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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 16th Mar 2018, 10:31 pm   #1
Whaam68
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Default Armstrong 625 receiver

Bit of a micro project while I wait for parts to finish the 521 amp. This was available very locally fo £35 on gumtree with the circuit and manual. A very nice chap who was the original owner from new. The tuner scale lighting was out. I listened on headphones at the vendors and it was working but didn’t sound amazing but I was curious and had driven over so bought it. Once home I popped the lid, very easy just a couple of catches and one of the Erie speaker output caps was bulging. I dug in the spares box but had to order a pair of caps. These were fitted along with a new main smoothing can, the output caps I mounted in closed cell foam I recycled from packaging which worked a treat. I also cleaned the pots and switches, fitted led’s in place of the blown 12v scale bulbs and got carried away and recapped the power amp board because I had all the values to hand and it’s one bolt to flip the board up....very nice to work on! The vendor had a CD player on the amp and the woodwork is darker in the middle. I tried to even it with coats of tops scratch remover but it’s still not there. Brought it up on the lamp limiter and just a very brief light of the bulb on switch on then off. I connected up to my record player and very nice sound sprang forth, it seems to have a very good phono stage. At some point I may re cap the preamp board if I can be bothered. I wish Armstrong lit the tuner scale all the time as it’s really pretty but only lights up on Tuner setting ...must’ve been saving leccy in the winter of discontent! Interested to see how it compares to the 521. Mike
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Old 17th Mar 2018, 2:49 pm   #2
Andrew2
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Default Re: Armstrong 625 receiver

That's a very sleek piece of gear. Stuff like this occasionally pops up at our local charity shop - I'll have to keep my eyes open, as our current amplifier does not have an input for a magnetic cartridge.
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Old 18th Mar 2018, 4:17 pm   #3
Whaam68
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Default Re: Armstrong 625 receiver

Hi Andy
Yes I think Armstrong were ahead of their time looks wise with the 400 and 600 series. I still think it looks “modern” today. Incidentally I found a cheap internal fm flexible dipole in the loft and am pulling in lots of stations. The tuner section sounds very good even with the £2 Dipole on the window ledge. Here’s a pic of the old school led tuning indicator.
Mike
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Old 18th Mar 2018, 6:25 pm   #4
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Default Re: Armstrong 625 receiver

I was saving up to buy the amp only version of this as a teenager (I couldn't afford the receiver). When I had done so I went off into Liverpool to buy one only to be told that there had been so many returns of faulty ones that the shop had sent their stock back to Armstrong and the supplier couldn't say when fresh stock would be available.

I now have a 625 in regular use. It came to me as not working and I found that one of the earth leads to the circuit boards had come away. I have since had to re solder several bad joints which suggests that poor build quality was the cause of the problems back in the early 70s. Another case of a good design let down by poor workmanship.

It looks and sounds great but will not, of course, fit into a standard 19" space.

As a matter of interest I bought a Rotel RX400A receiver with the same money and it has worked well ever since.
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Old 18th Mar 2018, 6:37 pm   #5
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Default Re: Armstrong 625 receiver

One odd point, If I remember rightly, the 600 series receivers with Long and Medium wave bands were up-converters on both bands. Quite unusual in a broadcast receiver.

David
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Old 18th Mar 2018, 9:34 pm   #6
Whaam68
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Default Re: Armstrong 625 receiver

Hi Paul
I’ve read that about these. This one looks fairly well put together to the extent I gave up trying to desolder one side of the smoothing can it was well put on the iron was struggling to melt the joint and in the end I cut the wire. That said a wire fell off the tuning LED when I touched it with my finger! Fingers x’d!

Excuse my ignorance David but what’s an up converter?
Mike
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Old 18th Mar 2018, 11:26 pm   #7
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Default Re: Armstrong 625 receiver

In a regular long/medium wave broadcast set the incoming signal frequency of whatever station it is tuned to, is converted to a fixed intermediate frequency (IF) of about 465kHz.

This is a handy frequency at which to provide most of the set's gain and the selectivity which separates the wanted channel from its neighbours.

455Khz is higher than the top end of long wave, and lower then the bottom frequency of the medium wave band. There's a convenient gap there with no broadcasters, just a few navigation beacons for aircraft.

The Armstrong 600 is odd in that its IF is placed above the top of medium wave, so that it converts all RF that is covers upwards to this higher IF, hence it's an 'upconverter'

So long wave and medium wave are covered in just one long, wide bnd with no need for a gap in the middle. The lack of a gap is only useful if you have an aeroplane to steer and you have a direction-finding loop antenna on top of the plane.

