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Bluejets

Oct 5, 2014
5,997
Joined
Oct 5, 2014
Messages
5,997
No need to be sorry, you're doing fine and in good hands with @Bluejets. He's just grumpy and only smiles on Sundays :D

Martin

Probably...... :)
Just had my fill of those calling themselves engineers as if to impress or make a difference somehow.
Had many of them at Gov. Water Resources many years ago who didn't know s***t from clay or as some say, their rrsss from their elbow.
 

sean5302

Sep 9, 2020
15
Joined
Sep 9, 2020
Messages
15
I have removed both bridge rectifiers and am satisfied that at least one needs replacement.
Before fitting the new ones when they arrive, what other circuit checks should I be doing so the new ones don't fry?
If another component is shorted, how will I find it without the rectifiers in place?
Is it just a question of following the power rails until I find something amiss?
 

sean5302

Sep 9, 2020
15
Joined
Sep 9, 2020
Messages
15
I'm straight back to square one.
Yesterday, I tested both Diode rectifier bridges and detected a straight short on one with the multimeter. Beeping on diode mode and no resistance on Ohms measure.
I also had 20v ac just everywhere around the board from left to right, checking pretty much every component.
No sign of dc voltage anywhere.
I removed both diode rectifier bridges and tested them out of circuit.
I'm sure that one of them had a direct short as I listed above, so I ordered 2 new ones from ebay.
I've just retested them now and both seem fine.
Thinking about it, something was charging and discharging when I used the MM and swapped lead polarity. Capacitors somewhere.

Today, with both bridges removed from circuit, I put my red multimeter lead on the -ve diode terminal and measured across to either of the ac terminals. It reads about 0.5V on each.
I put my black MM lead on the + diode terminal and got the same 0.5V connected to the ac terminals.
I put my MM leads across + and - on the diode and got 1V with red on - and black on +.
I got o/l with red on + and black on -
MM leads on both ac terminals gives o/l either way round.
This is all OK, isn't it?
I may well be going crackers. If my measurements yesterday revealed a faulty bridge but today all is fine, I must have jumped to the conclusion that the diode bridge was shorted but the short may be elsewhere on the board.

I'm so sorry to the knowledgeable. I am going up the learning curve but you guys must think I'm infantile. I'll have to send you a free car, or something.
 

Bluejets

Oct 5, 2014
5,997
Joined
Oct 5, 2014
Messages
5,997
Beeping on diode mode and no resistance on Ohms measure.

A full wave bridge rectifier consists of 4 diodes.
Transformers which use centre tap use only 2 diodes for full wave.

First, verify your red and black leads are the correct way around in the meter by testing on a known good diode ( say for instance a common 1N4004)
Assuming a digital meter set to diode test.......red lead to the anode, black to cathode ....circuit of perhaps 0.4 (actually it's voltage drop but anyhow)
Reverse should show open circuit.

Measuring on low ohm scale will verify a short and it will be in both directions.
Note once again, circuit components other than the device under test can give incorrect and misleading results.

Now move to the bridge out of circuit in most instances........there are four diodes as above,
All point from the negative output to the positive output

Bridge.jpg
Test each individual diode in turn, first say start at the top, test first on the left from negative (anode-red) to AC terminal (cathode) that is forward and expect somewhere around 0.4 on diode test range. Reverse should be open.
Continue for remainder, so next is red meter lead on top AC, black on Pos etc. etc.

One thing seems you are jumping the gun and determined to find a fault on the board somewhere before you verify the main transformer is working as it should.
 

sean5302

Sep 9, 2020
15
Joined
Sep 9, 2020
Messages
15
Bluejets, I don't think you're reading what I'm writing.
At post 15 I gave the results of my tests on the bridge rectifiers.
I could talk you through my voltage results but I don't think you're bothered, especially with your comment about "engineers". You imply that you're unqualified but experienced and resent people who are qualified.
I've worked in mechanical engineering for over 40 years, my chartered status is a matter of fact.
If nobody else on the board is prepared to assist me, I don't think I'm going to make any progress with you.
Thanks anyway.
 
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