DATEL Voltage Calibrator DVC 8500, Repair [SUCCESS]

[HighVoltage]

So I almost have it dialed back to zero...but it wanders about 0.000,010
on my 34401A. 

I wouldn't expect it to be accurate enough to match (let alone be better) than the HP34401A. Although it uses the same voltage reference - the LM399 - the HP34401A uses a combination of precision components combined with procedures (e.g. the auto zero) to compensate for the rest of the system. The Datel is good - but I wouldn't expect it to be as good - just look at the specs.

Very true..
The DCV-8500 is rated a 4 1/2 digit reference.
But then I was also surprised to find out how stable the reference really is.
Here are some measurements, taken over 19 hours with a 34461A
The drift is mostly contributed to the change of temperature during the night.

The short term drift of the DCV-8500 is claimed to be +/- 25ppm and that is about the same as the 34401A
So, one could probably use it as a short term transfer reference for even more than 4 1/2 digits.
 

[Assafl]

HighVoltage - what is the short circuit current (current limited) on your 8500?

Mine only gets to 50mA but the spec says 70mA. So I wonder if the sense resistor (or threshold) is off.

[Assafl]

For those that have issues with their 8500 I measured the voltage on the internal Test points relative the COM terminal.

1. 6.938V
2. 10.000,00V
3. 15.91V*
4. 31.54V*

* Voltage on TP 3 & 4 depends on the output voltage settings. Voltage set to +00000mV.

[USMC_Spike]

Assafl, HV:

Very good stuff.  I then must have misread the instructions on the DVC-8500.

I think your units are better equipped wtih resistors that my unit, they are mostly
I think 1/8 W carbon comp resistors...but the drift I think I saw was more to do
with Temp Co....but still I'd rather not have cc resistors though out.  It is what it is.

I was thinking of using either a 10 turn pot for the 46 pot or a 10 turn pot for the
vineer...I have 5K, 10k, and 20k ten turn pots, just not a 50k, with a smaller
shaft.. I think 3mm. 

Thanks for the explanation, Assafl and the data HV.

I'd like to upgrade to the 34465, or 70...but haven't. 
I also don't have any computer controlled devices yet
and need to figure out how to do it.

Thanks

Spike

[Assafl]

Few mistakes and a semi preliminary circuit analysis:

TP1: 10V to COM
TP2: 6.938V to COM
TP3: 32.65V to 40V supply ground (NOT to COM!)
TP4: Currently 31.54V to COM (when set to +0mV).

There are two power supplies:
Rear supply (transformer leads 3&4): 35VRMS, rectified to ~41VDC gets reduced (two power transistors, zeners and resistors) to 24V for the LM399 heater and to 32.65V (TP3). Use the capacitor - lead to measure TP3.

Forward supply (transformer leads 5&6): 42VRMS, rectified to 53VDC. Not traced these yet.

Bottom right side - a few resistors - only one of which is connected. Why? Is it trimming? Spares (as fuses)? Never seen that before. These resistors seem to work in conjunction with the bottom multiturn - the full range trimmer. So my guess is that entire bottom right circuit is setting the full range.

The 714 underneath the trimmer sets the balance between the +19v and the -19v.
The 714 under the lever switches (to the left of the single turn 0v balance) sets the zero volt point. My guess is it uses the 53v as +-19.999v and a floating zero (as opposed to a dual polarity supply).

[HighVoltage]

HighVoltage - what is the short circuit current (current limited) on your 8500?

Mine only gets to 50mA but the spec says 70mA. So I wonder if the sense resistor (or threshold) is off.

Short circuit current on my two 8500:
First unit (230V) is 51.2 mA
Second unit (110V) is 51.8 mA

So, it seems your sense resistor is alright.
It takes about 1 min to stabilize the short circuit current on both of them.
 

[HighVoltage]

For those that have issues with their 8500 I measured the voltage on the internal Test points relative the COM terminal.

1. 6.938V
2. 10.000,00V
3. 15.91V*
4. 31.54V*

* Voltage on TP 3 & 4 depends on the output voltage settings. Voltage set to +00000mV.

Here are the same values of my two units:

TP1. 7.2850 V
TP2. 9.9999 V
TP3. 15.000 V
TP4. 2.8849 V

TP1. 7.0488 V
TP2. 9.9999 V
TP3. 15.444 V
TP4. 2.9994 V

All settings to 0V and "minus" output polarity selected

[HighVoltage]

Here are some pictures of the resistors of my units

[Assafl]

They are also not connected. Nice.

