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Electronics => Repair => Topic started by: Vintagevalves on December 18, 2018, 10:38:23 am

Title: Sencore DVM56
Post by: Vintagevalves on December 18, 2018, 10:38:23 am
Recently acquired a free circa 1980 Sencore DVM56.    Didn't do much of anything with power applied when I got it, and after a fair amount of repairs only gets a little further.   

What it currently does: powers up, occasionally shows random characters on the display.  Most recently it seems to display a letter A (which I don't think is meaningful).   On the power supply board, there is a relay which switches a shunt in and out to change the range of the Ammeter.  This relay blips at a fairly even interval.  Let's just say every 1 second.     

Repairs done so far:  Found bad UA78M05UC 5V regulator on power supply board.  Replaced with LM3405 which I already had.  I think this substitution is OK; as the only difference seems to be the current carrying capability of the chips.   I now have rock solid 5V everywhere the schematic calls for it.     Fully recapped and replaced all carbon comp power supply resistors.    I replaced the resistors because some seemed to have drifted high and the 7.5V rail is metering low (6.8V).   After re-resistor it still meters low.  I've also tested most of the 2N3904 and 2N3906 Transistors (in circuit) as well as the MJE transistor for the 7.5V rail out of circuit. 

Other thoughts:
This meter is somewhat modular.  The various function cards can be removed to try to isolate problems.   The relay blipping seems to be tied to the controller board.  Removing it makes the relay stop blipping.   It still blips with all of the other cards removed.      Nothing seems to be getting hot.   The controller chips are warm to the the touch.     I think that the relay blipping is related to the reset function of the 8035 Microprocessor.    As the relay blips, I can watch various voltages around controller board pulse.  I'm not very well versed with digital things like this, so I'm reaching the end of my troubleshooting abilities.   

Here's a few grabs from the schematic:

Title: Re: Sencore DVM56
Post by: coromonadalix on December 18, 2018, 11:05:27 am
have you seen this video ? (

Unless you're the same guy ??

Some infos : (

Ohhhhhhhhhh the famous 100 megs range, i heard of it in the past  .....

Do you have the complete service manual  ?  i cant put my fingers on it  loll
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 18, 2018, 11:23:03 am
Saw that guy. I've done everything here has and more.   I have the full owners manual, which has calibration routines and full schematics.  Had to pay the $10 eBay fleecing to get it.  Check your DMs in a moment.
Title: Re: Sencore DVM56
Post by: coromonadalix on December 18, 2018, 11:32:54 am
thks received,   wow its a very good quality manual.

I dont think it would be a problem to share it publicly if you're not under a non disclosure agreement,  some members here could help you too  ?? 

I'll try to check the circuit a little
Title: Re: Sencore DVM56
Post by: coromonadalix on December 18, 2018, 11:51:11 am
Ld120 ad converter pdf : (

The first thing i would do is clean all the pcb's contacts slot,  check all the ground traces if they are okay.

Reseat any socketed chip once or twice.

Check every supply voltages lines,    you did that ;)  but there is lots of them.

Maybe some memory corruption may have occured :  ic304 rom   ??

Have you checked the page 55 for the improved temperature addendum, if it was applied in the meter ??

The power supply board  clicking shunt relay signal  "pin 6" is shared  on OHMS and AC board  and goes to  pin 27-28 of the cpu board,  to pin 28 of the cpu.

Maybe check the reset line composed of the ic309 and his surrounding parts, and c307,  the reset is active / done at low  "0vdc"  check if the voltage pulse at 0vdc at startup, and should stay up "5vdc" after the timing constant.

Check for the clock signal at pins 2 and 3 of the cpu, Should be around 12 mhz.
Intel 8035 cpu infos : (

Maybe try to read the ic304 rom with an programmer to see if there is some read errors ?  check his ic307 ic308 address latch decoders, and ic303.

Luckily, this meter has tons of basic parts / logic circuits
Just the ld120 ad converter, the cpu and the AD636 are more specialised.
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 18, 2018, 01:36:29 pm
Thanks! lots to chew on there. 

I'll try and pay a little more attention to the grounds.    I have noted that none of them seem to make it back to chassis, I think by design.

PCB contacts are clean.  No a hint of corrosion

Only two chips are socketed and I've had them in and out a few times.  Their contacts are also very clean.

There are definitely a few more voltages I can check and recheck.   I'm assuming that the power supply voltages are supposed to be in-circuit.  I checked them with all the headers disconnected and some were real crazy.   The +7.5 supply (7.1v) was only a little bit better that way, but I'm still concerned that it seems a little low.

Meter does have the Page 55 mod applied.

Your description of the reset scenario helps.   I'll investigate those voltages.

I'll check for clock with my scope.   I've paged through the CPU manual, but some of it is a bit too complex for me to understand.

