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Author Topic: Vox TB35C2 repair advice  (Read 264 times)

Fred187

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Vox TB35C2 repair advice
« on: July 28, 2019, 07:42:15 AM »
Had an issue yesterday with my valve amp - a Vox TB35 (Tony Bruno designed). It's a really lovely amp, I've had it for a few years and it's always done me well.

Had a shocker yesterday in a rehearsal studio with a dodgy power socket (realised too late it was cycling on and off) - when the amp was powered on, it made a horrible noise, and was turned off as soon as possible, but now won't turn back on.

I've done the basics - checked the fuses (all I could find were the mains fuse, and a fuse on the board that seems to be for the output transformer - but wondering if I've missed something) and they're good, so I'm worried the power transformer has blown - 'official' replacements seem to have been discontinued, but I'm sure I can get something with the correct specs from a third party.

However, there are no schematics available for this amp whatsoever. Hoping some of you fine folks could advise on how to proceed. I do actually happen to have a backup of this amp, which I expect could be invaluable if I need to take measurements.

First things first, is there anything else I should be checking before looking at the power transformer?

Full disclosure: I've never done a transformer replacement before, as it's always made me a bit nervous. Replaced pretty much everything else on the board myself quite successfully, but the only time I've had a failed transformer previously, I took it to my tech. However, my local tech these days is a bit of a bodger, and I'd actually feel more comfortable with the amp if I did it myself, but expert advice would be greatly appreciated.

alessandro_gfdp

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Re: Vox TB35C2 repair advice
« Reply #1 on: July 28, 2019, 08:22:16 AM »
I have the schematic, but before thinking about replacing a transformer I would perform some voltage measurements without tubes,that will tell me more if something is bad.

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Jazz P Bass

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Re: Vox TB35C2 repair advice
« Reply #2 on: July 28, 2019, 11:08:02 AM »
Before condemning the power transformer, try this.

Take the power cord and put an ohmmeter across the male tines.
Turn the power switch on & off.
With the switch On, you should be reading the power transformer primary coil.
If no reading is shown, you need to isolate that issue.

There are five components.
The cord itself
The mains fuse (110V/ 5 A__220V/ 2.5A)
Two in series thermistors
The transformer primary

If the reading is good, then you need to disconnect the power transformer secondaries and measure the winding resistance.
Mark down where they go!

P16_P15_P14 is the high voltage winding
P19-P17 is the heater winding

If all of this checks out and with the leads still unconnected, you should get the secondary voltages required.
If the voltages check good, then it's Not the transformer and it's time to move on to the circuit board.



 

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