iso Posted May 31, 2015 Report Share Posted May 31, 2015 It did no took too long to decide to use buy it now button, when I saw this quite original condition Armstrong 621 amplifier. Original mains cable has been replaced with orange one, but I decided not to replace it as it was soldered directly to mains switch... which may break if I try to replace cable again. The main reason for this purchase was that my younger brother Harri used to have Armstrong 600 series receiver in late 70´s. I was quite puzzled as his audio system sounded better than my system with ERA555 turntable, H/K 930 receiver and AR7 speakers. H/K 930 was supposed to be better amplifier than quite old fashioned Armstrong... but my ears told otherwise. This must have been the first time I trusted that the truth is in my ears, even this was pure hifi and stereo these days... not even heard term like "high end" or "golden ears".Pic 1When I received 621 it was sounding OK at low volume but struggled when more volume was needed. Pilot lamp was not working and left channel of tuner input was dead. I ordered 46 electrolytics for recap and found nice Armstrong site held by Jim Lesurf, http://www.audiomisc.co.uk/Armstrong/armstrong.html Jim was kind enough to give some advice and site was also quite helpful. It appears that 621 was quite conventional amp. RIAA stage was usual 2 transistor feedback design, line input has impedance buffer before passive tone control and gain stage of line amp used one transistor. Output stage used single rail 83 volt PSU and is capacitor coupled to load. Only unconventional feature is that inputs do use diode "cold switching". This means that there are no mechanical switches in the signal path. Pic 2Wiring harness is neat everything is soldered, no connector are used in the harness.Pic 3Original PSU and output electrolytics were of 1,75" dia which is in very short supply today. So... I decided to re stuff the original cans with modern CDE snap on electrolytics and therefore I was able to use original mounting hardware for output caps.Pic 4PSU board assembly almost finished. Only 2 wires do have tags made from masking tape to indicate soldering point. This was only board where it was necessary to unsolder wires (11 wires in total) to gain access for part replacement. Original PL4003 diode bridge was upgraded with 1N5408 devices as original 3300UF/100V PSU cap was replaced with 6800UF unit. Please note that original design did use spike suppressing caps in parallel to rectifier diodes. Input board on top right side of pic has been recapped with Wishay 021 series axial´s, Elna and Nichon audio radials were used. Pic 5Everything soldered and tied in place underside of chassis. Note vents drilled in the middle of output cap cases. Pic 6Chassis view from above. Re stuffed caps do not look exactly like originals... but close enough. Corroded fuse holder on top of transformer and new one on right side of transformer soldered in wiring harness. This was cause for nonworking pilot light and lack of power. Fused transformer winding was used to power pilot light and thermal delay switch on right side of output caps. As thermal switch was not powered there was always 47 ohm resistor in series to transformer secondary winding. This reduced 79 volt B+ voltage to 66 volts at idle and even further when more power was used. It seems that tuner input did have corroded pin to prevent signal flow. Now Armstrong is ready for cleaning of bias trim pots and idle current adjustment. Ready to rock´n roll... nice amplifier indeed.Best RegardsKimmoPS Armstrong story ended sadly after 600-series amplifiers, only batch of 50 pc next generation amplifiers was produced. Link to comment Share on other sites More sharing options...
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