I don't have a ROM programmer, and not sure if I'd know what to do with one if I had it. 

Here's  the manual.  Hopefully the eBay person who scanned it doesn't get too mad.   Their claim to copyright would only be valid if they had permission from Sencore to create a 'derivative work' anyhow. ;)
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 18, 2018, 03:30:20 pm
CPU pin 28 reads 5V at startup and remains constant through blipping. 

CPU reset pin 4 reads 3v at startup.  When relay cycles, it dips down to 0 momentarily. 

See photo below of CPU pins 3/4 for clock.   I think it looks right.
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 18, 2018, 03:36:35 pm
Here's a view of the clock on pin 2.  Pin 3 looks similar, maybe not quite as pure of a sine wave.  It remains constant through the resets.
Title: Re: Sencore DVM56
Post by: coromonadalix on December 18, 2018, 10:45:46 pm
The reset line should stay at 5volts and never drop until you power down and power up the dmm,   desolder the relay coil who's bliping, it will help diagnose.

The wave form  seems to be at 5 mhz  with 0.2 us sec in division ??  Do i read correctly ?? 
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 19, 2018, 10:31:22 am
Didn't have a lot of time to mess with this today.

Correct, that appears to be just over 5mhz.   Pretty fun to measure that on my scope.  I used to looking at audio frequencies, so I had to explore the other end of my time dial! 

With the shunt relay removed from the power supply board the reset voltage is still pulsing on pin 4 of the CPU.  I also faintly hear the other tiny relays for switching ranges pulsing along.   

I suppose I need to figure out why I'm not getting 5V to the reset pin?   It seems like there is quite a bit hanging off of it.   I'm not quite clear on what drives the reset circuit.  Is TR309 what turns IC309 on?     


Title: Re: Sencore DVM56
Post by: coromonadalix on December 19, 2018, 10:45:06 am
well  you could do an simple reset line (with the other parts removed ) with an pull up resistor say 4.7 k ohms to the 5 volts and shorting it briefly would do a reset.

If you say there is some pulses on the reset line its not good, unless the cpu is maybe dead or it's having problems with an corrupted rom,  you could try a test without the rom ic ??

I dont know why they did an complicated reset system like this, the 3140 may act as an comparator / trigger, since there is an 3.5 vdc on pin 3,  but the voltage value should be 2.5 volts since they divided the 5 vdc by 2 identical resistors values R322 R324 ...  an good "low" signal is under 2.5 volts.
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 19, 2018, 01:22:57 pm

With the CPU out I get a steady 2.9VDC at the reset pin (4) - No pulsing

IC309 Voltages as follows (CPU removed) {CPU Installed}
2.) (.875) {2V- 1.47} - when the CPU is installed this one is strange. It starts high, then my DMM seems to load it down.  When the volts are low it slows or stops the blips.    Should be 3.5V
3.) 2.62  {2.61} -  Should be 3.5
4.) gnd
6.) ({2.89}) - dips to 0 on blips  Should be 4.9
7.) {(4.98}) - Should be 5

The low pin 6 is caused by the low pins 2 and 3, correct?   

C308 has 5V on one side
CR303 zener has 2.66 before the 1 meg resistor
R322 has the 5v on one side by drops to 2.5v on the divider side as you allude to.   Do we think that the 3140 is bad and dragging down the voltages, or is he not getting the correct incoming voltages? 

Thanks for the continued advise.
Title: Re: Sencore DVM56
Post by: Chris56000 on December 19, 2018, 07:31:49 pm

Thanks for sharing the Manual – it's beautifully done and I'll come back with repair hints once I've had a chance to study it!

Chris Williams
Title: Re: Sencore DVM56
Post by: coromonadalix on December 19, 2018, 09:04:25 pm
You could remove the 3140, i think the reset could be done

The 3140  will garantie an low to high transition past the 2.5 vdc

Some cpu resets are done only with an capacitor and a resistor, or  with an specialised "reset" chip or a "watchdog" chip ..

have you changed the 2 caps  C307 and C308  ? 

Check tr309, it can affect the reset by the cpu RD signal "read" active at low, unsolder it  to do some tests ??

I hope the cpu is not busted or the eemprom content is fucked up.

Maybe some eevblog memeber can or could do a eeprom dump ??  Haven't found one on the web :(

Maybe Chris56000 could help ?
Title: Re: Sencore DVM56
Post by: Vintagevalves on December 20, 2018, 05:27:04 am
Electrolytic C7 has been changed.  C8 is/was a .15uf film cap.  I pulled it out and it tested good on my LC75.   One of the legs felt like it had some metal fatigue from having been bent over on the board.  I replaced it with a .10uf to go in it's place for now.    I've got to mention that the solder pads on this board are very delicate and prone to lifting / vaporizing.   :palm:

TR309 tests good with no leakage on my B&K 520B.   Reads 205 hFE on my cheapy DMM - in spec...

