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DM501A vs DM502A

Kevin Oconnor
 

I have both a DM501A and 502A DMM. These are identical except for the front panels being mirror images. Anyone know why Tek would create such a pair?

 

They are different albeit related designs.

DM501A
- Manual Ranging
- 4.5 Digits
- LD120/LD121A ADC
- 0.05% + 0.015% of FS basic DC accuracy

DM502A
- Automatic Ranging
- 3.5 Digits
- LD111A/LD110 ADC (same as DM502 and DM44)
- 0.1% + 0.05% of FS basic DC accuracy

On 28 Oct 2016 20:01:45 -0700, you wrote:

I have both a DM501A and 502A DMM. These are identical except for the front panels being mirror images. Anyone know why Tek would create such a pair?

Kevin Oconnor
 

I never noticed the auto ranging button on the DM 502A!
The DM501A is in need of repair. Display seems functional but none of the modes or ranges produce results that are expected. I know that this is a vague fault description. But maybe the failure mode is familiar.

Sent from my iPad

 

A good place to start is with the power supply voltages checking for
level, noise, and ripple.

On Sat, 29 Oct 2016 07:26:18 -0700, you wrote:

I never noticed the auto ranging button on the DM 502A!
The DM501A is in need of repair. Display seems functional but none of the modes or ranges produce results that are expected. I know that this is a vague fault description. But maybe the failure mode is familiar.

Szabolcs Szigeti
 

Hi,

I've recently fixed my DM501A, let me summarize what I did.

First the trivial stuff: Get and read the service manual. Note: DM501 and 501A are not the same. Check for any burnt, missing components. Reseat the ICs, the sockets aren't that good. You may have problems with the pots and the switches too. I had a use for contact cleaners a couple of times later, when the basic functions were fixed.

Then check the power supplies for proper voltages. Note: Ground reference is the GND (black) socket on the input, as the whole instrument is isolated from the chassis ground.

Do not bother to fix anything before you get the 200mV DC and 2V DC ranges working. All other functions and ranges are dependent on these two ranges, so until they work, nothing else will. When working, you should be able to zero and adjust these two ranges. Again, refer to the service manual for the adjustment process.

Check that the input voltage appears on the input of the LD121 IC in 200mV and 2V ranges. If it does not, then the problem is somewhere in the range switches/attenuators.
If it does, then check the waveforms on the LD121/LD120 ICs. Read and understand the service manual and/or the datasheet for the operation of these ICs.
Since you have proper digit display (i.e. no missing segments or garbage). high chance that the LD120 and associated display drivers are working.

Depending on what you see on the waveforms, you can figure out what is going on .
There are some high impedance notes around that IC, so when doing measurements, take that into factor when evaluating the values.

In my case, the LD121 (actually 122 in this unit) was faulty, during the autozero phase, it did not switch over to GND, so in essence the input was cancelled during the autozero and the unit was showing almost always zero. A symptom was that a quick change in the input resulted in a momentary value other than zero, then the autozero cancelled the new value and the display went back to 0. An overload (around 4 volts) on the input resulted in some display, as autozero was not able to compensate it.
My fix was to replace the LD121. After calibration the unit is fully operational.

Hope this helps,

Szabolcs





---In TekScopes@..., <kjo@...> wrote :

I never noticed the auto ranging button on the DM 502A!
The DM501A is in need of repair. Display seems functional but none of the modes or ranges produce results that are expected. I know that this is a vague fault description. But maybe the failure mode is familiar.

Sent from my iPad

Kevin Oconnor
 

Thanks Szabolcs,
Those are exactly the tips I'm looking for!

Sent from my iPad

Szabolcs Szigeti
 

Hi Kjo,

You are most welcome. You may want to read the thread on this, I did some mailing on the subject and got a lot of valuable advice from here.

Here is the link: https://groups.yahoo.com/neo/groups/TekScopes/conversations/topics/132469 https://groups.yahoo.com/neo/groups/TekScopes/conversations/topics/132469 If the link doesn't work just search for "DM501A problem".


Szabolcs

Kevin Oconnor
 

@Szabolcs /others

Fixing a DM501A:
So cleaning the switch matrix did the 1st order trick. I can calibrate the 200mv & 2v ranges. But when I switch to the higher ranges, there is a clear offset that cannot be removed. (3-7v on 20v range with 1v input). I lifted the top of the Caddock divider and can measure ~9.9Mohm. Top to common. All the AC caps seem ok. When I measure the the pot voltages on the attenuator PCB it is exactly the voltage on the display, (appropriately scaled) but wrong. It seems that a small current is being injected into the high resistance divider which raises the divider output voltage. This wouldn't effect the 200mv/2v range since the impedance is low on those ranges.

Thoughts?
Kjo
Sent from my iPad

 

It is possible that what happened is that you calibrated out the zero
error caused by leakage at the high impedance input to the LD120
(U1601) using the zero adjustment. R1613 (560k) is in series with the
input and allows leakage to cause an offset error even when using the
200mV or 2V ranges. Then the change in Thevenin equivalent resistance
when the ranges are changed would show up as offset.

You can test this by adding some series resistance while using the
200mV or 2V ranges and seeing if the measurement changes. The leakage
current is then the change in voltage divided by the series resistance
and should be significantly below 1 nanoamp. Leakage should be in the
10s of picoamps.

The leakage does not necessary have to come from the LD120 (U1601) but
it is the most likely source. Since they are difficult to replace, I
would rule out other sources first.

My DM501A displays similar problems but I have not gotten into it yet
to figure out what is going on. I think I am going to go the whole
refurbishment route including cleaning the boards and changing the
aluminum electrolytic capacitors before

On Thu, 3 Nov 2016 19:58:33 -0700, you wrote:

@Szabolcs /others

Fixing a DM501A:
So cleaning the switch matrix did the 1st order trick. I can calibrate the 200mv & 2v ranges. But when I switch to the higher ranges, there is a clear offset that cannot be removed. (3-7v on 20v range with 1v input). I lifted the top of the Caddock divider and can measure ~9.9Mohm. Top to common. All the AC caps seem ok. When I measure the the pot voltages on the attenuator PCB it is exactly the voltage on the display, (appropriately scaled) but wrong. It seems that a small current is being injected into the high resistance divider which raises the divider output voltage. This wouldn't effect the 200mv/2v range since the impedance is low on those ranges.

Thoughts?
Kjo
Sent from my iPad

Szabolcs Szigeti
 

Hi,

Have you checked the PCB or the components for contamination which could create leakage path? Clean it with IPA around the sensitive areas.


Szabolcs

Kevin Oconnor
 

After just setting for several hours it started to work! Must have adsorbed water from the IPA I used. Calibration good and back working.

Sent from kjo iPhone