Total noob with blown Pre-amp - diyAudio
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Old 11th February 2011, 08:40 AM   #1
Colday is offline Colday  United Kingdom
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Default Total noob with blown Pre-amp

Hi all,

I really am a complete noob to the site but I need help.

I'm fairly good with a soldering iron but knowing what to solder is currently beyond me!

I have a Pro-ject Debut II turntable running through a Pro-ject Phono Stage. It was packed away for a while during some renovations to the house and when I unpacked it I managed to plug the incorrect transformer into it. I plugged a 16v DC transformer in rather than the correct 16v AC transformer, resulting in a puff of smoke and no sound. I've opened up the casing of the phono stage and found what appears to be a blown resistor on the AC input.

I've tried to read through the thread about the Pro-ject phono stages but I'm afraid it is way beyond my knowledge levels.

I have included a photo of the circuit board, could someone tell me what value the resistor is that is blown. It's not hugely apparent as the flash from the camera bleached out the burn mark so I have circled it in red.
I have also included a photo of the circuit side of the board. Sorry that it is reversed but my brain isn't working too well and I can't get photoshop to reverse it.

If you could also suggest if anything else may have been damaged in the incident :-)

Cheers all
Attached Images
File Type: jpg Pro-ject Phono Box - comp side small.jpg (219.7 KB, 181 views)
File Type: jpg Pro-ject Phono Box - circ side small.jpg (250.4 KB, 174 views)
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Old 11th February 2011, 09:49 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
plugging in a DC voltage to a PSU expecting to be fed with an equal value AC voltage will not do any harm.

Something else happened.

Can you get a magnifying glass and examine the resistor to identify some or all of the colours?

Most of the circuit is two channels with identical component placement.
The bits that are different are at the input and the output.
At the input I cannot see any diodes. I suspect this circuit expected a DC supply and you plugged in an AC supply to kill it.
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Last edited by AndrewT; 11th February 2011 at 09:51 AM.
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Old 11th February 2011, 09:49 AM   #3
alexcp is offline alexcp  United States
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Colday: it is a diode that you marked. This thing appears to use two regulated rails derived from one AC voltage, and the two diodes in the upper (on the photo) part of the PCB are the rectifiers, one of them you say is blown. The regulators normally accept up to 35 volts, so 16v DC would not hurt them anyway. I wonder what might create smoke there... Try to plug in the normal AC adapter and measure voltages across the two bigger electrolytic caps in the upper part of the board. They should be identical, plus and minus 20V or so, maybe less. If one voltage is missing, you did blow a diode, replace it. If both are present, measure the voltages at the outputs of the regulators - those three-terminal TO-92 in the upper part surrounded by blue ceramic caps. Should be (I guess) plus and minus 15V.

AndrewT: Pro-ject Phono Box does require 16V 30mA AC (not DC).

Last edited by alexcp; 11th February 2011 at 10:07 AM.
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Old 11th February 2011, 10:07 AM   #4
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Looking quickly at the PCB layout, the two DIODES form a classic voltage doubler to produce + and - supplies from a single AC source.

This would not work if you attach a DC supply.

The circled component is a DIODE. I would simply replace the two diodes at the top centre of the board with 1N4002s and see if it works.
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Old 11th February 2011, 10:09 AM   #5
Salas is offline Salas  Greece
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Its a diode to replace its my guess too from as much as I can make out. Are those TO-92s something like 78L15 regs? Not likely to have been hit though.
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Old 11th February 2011, 10:16 AM   #6
AndrewT is offline AndrewT  Scotland
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I either need a bigger and higher resolution monitor or I need newer prescription spectacles.
Ignore what I said.
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Old 11th February 2011, 11:00 AM   #7
Colday is offline Colday  United Kingdom
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Thankyou all!
Including AndrewT, it sounded good anyhow! ;-)

So, if I go into Maplins and ask for some 1N4002s Diodes they will know what I mean?

Also, when I said I was a noob, I really meant it. Could someone translate Salas' line for me as I really have no idea!

"Are those TO-92s something like 78L15 regs? Not likely to have been hit though. "

Cheers all.
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Old 11th February 2011, 11:08 AM   #8
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The three legged black blobs at the top of the circuit board are most likely voltage regulators.

They will take the high voltage from the diodes and associated capacitors and turn it into + and - voltages suitable for the remainder of the circuit.

Looking into the circuit, there is no reason why the diode should have failed. Connecting DC across the doubler in either polarity would not cause the diode to fail unless one of the electrolytics is short circuit. (The large black capacitors).

Maplins stock of components is becoming limited. 1N4002 / 1N4003 / 1N4004 will all work, the increasing number is just increasing voltage handling, so even 1N4007s will do the job.
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Old 11th February 2011, 11:16 AM   #9
Mooly is offline Mooly  United Kingdom
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Burn mark ? or just heat discolouration which may be normal.

Yes 1N4002 diodes are common. 1N4001/2/4/7 are all the same series... any would do, higher number, higher working voltage but any OK here.

Can you couple it up to the tranny and do a couple of quick voltage checks first before condeming things...

Can you measure from "ground" (phono socket outers) to each end of the two diodes on a DC voltage range on your meter.
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Old 11th February 2011, 11:22 AM   #10
Mooly is offline Mooly  United Kingdom
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Can you measure and report the voltages at these points.
Also at the top of the picture are two 3 legged components that may as someone mentioned be regulators. What is the markings on each ?
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