Scotts Vintage AC30 re-build

I had an email from an old mate of Andy Guyton askling if I could take a look at a vintage AC30 he'd just purchassed. It had been in storage for quite a while and on power-up, was crackling and humming. A few further emails and photos exchanged and I decided to take in on.. A few weeks later, it was duly delivered to the office..

It looked in pretty good nick from the outside, so I removed the back and slipped the amp out.. and.... Oh, my good gawd!... I loaded it into the car and drove it back to the workshop to take a more detailed look at the old girl. This, ladies and gentlemen, is what I found on initial inspection...

So, the original output transformer had been removed and replaced with this lump. 

I'm not sure why it's been mounted this way, as it is a drop-through transformer and it's mountings match the appertures in the chassis! In fact, to mount it this way, they actually had to drill more holes in the chassis and struggle to find ways to get the wiring though to where it was intended..

Here's a good lesson in how NOT to mount a replacement output transformer in an AC30!

And here's why! Two of these wires had been trapped and chaffed against the chassis. One of them was shorting to the chassis..

The mains transformer had also got a little warm at some point!

I'm guessing the amp was run for quite a while with the voltage selector set to too low a voltage. When the old vintage power transformers get warm, they dribble out their wax potting! If they get too hot, they can lose their insulation on the windings, then start shorting. 

This unit had got off lightly.. Just the loss of wax and the insulation still in tact.. The wax loss, we can live whith. Worse case, the transformer might buzz a little bit more under stress! It does clean off with a little gentle heat from a heat-gun and a soft rag..

Other interesting foybles were as follows:

Earth wire removed from mains plug! Due to a faulty cap, the metal chassis was actually sitting at 320VDC!!

Output transformer secondary earth removed?

Poor solder joints and HT leads finger-twisted and wrapped in insulating tape!

Vib/trem switch with a broken contact.

The list went on, so I decided to just strip the whole lot out and start again! This, along with the power transformer and chassis members was just about all I could salvage..

 I measured every component on those strips! I had a 22K resistor that measured 580K (not good when it's supposed to be supplying power to an anode!), a 220K resistor that was around 2.4M, a few that were open circuit, then there were the caps! Well, pretty much all the electrolytics were replaced as they were either leaking or getting warm! I left as many of the original components in situ as I could, even if they were out-of-spec, as long as, in my opinion, they were doing a good enough job! I didn't want to lose the amp's heritage, but did want to make it safe and sound great!

The chassis was hand-cleaned in soapy water and the aluminium section, cleaned and re-finished with a DA.

I had a new output transformer wound to the 1960's spec and fitted that along with new ceramic valve bases. I've never liked the way Vox build their power section, so I built a new board to safely hold all those components in place.

A new fuse board was also added to offer aditional protection to the ageing power transformer.

The renovated boards were then bolted back in and wired to the new pots and valve bases.

A new filter cap board was also added along with a heater winding hum-nulling pot (seen on the left).

Scott had already purchassed an after-market TB module and asked me if I'd fit it for him whilst undertaking the works. The problem for me was that this unit was not of great quality either in build or component!

In for a penny, as they say, so I stripped the TB module back to it's metalwork and valve base and started again. This time, with a decent circuit board, good quality components and high quality CTS pots.

This was now ready to be mounted into the amplifier!

And final testing done to make sure all aspects work beautifully!

Then, back in the cabinet along with the newly routed rear panel (without removing the original tolex! 

She is now quite a lovely sounding and visually stunning example of a 1960's AC30..

And Scott's a happy bunny!!!

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