[Assafl]

HighVoltage - TP 3 must be measured to the ground of the 40V supply.

[Assafl]

Here are some pictures of the resistors of my units

Just saw this again - this time not on the phone. What the heck are they doing here? They seem to pick only one resistor out of the ladder - and not clear for what purpose. The values of the resistors double for each step of the ladder.

However, the resistors are not the same values between the units. Not even close.

Mine are (from the bottom) 155k, 77k, 38k, 19,5k, unknown, 2.12k and 10k. Look next to the resistors and you'll see a wire tying ONE resistor to ground. In your devices it is the bottom 640k resistor (red resistor picture) and it is the 2nd from the bottom 78k in the Blue resistor unit. For my device it is the 5th from the bottom resistor (about 10k but the value is underneath the resistor hence unknown).

My hunch is that these resistors create the 10V out of the 6.9V zener in the LM399. So these resistors will be picked for tempco and - more importantly - for the same tempco (hence always the same type of resistor). Why install 4 unused resistors? Don't know...

I am still a bit unclear how it works. Flipping the +- Switch seems to flip the LM399 from being referenced to COM at the Anode to being referenced to com at the Cathode (i.e. a +6.9v or a -6.9v supply). The 10V reference then multiplies that + or - 6.9v voltage to a + or - 10V Voltage. This same signal (B pin on the lever board) then seems to get the pair of MPSU05 and MPSU55 (NPN and PNP) power transistors (in the power supply section) biased which then seem to set the floating COM voltage at the proper level. To ensure the symmetrical nature of the "flip" - the two uA714 have their Offset adjust connected to the trim pots.

 

 

[HighVoltage]

However, the resistors are not the same values between the units. Not even close.

Mine are (from the bottom) 155k, 77k, 38k, 19,5k, unknown, 2.12k and 10k. Look next to the resistors and you'll see a wire tying ONE resistor to ground. In your devices it is the bottom 640k resistor (red resistor picture) and it is the 2nd from the bottom 78k in the Blue resistor unit. For my device it is the 5th from the bottom resistor (about 10k but the value is underneath the resistor hence unknown).

Yes, I have been wondering about that too.
Especially since they are in parallel connection and have huge difference in value.
May be done to fool people like us?
Would not be the first time I have seen that.
 

[Assafl]

However, the resistors are not the same values between the units. Not even close.

Mine are (from the bottom) 155k, 77k, 38k, 19,5k, unknown, 2.12k and 10k. Look next to the resistors and you'll see a wire tying ONE resistor to ground. In your devices it is the bottom 640k resistor (red resistor picture) and it is the 2nd from the bottom 78k in the Blue resistor unit. For my device it is the 5th from the bottom resistor (about 10k but the value is underneath the resistor hence unknown).

Yes, I have been wondering about that too.
Especially since they are in parallel connection and have huge difference in value.
May be done to fool people like us?
Would not be the first time I have seen that.

Well - they are always the same type and series. So my guess is tempco. Also, it doubles as the adjustment for 10V (which does not have a trim pot) - being wired in parallel with the main 10V/6.93V dividing resistor (which is the 5th resistor) - your DVC8500 needed less trimming than mine - hence the 1st or 2nd resistors were picked - while I had the 4th. My guess is that they solder them in, let them age a bit (or relax from the soldering heat) and then select which one's get as close as possible to 10V.  I think the 19.999V is set by the 7th resistor - also of the same batch/type - but with the Burns 10 turn trimmer (so with compromised tempco).

The 10V is guarded, as is the 'C' pin of the lever switch board. So my hunch is that the 10V reference (out the back) is at least an order of magnitude better than selecting 10V through the front. However, without access to a guarded COM - it is still not as stable as the 34401A's implementation of the LM399 (nor was it designed to be).

The floating/flipping COM (your TP4 measurement was for a "flipped" COM - hence you measured 2.88V for - voltages while I measured 31.54V for + voltages) is I think the main limiting factor in getting to uV accuracies. It will always "float" away from perfect uV level symmetry.

The flipping COM also makes the circuit substantially harder to decipher as even the reference point changes (53V ref, 41V ref, - voltages COM and + voltages COM). Each gives an obviously different reading (that really threw me off the first time I tried to take TP readings). Another obfuscation is that the uA714 OP Amps have pin 5 connected. But pin 5 is an NC pin. So one looks for substrate connections, etc. NADA. Perhaps it was meant to support different opamps (like the 741 which has pin 5 as Null)?

I think the mix of 741 and 714 is due to supply voltage. 741 only allows +- 15V input (and therefore handles the COM voltage) while the 714 allows full +-22V (hence handling the 19.999 range).

