyway. I really soaked them and worked them and left them to dry overnight.

The next day I tried testing some spare used12AT7's I had handy. At first I wasn't getting any readings on the Gm settings. I twiddled with the controls a bit and lo and behold I started getting readings. They looked a little low'ish, but the tubes were used. At least they were repeatable as I swapped tubes in and out.

SO, I decided to do a 2nd soaking in Deoxit overnight and had the same experience. But this time it took less jiggling to get the readings to start showing up. It seems to be the E-plate control that is most problematic.

At this point I decided to run through the calibration procedures again. Especially as I now had a scope to set the lower (1/100th) Gm setting. The Emission settings were still spot on. But, this time with the 1.6k resistor in place the setting of the high Gm setting was about 10-15% lower than before. I had to crank the pot almost to the end to get it to read 100.

SO then I followed the directions for setting the lower end of the Gm and while I could just get a decent square wave at the high settings, I could only get a distorted square wave to show up when I turned the F pot all the way down. And, the adjustment pot didn't seem to have any affect. Now bear in mind that I am still basically a Monkey banging on a typewriter when it comes to scopes. I do suspect my probes are shite so will try again when I have a chance to order some new ones. For now, I just set the lower adjustment pot to the middle of its range.

On a side note, the calibration directions are a little vague. It doesn't say to push in (engage) the Gm test switch when setting the calibration. Without the Gm switch engage I was able to hit 100 on the upper setting. If I pushed the switch in the meter would drop to 90 and I couldn't get it above that because the adjustment pot maxxed out. Thoughts?

Anyway. I am encouraged. I know that my unit seems to be conducting all the tests adequately beside the Gm test. And that now is showing signs of life.

Any suggestions for the next steps? I know @Brice mentioned that he had trouble setting the lower Gm calibration. What would be the best "default" position to leave that adjustment pot? Any other tricks for setting the Gm settings?

 

[Brice]

Hi Dog,

I've just re-calibrated mine and took notes as I get calibration questions quite often.

First make sure you have a good voltmeter on hand. Many DVM can't really integrate accurately this square signal.
My Fluke 117 can't, it just displays 0. I used my bench Tektronix for that.

And I set my Tester that way to have easy access to both sides:


And here is a picture of my board so you can see the pots I have:



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.



Calibration 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.


In your case, I think you have a mechanical or wiring problem, and I hope you find it soon.

I hope it helps.
Brice.

 

[dogwan]

Thanks @Brice for that clear and concise write up.

I see now a couple mistakes on my part...

• I was trying to measure voltage at calibration point to ground, not from wiper.
• I may have been trying to measure DC instead of AC (rookie mistake, now can't remember if I caught that or not).
• I didn't realize I can adjust the range of R24 by adjusting R22 (I did install pot there).
• I also suspect my DMM (Fluke 29) can't read this either. I will see if I can borrow a bench voltmeter.
  If I get lucky maybe I can find the optional DM44 multi-meter that attaches to the top of my Tektronix 465 scope.
  Or, maybe I will give it another go with the scope and try attaching the probe ground to the PCB ground.

 

[Brice]

I see.

Cool I am glad this will help you and others.

Funny I tried with a modern Tenma bench meter and it could not read correctly. Same thing with a el Cheapo Chinese DVM.

But I have an old 1980 Kontron DMM3020, vintage with red display which I use a lot, and that one can too.

Seems like the old early digital voltmeter can, but the new one can't.

I know the 5Khz signal exceeds my Fluke resolution (2.0% + 3 (500 Hz to 1 kHz), so maybe it is why above it reads nothing.

 

[Porthos]

I destroyed the switch for "B Load". Simply heating up the solder tab to replace a resistor caused the tab to separate from the switch. Looking at the service manual a 12P12P rotary switch is needed.

Can someone link me to a suitable replacement from Mouser or Digikey?

 

[Brice]

Looking at the schematic, it looks to me you need a 12 positions 4 decks rotary switch.
I've never changed one so I don't know which one would be the best.
Mouser shows a 30 degree angle, dunno if it will work.

