UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 21st Nov 2022, 3:37 pm   #1
Humber888
Heptode
 
Humber888's Avatar
 
Join Date: Jul 2012
Location: Bromley, London, UK.
Posts: 689
Default Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

I think I know the answer to this but seek confirmation. I am repairing the low voltage linear power supply in an old HP oscilloscope and have found that two of the four rectifier diodes (in a bridge) feeding the -12V regulator circuit are open circuit - probably because two of the metal bodies were just touching. The original diodes are old metal 'top hat' types of 3A, 50V PIV rating. I would normally go for something like the 1N5401 as a modern replacement in such an undemanding situation but find I only have the UF5401 ultra-fast version in my stock. This is slightly overkill but should not cause any problems.

With 1N5401s, I would probably just replace the two blown ones, but suspect it would be better to change all four if using UF5401s. Does the panel agree?
Humber888 is offline   Reply With Quote
Old 21st Nov 2022, 3:47 pm   #2
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 7,665
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Not worth the risk of not doing it. Possibly better if you can get a complete bridge and mount it on the chassis for better cooling

Ed
Ed_Dinning is offline   Reply With Quote
Old 21st Nov 2022, 3:48 pm   #3
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 25,189
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Diodes are cheap. If two have failed, then the other two may be on their last legs too.
paulsherwin is online now   Reply With Quote
Old 21st Nov 2022, 3:55 pm   #4
stevehertz
Dekatron
 
stevehertz's Avatar
 
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 7,527
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

From the learned answers so far, it would seem that replacing all four is the thing to do. FWIW - wrong as it seems! - I'd only replace the ones that are faulty.
__________________
A digital radio is the latest thing, but a vintage wireless is forever..
stevehertz is offline   Reply With Quote
Old 21st Nov 2022, 9:56 pm   #5
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,024
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Quote:
Originally Posted by stevehertz View Post
FWIW - wrong as it seems! - I'd only replace the ones that are faulty.
No....false economy! I used to think the same BUT a couple of times I did just that and either one or the other diodes would fail soon after. 40 years experience in the trade has proved that changing all four is the most reliable way and avoiding call-backs. As has been pointed out, diodes are cheap....only pence usually so not worth scrimping. Consider also that when a diode (or two) in a bridge go short, there is a tremendous current passed through the smoothing cap as the poor thing passes AC for one half-cycle. Before any fuse blows the two remaining diodes pass a current well beyond their design, tens of amps probably. That is not going to do them any good at all. Replace all four with the same type or better.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline   Reply With Quote
Old 21st Nov 2022, 10:04 pm   #6
US7IGN/P
Triode
 
US7IGN/P's Avatar
 
Join Date: Aug 2021
Location: Kyiv, Ukraine.
Posts: 12
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

New diodes will have different characteristics and the bridge will become "crooked". Change them all.
__________________
I'm not sad I'm sober
US7IGN/P is offline   Reply With Quote
Old 22nd Nov 2022, 12:37 am   #7
fetteler
Octode
 
fetteler's Avatar
 
Join Date: Jul 2011
Location: Staffordshire Moorlands, UK.
Posts: 1,223
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Yep, change 'em all
During the failure the diode pair in the 'good' leg will have been stressed.

Steve.
__________________
Those who lack imagination cannot imagine what is lacking...
fetteler is offline   Reply With Quote
Old 22nd Nov 2022, 3:49 am   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 20,865
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

The ultrafast diodes are NOT overkill. They avoid the problem of elderly diode types whic don't turn off cleanly and make RFI. Once you come across this, you'll be surprised at the amount some simple power rectifiers make, even if they're not in anything that receives radio signals, you don't want them in the same house/street.

Just treat it as good housekeeping.

David (woken by a rainstorm)
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 22nd Nov 2022, 8:55 am   #9
PaulDarzi
Tetrode
 
Join Date: Nov 2022
Location: Near Worcestershire/Gloucestershire border (in WR), UK.
Posts: 60
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Quote:
Originally Posted by Radio Wrangler View Post
The ultrafast diodes are NOT overkill. They avoid the problem of elderly diode types whic don't turn off cleanly and make RFI. Once you come across this, you'll be surprised at the amount some simple power rectifiers make, even if they're not in anything that receives radio signals, you don't want them in the same house/street.

Just treat it as good housekeeping.

David (woken by a rainstorm)
This makes sense, and looking at the poswer supply of (in my case R606-MB) Roberts radios, they place capacitors in parallel with their diodes in the bridge - I did wonder if it was to reduce RFI, but haven't seen it done in modern sets maybe due to using faster diodes?

As for replacing two or all four diodes, do we assume that a mains spike or short-time transient took out the pair that failed?
(Thinking one half-cycle of mains was over the top, but maybe not the next, so only two diodes were blown by the transient)
Just trying to gain an understanding of the "why" for this failure.
PaulDarzi is offline   Reply With Quote
Old 22nd Nov 2022, 8:57 am   #10
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 19,814
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Failure of one or more diodes in a bridge rectifier is quite common. I too would like to know the failure mechanism.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is offline   Reply With Quote
Old 22nd Nov 2022, 9:04 am   #11
Humber888
Heptode
 
Humber888's Avatar
 
Join Date: Jul 2012
Location: Bromley, London, UK.
Posts: 689
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Well that's 6 to 1 in favour of changing all the diodes, so pretty conclusive. That was my plan anyway, primarily (as stated above) because of possible over-stressing of the remaining diodes and the mismatched characteristics between old and new. I need to dismantle the PSU block further to be able to unsolder the old diodes. I will report back when the unit is repaired.

