Boogie - odd voltages

[txstrat]

Hi guys,

I have a Boogie MK3 on my desk that (according to a friend) cut out completely just before a gig (when else?).
I opened it and measured all the voltages and all seemed fine. No odds to find.
Anyway, I had to pause my work and next time when I measured again, all the voltages where way too low.
I found the voltages changed with moving the power cord. Even the primary side of the transformer showed a max of 180 vac (225 vac wall outlet) getting down to somewhat in the 90s or even to zero when I moved the cord.
OK, voltage yes or no with a broken power cord, but somewhere in between seems a little odd too me. Has anyone ever encountered this?

thanks

Matt

[Robert M. Martinelli]

Hi guys,

I have a Boogie MK3 on my desk that (according to a friend) cut out completely just before a gig (when else?).
I opened it and measured all the voltages and all seemed fine. No odds to find.
Anyway, I had to pause my work and next time when I measured again, all the voltages where way too low.
I found the voltages changed with moving the power cord. Even the primary side of the transformer showed a max of 180 vac (225 vac wall outlet) getting down to somewhat in the 90s or even to zero when I moved the cord.
OK, voltage yes or no with a broken power cord, but somewhere in between seems a little odd too me. Has anyone ever encountered this?

thanks

Matt

Hi Matt,
The cord could have been damaged by repetitively bending it, I think that the odd readings you' re getting are an average between the "powered" and "not powered" conditions that occur when you move the cord.

Have you already tried to change the power cord?

Does the cord go straight into the amp or does it have the ubiquitous pentagonal plug/socket system? If so, and if changing the cord does not fix the issue, the amp ( male ) power socket could be defective.

HTH

Best regards

Bob

[txstrat]

Hi Bob,
thanks for your quick reply.

...The cord could have been damaged by repetitively bending it

Yeah, that's what I thought.
The cord goes straight into the amp and I took the readings right from where the hot wire is soldered to a terminal strip. Haven't changed the cord yet.

I think that the odd readings you' re getting are an average between the "powered" and "not powered" conditions that occur when you move the cord

I had that in mind too, but wasn't sure since I held the probe to the terminal strip for quite a long time. The voltage rose to 180 vac and that was it. I was wondering if that could only be the reading 'in between'.

the amp ( male ) power socket could be defective

I might fit a socket to the amps chassis. If only I could find a free space.

Matt

[Enzo]

Pull the plug from the wall and clip your ophm meter leads to the blades of the plug. Turn the power switch on. You should now get a reading that is the resistance of the powr transformer primary winding. I have no idea what it will be - 10 ohms? 150 ohms? - but that isn;t impoprtant. WHat is important is finding out if it changes as you move the cord around. (That is why I want you to use clips on the plug, you will get a reliable connection to your meter.)

If the resistance changes as you move the cord, your cord needs replacing.
In fact you could go to the terminal strip inside and measure across the transformer primary wires so you know the resistance it has first. Then compare that to the end of the power cord.

And try this experiment with your meter. Where the mains voltage come into the amp - your terminal strip - put just the red meter probe on a terminal with the meter set for AC volts. How much do you read? And the other post? Your meter is sensitive enough that it can "pick up" the mains voltage. Like when your amp makes noise if you touch the tip of a guitar cord plugged into it.


I one time confused myself trying to sort out an open heater winding. My tubes wouldn't light, yet I got 6VAC across the transformer. The winding was open, yet the meter sensitivity still gave me a reading that was coincidently 6V.

[Robert M. Martinelli]

.....And try this experiment with your meter. Where the mains voltage come into the amp - your terminal strip - put just the red meter probe on a terminal with the meter set for AC volts. How much do you read? And the other post? Your meter is sensitive enough that it can "pick up" the mains voltage. Like when your amp makes noise if you touch the tip of a guitar cord plugged into it.

I one time confused myself trying to sort out an open heater winding. My tubes wouldn't light, yet I got 6VAC across the transformer. The winding was open, yet the meter sensitivity still gave me a reading that was coincidently 6V.

Good point Enzo!

DMMs have an ( almost ) infinite input resistance/impedance when they're set to measure voltages, so they're likely to pick up "fake" readings; that's why I use an analog ( needle ) multimeter ( a rather expensive one ) to confirm or deny the readings every time I'm in doubt. The input resistance/impedance is lower enough to avoid "fake" readings ( maybe it's a tad less accurate, but "truthful" ).

Best regards

Bob

[Enzo]

WHich is a different way of doing what I did. I suggested checking resistance as an alternitave to voltage readings. You chose using different volt meters. All boils down to checking things more ways than one.

[txstrat]

It was the power cord.
Even the wire isolation was broken inside the external insulation of the cable (hot and neutral were broken - ground was ok).
I'll fit a power socket to the chassis to rule this out for the future.

Thanks guys for your help.
BTW Enzo, I haven't tried the DMM trick yet. Will do when I'm done with the repair.