General Resistance DAS56A - tear down and repair

[MosherIV]

Hi

I got one of these a couple of weeks ago 


The specs are here :
www.eevblog.com/forum/testgear/i-don't-need-no-stinking-calibration/?action=dlattach;attach=51658

The tear down is to come (basically I will document it as I open it up to service it).

In the mean time, I would like some advice please.....

It does not seem to working properly. While I can dial in any voltage to the nearest 1mV
It should be able to dial in any voltage in the 0.1mV (100uV) range but it does not work properly.
It can set 0.1mV when the total voltage is below 4V, as soon as it goes above that the output is 0.2 to 0.3mV low.

Now I suspect the contacts on the rotary dials are dirty.
Anyone got any tips/hints on how I go about cleaning them?
(I read somewhere on the forum that someone cleaned similar Datel device by soaking the while switch assembly - not sure I can do that with this one)

Does anyone think it is not the switch contacts and may be something electronic in nature, eg op-amp output drive ?

Thanks in advance

[Vgkid]

Look into one of robbenz post on the hp 6114/6115 about cleaning those thumb switches. Unfortunately you will need to open it up to further troubleshoot the unit. 

[MosherIV]

OK, so here are some tear down photos

[MosherIV]

From the sides

[MosherIV]

Unscrewed the front panel to get access to the KVD

[MosherIV]

The money shot - inside the shield of the KVD 

[MosherIV]

Finally, I opened the shield can where the Vref is

[jonovid]

good images keep it focus  if u can

[MosherIV]

Yeah, sorry about the blurry ones. I took them with my cheap phone camera which is fixed focus. They look OK on the phone but when you load them to a computer you can see how bad they are. 

[Gyro]

As you'll be mechanically disassembling to clean the thumbwheel contacts (they do look a bit black in places), I'd take the opportunity to respray the black front metalwork to get rid of the wear marks. I noticed them on the original listing but assumed that it was white plastic showing through.

I think the power neon originally had a cover over it too, so probably worth replacing.

[Vgkid]

That low ohms resistor board is interesting. An L&N type 31 switch with their l&n resistors.

[edavid]

MosherIV, does your unit produce loud transformer buzz?  I have a couple of them that are quite irritating (which is why they are in the back of the storage unit at the moment).

[Seekonk]

Inspiring....... I have two of those.  At least one of them appears to give the right voltage if you move the wheels back and forth a few times.  Good to see the switches are open and can get some spray cleaner into them.  Waiting for more.

[MosherIV]

MosherIV, does your unit produce loud transformer buzz?

Yes it does. Sometimes it stops after it has warmed up. I have a feeling that after I took it apart it will buz permanently now 

Good to see the switches are open and can get some spray cleaner into them.  Waiting for more.   

It will be next week before I can check if the clean has made any difference.
What more did you want?
The only thung I did not open was the mains transformer box.

[Vgkid]

That is a mice design for those KVD switches. much better than the average thumbwheel switch.

[MosherIV]

Hi

I had some response in the metrology forum with ideas for fixing my DAS56A.

I have had some success cleaning the contacts of the KVD.
However, it appears the PSU is noisy :

https://www.eevblog.com/forum/metrology/general-resistance-das56a/

[MosherIV]

So I made some progress. I found the old caps in the PSU rectifier were not up to the job as GeneM suggested.

I replaced them, see https://www.eevblog.com/forum/metrology/general-resistance-das56a/msg1069255/#msg1069255

but the noise did not go away 

This weekend, I did some more investigating and I think I have found the source of the problem - the noise is not coming from the DAS56A itself, it is coming from the mains it is connected to 

So I had a look in the main input filter, here

My progress is being documented in the metrology forum
https://www.eevblog.com/forum/metrology/general-resistance-das56a

[edavid]

My progress is being documented in the metrology forum
https://www.eevblog.com/forum/metrology/general-resistance-das56a

Bad link... apparently you need the trailing slash: https://www.eevblog.com/forum/metrology/general-resistance-das56a/