woopwoop360 Posted December 20, 2024 Posted December 20, 2024 (edited) Merry Xmas to all I recently purchase this Amber 50c amp and it's in need of a service/repairs. I always wanted a Class A amp. It powers on but nothing from the preamp. I connected my ME25 preamp and I get sound from both channels, not real loud and distorted. I researched everything I could find here on SNA and the web, not much out there. I rang everyone local to me in Melbourne, they are either no longer in business or not interested in taking the job on except for Paul at Zamtek, who will help me out only if he has a service manual to work with. Can anyone please help me with my quest? The unit is in a nice and clean condition I think for its age and it look different to any of the other Amber 50c pics I see here on SNA. If anyone has info to share regarding this amp it would be much appreciated. I've included a couple of pics for your reference. Let me know what you think. Cheers. Edited December 22, 2024 by woopwoop360 spelling
SonicArt Posted December 31, 2024 Posted December 31, 2024 From memory Nick used to build Amber amps in the back of the shop. Late 80's I was doing a design project for him for speakers, and spent some time with him, bought my first NAD amp from him too, still have it! The 50C was made here, from memory the C denotes Aussie built versions but dont quote me . Its a 50wpc class A/B amp, I am not sure if its a similar/same circuit as the 70, but could be. My understanding is many of the 70 power amps were set with higher bias than they should have been, 150ma instead of 50ma, and the emitter resistors were not really big enough to cope with this long term. Could be the 50 was running higher bias than it was really designed for too, unfortunately going to be hard to find out anything now. All the service paperwork has likely been thrown out too, a real pity as I reckon Nick would have had a lot of very useful paperwork filed away.
VanArn Posted January 1 Posted January 1 The Amber amplifiers use the National Semiconductor LM391 chip and if you look up the data sheet for that item it will assist in following the circuit details. If you have access to the hifi engine site you can download details of a similar amplifier. The amplifiers are designed for class B operation and the bias setting should be around 6mV measured across the .22 ohm emitter resistors after the heat sinks warm up. Initial testing of the amplifier should be via a Variac or another mains voltage dropping method.
woopwoop360 Posted January 2 Author Posted January 2 On 31/12/2024 at 2:48 PM, SonicArt said: All the service paperwork has likely been thrown out too, a real pity as I reckon Nick would have had a lot of very useful paperwork filed away. Yeah, a real shame.
woopwoop360 Posted January 2 Author Posted January 2 I don't really know what I am doing, but I've started to investigate possible issues. I thought I'll try myself to repair. Check Dc offset on speaker terminals and getting about 12mV. Desoldered a couple power capacitors with a ESR meter and getting within spec. Next I was going to check the TO3 transistors on heat sink. Relay clicks when starting up. All fuses are intact. I'm looking at used oscilloscope and isolation transformer. 7 hours ago, VanArn said: Initial testing of the amplifier should be via a Variac or another mains voltage dropping method. and one of these.
woopwoop360 Posted January 2 Author Posted January 2 7 hours ago, VanArn said: The Amber amplifiers use the National Semiconductor LM391 chip and if you look up the data sheet for that item it will assist in following the circuit details. If you have access to the hifi engine site you can download details of a similar amplifier. The amplifiers are designed for class B operation and the bias setting should be around 6mV measured across the .22 ohm emitter resistors after the heat sinks warm up. Thanks for the info. I don't have access to hifi engine, I thought I had registered years ago but no credentials are working.
Recommended Posts