DEC VT100 teminal power supply repair - Page 1
Forum section: electronics / repair

author: tlt
author_rank: Newbie
timestamp: May 25, 2025, 03:50:21 pm
content: Hi,I'm new in the forum. Looking for some help repairing a power supply for a Digital VT100 terminal. Right now I can start the terminal but after ~20 seconds smoke starts coming out of the power supply. I have located it to R27 a 1kohm power resistor that is part of the "snubbing network" according to theTechnical Manual. I think this is more or less the rightschematics. I have tried to measure the components in the snubber circuit and around the area but I can't find any obvious faults. What could cause this behavior? Where should I look next? Thankful for any suggestions.

author: m k
author_rank: Super Contributor
timestamp: May 31, 2025, 03:57:38 pm
content: 20 sec. is quite a long time.How different supply voltages behave during those early seconds?5V and 12V have feedbacks.It has RS-170 composite video out and disconnecting internal monitor should be easy.So with NTSC video input somewhere you can eliminate overloading of internal monitor.Keep also in mind that certain parts of the monitor board have a tendency to eat their joints away.Just look, you can't avoid it if it has happened.I've seen it so many times that I should know, but unfortunately can't pinpoint it anymore.Brown or black power resistor came up, but no recollection how it possibly affects the PSU.

author: D2236
author_rank: Regular Contributor
timestamp: May 31, 2025, 06:39:35 pm
content: C19 may be breaking down under high voltage spikes after some time. Replace C19 and D27 to eliminate these suspects. Replacing C19 most probably will fix your issue.

author: tlt
author_rank: Newbie
timestamp: June 01, 2025, 02:28:14 pm
content: Quote from: m k on May 31, 2025, 03:57:38 pmI don't really know how to check them, should I look with a scope?QuoteThat's a good tip, thanks!QuoteYes, I found some dried and cracked solder joints on the cable connector of the monitor board.

author: tlt
author_rank: Newbie
timestamp: June 01, 2025, 02:29:15 pm
content: Quote from: D2236 on May 31, 2025, 06:39:35 pmI'm going to replace C19 and D27 and report back, thanks!

author: m k
author_rank: Super Contributor
timestamp: June 01, 2025, 07:25:29 pm
content: Quote from: tlt on June 01, 2025, 02:28:14 pmMultimeter is enough for now.Measure first that +5V and +12V are high enough.Technical Manual page 163/454 and on has details.

author: Poroit
author_rank: Frequent Contributor
timestamp: June 02, 2025, 07:52:53 am
content: R27 is 9W resistor so it probably runs warm.The attached shows all the voltages.

author: m k
author_rank: Super Contributor
timestamp: June 03, 2025, 09:05:44 am
content: R27 for revision H, later it is 500R and 13W.Quote from: tlt on May 25, 2025, 03:50:21 pmIs it a vanilla version?Only one 25 pin connector at the back and only one light green logic board inside, without any daughter boards.Newer board is dark green and marked as 54-14185, old is 54-13009.E,R27 as 1k is with T1 as 16-15668-00, later T1 version is -01 and R27 is 500R.With luck the part number is stamped on the transformer core.

author: tlt
author_rank: Newbie
timestamp: June 15, 2025, 01:10:04 pm
content: Quote from: m k on June 03, 2025, 09:05:44 amYes, only one 25 pin connector and a gray PCB.I got another VT100 (in worse condition) to get parts from. It is of a newer revision. I replaced D27 with a new UF4007 and took the C19 from the other PSU, it was the same part. I also took R29 from the other PSU, so I replaced the 1k 9W resistor with a 500 ohm 13W part. This seems to work. R27 is running at around 145° C. My guess now is that 9W on that resistor was too low and that is way they changed it in later revisions. Maybe it was failing of age and stress. Do you think its ok to run it with different values on R27?I have changed out all the glass type diodes on the other power supply that I have now as they where falling apart. the start circuit seems to give the correct voltage ~10.5 V but the 5 volt rail is not working , its at 0.25V. I'm thinking of following the trouble shooting guide from theTechnical Manualto try to fix it.Any tips on how I can get hold of a new Q7 transistor that will work in these PSUs?In the documentation I found that Q7 is a 2n6547 but my older revision PSU have a BUY69B and the newer one have a 1517910. Are these all exchangeable in these PSUs or is more of the design revised as well?

