Any electronics gurus here?

Hey all, it was 20 years ago I did my electronics degree and the brain has got all fuzzy since then since all I do is software management nowadays.

I have a small combo, practice, amp that has failed. It just gives out a 50hz hum. The power supply is via a 4 diode bridge rectifier with some caps to smooth it out. I’ve measured the DC output of the rectifier circuit and all seems well, HOWEVER, I get about 40V AC too. That doesn’t sound right and is probably my hum-source.

I can’t remember the failure modes of BR power supplies. Would a shorted/open diode cause this or is it one of the caps that is the likely suspect.

Any real world experience or theory appreciated.

Cheers


Xon

If you’re getting AC out of a bridge rec, the diodes ain’t dioding! :)

So yes, at least 1 diode is O/C.

The caps are only for smoothing, so if they were gone you’d have increased ripple on the DC, but they wouldn’t produce AC.

I assume the caps are big electrolytics; if they are old, they might have dried out, but if they’ve blown it’s usually pretty obvious.

Have you a 'scope?

the bridge rec should produce a fully rectified 100Hz waveform, the caps then smooth that to a smooth DC.

Thanks gizmo. No access to a 'scope unfortunately.

I’ll grab some diodes and replace all 4.

Cheers

Well I think I’d analyze the problem more before replacing things. :)

Lift a leg on each diode, and then you can test them with a multimeter, should be 0.6-0.7 of a volt drop one way, and O/C the other way.

And an important question is, if it’s not an old amp, and a diode is dead, why did the diode die?

What Gizmo said…if it IS an old amp then I bet it’s a dried out cap.

Quote (Guest @ Mar. 27 2007,13:34)
Well I think I'd analyze the problem more before replacing things. :)

Lift a leg on each diode, and then you can test them with a multimeter, should be 0.6-0.7 of a volt drop one way, and O/C the other way.

And an important question is, if it's not an old amp, and a diode is dead, why did the diode die?

Fair point, but diodes are cheap so I'll get a few and test/replace as I go.

I have a theory as to why it failed. I laid the amp on its back and turned it on. It failed at that point. When I stripped it down, I found a few bits of solder and metal floating around in the chassis so my guess is that something may have got shorted. Sure, something else may have failed and taken out the diode, but I won't know until I try to sort out the PSU. The amp is worth peanuts when working, and less than peanuts when not, so it's not worth too much effort... I just don't like to waste gear!


Thanks for the advice

I would guess the amp is >5 & <10 years old.

You may have a blown output transistor(s)… I’d check/measure to see if there is any DC Voltage accross the speaker termimals. If it’s a 100 watt amp, put a 100 watt bulb in series with the AC Mains… If it’s a 50 watt amp put a 40/60 watt a series with th the AC Mains. Ya don’t want to dump any DC voltage into the speaker’s voice coil… You’ll burn the speaker’s voice coil… A 100 watt bulb is equal to approx. I amp at 115 v ac. If the amp is working right the idiling current of a 100 watt type amp is in the order of 10-25 watts… You wouldn’t see the light bulb light-up… If the output transistors are faulty you have a welding machine dumping current accross the speaker termimals…


What you you think caused the amp to behave the way it’s working… now??

Bill…

Quote (woxnerw @ Mar. 27 2007,15:58)
You may have a blown output transistor(s)… I’d check/measure to see if there is any DC Voltage accross the speaker termimals. If it’s a 100 watt amp, put a 100 watt bulb in series with the AC Mains… If it’s a 50 watt amp put a 40/60 watt a series with th the AC Mains. Ya don’t want to dump any DC voltage into the speaker’s voice coil… You’ll burn the speaker’s voice coil… A 100 watt bulb is equal to approx. I amp at 115 v ac. If the amp is working right the idiling current of a 100 watt type amp is in the order of 10-25 watts… You wouldn’t see the light bulb light-up… If the output transistors are faulty you have a welding machine dumping current accross the speaker termimals…


What you you think caused the amp to behave the way it’s working… now??

Bill…

Hi Bill

It’s basically a little tranny, 20w practice amp. It’s more or less an op amp preamp stage and a single tranny output stage. Cos it’s giving me a nice amplified 50Hz hum, I’m assuming that the output stage is at least trying to work.

I’ll try your suggestion and look for DC on the output.

I’m also going to pop to Maplin at lunchtime and grab a few diodes and caps. The “investment” should be less than a £1. I don’t want to spend too much more time on it so if that doesn’t work, I’ll probably give up.

I’ve just discovered that the schematic/circuit diagram that I have is a slightly different revision to the amp’s board. It’s very close but a few components have been renamed etc.

Thanks for the help guys.


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