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Old 8th Jan 2022, 7:42 pm   #1
cooperman
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Default Armstrong 621 amp - flat high wattage resistors

The 0.47 ohm high wattage resistor at location 227 on the A17 amplifier PCB has gone open circuit on my Armstrong 621 amplifier. The resistor is of flat construction (15mm x 12mm x 2mm) with two pins at the bottom edge. I've not been able to find anyone selling this type of resistor currently, so I need to replace it with a standard cylindrical one. Would anyone be able to suggest what wattage of resistor would be required. Circuit diagram is here: http://www.audiomisc.co.uk/Armstrong/600/600faq.html. Resistor 227 is the one connected to the emitter of the output transistor at location 525.

Alternatively maybe someone knows where I can obtain the original flat type.
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Old 8th Jan 2022, 8:06 pm   #2
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Default Re: Armstrong 621 amp - flat high wattage resistors

Those sound like "thick film" resistors.
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Old 9th Jan 2022, 10:41 am   #3
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Default Re: Armstrong 621 amp - flat high wattage resistors

I replaced mine with no problems. Here is a photo. I can't remember the wattage, though
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Old 9th Jan 2022, 11:37 am   #4
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Default Re: Armstrong 621 amp - flat high wattage resistors

At a guess I'd say they look to be around 3 watts.
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Old 9th Jan 2022, 12:58 pm   #5
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Default Re: Armstrong 621 amp - flat high wattage resistors

That sounds about right.

As a matter of interest, when I put the board back there was an annoying hum. It turned out to be caused by the leads that connect it to the previous stage. They are just single core wires. Replacing them with coaxial leads cured the hum.
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Old 9th Jan 2022, 4:12 pm   #6
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Default Re: Armstrong 621 amp - flat high wattage resistors

I found an old A17 pcb in my "bits box". Interestingly it has 0.68 ohm flat resistors instead of the 0.47 ones in the board I'm repairing. Seems Armstrong must have changed the value at some point in the production run. I also note that your PCB (and my spare one) has extra resistors (above 219 and adjacent to 225) not shown in the widely circulated diagram.
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Old 9th Jan 2022, 5:06 pm   #7
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Default Re: Armstrong 621 amp - flat high wattage resistors

They must have changed things during the production run. This one had been messed around with by a previous owner who scratched the various things on the board. Fortunately it works well now except for a tendency to oscillate as a "motorboating" frequency if the volume is turned up high on the phono setting.
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Old 10th Jan 2022, 1:05 pm   #8
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Default Re: Armstrong 621 amp - flat high wattage resistors

Poor supply decoupling or tired electrolytics, perhaps?
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Old 10th Jan 2022, 5:00 pm   #9
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Default Re: Armstrong 621 amp - flat high wattage resistors

Yes, you would think so but I replaced the main power electrolytic, the output ones and all those on the power amp board. I don't remember doing the pre amp boards though.

I used 2N3773Gs instead of 2N3055s though.
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Old 10th Jan 2022, 6:18 pm   #10
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Default Re: Armstrong 621 amp - flat high wattage resistors

You could give Armstrong a ring. The are located near me in East London.

https://www.armstrong-audio.com/

Hope this helps
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Old 12th Jan 2022, 8:38 pm   #11
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Default Re: Armstrong 621 amp - flat high wattage resistors

Quote:
Originally Posted by cooperman View Post
At a guess I'd say they look to be around 3 watts.
I ordered some 3 watt ones - fitted one and it works just fine (i.e. it does not get hot).

On a separate subject (but A17 PCB related), does anyone know where I can get "tall" heatsinks for the TO-39 can transistors? The ones fitted are about 11mm high, but there is plenty of room to fit taller ones (to dissipate more heat).
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Old 13th Jan 2022, 12:35 am   #12
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Default Re: Armstrong 621 amp - flat high wattage resistors

Quote:
Originally Posted by cooperman View Post
I found an old A17 pcb in my "bits box". Interestingly it has 0.68 ohm flat resistors instead of the 0.47 ones in the board I'm repairing. Seems Armstrong must have changed the value at some point in the production run.
The original design was for .68 ohm. A disgruntled customer took Armstrong to Trading Standards and won because his sample came up a Watt short on output. Armstrong changed to .47 ohm to give some margin on output in production, sacrificing some thermal stability in the process.
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Old 13th Jan 2022, 7:28 am   #13
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Default Re: Armstrong 621 amp - flat high wattage resistors

Quote:
does anyone know where I can get "tall" heatsinks for the TO-39 can transistors
I might have some spare, will look.Drop us a PM if you haven't heard from me in a couple of days.

Andy.
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Old 13th Jan 2022, 10:14 am   #14
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Default Re: Armstrong 621 amp - flat high wattage resistors

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Originally Posted by Ted Kendall View Post
The original design was for .68 ohm. Armstrong changed to .47 ohm to give some margin on output in production, sacrificing some thermal stability in the process.
Worth changing them back to 0.68 ohm then?
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Old 13th Jan 2022, 11:29 am   #15
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Default Re: Armstrong 621 amp - flat high wattage resistors

If you are doing anything to the output stage, I'd say so.
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Old 13th Jan 2022, 2:06 pm   #16
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Default Re: Armstrong 621 amp - flat high wattage resistors

I have two examples of the A17 PCB. One is in the amplifier and currently working, the other is used for spare parts. The working one has the 0.47 ohm resistors and the non-working one has the 0.68 ohm ones. Interestingly the non-working one has additional resistors in a couple of places - whereas the working one has wire links instead. The working one follows the circuit diagram as seen on Jim Lesurf's web site.

I'm wondering if the version with the 0.68 ohm resistors need the other resistors as well.
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Old 18th Jan 2022, 6:04 pm   #17
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Default Re: Armstrong 621 amp - flat high wattage resistors

I had removed my 621 in order to test another amp. As I had it out I increased the resistors to 0.68 ohms and adjusted the quiescent current to get just over .01 volts across them (20mA is quoted for the quiescent current). After being used all afternoon it is working well.
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Old 29th Jan 2022, 8:05 pm   #18
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Default Re: Armstrong 621 amp - flat high wattage resistors

Still motorboating at higher volumes despite having substituted 4700uf cap for the original 3300 one in the power supply.
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Old 30th Jan 2022, 10:03 am   #19
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Default Re: Armstrong 621 amp - flat high wattage resistors

Motorboating is LF oscillation, a 4700pf cap will effect high frequencies, you need to go the other way EG lots of micro farads and or resistance, but then you'll run into issues with your mains transformer. LF oscillation or motorboating is a complex phenomenon and is a right sod to cure.

Have a look at the grounding scheme, unsolder grounds and move them after first checking that all grounds present have good continuity. You could also try a bit more capacitance in your power supply, try tacking in a biggish cap at the correct rated voltage. Either way you could be in for hours of frustrating work, good luck.

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Old 30th Jan 2022, 10:52 am   #20
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Default Re: Armstrong 621 amp - flat high wattage resistors

Thanks Andy

Fortunately the motorboating only occurs at higher volume levels so I can probably live with it. I only noticed because the neighbours were away and I decided to let it rip! I put this down to the power supply being inadequate, maybe in view of the 2n3773s I used in place of the 2N3055s already there. Having said that it did it with only one speaker connected.

The amp had lots of work done to it before I got it leading to blown output transistors and damaged tracks on the amp board so messed up grounding is a distinct possibility. In view of the fact that it works at reasonable volume levels I think I will leave it in place for the moment and look at it further when the time comes to swap it out to give another amp some use.

Thank you
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