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Is it my transformer or rectifier?

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Discussion in 'Troubleshooting and Repair' started by foTONICS, Oct 9, 2015.

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Oct 9, 2015 #1

foTONICS

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Sep 30, 2011

Hey everyone,

I have a power amplifier that has not been working lately, more accurately it shorts hot and neutral together when I turn it on via the mechanical switch on the front of the unit.I used a multimeter to trace the problem to either the transformer or bridge rectifier.

transformer ratings:
I/P 0V-110V 0V-110V
O/P 59V-0V-59V/22A
62V-0V-63V/0.5A
17V-0V-17V/0.5A
22V/0.5A

bridge rectifier:

I'm not really sure the part number associated with this bridge rectifier, the only identifying marks on it is that symbol and "QL 14-3" which isn't giving very promising google hits.

A close up of the diagram on the label:

So by this diagram I shouldn't see a short on the to AC inputs but I do. I guess I could just desolder the connections and test for continuity.

I know that the transformer ratings are for 110 but this unit did have 220 applied to it's transformer input through miscommunication. What kind of damage would 220 do to a 110 transformer, did some of the enamel on the magnetic wire become faulty? or did I just most likely fry the input to the bridge rectifier?
Oct 9, 2015 #2

Minder

3,044

644

Apr 24, 2015

I would disconnect the rectifier and apply power to the transformer and let it run for a while to see if there is any excess heating, meanwhile do a continuity check on all the bridge diodes.
Incidentally if a metal enclosure cover touches the centre bolt, it will act a s a shorted turn on the transformer and over heat.
Max.

Martaine2005 and Arouse1973 like this.
Oct 9, 2015 #3

duke37

5,364

769

Jan 9, 2011

If you have a dead short, it is difficult to measure things when power is applied. A bulb (lamp) in series will let some supply be applied without the fuse blowing.

The transformer could have failed due to the core saturating and excessive current being taken.
The rectifier could have failed due to excessive voltage but these often have a lot of voltage capability spare.
The main smoothing capacitors or amplifier transistors could have failed due to excess voltage.

Do as Minder recommends and disconnect the rectifier, you may have to go further and disconnect nearer the amp.

Martaine2005 and Arouse1973 like this.
Oct 13, 2015 #4

foTONICS

332

9

Sep 30, 2011

duke37 said:

The main smoothing capacitors or amplifier transistors could have failed due to excess voltage./QUOTE]

I checked the capacitors and they're not ruptured or leaking, can caps fail and still look functional?

I haven't pinned down the location of the power transistors yet but when I do I'm going to run a diode test on them. Am I correct that I may see 1.4v if they are darlington?
Click to expand...
Oct 13, 2015 #5

Minder

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644

Apr 24, 2015

Have you done the transformer & bridge test yet?
M.
Oct 13, 2015 #6

foTONICS

332

9

Sep 30, 2011

Minder said:

Have you done the transformer & bridge test yet?
M.
Click to expand...

I am back at work tomorrow and will perform the bridge test. In response to the transformer test, the secondary splits of to 3 different voltages. Should I disconnect all secondary loads when I attempt to power the transformer input or just the ones leading to the bridge?
Oct 13, 2015 #7

foTONICS

332

9

Sep 30, 2011

I ask because the secondary wires branch off and disappear under some paneling and I want to be as least invasive as possible
Oct 13, 2015 #8

Minder

3,044

644

Apr 24, 2015

At least disconnect the bridge which would be a start, it appears accessible.
Do you mean three different supply conductors or three different secondary/supplies?
M.
Oct 13, 2015 #9

foTONICS

332

9

Sep 30, 2011

Minder said:

At least disconnect the bridge which would be a start, it appears accessible.
Do you mean three different supply conductors or three different secondary/supplies?
M.
Click to expand...