author: m k
author_rank: Super Contributor
timestamp: June 15, 2025, 06:04:29 pm
content: 145°C is pretty high, better use multiple resistors if actual value is difficult, there should be plenty of space.R27 and T1 seem to go hand in hand, can you see what part number your main transformers are, old and new?Metal plate at the outer end of the power has a tag or stamps, somewhere is a power type and revision, can you find them?Q7 is a simple switcher transistor, so nothing fancy.Just get something with enough CE voltage and current, and overall power rating and you're most likely fine.2N6547 and 15-19710-00 are the same thing.Older rev H manual has Q7 as 15-12782-00 NPN 100W S1325 15 and schematics page has BUY69B.Revisions are linear in time, later alphabet is newer.VT100 was very successful construct and used and modified in many ways, for example VT103 adds a TU58 tape drive.So later revision is generally more capable, even that both have same ratings.My guess is that S1325 of rev H manual is an error, it should be SI for silicon and then voltage and current limits.But values are still not good, they should be either 325 and 10 or 400 and 15.So revisioning is upgrading Q7 from rev H as Vce from 325V to 400V, Ic from 10A to 15A and Ib from 3A to 10A.My guess is that 325V of BUY69B was too close to limit.If you have an old hard drive you can use it as an artificial load.Very low +5V rail seems to be a short somewhere.That C31 reservoir capacitor is pretty old and voltage limit quite low.

author: tlt
author_rank: Newbie
timestamp: June 24, 2025, 07:25:39 pm
content: Sorry for slow replies, I try to work on this when I have some time to spare.Quote from: m k on June 15, 2025, 06:04:29 pmold: MMC 6389-4new: COILCRAFT-T 16-15668-01 REV-00 8102 2500V HIPOTTEDQuoteI'm having a hard time to locate the partnumber of the whole PSU..old: PN10686, 3613210-02, PCB says 5012995D stamp on pcb: 70-14979-00 PN8450235Bnew: *AB1380IWI2*, 3613210-02, PCB says 5012995E-2

author: tlt
author_rank: Newbie
timestamp: June 24, 2025, 08:01:37 pm
content: Quote from: m k on June 15, 2025, 06:04:29 pmYou where right! I had not checked the 12V rail and it was shorted. I removed C26 (560 uF 20V) and the short whet away. But the cap seems to test ok now so I'm a bit confused. It has three legs, I have not seen that before. I tested the bottom two legs. It's not shorted but the PSU started up without it.

author: m k
author_rank: Super Contributor
timestamp: June 25, 2025, 12:58:12 pm
content: Capacitors are not straight forward things, much depends how you do what you do.Means that testing with a multimeter is a different thing than live circuit, and the meter is most likely the more gentle one.It's of course also possible that mechanical movent removed the short.But the capacitor is also old, so it is its time to go.Capacitor's case can be connected or not.Three legged 627D type can be placed in different orientations.Despite that the C26 is just a simple 560uF capacitor.DEC was notorious with its part numbers.If you add something, like screws, ding, you have a new part number.It's also possible that there are a number for the plate and another for combined thing.There can also be completely different sales numbers.50-12995 is an etch number, so just a board, old is rev D and new is E2, add components and it changes to 54- something, usually next to etch, so 12996 in this case.Then when you add the end plate you get an assy and number changes to 70- something, 14979 in this case.36- number is a label, it's sort of a floating thing, different label and assy number remains.So both powers are the same and interchangeable.I'd say that repair the newer one, if possible.Newer board should have an extra wire from heatsinked 12V regulator Z1 to near the edge connector and small capacitor, possibly C35.It can be a trace strengthening thing that is found necessary.That is one of those things why you should repair the newer one.New transformer is the upgraded one, it has that -01 number, REV-00 is a manufacturer issue.2500V is used Hi-Pot voltage for insulation testing.Old transformer is assumed to be the original version.So old model should have that 1k resistor.

author: tlt
author_rank: Newbie
timestamp: June 29, 2025, 03:04:00 pm
content: The newer PSU seems to work now. The voltages are ok but the ripple is almost 400mV peek to peek on the 5V rail and 700mV on the 12V rail. Seems high to me, do I need do a recap of the PSU to get it down?I messured the older PSU and it has ~220mV on both rails.The way I did it was just putting the scpoe on the rail and look at Vpp, not sure that is correct way of messuring.

author: m k
author_rank: Super Contributor
timestamp: June 29, 2025, 04:07:47 pm
content: So around triple of what manual has.Is it standalone measurement or whole machine?How fast it is, is -12V any different?You can change those small filter caps from outputs, but check some logic side points first.On the other hand, it's a digital machine, so that part is fine if it's fine.Though obviously better be without any ripple.It has a 12V monitor, is picture steady?

author: tlt
author_rank: Newbie
timestamp: June 29, 2025, 05:57:13 pm
content: Quote from: m k on June 29, 2025, 04:07:47 pmstandalone with a load 3ohm on 5V and 11,7ohm on 12V should be close to the minimal load required.QuoteThe older PSU looks like 31kHz on both 5V and 12VQuoteYes it looked fine when I tested it earlierI thing all ripple on both PSUs are around 200-250 mV. I can't calibrate my osilloscope probe properlly, I think it's broken. Some of the higher messurments where probably wrong.