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Old 6th Jul 2019, 3:37 pm   #1
vishalk
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Default Armstrong 621 and 625 rebuild and transplant

Hi All

Like many others on here i have some armstrong audio love to rekindle, i remember as a child my uncle playing his 625 receiver through some kef speakers, at that time it was so lovely to hear and look at, this was my first introduction to hifi.

Even today the looks of these amplifiers are brilliant! So i went and searched and bought these two units, the 625 and 621.

https://i.imgur.com/onqVrTY.jpg

https://i.imgur.com/ZDDn1zf.jpg

I want to rebuild the integrated 621 amplifier, the 625 was thrown in for the deal as it was damaged in transit. For me this was great as i can use it for spares, they share the same makeup right?

So this is my plan, i would love to upgrade the boards with better components such as doing a full recap of all the boards such as the preamp, amplifier board and psu etc

I also want to change any transistors and diodes which i have read can go faulty. I would like to build this amplifier to be like brand new and to last another 20-30 years.

So my questions are:

Is it worth replacing resistors? What resistors were used (type carbon?Metal film etc) The amp board would be easy to do i could replace all the components with new parts (not to sure about the bias resistors) they look odd and special heat disspersation types.

The amp board has other tiny driver transistors, do these tend to go and would it be a good idea to replace these?

The preamp and input boards have far too many resistors so i am hoping i can leave those. I know resistors can drift so wondering if they will all be ok.

I am slowly going through the circuit diagrams to understand it all better, i also know over the many years they changed components and always upgraded or made changes.

I will start with the 621 by just replacing all the capacitors, diodes and small transistors to get it working and running as it should. The 625 will be where i play around with it and try to upgrade the components.

Any advice would be appreciated such as what the bias should be, where to measure the bias, known things that should be checked, changed, replaced etc.

One other thing is the potentiometers on the 621 are very stiff unlike the 625 which feel smooth to turn, the input selector on the 621 takes quite a bit of force to click into place, where as the 625 is smooth and doesn't take me to squeeze my fingers hard to turn it. Wondering if spraying detox in there will solve this issue.

As i start to build i will post on here, cant wait to get started! Thanks in advance

https://i.imgur.com/MFhBagn.jpg

https://i.imgur.com/E5rnUZw.jpg

https://i.imgur.com/fIHILn9.jpg
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Last edited by vishalk; 6th Jul 2019 at 3:41 pm. Reason: added more info
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Old 6th Jul 2019, 4:06 pm   #2
Trevor
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Default Re: Armstrong 621 and 625 rebuild and transplant

First thing of note all output capacitors on both units are showing signs that they need to be replaced I would also expect the mains supply cap to be in a similar state Not a big job to change though
What test gear do you have ?
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Old 6th Jul 2019, 4:46 pm   #3
Whaam68
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Default Re: Armstrong 621 and 625 rebuild and transplant

I refreshed a 625 and it wasn’t that tricky. If you have a lamp limiter it might be an idea to see if it’s working.....definitely worth making one if not to avoid very frustrating mishaps. My 625 had a lot of bulbs out which I replaced with led’s, the main smoothing cans were bulged and leaking (common on these) and needed replacing. Cleaning the switches with servisol (carefully) is always one of the first things to do. Again from memory there weren’t that many caps on the 625 so you could just do the lot if planning to keep it. I think the resistors are film so should be fine but measure with a dvm if unsure. The transistors on mike were all fine. One “feature” is the tuning scale only lights up when the tuner input is selected. These were never high end so I wouldn’t go bonkers on “audiophile” components...sound wise I preferred the earlier 500 series.....they do look nice though!
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Old 6th Jul 2019, 6:07 pm   #4
PaulR
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Default Re: Armstrong 621 and 625 rebuild and transplant

I would concur regarding the output and smoothing caps. I have been trying to solve a buzz/hum in one channel on my 625 and discovered that several of the electrolytics on the amp board were beginning to weep so I replaced them all. This has helped but not completely solved the problem.

When I reassembled it, it had a hum that varied with the volume setting. This turned out to be the position of the leads from the pre amp board to the main amp board. They need to be run in the "well" in front of the main board otherwise they pick up hum from the mains components. I think that screened leads would have been a better choice here.

As all the electrolytics are the same make then you could consider replacing them all for long term reliability but otherwise I would leave well alone as it sounds great to me as it is.
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Old 6th Jul 2019, 9:24 pm   #5
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Quote:
Originally Posted by Trevor View Post
First thing of note all output capacitors on both units are showing signs that they need to be replaced I would also expect the mains supply cap to be in a similar state Not a big job to change though
What test gear do you have ?
Trev
Hi trevor i have two DMM, a variac, hand held oscilloscope (which i am still learning to use) i can make a light bulb tester, got a component tester for the transistors and caps, all cheap amazon ebay buys.
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Old 6th Jul 2019, 9:37 pm   #6
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Quote:
Originally Posted by Whaam68 View Post
I refreshed a 625 and it wasn’t that tricky. If you have a lamp limiter it might be an idea to see if it’s working.....definitely worth making one if not to avoid very frustrating mishaps.
Agreed Whaam i have a variac but would mean measuring voltages, this is a quick simple way and i can avoid any further damage. lucky we can still but incandescent bulbs, better stock up.

Quote:
My 625 had a lot of bulbs out which I replaced with led’s, the main smoothing cans were bulged and leaking (common on these) and needed replacing. Cleaning the switches with servisol (carefully) is always one of the first things to do.
All caps will be replaced and yes if i clean the switches and it makes no difference i have spare, also i noticed they are ALPS potentiometers very posh!

