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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 22nd Jul 2015, 11:39 am   #1
'LIVEWIRE?'
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Question Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

The Capacitor C20 (A metal clad 2uf 260VAC axial type) has failed both electrically leaky (measures ca 79 ohms on my DMM)and physically leaky, on one of the above machines. It is wired in parallel with the motor windings. Would it be OK to replace this with a 275VAC Class 'X' type or the nearest available 'Motor Run' capacitor? Industrial Capacitors(Wrexham)Ltd make a 2u2 250VAC Axial polypropylene capacitor which would be suitable, but I cannot find a retail stockist of this part. CPC only list the 1uf version. BTW the motor is fed from the 100VAC tap on the mains transformer primary so a 250 or 275VAC capacitor would be adequately rated. The original has lasted since 1966!
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Old 22nd Jul 2015, 11:51 am   #2
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Hi,
Couldn't you put two of the 1uf versions in parallel if all else fails?
Stuart.
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Old 22nd Jul 2015, 12:05 pm   #3
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

I could do that, Stuart,but at over £5 each from CPC, I'd rather not!
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Old 22nd Jul 2015, 7:22 pm   #4
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Since posting the above, I've found a 2.2uf Capacitor, type SA2u2J630V, in one of my spares boxes. These are (were) sold by Maplin as 'audio grade; This is rated at 630vDC, which should be, IMHO, more than adequate, since the motor runs from 100-120VAC. I've fitted this cap., but haven't yet reconnected the deck mechanism to the amplifier, but will do so sometime tomorrow. Hopefully the recorder will then work OK
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Old 22nd Jul 2015, 7:51 pm   #5
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

you'll need a metallised polypropylene ideally. If it's audio grade it may be polyester in which case it won't last long as a motor run capacitor. It will normally be in series with one of the windings. If it's in parallel it's a pfc capacitor which wouldn't be normal on a tape motor.
A motor run capacitor may well have more voltage across it than the supply.
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Old 22nd Jul 2015, 11:40 pm   #6
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

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Originally Posted by McMurdo View Post
A motor run capacitor may well have more voltage across it than the supply.
Absolutely! One of the safety tests we have to do when faced with a motor run cap is ensure that the capacitor is rated at 1.5 x the supply voltage. 630V DC rating is about 250V AC
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Old 23rd Jul 2015, 12:44 am   #7
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Hi,
The 1710 has two motor run capacitors, the 2uF you're trying to replace and a 0.5uF. The 0.5uF should also be replaced as it has a high failure rate, especially considering its age.
As already stated by others, the 'audio grade' 2uF you have is not suitable as a replacement.


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David
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Old 23rd Jul 2015, 8:43 am   #8
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Thanks, everyone for your answers. Two points (1) as it happens I had aready replaced the 0.5uf cap. about 3 years ago with a Lorlin 1000v type. (2) As I previously noted, the motor is run from 100vac on 50Hz & 120vac on 60Hz, and the original capacitor, a metal cased MP Type is (was) only rated at 260vac, and it has been doing it's job for 49 years!
By my calculations also, a 630v(DC) rated capacitor should be good for up to >400vac. However, if you all feel that the cap. I've fitted isn't suitable for this purpose, I'll look for a suitable alternative before powering up the deck, and, in the meantime, look up the data for the SA2u2J630V capacitor out of interest.
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Old 23rd Jul 2015, 9:30 am   #9
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

As far as I can establish, the SA2u2J630V is a Polypropylene Capacitor designed for 'high power crossover networks'
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Old 23rd Jul 2015, 10:50 am   #10
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

2uf 440V motor run capacitors are £3.49 from Maplin http://www.maplin.co.uk/p/2f-440v-mo...apacitor-n38kg

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Old 23rd Jul 2015, 10:57 am   #11
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

I did think of using one of those, David. I have a couple here somewhere, but can't find them or remember their capacitance. Fitting it may be a problem, as the original is bolted to the motor case with an aluminium clamp bracket, but it may be possible.
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Old 23rd Jul 2015, 11:46 pm   #12
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

I've replaced these using the appropriate capacitors from RS, used the original capacitor clamp
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Old 24th Jul 2015, 12:58 am   #13
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Hi,
I often use these as replacements for motor run capacitors when repairing reel to reel decks. They are especially designed for this task, (phase correction or motor start/run). I'm in Australia, so I source these locally from AZtronics or Altronics.


