Bridge Rectifiers from 1976 now blowing short! - Page 1
Forum section: electronics / repair

author: RNA
author_rank: Regular Contributor
timestamp: August 16, 2025, 04:51:17 pm
content: I'd thought I would share my experience with working on old studio equipment. On the bench a had an EMS Vocoder 5000 that was blowing the external fuse.I found a blown bridge rectifier dated 1976 that had shorted for no other reason than its age. During the repair I came across another (same batch by the look of it) that was intermittent and on its way out.Normally one would ask why it would blow short but after some checks (no service manual so working blind) I came to the conclusion it was just ag

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 05:21:03 pm
content: I have come across shorted bridge rectifiers like that in 40+ year old test instruments where there was no apparent reason. I do not have an explanation.

author: Gyro
author_rank: Super Contributor
timestamp: August 16, 2025, 05:35:43 pm
content: Number of thermal cycles and mechanical stress? When not heatsinked, they tended to be run pretty hot in older equipment (from my experience anyway).Edit: Bridges made up of discrete diodes seem to be more reliable.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 05:43:30 pm
content: Quote from: Gyro on August 16, 2025, 05:35:43 pmThe failed bridge rectifiers I found all ran cool.QuoteIndividual diodes do seem to be more reliable. Maybe the difference has to do with construction?

author: picburner
author_rank: Frequent Contributor
timestamp: August 17, 2025, 09:09:47 am
content: The only document you can find for this old dinosaur is the user manual. Unfortunately no schematics.Edit: I'm just realizing this now: Look who owned the manual.

author: trobbins
author_rank: Frequent Contributor
timestamp: August 18, 2025, 12:33:20 am
content: Be mindful of the very small sample size of your failures - it's a very difficult topic to get tangible data on.There may also be some relationship to how often the equipment has been operating over that 50 years. That age and use profile relates to exposure time to random electrical mains stress events, and to aging effects of other parts within equipment that can add stress to a rectifier - such as e-cap leakage where turn-on leakage can be very high until the cap insulation reforms over seconds/minutes (after which the cap may test healthy). Such vintage equipment may also have changed location, which could exacerbate moisture ingress and show up age related material degradation/cracks/delamination.I've noticed individual vintage diode degradation in the form of lowered insulation resistance at PIV for 1960's Si diodes, and would typically replace them in a restoration due to a general perception that early Si diodes have a likely higher failure rate due to immature manufacturing consistency, along with the ravages of time.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2025, 02:59:46 am
content: The puzzling part for me is that I have not seen failures of individual diodes in the same application.My bridge failures were all on low voltage secondaries with significant voltage and current derating. Nothing was shorted except for the bridge rectifiers.

author: Seekonk
author_rank: Super Contributor
timestamp: August 18, 2025, 12:20:52 pm
content: I notice that the case is plastic and that ageing of the plastic could be an issue. Purity of the silicon maybe. ORIGON made diodes that had a high failure rate. Like that run of bad electrolytic capacitors years ago. Stuff just happens.

author: Synthtech
author_rank: Regular Contributor
timestamp: August 18, 2025, 12:26:46 pm
content: Last month I had a burned rectifier bridge in the PSU of a Yamaha CS-50 synthesizer and today I replaced the 4 diodes in the PSU rectifier bridge of a 1970’s Minimoog as these are a very common failure point In Minis and I replace them as a matter of course. I recently has a failed rectifier bridge in a PPG Wave 2.3 synth as well.

author: Seekonk
author_rank: Super Contributor
timestamp: August 20, 2025, 03:47:38 pm
content: It is obvious that rectifiers are sensitive to music and prefer suicide to what is being produced.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2025, 05:01:11 pm
content: Everyone knows that 1960s pop music was better than 1970s pop music, presumably because of the use of Allen-Bradley carbon composition resistors.Real music has been going downhill since the death of Monteverdi in 1643.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: August 20, 2025, 08:13:13 pm
content: Edit: I'm just realizing this now: Look who owned the manual.I've never heard of Otto Joachim, so I asked Uncle Google about him. Seems he is/was quite an accomplished Canadian musician...very kewl, (nice catch)