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.
This interesting design is does not use an output transformer but is direct coupled to the speakers, which makes it a member of a very exclusive club of amplifiers operating in this way. I’ve been commissioned to build two of these mono blocks and will report back on sound quality once complete. For the moment I can say the kit which is sourced from the States contains quality components, and is complete with very comprehensive instructions. The chassis layout while appearing somewhat messy is actually a case study in layout where many wires cross but they all cross at right angles and ac lines are twisted, to keep noise and residual hum down. The instructions show great detail for these the wiring routes. However, as standard the kit has no safety earth connection which is in breach of UK /EU safety regulations and a state for a metal chassis amplifier with high voltages in which I would not release the amplifier. Once complete, I will test for hum with the chassis earthed and depending on results consider implementing a star earth system with earth lift.
My only gripe with the kit would be the aesthetics: the chassis top holds naked toroid transformers and chokes, and the front panel is a piece of vinyl type material. Of course, sorting out those issues would cost, but if one is truly building the ‘Pinnacle’ of audio reproduction why should it not look the part too? As well as adding to pride of ownership it would make selling the amps to one’s partner much easier! More later…..
As the capacitor shows here, it’s an original electrolytic capacitor from 1964! This Radford didn’t even sound too bad despite the aged electrolytic capacitors, with many originals dating from the 60s still in place. But an enthusiastic owner had upgraded the coupling caps and also added some large film capacitors which meant the bottom plate wouldn’t close and spacers had been added to allow the bottom to be fitted.
Some of these capacitors were placed in on critical positions so it was an easy task to swap them for the correct ones. All the electrolytics were replaced, with Elna Cerafines on cathode bypass duty. The bias pots were also knackered and therefore replaced.
Above is a picture of the refreshed amp, and now the bottom cover fits back in place.
This Rose RV23 preamp dates from the 90s. As standard, it already sounds good and it came to me a few years’ ago for some upgrades to the power supply, and improved coupling capacitors. With those mods it sounds really special!
This time it was in for a new volume pot, and selector switch, along with a couple of new phono sockets as the originals were very loose. A TKD pot was chosen along with a quality Elna selector switch and teflon /gold plated phono sockets. I always enjoy listening to this pre which you’d have to spend several thousand pounds to beat! No circuit digram is available and no one is even sure whether the valves should all be ECC82s or a mix of 82s and 83s…..it works with both!
It’s not every day that a pair of amps costing £23k arrive in my workshop but I have come across Shindo equipment a few times in recent years. Very iconic, high level, hand built Japanese hi fi for those with plenty of cash. These mono blocks have many things I hold dear to heart, such as parallel single ended operation and zero feedback. They are also specially voiced using some NOS valves and capacitors in key places. Nothing too seriously wrong with these boys, they are in for a check over and rebiasing.
The retro green styling I guess will please some and leave others cold. It has grown on me over the last week! And they certainly sound beautiful, being close in character to my Mastersound 845 PSE mono blocks!
As the photo below shows, build quality is exemplary.