General Resistance DAS56A - tear down and repair - Page 1
Forum section: electronics / testgear

author: MosherIV
author_rank: Super Contributor
timestamp: October 26, 2016, 10:09:44 pm
content: HiI got one of these a couple of weeks agoThe specs are here :www.eevblog.com/forum/testgear/i-don't-need-no-stinking-calibration/?action=dlattach;attach=51658The 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 1mVIt 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

author: Vgkid
author_rank: Super Contributor
timestamp: October 26, 2016, 10:56:25 pm
content: 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.

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 10:12:10 am
content: OK, so here are some tear down photos

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 10:14:01 am
content: From the sides

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 10:15:57 am
content: Unscrewed the front panel to get access to the KVD

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 10:18:15 am
content: The money shot - inside the shield of the KVD

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 10:20:07 am
content: Finally, I opened the shield can where the Vref is

author: jonovid
author_rank: Super Contributor
timestamp: October 27, 2016, 10:27:20 am
content: good images keep it focusif u can

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 12:56:34 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: October 27, 2016, 05:59:04 pm
content: 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.

author: Vgkid
author_rank: Super Contributor
timestamp: October 27, 2016, 06:11:50 pm
content: That low ohms resistor board is interesting. An L&N type 31 switch with their l&n resistors.

author: edavid
author_rank: Super Contributor
timestamp: October 27, 2016, 06:21:58 pm
content: 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).

author: Seekonk
author_rank: Super Contributor
timestamp: October 27, 2016, 06:38:47 pm
content: 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.

author: MosherIV
author_rank: Super Contributor
timestamp: October 27, 2016, 07:31:08 pm
content: QuoteYes it does. Sometimes it stops after it has warmed up. I have a feeling that after I took it apart it will buz permanently nowQuoteIt 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.

author: Vgkid
author_rank: Super Contributor
timestamp: October 27, 2016, 08:35:09 pm
content: That is a mice design for those KVD switches. much better than the average thumbwheel switch.

author: MosherIV
author_rank: Super Contributor
timestamp: November 07, 2016, 06:01:48 pm
content: HiI 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/

author: MosherIV
author_rank: Super Contributor
timestamp: November 21, 2016, 04:22:01 pm
content: 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, seehttps://www.eevblog.com/forum/metrology/general-resistance-das56a/msg1069255/#msg1069255but the noise did not go awayThis 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 toSo I had a look in the main input filter, hereMy progress is being documented in the metrology forumhttps://www.eevblog.com/forum/metrology/general-resistance-das56a

author: edavid
author_rank: Super Contributor
timestamp: November 21, 2016, 04:41:59 pm
content: Quote from: MosherIV on November 21, 2016, 04:22:01 pmBad link... apparently you need the trailing slash:https://www.eevblog.com/forum/metrology/general-resistance-das56a/