Still in the dark on how and where the current limiting happens. The current limit LED is operated by the TO92 transistor next to TP4. It seems to work in conjunction with the TO220 next to the output binding posts.

I admit that this circuit is proving to be taxing on my reverse engineering skills - unlike the DVC-350 which is rather easier to decipher albeit it has 3 boards, a CPU, display and parts that have no available datasheet... C'mon - no ovens to control, no batteries to charge, nada. 3 opamps, a handful of transistors and a strange double sided layout. Geesh!

As for deciphering the lever switch: A - COM; B- (+ or -) 10V; C- Guarded (output?); D- Out Sense(?huh); E,F,G,H- Zener Flipper (COM and 6.934 volts as TP1 and TP2). Pondering over why are they guarding point 'C' (and decoupling it with two huge capacitors)? And why does the lever circuit need the sense terminal to be routed back to it???


   

[wictor]

Hi,

I have been doing some reverse engineering stuff lately. Here is "raw" schematic of Datel 8500 without front panel dividers. Most of the components are placed similarly how they are on the board and that's why it's a bit messy.

[HighVoltage]

Hi,

I have been doing some reverse engineering stuff lately. Here is "raw" schematic of Datel 8500 without front panel dividers. Most of the components are placed similarly how they are on the board and that's why it's a bit messy.

WOW!
Thank you for putting the work in to this. I am traveling and will have a closer look, when I am back in the lab

[Assafl]

Thank you for reverse engineering the 8500. I think you may have reversed the TP1 and TP2 (TP2 is the 10V reference and TP1 is the LM399 reference Voltage + or - 6.9..VDC).

DIV_E, DIV_F, DIV_G and the DIV_H (COM) set the polarity by flipping the anode/cathode of the LM399 between COM  and U1. It uses a crossover configuration switch (2x2 matrix).  This then in turn flips the 10V reference from a -10V (for positive voltages) to +10V (for negative voltages).

This gets amplified by the 714 to the +-19.999V.

Does Q13 miss a link to +V2?

Have you figured out what do all of the transistors on the right do?

[wictor]

Thank you for reverse engineering the 8500. I think you may have reversed the TP1 and TP2 (TP2 is the 10V reference and TP1 is the LM399 reference Voltage + or - 6.9..VDC).

DIV_E, DIV_F, DIV_G and the DIV_H (COM) set the polarity by flipping the anode/cathode of the LM399 between COM  and U1. It uses a crossover configuration switch (2x2 matrix).  This then in turn flips the 10V reference from a -10V (for positive voltages) to +10V (for negative voltages).

This gets amplified by the 714 to the +-19.999V.

Does Q13 miss a link to +V2?

Have you figured out what do all of the transistors on the right do?

Thanks for comments! Maybe I have done too much reversing lately  and reversed some points... I'll check those.

[wictor]

I think you may have reversed the TP1 and TP2 (TP2 is the 10V reference and TP1 is the LM399 reference Voltage + or - 6.9..VDC).

OK, now it starts to go interesting... My board has TP1 and TP2 located as yours, TP2 closer to board edge. But voltages are really reversed. My test point voltages relative to COM:
dials -00000
TP1: 10.000
TP2: 6.888
TP3: 16.201
TP4: 2.9644
dials +00000
TP1: -9.997
TP2: -6.888
TP3: 16.811
TP4: 31.201

[Assafl]

It might be my mistake. I'll check.

[wictor]

I did some cleanup of the schematic. Here is also image of my board.


Reason for repair:

[Assafl]

You  were correct about TP1 and TP2.

So the "extra" resistors are for trimming the 10V reference voltage. Funny that they never seem to use more than 1 trim resistor.

Excellent work reverse engineering & cleaning up the schematic. Makes much more sense now!

A question - how does the output voltage loop close around U3?

 

[Assafl]

Okay - U3 was a puzzle to me. I think I know how it works. So a bit of a circuit description:

The inputs to U3 are Div_C and COM_Sense (two "grounds"). The output of U3 is the output voltage (+ the voltage drop of the buffer transistor).

How does it work?
Div_C is the "wiper" of the divider.
Div_D is the OUT_SENSE line. It has the output voltage on it.
Div_B is the reference voltage: -10V for positive voltages and +10V for negative voltages (opposite polarity).

The ratios of the divider are calculated so that the voltage at Div_C nulls, when the correct output voltage and an opposite polarity reference voltage are applied to the divider. U3 - a uA714 (OP07) is responsible for nulling Div_C by creating the proper output voltage. C6 1uF and C7 0.1uF (the two big metalized film caps connected to Div_C) are there to probably ensure the output reacts fast to dynamic loads (given that the entire system is littered with RC pairs for stability).

