1% capacitors source UK

It is for the repair of a Revox A77 (reel to reel tape recorder circa 1970) capstan servo. It uses a series LC discriminator with 1% caps to set the motor frequency. The inductor is adjustable but since the fault is both speeds running high I suspect the caps have drifted - anyway I can replace them but the L is no longer available.

I suppose it is possible the inductor core has moved but I am reluctant to adjust the core of such an old component.

Ian

Later version use and RC circuit and a 555 timer ;-)

Ian

"Eeyore" wrote in message news: snipped-for-privacy@REMOVETHIS.hotmail.com...

?? The gm of tubes usually increases for the initial period of operation. Then it slowly drops over the life of the tubes. I assume they tested the sets with brand new tubes.

I assume you have the schematics for the A77?

It is possible the inductor slug has been moved but the inductor may be in good condition. The symptoms suggest a single common component as the first choice cause, rather than the caps, so the inductor would be a good culprit. In the A77 era it was usual for close tolerance caps for applications such as this to be of the axial polypropylene type, and in my experience these were very reliable. Since both speeds are similarly affected it is unlikely that ALL of the 1% caps have drifted in the same direction, which would be the case if speed is high in both cases. Have you tested the 3.5uF motor cap? Also, if you have an oscilloscope that would very soon tell you if the frequency sync circuit was working correctly and would also allow you to correct it.

My thoughts too btw.

Graham

Yup, complete service manual.

Agreed.

Except two caps are in circuit all the time and a third is switched in for the lower speed so it could be caps.

Agreed

See above.

No, but I will now.

Indeed. I have various things to check out like the sensor spacing for example. The only problem with that is this board is rather inaccessible unlike everything else on the A77. My manual conveniently ignores removal of this board.

Ian

I tend to agree but this recorder is nearly 40 years old.

Ian

It's rather rare for good non-electrolytic capacitors to drift in value. Get leaky, sometimes.

I'd first check the electrolytics for leakage and dissipation factor. Quite often those nice-looking German electrolytics will dry up and become poor resistors.

looking at the schematic, if Q209 or the capacitor by it short out, you'll get high speeds. Much more likely than the capacitors drifting.

Also there's a 10uf capacitor on the driver transistor base-- just a teeeensy bit of leakage there and the speed will go wheeee!!

And there's those 250uF capacitors on the discriminator output-- electrolytics don't like working at such low voltages-- they're likely to be leaky or open by now.

Also curious circuit, with a 0.022uf "1%" capacitor in parallel with a

"Ancient_Hacker" wrote in news: snipped-for-privacy@e3g2000cwe.googlegroups.com:

Ok, I'm tired, and not to fond of maths, so I'll ask.. Is this true?

I know that a 1M resistor has a 1% portion way bigger than the 1% portion of a 10K resistor, yet the standard advice is that to maintain a 1% tolerance in a network, ALL resistors in that network should be at 1%.

Isn't it the same for capacitors? If not, why not?

Yes the output of the discriminator is smoothed by a couple of electrolytics so that could well be a factor.

Ian

Yes it is probably worth me replacing all these.

Pass ;-) I suspect the designers retired long ago.

Ian

Standard advice can be wrong in some cases. It's okay if the values are close to each other. But:

Imagine putting a 1megohm 1% resistor in series with a 1K ohm 1% resistor.

The 1 megohm resistor could be plus or minus 10K.

The 1K ohm resistor could be plus or minus 10 ohms.

So you can see there's no point in specing the 1K resistor at 1% if the

Same with capacitors in parallel, the values add, so the smaller one need not be any better tolerance than their mutual ratio. i.e if the capacitors are 10:1 in capacity, the smaller one's toelrance can be 10 times the larger one's. (or vice-asrev)

"Ancient_Hacker" wrote in news: snipped-for-privacy@d34g2000cwd.googlegroups.com:

Ok, agreed, but it can't be any worse. :) I think the general advice makes sense, in that if you have a stock of parts you might not know exactly where any value might go in a new design till it exists, and if it's easier to buy a batch of 1% parts, that's what happens, it eliminates time consuming choices, and the designer may just specify a part that they know is in stock so that parts selection is correct.

Point taken re that capacitor though, unlike resistors, standard caps rarely come at 1%, so it's a bit odd seeing it specified there, but again, it might just be someone's way to identfy an exact part they had in stock at design time.