Armstrong 400 series question

[cooperman]

I've read (on Jim Lesurf's fine website) that the Armstrong 400 series and 500 series all used the same PCB's and output transistors. I wondered if they were interchangeble between the two series? main reason I'm asking is that the 400 is rated at 15 watts output, but the 500 is 25 watts - so there must be some difference between the two to explain the improved output.

[Edward Huggins]

The power supply circuitry on the 500 is uprated to allow for increased power handling.

[cooperman]

So in theory if I had a 400 series with good PCB's (and a faulty chassis) i could use the PCBs unmodified as spares for 500 series?

[Trevor]

There are two different power modules the all germanium A14 that was used in the 400 series and the silicon driver but germanium output A15
i think you will find the 400 series rated into a nominal 15 ohm and the 500 into 8 ohm the power supply voltages are very similar in both units
the excellent web site shows these
I ask why would you use the much rarer 400 parts as spares for the common 500 models
there is a lot of information on the vintage radio site re fitting silicon devices in place of the now rare and expensive germanium
Trev

[cooperman]

Maybe my question was the wrong way around and should be "could I use 500 series cards in 400 series chassis". Just intrigued about the compatibility of components between the two types. I just love the whole card cage arrangement of these units - it reminds me of mainframe computer systems of the era - ICT 1900, EE System 4 and the like. The same ideas from the same decade go into the construction methods.

Off topic - almost - I see that the repair facility in Walthamstow that was all that remained of Armstrong is turning itself into a coffee shop.
https://www.google.co.uk/maps/uv?hl=...ULN_sXC_z5hxOo

[Trevor]

Yes you can and i have already done so ! A very strange story Armstrong how they manged to grab failure from the jaws of success has always amazed me
Ted Rule the head designer was always a hero of mine and i always thought highly of his designs the 400 series is almost a clone of a 1964 harmon Kardon design all silicon and 50 watts a channel into 8 ohms I have one in my collection Although te Armstrong modular construction was to be an achillies heal in the end what with poor contacts
Trev

[Nickthedentist]

Wow, I almost feel like paying it a visit!

[cooperman]

I guess Ted and his team were always up against the cost of components. From what I've read the business aimed to make money - not necessarily quality product. The modular idea for the 500 series is excellent with regard to fault finding - if the connectors had been properly plated they would have been perfect. Cost constraints I guess. Having said that - who would have thought that they would still be around (and working) 40 years later.

[Nickthedentist]

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