An unusual quirk.

David
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Old 19th Mar 2018, 6:28 am   #8
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Default Re: Armstrong 625 receiver

Armstrong evidently “borrowed” the upconversion approach from communications receiver practice. As I recall, one reason for doing it this way was to facilitate the use of varicap tuning for AM as well as for FM. At the time it was designed, I think that varicap diodes with a wide enough tuning frequency range (>3 to 1) to cover the MW band with the normal IF (455 kHz or thereabouts) were not readily available. But the combination of an aperiodic RF amplifier (of the feedback type) with a 3.1 MHz IF put the required local oscillator range (about 1.5 to 1 for LW + MW combined) within varicap reach, the oscillator being the only tuned element in the AM front end.


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Old 19th Mar 2018, 2:00 pm   #9
Whaam68
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Default Re: Armstrong 625 receiver

Cheers Gents
Interesting to know.
Mike
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Old 21st Mar 2018, 10:59 pm   #10
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Default Re: Armstrong 625 receiver

I remember my Armstrong 626 - the AM sounded very muffled - indeed the audio frequency response began to roll off at 1kHz. I took it back for re-alignment - no change. There is nothing anywhere in the spec about the AM frequency response.

I also found that the 20volt stabiliser for the varicap tuning voltage drifted (temperature ??) and replaced it with an SN76552 (the 22volt version of the usual 33volt stabiliser in TV varicap circuits) cured that problem
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Old 22nd Mar 2018, 7:25 am   #11
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Default Re: Armstrong 625 receiver

The AM tuner is very unusual.

The ferrite rod is untuned and feeds a broadband buffer amp. The external antenna connector bypasses this.

A fixed lowpass filter feeds a single-balanced diode mixer which converts the wanted signal to about 3.1 MHz where it gets filtered by two IFTs with an AGC'd amplifier between them.

Next comes a dual gate mixer taking the signal down to 455kHz and a ceramic filter. The second LO is a free-rinning LC job.

The first LO is tuned by a single varactor with voltage from a pot or a bank of preset pots.

It ought to sound OK, but if the second LO is off frequency, then the ceramic filter and the 3.1MHz selectivity could fail to coincide, or there could be something wrong with the ceramic filter.

Designed with 9kHz channel spacing in mind and with a tight-ish ceramic filter, it's not going to give the audio bandwidth of classic AM sets, but 1kHz drop off sounds like there's a fault.

A mini-communications receiver on the broadcast band! The front-end is wide open to overload from any strong signals.

David
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Old 22nd Mar 2018, 8:19 am   #12
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Default Re: Armstrong 625 receiver

In the 70s this was the receiver that I lusted after. Never got it, I bought a Goodmans Module 90 from the company (Thorn Automation) staff sales shop. BTW, you must be able to (quite easilly) modify the source switching to allow the tuner scale to be backlit at all times? Like you say, it's not a great look otherwise when using a source other than the tuner. Nice set and lovely veneer, well done.
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Old 22nd Mar 2018, 3:09 pm   #13
Whaam68
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Default Re: Armstrong 625 receiver

Thanks, messing with old audio (particularly amps) can be frustrating but mostly I get a lot of pleasure from it. Resurrecting bits from long defunct British companies is interesting from a historical perspective, I was about 5 when this one was made but we couldn't afford this kind of thing back then instead having a big teak pye "stereogram". Almost all the amps I've messed with sound very good indeed, often despite the collective "wisdom" from other "audio" forums. I don't have a very high knowledge base only picking up a soldering iron for the first time when recovering from a serious illness a few years ago but can do the basics. Having half an idea I often need a steer from the very generous folks on this forum who are collectively very knowledgeable.

I can't comment on the AM side but the FM on the 625 sounds very clear to me. I was toying with modifying the Tuner scale lighting to always on and I will finish the re cap when I get time.

Can anyone suggest a mod for the slight zhhhht noise on switch on? I think this is the "Mike Solomons" mod mentioned on the audiomisc pages.

Re the Goodmans module 90 I have a module 150 in the pile....sounds very powerful but some tuner functions are U/S and it needs a refurb & a touch up.

regards all

Mike
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Old 18th May 2018, 5:19 pm   #14
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Default Re: Armstrong 625 receiver

One post moved here: https://www.vintage-radio.net/forum/...d.php?t=146670
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