Quote:
Again from memory there weren’t that many caps on the 625 so you could just do the lot if planning to keep it. I think the resistors are film so should be fine but measure with a dvm if unsure. The transistors on mike were all fine. One “feature” is the tuning scale only lights up when the tuner input is selected. These were never high end so I wouldn’t go bonkers on “audiophile” components...sound wise I preferred the earlier 500 series.....they do look nice though!
Thanks Mike, these were popular amps and there are some lovely reviews on them. Personally i think for there age they are brilliantly built and well laid out. Seems like it will be a pleasure to work on, i also believe this will sound better then much of the modern integrated amps i see selling!

Mike

Last edited by AC/HL; 7th Jul 2019 at 12:36 am. Reason: Quote corrected
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Old 6th Jul 2019, 9:42 pm   #7
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Quote:
Originally Posted by PaulR View Post
When I reassembled it, it had a hum that varied with the volume setting. This turned out to be the position of the leads from the pre amp board to the main amp board. They need to be run in the "well" in front of the main board otherwise they pick up hum from the mains components. I think that screened leads would have been a better choice here.

As all the electrolytics are the same make then you could consider replacing them all for long term reliability but otherwise I would leave well alone as it sounds great to me as it is.
Notes taken, i should probably screen these cables, yes will change all the caps as i plan to tear it all apart and rebuild it for long term reliability, this amp deserves to be repaired and used.

Last edited by AC/HL; 7th Jul 2019 at 12:32 am. Reason: Quote corrected
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Old 6th Jul 2019, 10:02 pm   #8
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Default Re: Armstrong 621 and 625 rebuild and transplant

Is the unit motor-boating when turned up to a high level but without any signal input
If this is the case check the pre-amp regulated supply This is located underneath information re the digram can be found onthe web
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Old 8th Jul 2019, 9:18 am   #9
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Hi All

So i've started planning this rebuild and have a few questions.

The 625 i am stripping for parts seems to be more modern and with different parts which seem better then the 621.

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As you can see the difference, my question is could i work on the 625 board by upgrading the parts and then just swap it over to the 621?

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These are the 621 potentiometers which feel like they have had better days, when i turn some of them the actual connecters which are soldered move, so not a good sign.

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These are the pots from the 625 which are alps and are in much better condition, could i swap these over to the 621?

Plan is to make the 621 perfect and with any known problems sorted.

Another question is about

http://www.londonsound.org/Armstrongrepairs.htm

he quotes

The Armstrong 600 series
I reserve my best to last - the modifications applied to all 600 series amplifiers and receivers. The power supply capacitor can fail, spraying corrosive fluid onto a circuit board, causing massive damage. Often the volume control is noisy, (due to a design defect). The output circuit is slightly unstable, and cannot, therefore, be set to the minimum distortion levels achievable by this circuit. My modifications to resolve all three issues was so modest in cost, it was included in all repairs! Unfortunately, now over 30 years on, I find that too many of these capacitors fail despite the above mentioned modification that I now replace all of these capacitors as a routine service item, but by bulk buying of these parts I have still managed to keep the price down."

any idea on the sort of mods he does?

Thanks.
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Old 8th Jul 2019, 8:08 pm   #10
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

http://ukhhsoc.torrens.org/makers/Ar...ets/index.html

Excellent resource for armstrong products. The circuit diagrams are also very good quality and can be read easily and zoomed in without getting blurry
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Old 9th Jul 2019, 12:32 am   #11
jamesperrett
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Default Re: Armstrong 621 and 625 rebuild and transplant

Jim Lesurf's website would be my first port of call for anything Armstrong - here's one of his pages for your amplifier.

http://www.audiomisc.co.uk/Armstrong/600/600faq.html

and here's the main Armstrong page with links to more useful information

http://www.audiomisc.co.uk/Armstrong/armstrong.html
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Old 9th Jul 2019, 7:05 pm   #12
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Thanks James already been through there and read it. Im sure swapping boards will be fine.

Another question what are the rated power ratings of the resistors? 0.25w or 0.5w? Some have drifted quite a bit upon checking them so would like to replace those which are not in spec.

Thanks
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Old 9th Jul 2019, 7:07 pm   #13
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

anyone got a good scan of this review of the 621 amplifier

1977 Hi Fi Choice No 6 – 621 Review by Hugh Ford? Would really like to read it, but cant find it anywhere.
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Old 11th Jul 2019, 5:19 pm   #14
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Any word on the resistor power rating? Also need a power rating for new trimmers on the amplifier board for the bias. Choosing Bourns trimmers which can be 1/5w to 0.5w, any ideas in replacement diodes for ITT44?

Thanks

Last edited by vishalk; 11th Jul 2019 at 5:25 pm.
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Old 14th Jul 2019, 7:26 pm   #15
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Hi guys need to replace the noisy phono transistor being a BC384L, anyone know what the equivalent part to change this to is?

Thanks
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Old 14th Jul 2019, 8:01 pm   #16
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Default Re: Armstrong 621 and 625 rebuild and transplant

It seems to be within the ratings of a BC547 (or the lower-noise BC549), but watch out for the pinout. BC547/549 have the base in the middle.
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Old 15th Jul 2019, 9:16 pm   #17
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Thanks Julie

I shall check those recommendations out and not the pin locations
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Old 16th Jul 2019, 2:42 pm   #18
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Default Re: Armstrong 621 and 625 rebuild and transplant

There is an interesting thread on Armstrong 600 series here https://www.diyaudio.com/forums/soli...ease-help.html
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Old 18th Jul 2019, 5:36 pm   #19
vishalk
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Default Re: Armstrong 621 and 625 rebuild and transplant

Thanks Paul, it's a good thread!

Just replacing all the electrolytics in the 621 and also the recitifiers, may also do the bias potentiometers aswell just incase.

From there will get it working and check for any issues.

Got the three boards out of the 625 amp board, input board and control board, stripping the chassis spare for parts.
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