Vision Engineering Alpha Microscope PSU - Page 1
Forum section: electronics / repair

author: Smelter
author_rank: Newbie
timestamp: May 14, 2017, 09:39:38 am
content: Looking for a schematic/circuit for its main PCB.I salvaged a much used and abandoned microscope for home use.Initially the lamps were flickering at about 2Hz which I cured by replacing the main Electrolytic, but now at power up, the circuit takes several attempts to ramp up to full light level.I have gathered up data sheets for the ICs and I am hoping that someone has a schematic/circuit diagram before I have to trace out the board.I believe the problem area is the Triac (MAC223A-8-ON) driven by a Zilog (Z86E0412PSC) that produces the lamp level PWM?, so anyone with experience of this type of circuit I would appreciate hearing from you.I have yet to start probing the board with my scope, but this will be my first use of it, so I am doing a lot of reading up before I start poking into a live board.Thanks in anticipation.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2017, 10:28:47 am
content: Some pics of both sides of the PSU will help. Reduce them in size to a few hundred Kb's max and add the files in attachments at the foot of the posting page.They will show up as thumbnails in the post but we can click on them to make them larger or download to magnify more.

author: Smelter
author_rank: Newbie
timestamp: May 14, 2017, 10:58:46 am
content: Hi TautechThanks for the prompt response. Photo's attached.The 4 caps replace the original 1. I only had 470µ-25V to replace the 470µ-35V and space is not a problem.It appears that C9 Top RH has been pulled off as the pads have grooves from the pins, so I have no idea of what value to use to replace and repair.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2017, 11:04:23 am
content: C9 is probably 10uF 16V, like all the other small SMD ones on the board.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2017, 11:12:49 am
content: Sean's most likely correct but for now to check it gives the circuit functionality drop a 100uF 25V in there as I don't think the value is crucial. Be sure to check for correct polarity.

author: Smelter
author_rank: Newbie
timestamp: May 14, 2017, 12:12:23 pm
content: Thanks for getting back to me guys. I can not believe I've got it to work this quick.The next problem I have to solve is the gross over heating Halogen bulbs.The internals were so bunged up with years of dust drawn-in by the fans, that the case parts are badly blistered and split.These are the offending items:-http://docs-europe.electrocomponents.com/webdocs/1374/0900766b81374d75.pdfI plan to replace them with these:-http://www.farnell.com/datasheets/2171949.pdfSo the main problem solved, I'm now confident to buy the replacement bulbs.Thanks once again!Best RegardsAlan M.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2017, 12:56:44 pm
content: Not going to work too well, you need a unit with a single emitter, not a 3 spot one. However if the original lamps are 50W, replace with 20W ones, which are a lot lower power, cooler running but only a smaal amount of light less in output.

author: Smelter
author_rank: Newbie
timestamp: May 14, 2017, 02:01:12 pm
content: The Halogen original bulbs are 20W @ 580cd/205lm.The LED bulbs are "Claimed equiv. conventional lamp power 20W" rated @ 500cd/184lmI have not found a single emitter or higher equivalent wattage LED version.In the microscope there are no lens's involved, just flat mirrors and clear windows.I can always add diffusers if I have shadow problems.It is heat that is the real problem, as the original bulbs, even in a clean set-up, cause the thermal trip to kick in.Any further thoughts?

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2017, 02:22:17 pm
content: Then get the LED units and use them, they will be cooler running, but be prepared to place some diffuser in front of them.