With IC9 removed, I get 4.6V on the reset pin 4.   No more blipping from the relays, but still no device functionality.   

I just noticed that some of the LED numeric displays are slightly warm although they are not illuminating which may be a clue as well. 

Chris56000 - appreciate you taking a look! 

If it comes down to a bad CPU, I think this thing might be destined for the trash bin.   It looks like a used one can be purchased very cheap.  Of course even a very cheap modern bench DMM is almost certainly superior in every way and measurement.      I'm sure tired of chasing batteries for my Fluke and Amprobe DMM's. 
Title: Re: Sencore DVM56
Post by: coromonadalix on December 20, 2018, 08:15:35 am
send a pm to chris56000  maybe he has one working ?? he could make an eeprom dump ??
Title: Re: Sencore DVM56
Post by: Chris56000 on December 20, 2018, 11:11:40 am

Regrettably I don't have one of these meters myself I can offer a dump for, but it's very unlikely you've got a total CPU failure - the Siliconix LD120/LD130 series of A/D converter devices are notorious for developing internal faults in their I/P stages causing a permanent overflow, which blanks out the display or causes other silly tricks!

Referring to Schematic no. 400 for the A/D board, begin by linking pins 3 & 6 of IC403 (TL081) with a piece of wire or a 1k resistor - this should cause the LD120 to read a zero input which should be shown on the meter's display - if there is still no display or nonsense, you need to check the "Integrator" output of the (pin 11) LD120 is ramping up and ramping down - this is typically over 32 clock cycles, with the U/D pin 4 going high to produce a rising ramp and then low to produce a falling ramp - a good dual-beam oscilloscope should be able to synchronise to these.

If either the integrator ramp or the U/D control is missing, this could be either a micro or an LD120 fault, the way to find out which is to lift pin 31 (P1.4) of the micro temporarily (I'm assuming the micro is socketed), then apply a TTL square-wave of about 100kHz to pin 4 (U/D) of the LD120 via a 1k limiting resistor, whilst looking at it's integrator O/P pin 11 - if there is no triangular up/down ramp, when a separate TTL input is applied to "exercise" the LD120, the LD120 is suspect and you'll need to source another one!

If there IS a triangular integrator signal on pin 11 and it appears to match the U/D signal on Pin 4, then the A/D converter is at least partly functioning and you need to check the Digit and segment drives from the 8035 microcontroller, and also the /INT signal coming in on Pin 42 of the 300 controller PCB - the micro needs the /INT signal to allow a measurement to begin!

Try these tests if possible and I'll get back later!

Chris Williams


I've assumed you've checked all the PSUs, reference voltages and all the function-switch contacts - this is basic diagram-work!

Title: Re: Sencore DVM56
Post by: Vintagevalves on December 20, 2018, 02:02:41 pm
Hi Chris.  Thanks so much for the suggestions.  That's a lot for me to process, but I think within my capability (barely).

I did go through all of the power supplies.  The PSU is fully recapped and has had all the carbon resistors replaced.  I mentioned earlier that the +7.5V supply seems to be a tad low (+7V).   I initially had a dead 5V supply, which has been rectified.

It may be until after Christmas before I get a chance to work through all of this, but I promise to report back.  I hate it when I give people advice and then they disappear.  I won't be that guy if I can help it!  :)
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 16, 2019, 03:29:08 pm
Hey all.   Hope you had a good holiday season.  I'm finally circling back around to this project after getting sidetracked with two Hickok tube testers. 

Did some of Chris' troubleshooting.
Jumped pin 3 to 6 on IC403 with a 1k resistor.  Result:  blank display as opposed to on random character before.   Relays still blipping.   

LD120 scope photos in next post.

I decided to re-investigate some of the power supply anomalies.  I've found that they vary quite a bit depending on if the power supply is isolated or connected to the other boards.  I was wondering if you guys could comment on these and if they are close enough' to correct.

No load:
+15 = 14.99
+7.5 = 7.14
-7.5 = -16.3 (!)
-12 = -11.97
+8 = 12.59
+5= 4.99

With all boards connected:

15V = 14.99
+7.5 = 6.81
-7.5 = -7.39
-12V = -11.96
+8 = 10.99
+5 = 4.99
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 16, 2019, 03:57:53 pm
Here's the scope on pins 11 and 4 of the LD120.  Doesn't look like much of anything to me.  It pulses with the blips.
Title: Re: Sencore DVM56
Post by: helius on January 16, 2019, 04:26:04 pm
I'm sorry I don't have anything to contribute, but I did notice that Sencore chose to make their voltmeters with a 20 MΩ input impedance, unlike the rest of the industry that went with 10 MΩ. Other than a scheme to sell proprietary accessories and probes, why would you do this? Is there some advantage on certain types of circuits?
Title: Re: Sencore DVM56
Post by: Chris56000 on January 17, 2019, 12:49:36 am

All you're seeing is random noise and ripple on the U/D and Integrator pins of the LD120 – the up/down ramp should be at least 2–3V pk–pk in amplitude!