Three sets of wires come from underneath the wrappings on the secondary side. One branch goes to the bridge and the others go under paneling
Oct 13, 2015 #10

Minder

3,044

644

Apr 24, 2015

Then start by disconnecting the bridge and go from there.
M.
Oct 14, 2015 #11

dorke

2,342

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Jun 20, 2015

The failure process if you apply 220V instead of 110V would be the following.
In general the "weakest links" are damaged first:

1.Momentarily the power supply DC unregulated voltages will all double.
that can instantaneously damage all the semiconductor devices in the unit
(those that get their absolute maximum voltage rating violated) .
If the semis blow up in a way which doesn't create a short on their "supply pins" they
become disconnected from the supply voltage and don't "effect"/load it anymore.
They may become a permanent short on the supply voltage as well, in case they are
damaged and short their " supply pins".
2.The filter caps get double the voltage they are planed to work with ,
they are the 2nd in line to fail ,mostly vent and short out.
3. The last would be the Transformer, the over-current created by the above 2 processes
will generate heat and usually cause wingdings to disconnect.

The above didn't take into account fusing which is there to prevent fire from overheating and mostly to protect the main real-estate in an AMP=the transformer!

with correct fusing:
stage 1 damage is a given
stage 2 is possible
stage 3 will not occur

However,
with an incorrect fuse value( way to high),
stage 3 can happen as well.

So,
in your case you don't need to disconnect a thing.

Use a DMM in resistance mode(not diode mode!)
check the resistance on the +/- of the bridge(be sure there is no voltage on them before you measure resistance).
if you get zero ohms or about the cause is most likely the semis in the amp.
Next disconnect the amp PCB (I assume the PS is on it too) and check again:
It is almost a given that PCB is shorting.
but,If you still get zero
you need to check the filter caps as well(this time disconnected).
Alternatively with the PCB disconnected you can power the unit and check voltages on the filter caps.
Oct 14, 2015 #12

foTONICS

332

9

Sep 30, 2011

Minder said:

Then start by disconnecting the bridge and go from there.
M.
Click to expand...

I desoldered the secondary of the transformer to the input of the bridge rectifier and the resistance between the two AC inputs of the bridge rectifier is now 1.7Mohms whereas it used to be 0.6ohms.

I mentioned how there are some wires that disappear under some panelling coming from the secondary of the transformer, this is the bundle of green/white/orange/red/brown wires I'm pointing out with my pen:

The transformer stage of this P.A. can slid out so I set it on the work bench and flipped it on it's side and saw that the secondary side is a lot more accessible than I previously thought:

So it's obvious that the transformer is not operating correctly. I'm going to start tracing out exactly where the rest of the secondary's wires go. Crossing my fingers that the lower voltage secondarys didn't damage any of the circuitry

It's not really relevant but he's a pic of the entire unit, it's made to be disassembled rather easily

I have yet to figure out what the lower half of this P.A. does, I see a set of three fans on the front but I haven't even looked too much into it's operation.
Oct 14, 2015 #13

foTONICS

332

9

Sep 30, 2011

dorke said:

Use a DMM in resistance mode(not diode mode!)
check the resistance on the +/- of the bridge(be sure there is no voltage on them before you measure resistance).
if you get zero ohms or about the cause is most likely the semis in the amp.
Click to expand...

The bridge rectifier showed a resistance of either OL or around 3-5Mohms, it bounced around but I think that just may have been the way I was making contacts with the multimeter's probes.

I finished tracing out the secondary outputs and this is what I got:

There are three pcb's with edge card connectors on them, here is a photo:

I labelled them so I don't accidentally insert them in wrong after inspection. I started with card 'A' which houses four silicon bridge rectifiers, caps, a 555 timer, and some fuses. Here is a picture of 'A':

The green wires (62V) go to D1 on the right side. The orange wires (22v) go to both D2 and D3 which have their AC inputs in parallel with each other with one of the inputs fused by the left most fuse. Brown wires (17v) go to D4.

