In article , RJH
wrote:
On 31/07/2015 11:14, Jim Lesurf wrote:
I have one of those - very nice too, and still works perfectly. It
hasn't been in daily use for some time now, though.The only slightly
annoying thing for me is the DIN speaker sockets.
What are the mods you're having done?
I plan to write something about this in detail for my website. However I
should say up front that Mike was happy to give me full details of the mods
he makes *on condition* that I don't disclose all the details because they
are his ideas and his bread and butter. That seems fair enough to me, so I
agreed happily. Journalists have to respect their sources, particularly
when they are very helpful.
All that said...
The standard changes are simply to restore and repair. e.g. what I sent to
him had meters that didn't work properly and lamps that had failed. Some
switches/sockets were intermittent. All the predictable signs of a long
life of use for such old kit.
The returned set I now have here has all the meters, lights, etc, working
'as new'. (If I can manage I'll take some photos for when I write about
this, but I'm a lousy photographer.)
He also checked things like the IF/RF alignments, but since the set had
been owned by Ted Rule, then me, found these were spot on OK.
I think he also installed new heatsink compound, etc, for the output
devices to ensure the thermal sinking. And old electrolytics may get
replaced.
Essentially where possible he fixes/replaces what has gone wrong or seems
likely to go wrong soon. The main area where this is limited is if there is
physical external damage and he doesn't have a suitable replacement lid, or
whatever. Meters can be a problem. For my set he actually had to shave a
small bit of plastic and slightly bend the meter 'hand' as it had been
sticking because the plastic had slowly warped with time from the heat of
the pea bulbs. This perhaps gives you some idea of the detailed work he is
prepared to do when needed, and that it all depends on the specific set and
the wishes of the owner.
The main mods are that:
1) He fits a circuit to suppress the 'whoosh' which can arise if you change
the volume too soon after switch-on. This tends to occur eventually with
the earlier sets with the carbon based tracks. FWIW I used conductive
plastic pots in a later version and these are less likely to get the
problem. So if a set is one of the later versions you may not experience
the problem. But earlier sets are very prone to it. This mod also tends to
reduce the switch-on thump I found.
2) He also modifies the power amp to be more thermally stable and less
likely to develop crossover due to thermal tracking differences. The result
seems to be more consistent and lowered distortion in normal use. This
change means he can set a higher bias current safely, so get smoother
results.
3) Along with (2) he tends to fit better drivers and splitter transistors.
So the amp should finding driving low loads easier, etc.
I'd expected him to remove the old thermal delays and change to using
higher rated diodes in the PSU. However he feels that this isn't necessary
if the delay still works. On reflection I ended up agreeing with him. I
changed the sets to lose the thermal delay because these were causing most
of the set returns under guarantee. However talking to Mike I realised this
was because some delays were dud. Once they've worked for a few years
they'll last forever. So by this stage they are unlikely to become a
problem as the survivors are the good ones! :-)
He may make other changes depending on the state of the set and the precise
details of the version. Bear in mind that the actual production circuit was
changed in detail many times as it 'evolved'. Alas, no-one now has any
clear record of them. [1] All minor, but each designed to tweak an aspect
of performance. e.g. The output caps and reservour cap values were
increased more than once to get more power at LF and into low loads - once
the drivers, etc, could cope!
FWIW As per an earier posting I'm using a set of DIN speaker adaptors I
made, based on some 'new old stock' plugs Dave kindly found and sent to me.
(Again, thanks Dave for those! :-) )
Currently listening to Radio 3 using the 626's FM tuner. Even though I've
not yet sorted out a sensible VHF antenna it sounds very good to me.
(Currently I'm using a set-top UHF TV antenna in the loft as the VHF
antenna! Need to get into the loft. 8-] )
Cheers,
Jim
[1] You may have noticed that there are two different Armstrong circuit
diagrams that were released with the 600 range sets. The change that
confuses most people is the change to the output bias current from 5 to 20
mA. But there are other changes that people miss. And in reality there were
many more 'versions' than two! One of my biggest regrets now is that all
the production paperwork was discarded and so I can't tell now how many
sets were made, and which changes were made, when! If I'd realised in time
I'd have tried to rescue the info. But it was too late by the time I
started writing the Armstrong webpages and asked others about this, and
other details now lost. :-/
With that in mind I wonder if it is worth asking people who have a 600 set
what serial number it is and when they bought it. Might be possible to
recover some details if enough sets are about. Should ask Mike as well as
he might have worked it out...
--
Please use the address on the audiomisc page if you wish to email me.
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio
http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc
http://www.audiomisc.co.uk/index.html
Armstrong 626 nenewal!
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