Help with a Pace MBT 250 as my digital display went out. - Page 1
Forum section: electronics / repair

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 05:30:18 pm
content: Hello, I have a Pace MBT 250 that my display went out on abt 2 years ago. So i mothballed the equipment until the other day. I went ahead and plugged up just the station and turned it on. In which i was pleasantly met with the standard "E - 1" on the display aka no hand piece attached. I thought well that's cool and shut the unit down. I plugged in my SX-80 and to my surprise the hand piece warmed up to temperature abt 700F. So all that was left to do was pull the trigger to see if i have vacuum. Yep sure enough it worked and then it didn't. The display went down again but the vacuum pump works fine. Also the station will no longer heat the SX-80 and or a PS-90 wand. So here i am and i have a picture on this unit I would like to attach. I have looked around for a service manual with some flow charts but cannot seem to find anything for my unit. However i did find this older service manual hum.http://www.paceusa.com/sites/default/files/MBT%20250%20Service%20Manual_0.pdf?fbclid=IwAR0lpmagtwAdxxambYONiy4EU5bO2PsBpudfcRZve29NhiTZaMrhFZ32yWc

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 23, 2019, 05:42:44 pm
content: I'd start by measuring the voltages as shown on page 55.

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 05:53:07 pm
content: Quote from: PA0PBZ on October 23, 2019, 05:42:44 pmWell you see that manual is for a much older station & i can't seem to find the correct manual. Thank you for your time PAOPBZ. It's much appreciated!---Anyway I opened up the drawer on power entry module to expose the two fuses to my meter and they are fine. I can also hear the tone the station makes when you turn it off. So here i am with no display and no heat to the hand pieces. Here is another picture of the back on the unit. Does anyone have any idea how old the unit is based on the numbers?I have this strange feeling that pace hasn't released the service manual on my unit. That's why i am asking about the date code.Thanks in advance for any wisdom.Bill

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 06:30:30 pm
content: I will take it apart and measure the voltages across the transformer and see what i have.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 23, 2019, 06:39:30 pm
content: Quote from: BillThomas on October 23, 2019, 05:53:07 pmSorry, I didn't realize that. Unfortunately I have a MBT 350 so I can't do any measurements for you.

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 07:22:05 pm
content: Hey it's okay i'm sure will get it all sorted. I almost have it apart.

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 08:20:33 pm
content: Okay here are the snaps from taking the station apart. I will go back and document them a bit later for others who might stumble upon this thread.

author: GLouie
author_rank: Regular Contributor
timestamp: October 23, 2019, 08:50:53 pm
content: Have you tried contacting Pace about a current manual? Or perhaps there are minimal or no changes in the MBT 250 except for the front panel decoration? Does yours match the PDF at all?My MBT 250 is even older than the PDF manual.

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 09:49:00 pm
content: Quote from: GLouie on October 23, 2019, 08:50:53 pmHere is the correct manualhttp://www.paceusa.com/sites/default/files/Manual%20MBT250.pdfhowever it's not the service manual. So no schematics flow charts / truth tables.Pace would certainly have the correct service manual listed on their site if they were done with the system. Maybe i can purchase the service manual but somehow i doubt it.Some of it certainly matches up with the older service manual. Anyway i have a very old Rat Shack Meter Part#22-801 that i need to figure out how to calibrate or test against something. I also have a few brand new Harbor Freight Centech Meters here new in the packages. These will half to do i just need to trust what they are telling me before i go any further.So here are some voltages from the TP's.-5v = 4.85v = .6 to .712v = 11.8Hum no 5v

author: BillThomas
author_rank: Newbie
timestamp: October 23, 2019, 10:00:55 pm
content: Now i just need to prove this

author: tooki
author_rank: Super Contributor
timestamp: October 23, 2019, 11:25:29 pm
content: Quote from: BillThomas on October 23, 2019, 05:53:07 pmWhat makes you think that that service manual wouldn't apply to your unit? All I see is that a minor cosmetic change was done to the front panel, in that they added a bezel. But the front panel layout is identical, as is the back panel, and everything inside looks identical, too. So I'd be very surprised if there were any real changes to the internals. (And given that they must remain compatible, it'd be exceedingly unlikely that the circuitry would be changed in any meaningful way while retaining the same model number.)

author: BillThomas
author_rank: Newbie
timestamp: October 24, 2019, 09:19:29 am
content: Well it saysLM340LAZ-12 // +12v in U12LM320LZ-12 // -12v in U13LM340T-5 // +5v in U14however i have-5v = 4.85v = .6 to .712v = 11.8

author: Shock
author_rank: Super Contributor
timestamp: October 24, 2019, 11:23:12 am
content: The fact it was working and then fell over indicates a part that has failed could now be thermally sensitive.Check input/output to all voltage regulators and voltage to ICs if you have to. Then remove regulator if incorrect voltage, while it is out you can test for short between rail and ground and also carefully feed appropriate current limited voltage from a bench supply where the regulators output to supply the circuit was (voltage substitution method). If doing that monitor current closely to see if the circuit is stable and if you can restore reliable operation.If everything follows the voltage regulator then replace it. Sometimes other components down stream from the voltage regulator can put undue stress on it. So check caps as well, then make sure it had good thermal contact to the heatsink, this is to prevent it failing again.