Restoring a Vintage Vox Guitar Amp Part 1: Testing and Re-cappingFollow article
Replacing the capacitors and potentiometers on a vintage Vox Escort guitar amplifier.
The Vox Escort amplifier was first introduced in 1974 and the first version ran on 2x PP9 batteries, while the second version — christened the BM1 — which is the one I have, could also run on mains. The amplifier is rated at 3.4 watts and uses a TBA810 power amp chip connected to a single 5.5" speaker. Dual inputs allowed both a guitar and a high impedance microphone to be used simultaneously.
Mine is probably from the late 70s or early 80s and has, over the years, been thoroughly messed about with. The original battery holder has been replaced with 4x AA battery holders that hold 3 batteries each.
A domestic mains socket has been added to the back panel, but I am not clear how or why; it was not wired up when I received the amp. The front speaker cloth has been removed and replaced with a grill that looks like it was intended for a car sound system, and there are various holes all over the place where sockets or switches have been added and then removed.
On the plus side, the electronics are intact and it has a nice vintage Celestion loudspeaker.
My intention is to test and replace any capacitors that need it and replace the noisy volume and tone potentiometers. I am also planning on using a standard desktop 18V power supply to keep things safe. When I am happy with the internal workings I can attend to the case. A perfect lockdown project!
As it was a sunny day I sat out on my terrace and carefully dismantled the amp, taking lots of pictures as reference for putting it back together again.
Identifying the capacitors
Once I’d taken plenty of photographs and I had the circuit board free from the case, I took it inside to my makeshift workbench and began cleaning it, and identifying and testing the capacitors. I did discover that cleaning dirt off old capacitors in an attempt to make the labelling clearer can be counterproductive as it can also remove the print!
Vox is now owned by Korg who keep a comprehensive online archive of schematics and manuals for many of the old Vox products, where I managed to find an old schematic for the amplifier. Although it looks like it was hand-drawn and is a little unclear in places, it will help identify some of the components.
First off I replaced the tone and volume potentiometers, as they were noisy; using 1M ohm logfor the tone and a 100 K ohm for the volume.
Then I started checking the capacitors as I know they can deteriorate over time and particularly the electrolytic ones, so my plan was to test those first and then replace as necessary.
I have an Atlas ESR Meterat home with me, so I can test the capacitors using this. It measure’s a capacitor’s equivalent series resistance (ESR), which, as the name implies, is a measure of the resistance that is in series with a typical capacitor. An increase in ESR (due to age, abuse or temperature cycling) can result in poor capacitor performance. The capacitor becomes less “ideal” and starts to dissipate more power. The meter measures elevated ESR which is a sure indicator that the capacitor is failing and likely to dissipate heat and perform less like a capacitor and more like a resistor at high ripple currents. For any particular capacitance and voltage rating, a lower ESR reading is generally better than a higher one.
An internet search for ESR Table will bring up loads of examples of tables and there is one in the user guide that comes with the Atlas ESR Meter. For good quality capacitors, it is common for the ESR readings to be lower than the figures shown in the chart.
I found a few capacitors I was not happy with and replaced them, deciding to also replace any more with the same values to keep things consistent. I used solder wickand a solder sucker ( 771-9543) to remove all the old solder and then gently removed the old capacitor from the PCB, doing my best not to damage the copper. I then replaced with capacitors with the same capacitance and voltage rating:
- 3 x 100uF 25V
- 1 x 47uF 16V
- 1 x 10uF 16V
- 1 x 1000uF
The last one of these I was a little hesitant about replacing: its ESR reading was about right according to the table, but the capacitance was about 30% higher than expected, which seemed a lot. Also, the replacement capacitor I have is an awful lot smaller, which is to be expected given the changes in capacitor production over the past 30 years or so, but still made me wonder if it was a good idea to replace it. Anyway, I removed the old one extra carefully so I could put it back if necessary and soldered the newer, physically smaller one in its place.
Once all this was done and I had tested the amplifier was still working OK, I removed the transformer and associated components that converted the incoming supply to 18V DC, as I was going to be using an 18V desktop power supply instead, so that I would not need to be working with mains voltage at home and I could keep myself safe. For the time being, I am powering the amplifier from the bench power supply I have borrowed from work.
My next job is to attend to the cosmetic appearance of the amplifier – it does look a bit of a mess at the moment. I know it is never going to look as it did when it left the factory all those years ago, but I am going to remove some of the uglier additions and plug and cover some of its many holes. I am also going to investigate options for powering it, other than the big PP9 batteries that it used originally, or the collection of AA batteries that the previous owner had replaced them with. Something rechargeable would be good.
As an aside, one of the things I have noticed working from home, especially with this project, is the lack of space. I am not the tidiest of people at the best of times and working on something like this — where there are lots of bits and pieces, as well as the hand tools for dismantling the amp and the soldering iron, power supply etc. — my little back room working space got in a bit of a mess and I did spend 10 minutes trying to find the small pair of snips I had buried under a load of other tools! I miss the big workbench and shelving at work.