Help with MCP psu.  - Page 1
Forum section: electronics / repair

author: Emi
author_rank: Contributor
timestamp: March 07, 2016, 08:56:01 am
content: HiI'd like to do some improvements on this psu :Www.mcpsh.com/M10-QR_Series.htmlDoes anybody own its schemary?My issues are :-poor handling due to very tight and coarse knobs ( i would change them or add a fine regulation pot/trimmer. .)-Displayed voltages are even 0.5 v wrong in ch1 but very accurate in ch2 !I 've to tune Vmeters (luckly my reference is an 87/V. .)-I would find a way to "turn on" last 7segm Dot or add a separate led showing when the precise 0.00 or 0.05 Is reached.Now Is very tricky to set voltages precisely; i must try a lot of times before i catch the right number!And obviously there's no difference if voltage is 0.09 or 0.19 , it still shows 0.1-I also need to Dim that strong blinding blue display. ..I Hope in your help ..i really need schemes. .

author: Agent24
author_rank: Regular Contributor
timestamp: March 07, 2016, 08:41:23 pm
content: If you post some photos of the inside if your PSU someone may recognise the design.

author: Emi
author_rank: Contributor
timestamp: March 09, 2016, 06:50:15 am
content: Here's some pics...There's a Lot of stuff on that pcb !Display drivers /Vmeters are DH7107GN ..I've found some quality issues in general assembly. .-5v out adj trimmer was broken. ..fixed.-One of the little diodes in the power board was desoldered ! Fixed. ( They are all put in so badly!!!!)-Changed 4 x 27 ohms segments resistors with 150 ones. .Less blinding brightness. .fixed.-Pot Knobs was too tight to the panel. ..fixed. .-internal panels are very sharp. .like a blade!some wires appears worn due contact with It. ..Now i'd like to know If its possible to Hack Vmeters to make right Dot turning on when decimals are precisely at .x0Its a shame i Can only See one single decimal even under 10V !

author: macboy
author_rank: Super Contributor
timestamp: March 09, 2016, 02:34:26 pm
content: The meters are based on a clone of the ICL7107 3.5 digit meter IC. This is a very common IC, and the documentation is very good. Look for Intersil or Maxim docs, they are almost identical. Also look for Intersil app note AN023. The short answer is yes, you can modify the meter to do auto-ranging. You will need some circuit to detect a voltage below 10 V (a comparator, preferably designed with a little hysteresis), and it will need to drive something like a relay to switch between two sets of resistors in the divider network that supplies the measured voltage to the 7107 inputs, so that the voltage is 10x higher when in the low range. You also need to switch the decimal point. The 7107 does not drive the decimal point, it is probably just hard wired on the board. Another modification you can do is to increase the measurement speed of the 7107, it can easily do 10 readings/second or more before issues start. If you do this, follow recommendations in the datasheet for sizing of the integrator capacitor, and use a good quality polypropylene cap, NOT a ceramic one.

author: Emi
author_rank: Contributor
timestamp: March 09, 2016, 03:49:51 pm
content: Thanks Mac. .I've Just given a look at docs. .It seems there's nothing to do in my case..Even adding another 7segm (impossible...)i could not obtain What i need. .These IC allows 1.999 and 199.9 layoutsBut my target would be 19.99I thought It was possible a trick to set voltagesas precise as possible in this "toy" ..If i need , for example, 12.5 v and i set It on display and then i plug my fluke 87v reading may be 12.45 or 12.56...I know that my circuits will survive anyhow. .Is just my paranoia

author: macboy
author_rank: Super Contributor
timestamp: March 09, 2016, 04:19:44 pm
content: Quote from: Emi on March 09, 2016, 03:49:51 pmLook again at the documents. The recommended full scale voltage (i.e. input voltage to give 1999 counts) is either "2 V" (actually 1.999 V) or "200 mV" (actually 199.9 mV). This gives one count per 1 mV or per 0.1 mV, which makes sense. The IC doesnotdrive the decimal point, so you can put it anywhere by driving it directly. You can make the chip work at any scale you want by dividing (or amplifying) the input appropriately. So 19.99 V is easily achievable. You will need to determine the voltage reference input (measure voltage across pins 35/36), it will typically be either 100 mV or 1 V, but if the input is being scaled anyway (and it is here) then the reference voltage could be just about anything.

author: Emi
author_rank: Contributor
timestamp: March 09, 2016, 04:57:23 pm
content: ok..i'm very grateful for your clear explanation but i guess it would be an hard job to perform for memy skills are quite basic ! and smd is a terrific world !ps: pheraps i've written wrong but at the moment my displays are only 3 dgt, not 3 1\2 ..this mod is too difficult for me...