If you have a suitable function gen, use the TTL output of this to feed the u/d pin 4 of the LD120, via a 1k safety resistor and see if you get the up/down ramp on pin 11 – you'll have to experiment a bit with the square–wave frequency on pin 4 a bit but 5–10 Hz is a useful starting–point.

If you can't get the integrator o/p of the LD120 to respond to anything on pin 4, make sure the device's power supply is correct & free from ripple, it's earth pins are correctly connected back to the general meter 0V rail, and there are no leaky or disconnected small timing caps, etc., then I would source a replacement LD120 device.

If your function generator can provide a triangle wave, you could try lifting the integrator pin of the LD120, and feeding the triangle into the microprocessor input circuitry – about 5Hz @ 4V pk–pk (use the TTL output) – this should produce some sort of response on the display.

Have you tried momentarily connecting the /INT pin of the micro to 0V whilst monitoring the display and/or the LD120's u/d pin 4?

Chris Williams
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 17, 2019, 02:36:56 am
Hey Chris,
Thanks for the continuing guidance on this.  Any thoughts on the PSU voltages above?

I don't have a 'suitable' function generator with a TTL output, but I do have some audio generators which I think will swing that kind of square wave.     It doesn't do triangle though.  I suppose I could use the function generator on my computer through an iso transformer for that.    I'll have a look at it in a bit this morning.   

Looks like the LD120 is not too expensive on eBay if it comes down to it.

Helius - I'm not sure why either.  Obviously the higher the impedance the less loading.  These meters were marketed to TV repairmen, so maybe there was something on CRT TV's that benefitted from it?

Title: Re: Sencore DVM56
Post by: Chris56000 on January 17, 2019, 02:52:13 am

I'll have to get back to the circuit again but I think the –8V rail is reading high because some part of the circuit is non–functional when it shouldn't be – it might be worth having a look at the drawings carefully and seeing if any of the i.c.'s appear inoperative – I'm assuming Sencor wouldn't mark "–8V" on their circuit if they weren't satisfied a good working instrument actually provided that voltage, or fairly close to it!

It might be worth your while buying one of these:– (

. . .they're only 99p (UK prices!) Free P & P and they'll give you square and triangle o/ps which are frequency & amplitude adjustable, and you might be able to coax something out of the uP circuit by feeding in a triangle in the appropriate places!

Unfortunately the last DVM56 on fleabay went for far more than I could have afforded so I was unable to buy it to get comparison voltages, waveforms, etc!

Chris Williams
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 17, 2019, 04:04:05 am

On the schematic, it looks like only the segment driver transistors run off the 8V supply.  I guess it makes sense that this supply would be high when no segments are illuminated? 

What about the low'ish +7.5 supply?   

I'll see what I can do with my audio generator and order up an eBay function generator in the meantime.
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 17, 2019, 04:06:48 am
Also, let me rewind a bit and restate that when I got this thing the +5V regulator was dead.  I'm not sure how or why it became that way.  This could be part of the solution. 
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 17, 2019, 08:49:13 am
 5V  5-10hz P-P square(ish) wave out of my old Leader audio generator into pin 4 of the LD120 still results in no output on the LD120.   +15 an -12V present on correct legs and grounds trace out.  Dead LD120???
Title: Re: Sencore DVM56
Post by: Chris56000 on January 23, 2019, 08:41:42 am

I would obtain another LD120 before you go any further with your meter!

A quick check you can try is to lift pin 15 of the LD120 and leave it floating temporarily, this should cause the display to flash "888.8" continuously – this device is noted for internal failure!

Chris Williams
Title: Re: Sencore DVM56
Post by: Vintagevalves on January 27, 2019, 09:15:27 am
Swapped the LD120 with no improvement.  :--

I think I'm ready to give up on this thing and move on to more fun projects.  As cheap as used much more modern DMM's are on the used market I don't see any reason to put more time into this thing. 

I'd probably be willing to ship this thing to anyone who wants it for the cost of shipping.  Mostly re-capped, PSU has had all the carbon comp resistors replaced.  I have lifted and repaired a few traces on the function boards.  :-[
Title: Re: Sencore DVM56
Post by: Chris56000 on January 28, 2019, 06:53:20 am

PM sent - I've decided to take it on as a challenge I can't resist!

Chris Williams