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Armstrong 621 rebuild

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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 1

When 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 2

Wiring harness is neat everything is soldered, no connector are used in the harness.

Pic 3

Original 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 4

PSU 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 5

Everything soldered and tied in place underside of chassis. Note vents drilled in the middle of output cap cases.

Pic 6

Chassis 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 Regards


PS Armstrong story ended sadly after 600-series amplifiers, only batch of 50 pc next generation amplifiers was produced.







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I was asked to give shopping list for caps as PM. However, when I asked from Jim Lesurf, man who was responsible for production updates of amplifier, about cap values that differed considerebly from published scematic on Jim´s  http://www.audiomisc.co.uk/Armstrong/armstrong.html  site. Jim said that there were several updates due new semiconductors etc. during 600-series Armstrong production and there is no documentation available about updates.  He suggested that safest way to rebuild amp is to use similar cap values that were used in amplifier to be rebuilt. For example there was 4-5 empty cap places on preamp board of my 621 and many caps differed 500% from values on published scematic.

So... there is no easy way to get proper shopping list of caps, as every cap value and lead spacing must be checked anyway. 


PS 3300 uFmain filter cap was upgraded to 6800 uF with 1N5408 PSU rectifier diodes

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