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Ant
15th May 2013, 19:54
One for the experts here:

I'm attempting a repair on an audio amplifier where a faulty Bridge Rectifier was suspected of causing hum.

The faulty item, a PBU1001 rated max 50 volts and 10 amps had to be replaced with the only alternative available, a PBU1004, (see table here (http://www.diodes.com/_files/products_inactive_data/ds21309_R8.pdf) ) which while it has the same physical size has a greatly increased capacity for voltage and amperage.

At first glance this substitution should work, because it does exactly the same job of AC to DC rectification, just with a greater overhead margin.

However, the amp is lifeless at the present time.:{

Is the PB1004 the wrong man for the job?

Lonewolf_50
15th May 2013, 20:02
Have you examined the entire circuit to see if there isn't a power clip or filter in line with that part? :confused: It's been years since I was digging about inside my Hi Fi systems, but I seem to recall that there are a variety of protections built into some of the systems that are not immediatly obvious unless you have the circuit diagram in front of you.

green granite
15th May 2013, 20:04
Should be fine, suspect the failed one has killed other things as well.

Tone
15th May 2013, 20:18
If the amp was working but humming it may be that one diode has gone and the bridge was operating as half wave rather than full wave. Substituting a bridge with higher voltage / current capability should perfectly OK.

belfrybat
15th May 2013, 21:33
The substitute should be perfectly OK. How did you come to suspect the rectifier? Did the amp work, albeit humming, before the replacement?

Flash2001
15th May 2013, 21:52
Have you got the AC on the correct terminals?

Is AC present?

If so is there DC on the other 2?

Any warmth?

etc.

After an excellent landing etc...

Dushan
15th May 2013, 22:35
What time is it in NZ? Loose Rivets should be by, shortly, to sort this out.

Bushfiva
16th May 2013, 01:09
IF you mean the system has gone from humming with the defective component to silent with the new one, have you put it in the right way round? You can't get the ac input wrong, but you can get the output polarity wrong.

If you're not sure from the specs whether the new item is a drop-in replacement (it is), are you sure you have correctly determined the old unit has lost a diode? You might want to check for any odd-looking electrolytics on the failed line.

Re LR: upside, he may be able to help. Downside, only if you let him disassemble something bigger and functional first, such as your telly.

EDit: I mean silent dead, not silent fixed. Derpy derp derp

Keef
16th May 2013, 01:10
If you've replaced a bridge rectifier with another with a higher power rating, and connected it correctly, then that isn't the problem.

The next step is to measure voltages. There are many things that could be wrong, starting with connecting the rectifier the wrong way round.

Msunduzi
16th May 2013, 03:55
What comes after the rectifier?

The original problem could have been a faulty transistor or regulator (if it is a regulated supply), or smoothing capacitor.

If the power supply is a separate part of the amp, disconnect it from the amp circuit until you are sure the power supply is working as it should.

A 2 DMM is worth buying, I'm assuming you don't have one, as you didn't mention doing any checks.

Also worth doing a search for the make of amp, some like the old NADs are known for capacitor problems. It may be a common fault, where you would find mention of it.

Loose rivets
16th May 2013, 06:15
the amp was working but humming

In tune I hope.

has a greatly increased capacity for voltage and amperage.

Mmm . . . has it got any protection circuitry? It may be looking for a higher minimum residual current, and not seeing this, switch off.


Funny thing about those power electrolytic capacitors: they can sometimes be revived after a long period of non-use. Powering up via a lower voltage sometimes works . . . or stick your finger in your ears. (hard to do when they're crossed.)



What time is it in NZ? Loose Rivets should be by, shortly, to sort this out.

I was Skyping a pal in NZ today and he informed me, reliably I hope, that I wasn't there.:p


In fact, I'm the last person to pontificate about radio power supplies. I failed my radio servicing practical in about 1958. The b:mad:ards had hidden the centre tap of the 300 0 300 winding, but I'd still got those voltages on the anodes of the rectifier valve (tube) compared to the chassis. Never really understood how that came about.

By the time I'd found it the time was nearly spent, and the radio looked like I'd been jumping up and down on it.