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David
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Old 24th Jul 2015, 8:27 am   #14
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

David (dmaillard) I've not come across those on any UK stockists website, however I'll check out RS , CPC, and possibly eBay, to see what is available.
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Old 24th Jul 2015, 8:49 am   #15
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Michael, which exact RS Motor run Capacitor(s) do you use? AKAI specify a 2uf 260vac 'MP' type, which is a metal can capacitor with a tag at either end (i.e. basically an axial type)

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Old 24th Jul 2015, 6:11 pm   #16
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

You will find that the smaller 0.5uF capacitor is switched into the circuit when you select 50Hz operation.

That being the case, you could fit a 2.5uF capacitor such as this one or this one
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Old 24th Jul 2015, 7:20 pm   #17
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

I may do that, Michael. The 0.5uf, which is indeed switched into circuit for 50Hz operation, could then be removed, since the recorder is unlikely ever to be used on a 60Hz supply.
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Old 28th Jul 2015, 10:10 pm   #18
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Default Re: REplacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

Quote:
Originally Posted by 'LIVEWIRE?' View Post
By my calculations also, a 630v(DC) rated capacitor should be good for up to >400vac.
That is definitely not right. It's always best to consult the data sheet, but in practice a factor 2.8 (actually 2 times the square root of 2, or 3 for easier calculation) is realistic which would rate a 630VDC capacitor at around 225VAC. Since the motor is run on 100V, a 150VAC capacitor is the minimum required (the factor 1.5 that was mentioned before).

Conclusion: a 630VDC capacitor would be fine, however it needs to be an MKP (polypropylene) intended for motor run or pulse use. Using one of the capacitors that Michael Maurice links to, would virtually guarantee that even after all other parts in the deck have broken down, the capacitors will still be fine.

P.S. While an X2 capacitor will withstand the tension, it will also not last too long and go open circuit sooner or later. These capacitors are specced at the minimum they can get away with for testing, and mostly not intended to carry current, unless they were especially constructed for that (and even then they might not handle the relatively large motor current very well).

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Old 29th Jul 2015, 9:08 am   #19
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

I've now fitted a 2.5uf 440vac Motor run capacitor, so should have no further problems with the motor. As to the calculation I've always used a round figure of DC divided by 3 = AC Volts, based on the fact that peak to peak AC =2.828 x RMS AC, thus a 630vDC Capacitor should be good for 210vAC. On this basis the 440v Motor Run cap. should never fail. I've yet to reassemble the 1710, having also replaced the main reservoir/smoothing can on the grounds that, although it reads close to nominal capacvitance, and has a low ESR on a tester, it's 50 years old, and could fail at any time. On this basis maybe I should replace all the other electrolytics in this valved machine. Fotunately there are no 'waxies'. All the coupling/decoupling caps. appear to be ceramic disc types, so should be OK.
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Old 7th Aug 2015, 8:51 am   #20
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Default Re: Replacing Motor Run (?Motor Start?) Capacitor in AKAI 1700

The Machine is now 'up and running', but the rod which links the rec/PB switch knob to the actual rotary switch keeps dropping out, and it's a ***** to refit with the deck screwed to the amp/speaker assembly. The rod doesn't appear to be bent, and the two ends are shaped as shown in the service manual - i.e. one end formed into a hook, and the other having the last 0.5" at right angles to the rest of the rod. Any tips on how to (a) get it into place, and (b) keep it there gratefully received!
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