I’d been wanting to do this for a long time. Upgrade an AI 500 by converting to a choke power supply along with rewiring, capacitor and resistor upgrades, and upgraded selector switch and volume pot.
Instead of mounting the components in a separate case a la Border Patrol, I mounted the choke on the rear of the case. The sound quality is the best yet. While I don’t have a Border Control power supply for direct comparison this version is right up there, or maybe even better!
You don’t see many of these around but one turned up for a few upgrades recently. A high quality TKD pot was installed to replace the cheap original, the balance control was taken out of circuit and a chassis earth post was fitted.
The result is a very decent sounding ECC88 based pre.
This Radford came into the workshop for a few reasons. Firstly the original cathode resistors had literally dropped out from their solder, through the action of heat over the years and finally overload with some dodgy valves. These were replaced with the superb Mills wirewound resistors.
Secondly the owner wanted attenuators fitted instead of the variable output impedance switch. I fitted the high quality Tocos carbon track potentiometers here.
Thirdly, removing the impedance switches meant hardwiring the output transformers to the speaker binding posts.
What I wasn’t prepared for was the great increase in sound quality that removing this 50 year old switch gave. Bass was much firmer and better defined, and there was greater clarity across the frequency range.
Another AI500 in for some resistor upgrades, rewiring, and new valve sockets. This one has already had my regulated power supply for the line stage along with cap upgrades and improved power supply.The original wire gets heavily oxidised with the internal heat and literally crumbles.
The resulting sound can hold its own with any modern valve amp.
So, this interesting build nears completion. A few reflections:
- One amp worked perfectly from switch on, the other was blowing fuses after 5 mins. Tracing this to a hot-running resistor meant the only cause of excess current could be 2 Zener diodes fitted the wrong way round or failure of those diodes. The diodes were fitted correctly and when I switched on with a multimeter connected across them it was easy to pick up that they were failing once warm, i.e after around 4-5mins.
- As supplied and according to the circuit diagram, the metal-chassis amp has no safety earth connection. This is not only highly dangerous but also illegal in the E.U. On testing the amp there is no difference in hum with the chassis connected to safety earth, so it’s even more strange the amp is supplied in this way. Of course, if the amp hummed with other equipment connected to it, then I would implement an earth lift or similar arrangement but so far no hum has been apparent.
3. And there’s another safety / reliability issue. Switch on surge or failure of the HT fuses means 500v is applied to 400v rated caps and 400v to 350v rated caps. These caps should all be rated at 500v for long term reliability /safety.
This amp was distorting badly along with significant levels of hum. As is often the case with these kit amplifiers, most of the problems were caused by a previous over-enthusiastic owner who had modified the basic amplifier leaving quite a few dry joints.
Some new capacitors were fitted and the dry joints and broken PCB tracks repaired and the amp was returned to its full, highly satisfying performance.
This amp came into the workshop for a recap. AVI use no name brands which tend to dry out, causing the sound
to become harsh and brittle.
I’ve serviced and upgraded virtually all the AVI legacy pre and power amps in my time. Power supply caps are one of the commonest repairs. I replace with larger cspacity, Nichicon capacitors which improve bass response and overall sound quality.