Many thanks to all.

Mike
Humber888 is offline   Reply With Quote
Old 22nd Nov 2022, 9:16 am   #12
Humber888
Heptode
 
Humber888's Avatar
 
Join Date: Jul 2012
Location: Bromley, London, UK.
Posts: 689
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

In part answer to #9 and #10 of the thread, I did say in the original post that it looked as if two of the diodes were just touching (all metal bodies). It is unlikely that the downstream regulator board caused the fault as the fast blow fuses on its inputs are all intact. So that just leaves old age, the big reservoir capacitor or a wiring short. I will check the latter as I go.
Humber888 is offline   Reply With Quote
Old 22nd Nov 2022, 9:42 am   #13
mark_in_manc
Octode
 
Join Date: Nov 2011
Location: Manchester, UK.
Posts: 1,694
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Quote:
Originally Posted by Radio Wrangler View Post
David (woken by a rainstorm)
Ohhh, you have my sympathy. My half-fixed was-covered-in-kitchen-fat Hitachi radio-cassette street find (subject of perhaps the least memorable thread ever to appear on UKVRR) is currently doing sterling service providing FM static noise for just such nocturnal reasons (tinnitus in my case). In fact it's a nuisance that it pulls in so many stations; clean static is hard to find!

OK, back to diodes. I'm a tight-wad, so reading this thread is a good corrective for me.
__________________
"The best dBs, come in 3s" - Woody Brown
mark_in_manc is offline   Reply With Quote
Old 22nd Nov 2022, 11:08 am   #14
HamishBoxer
Dekatron
 
HamishBoxer's Avatar
 
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,654
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

I have always changed all if one failed in a bridge, no come backs that way.
__________________
G8JET BVWS Archivist and Member V.M.A.R.S
HamishBoxer is offline   Reply With Quote
Old 22nd Nov 2022, 11:39 am   #15
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 25,189
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

I've just checked eBay and you can buy 10 UF5408s for less than 3 including postage. 1N5408s are 1.50 for 20. It's what the Americans call a no-brainer really.
paulsherwin is online now   Reply With Quote
Old 23rd Nov 2022, 10:30 am   #16
Sinewave
Octode
 
Sinewave's Avatar
 
Join Date: May 2016
Location: Oxfordshire/Bucks borders, UK.
Posts: 1,485
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

I'd suggest replacing smoothing capacitors too, as they may either be aged/incorrect value and have stressed the rectifier diodes.
__________________
Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.
Sinewave is offline   Reply With Quote
Old 23rd Nov 2022, 11:38 am   #17
stevehertz
Dekatron
 
stevehertz's Avatar
 
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 7,527
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

Quote:
Originally Posted by Humber888 View Post
Well that's 6 to 1 in favour of changing all the diodes, so pretty conclusive. That was my plan anyway, primarily (as stated above) because of possible over-stressing of the remaining diodes and the mismatched characteristics between old and new. I need to dismantle the PSU block further to be able to unsolder the old diodes. I will report back when the unit is repaired.

Many thanks to all.

Mike
Not really, if you read it, my "only change two" response was a rhetorical one because previous to the thread I knew no better. Given the much better knowledge levels than me on the forum, I would always change all four now. So no, I would not advise changing just two.
__________________
A digital radio is the latest thing, but a vintage wireless is forever..
stevehertz is offline   Reply With Quote
Old 23rd Nov 2022, 3:49 pm   #18
Humber888
Heptode
 
Humber888's Avatar
 
Join Date: Jul 2012
Location: Bromley, London, UK.
Posts: 689
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

All 4 diodes changed, ancillary items checked and LV PSU refitted to scope. Everything is now working and all dc supplies are within spec. I do not replace the big Sprague smoothing electrolytics on sight as previous experience suggests they are quite reliable unless the pressure release pip has aged and leaked some of the contents. What caused the diodes to fail? As nothing downstream was faulty, I guess it's down to either age or two diode metal bodies touching - although why this should occur now after years of being like that is not obvious.

Again thanks to all who contributed to my further education.

Mike
Humber888 is offline   Reply With Quote
Old 24th Nov 2022, 9:54 am   #19
Hybrid tellies
Nonode
 
Hybrid tellies's Avatar
 
Join Date: Jan 2004
Location: Bristol, UK
Posts: 2,854
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

If any of the bridge rectifier diodes fail, open circuit, short circuit or leaky we always changed out all four diodes.
__________________
Simon
BVWS member
Hybrid tellies is offline   Reply With Quote
Old 1st Dec 2022, 5:23 pm   #20
Chris R
Pentode
 
Join Date: Nov 2021
Location: Laurencekirk, Kincardineshire, Scotland, UK.
Posts: 112
Default Re: Should I Replace Just Two Blown Diodes in a Bridge Rectifier or all Four

On the same theme - I have just restored a Dansette Musique record player and in the process I had to change the original (copper oxide?) bridge rectifier. I used a new KBPC 802, which I had in my spares. I believe this is a silicon device.
A number of bewhiskered transistors and all the electrolytics on the amplifier board were changed. The resulting sound quality is very good - except that that there is a 100Hz buzz when the volume is at higher settings. This disappears when the volume is reduced, so it appears to be picked up in the early stages. I changed the quite substantial (4700 microfarad) smoothing capacitor, to no avail.
I wonder if is the effect described by Radio Wrangler and if so, is there a more suitable bridge rectifier?
Chris R is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 11:09 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2023, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.