I ran a test on all rectifiers in the diode setting and all passed except D3. D3 did pass basic forward voltage tests, giving me values of about 0.5v but when I tried to test the (+) to (-) I didn't see the OL I was expecting but rather 1.812v. I noticed then that D3's DC outputs were also shared by C4, C5, V1, V2, V3, R1, R3, and N1. N1 is a 555 timer and after removing it I noticed that D3 no longer gave me funny readings. I'm not sure if the 555 is defective as it could have just been interfering with my diode tests.

The four silicon bridge rectifiers are part number 2W10 which can handle:

max peak reverse voltage: 1000V
max RMS voltage: 700v
max dc blocking voltage: 1000v
max average forward current: 2A

The only one of those parameters I'm worried about is the 2A max, if this transformer went into saturation from over voltage I could have exceeded that, although they do seem to be giving proper readings.

I plan on leaving the cards disconnected and powering up the unit to see if it still trips the breaker, if it doesn't I'm going to test the various voltage and see if there is a problem on these cards.
Oct 14, 2015 #14

dorke

2,342

665

Jun 20, 2015

What is the Make and Model of the AMP?
Oct 14, 2015 #15

dorke

2,342

665

Jun 20, 2015

BTW,
The 0.6ohm secondary you measured sounds normal for a transformer that size.
what is the units fuse value?
Oct 15, 2015 #16

foTONICS

332

9

Sep 30, 2011

I'll look at the fuses when I get back to work tomorrow. What kind of resistance would a transformer see across it's primary? Would a multimeter on the continuity setting start beeping?
Oct 15, 2015 #17

dorke

2,342

665

Jun 20, 2015

On a transformer like you have(step down) ,
the resistance of the primary would be higher than the secondary.

The resistance would be small ,can be a few ohms .
The meter it will beep.
Oct 15, 2015 #18

foTONICS

332

9

Sep 30, 2011

so I've had a bit of a break through. I noticed that when I turn on the unit, the split second before the circuit breaker trips, a few of the indicator lamps flicker and then fade away which has led me to believe that at least something is going on in the secondary.

I set my meter up to catch the highest value it saw, placed it on the secondary, and turned the unit on....and to my surprise the unit stayed on, it didn't trip the circuit breaker.

At this point I had removed the three PCB's from inside the unit and disconnected the bridge rectifier and the unit now works. I checked the secondary's voltages and got:

green-red-green: rated 62V; measured 72V
brown-white-brown: rated 17V; measured 19.6V
orange-orange: rated 22V; measured 26V

I'm assuming my measured values are higher because the only load on the secondary are two lamps.

The funny thing is when I first tried turning it on the unit tripped the breaker. I set it on its side to see what kind of voltages I got and the unit stayed on. Could this be just a loose connection somewhere that is shorting something out or was the first turn on attempt just charging the caps?

After this successful run I re-inserted the bridge rectifier, attempted to turn the unit on again and it started tripping the breaker. I'm going to disconnect the DC output of the bridge rectifier and see if the unit stays on, hopfully this will allow me to determine if it's the rectifier or not.

dorke said:

what is the units fuse value?
Click to expand...

The fuses are all 2A 250V which matches the 2A max on the silicon rectifiers datasheet
Oct 15, 2015 #19

Minder

3,044

644

Apr 24, 2015

Any fusing on the primary side needs to be slo-blo fuses.
The no load voltage is normal, depending also on the supply voltage.
Toroids particularly have a high inrush current, this is compounded when large capacitors are powered at switch on, this prolongs the inrush current.
M.
Oct 15, 2015 #20

dorke

2,342

665

Jun 20, 2015

Boy oh Boy,
This all debugging process is so not methodical !
Why complicate things when they can be simple?

Can you please answer some simple questions:

1.What is the Make and Model of the AMP?

2.What is the value of the mains fuse on the unit value?

3."trip the circuit breaker",
Does the unit have a circuit breaker on it?
Or,are you talking about the one on the wall?

4.Try operating the unit with all PCBs pulled from their sockets,
what happens ?

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