1984 Synthesizer - Looking for fried diode spec - Page 1
Forum section: electronics / repair

author: dr_theopolis
author_rank: Newbie
timestamp: October 10, 2025, 06:51:06 pm
content: Hello all, I have a vintage synthesizer (Sequential Circuits Six-Trak) that recently experienced a failed voltage regulator, which I have replaced. However, down the line a diode which looks to be part of a 4 rectifier series, split in half and I'm trying to determine a suitable replacement for it or all four of them even though the remaining three test ok on a multimeter. I have a schematic from the service manual but it isn't clear enough to read any specs. My multimeter shows about 0.58v voltage drop on the good ones. The diodes themselves are black casing with "4•B" in blue along with a blue cathode band. I've also seen this series in photos with the more common silver band as well (I have an early edition). Anyway, if anyone has any thoughts or suggestions for a replacement I'd really appreciate it. Photo attached for clarity. Thanks!

author: free_electron
author_rank: Super Contributor
timestamp: October 10, 2025, 07:21:52 pm
content: simple rectifier diode. most likely 1n4004. you can stick anything in there from 1n4001 all the way to 1n4007. it'll work. the T logo is most likely Teccor

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2025, 07:42:45 pm
content: All 4 diodes are the same. Desolder a nearby diode and read what type it is.By the size/look of the diode, and by the number 4 at the end (on the nearby one), that's probably a 1N4004.

author: dr_theopolis
author_rank: Newbie
timestamp: October 11, 2025, 12:01:52 am
content: Great, thanks for the replies. I have some 1N4002's on hand that I can slot in there.

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 11, 2025, 06:10:12 am
content: The schematics I found state 1N4002 for the diodes near the battery.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: October 11, 2025, 07:23:47 pm
content: It is very unusual that a regulator to fail. Check for any additional downstream components that could have caused the failure in the first place.

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2025, 07:44:25 pm
content: Looking at the schematic it's pretty much all 1N4002's (8 of them) and the power transformer is 18VCT 1A.But no simple 4-diode bridge. They use a voltage-doubler to supply a +15VAregulator. The 47uF capacitor sees high stress and might have shorted. You have to use a cap there rated for high ripple current.This would not damage the regulator IC though, but not sure which IC failed.I would be replacing all electrolytic/tantalum capacitors in the power supply given the age.

author: dr_theopolis
author_rank: Newbie
timestamp: October 11, 2025, 10:41:56 pm
content: Probably a prudent precaution while I'm in there, yes it's pushing 41 years now. No evidence of leakage but at that age they should really go now rather than risk it. I'll have to work my way around and see if I can find any further clues as to what might have killed the LM337.

author: floobydust
author_rank: Super Contributor
timestamp: October 12, 2025, 01:44:44 am
content: A LM337 usually has two (three needed here) protection diodes added but they are not present. Also the IADJfeedback coupled to the 5VAthrough C110 can wreck it too, giving +ve voltage spike to that pin easily kills the IC I think.LT337T(obsolete) datasheet shows the diodes that you might add.

author: dr_theopolis
author_rank: Newbie
timestamp: October 12, 2025, 05:55:03 pm
content: Interesting, it would be a pretty simple modification to add some cheap insurance there. I've got a parts order coming in this week so once I go back in I'll report. Thanks!

author: dr_theopolis
author_rank: Newbie
timestamp: October 16, 2025, 03:40:05 am
content: After replacing the bad LM337T and all 4 rectifier diodes with the 1N4002’s, I can report that everything seems to be working correctly once again. I didn’t find any other component issues or shorts anywhere else. I’ll next go ahead and re-cap everything while I’m at it just to future proof. Very odd indeed, but it appears to be fine after leaving it on for a couple of hours and testing all functions. Thanks for everyone’s feedback on this.