As an example, for a selected +10V output, the "Wiper" is centered, and when U3 settles the 10V output (on Div_D) and -10V (on Div_B) cancel each other out.

Also, since U3 tries zero Div_C - it does not really matter if OUT needs to be a source or a sink (the NPN or PNP buffers are active). Of course as long as the current is within compliance (+-60mA or so).

The ability to have the 10V at one polarity and the 19.999V at the opposite polarity is the reason V and V2 are floating relative to each other. Note - Since both are regulated to 33V (minus the BE forward bias voltage of the MJE340) - I don't really understand why the RMS voltage of the V and V2 windings are different. The power supplies consist of a current source (2x 2N2907) driving a 33V Zener (1N4752) buffered by an MJE340 power transistor. V2 power supply is a bit different in that Q7 detects the V2 current flow (mainly U3) and lights the Overload LED.

Also - as per the previous posts, DIV_E, DIV_F, DIV_G and the DIV_H (COM) connect to the polarity lever and set the polarity by flipping the anode/cathode of the LM399 between COM  and U1. It uses a crossover configuration switch (2x2 matrix).  This then in turn flips the 10V reference a negative 10V (for positive output voltages) or to a positive 10V (for negative output  voltages).

Since the 10V and the main divider are the most important in terms of TC - the resistors used are part of matched sets - hence the weird markings on the resistors. (HighVoltage has a red set of resistors from these guys: http://prpinc.com/products/leaded-metal-film-resistors/pr-series-leaded-ultra-precise-metal-film-resistors). I don't know who makes the other style resistors. I also don't understand why the values are always different (and thus requiring a different R32-R37). Perhaps they are trying to match the Tempco of the lever resistors?
I also have no idea who makes the lever divider resistors. They look like PWW resistors, edge sealed with epoxy. The switches are Cherry but I cannot find them in any catalog. 

COM level set: Q3 and Q4 using two diodes D18 and D19 respectively (I think they are ~3V Zeners) - at 3V below TP4 (for negative voltages) and at 2.8V above -V2 for positive voltages. TP1_10V is the trigger that causes Q3 and Q4 to switch the voltage. 

Don't know what the difference between COM and Div_A is (and if the common ground is in the divider - which would make sense).  Div_a and Div_H measure like a short...

[USMC_Spike]

This is all really good stuff Assafl, wictor, HiVoltage.

Wictor, looks like someone was in there changing the
resistors or pulling and replacing a bad one as that
is the first I've seen pics of them out of alignment
with each other on the board.

I thought I posted pics of my unit but I guess I didn't.
School hit also so I am just learning about solid state devices
and the other class is instrumentation and measurement.
The I and M should be a better class but the guy teaching
it seems to like the challenge of figuring out the answers in
his head instead of teaching the process to properly figure it out.
The class doesn't use a text book, it has some self written text
from another teacher.

ON the good side, I found an actual older textbook that teaches it
better than the prof and has better examples too. 

I appreciate the effort from y'all and I'm certain it will benefit
many folks for years to come too.

[HighVoltage]

Wictor
That was a great cleanup of your schematics, Thanks so much.
I will have a closer look with one of my 8500 open.

[wictor]

This is all really good stuff Assafl, wictor, HiVoltage.

Wictor, looks like someone was in there changing the
resistors or pulling and replacing a bad one as that
is the first I've seen pics of them out of alignment
with each other on the board.

Thanks! I don't know about those resistors... The solder joints seems to be original. Anyway, you are right that they are not so nicely aligned, maybe I have monday piece 

Wictor
That was a great cleanup of your schematics, Thanks so much.
I will have a closer look with one of my 8500 open.

Thanks for good pictures and repair tips! I'm getting help from this forum so I'll try to help others too. I'm not an electrical professional, but I wan't learn more and that was also one reason to reverse engineer the unit. When I was doing the schematic, I was doing the layout also to verify all the nets are correct. I'm quite confident that nothing is missing now except the divider itself. But if you find any mistakes, please let me know.

I think Assafi has done the biggest work trying to understand the operation of all the parts. I may publish the whole Kicad project(schematic, pcb, footprints etc), but first I would like to have it "ready" and probably have better names for all the nets and power rails etc.

I have tried to calibrate the unit and the common mode adjustment seems to be quite coarse. Maybe multiturn trimmer would be one improvement for this unit.

Wictor