 

[Porthos]

I see 12 Position switch where I can select 4 Decks. Then I see 12 Position, 12 Position switch which only has two decks with a confusing selection. I hope 12P12P in the MU150 service manual is not the latter.

If the plain 12 position switch with four decks is needed do I need a "Shorting" or "Non-Shorting" contact style?

Thanks

 

[Sencore1]

View attachment 1903165 Hello again

I am uploading a photo that is more clear. Sorry for the confusion the black insulation tube
on the resistor may have gotten in the way.

I think the connections are properly made as in Brice's pic. (corrections, I misunderstood your sentence.. The bulb and r47 connection should not touch any pcb trace. I connected them to the trace.. I will disconnect now and see if it works)
Meanwhile I will visit a local electronic s store
and find a new signal pot. Will buy two and swap the wafers. 5k + 50k.

Thankyou so much
Really appreciate all the help provided!

The official schematic for the MU150 does not appear to match the posted photos of the underside of the MU150 PCB and the connections for the NE2 bulb. When I follow the schematic to wire the NE2 bulb and R46, R47, the emission won't calibrate correctly--I can only get to about 72 on the meter, even with R36 fully adjusted. I have replaced almost all the components on the PCB, so I'm fairly certain it's not defective components. Any thoughts? Should I try to wire the 2 resistors according to the posted photos, because they do not match the schematic on all the connections.

Thanks

 

[Brice]

What MU150PCB photos are you referring to? The original PCB or my new one?
The schematic shows NE2 connected to PC9 (-210V) and R47 (3.3M).
R46 is not connected to the NE2 directly.

 

[Sencore1]

The photos that show the underside of the original PCB. There were a few photos posted from this thread showing the underside of the original PCB(s). I have looked at the photos of the replacement PCB, and it appears to match the connections for the schematic. As I said previously, the schematic connections do not match the connections for the NE2 bulb. The connections for R46 appear to match the schematic and the photos, but almost everything else does not. Thanks

 

[Brice]

Ok I had to dig my old PCB and look at it. After tracing it, it is as the schematic.

Just the order is reversed between the NE2 and the R47 but that does not matter as they are in series.

What I traced is: PC9 -> R47 -> NE2 -> trace between R17/R18/ Anode CR8.
The R46 jumps from that trace to pin 1 of the tube regulator as schematic.

I took a shot of the view so you can compare with your PCB.

My PCB was working before I removed it and made a new one.
I hope it helps.

 

[Sencore1]

Ok, I'll take a look again because your board connections for the NE2 are different from the the other board photos that have been posted. Thanks for going out of the way to check on this.

 

[Sencore1]

Ok, I completed the emission calibration with your help! Now, when I go to calibrate the Gm, the needle only goes up about 1/4 scale on the meter. I'm assuming the calibration module still needs to be inserted in socket 1 and the previous emission settings stay the same except for the signal pot to max and B Load switch is set to 1. A couple things I want to mention...I found out that there are 2 schematics for the MU150--one that includes 2N5172 transistors for TR1, TR2 and TR3, and another schematic in the manual that lists these transistors as 2N2923's. The 2N5172's are the correct ones, right? Also, I noticed that you installed a Schottky diode in series with a 1N4007 for CR12. I currently have a 1N4004 installed, so do I need to add the Schottky for the Gm to calibrate? Thanks

 

[Brice]

Oh, that's a long time ago and I don't remember clearly the details.

Yes you need that schottly diode in serie with a 1N4007 for the meter protection. I think I used a 1N5817.

For the transistors, I've used Mouser #: 610-2N2923 and had no issues with the oscillator.

For the gm calibration, I am under the impression that the module has to be removed.
It is only used to adjust the correct volt meter function of the tester, so after you measure it at 60, you know the meter measure voltage correctly.
Then on to the gm part.

 

[Sencore1]

Do you have any recommendations for cleaning the F signal pot (R20)? My readings are not correct for these--one is supposed to be 5K and the other is 50k.

 

[Brice]

You can try with Deoxit F5 but in my case the pot was shot and I couldn't find one.

So I built one with a rotary switch 2 poles 24 positions with resistors like a stepped attenuator. Please read post #12.