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Old 4th Nov 2019, 12:16 am   #1
Lef de art
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Default Different capacitors in the chassis and different values in the schematic !

Hello guys.
Recently i finished my first restoration in a 61 years old tube reel to reel recorder (Geloso G258) .
Thanks God the process ended positively and the recorder works again very good.

In this process i met a big problem by trying to figure what value in capacitors to use.

I say this because all the capacitors excluded the electrolitic had different values from the capacitors in the sassi !!!
Not one or two capacitors. It's about 12 capacitors !!!
For sure i have the correct schematic.

At the end i have used the values from the sassi and everything works perfectly.
Any idea why this happens?
I believed that the schematic always corresponding with the real machine.

I really need to hear your opinions .
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Old 4th Nov 2019, 12:54 am   #2
Maarten
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Default Re: Different capacitors in the sassi and different values in the schematic !

Circuits can be revised over time, and they don't correct schematics that are already printed. What's in the chassis, is

Small changes (50uF to 47uF for example) are usually because of availability, large changes are usually because of improvements to the design.
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Old 4th Nov 2019, 2:24 am   #3
Lef de art
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Default Re: Different capacitors in the sassi and different values in the schematic !

Thanks Maarten
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Old 4th Nov 2019, 12:13 pm   #4
Uncle Bulgaria
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Default Re: Different capacitors in the sassi and different values in the schematic !

Kalimera! Often in old equipment that has been repaired, one finds newer components. As Maarten says, availability plays a part as more modern parts have different values from older ones, because of changes in the spacing between values in each decade - the "E Series of Preferred Numbers".

In this case, especially small changes as between 50F and 47F, use the next available modern value. In other cases, it can make very little difference depending on where in the circuit the capacitor is used. In an RC filter or in a valve rectified power supply, there will be a requirement for certain values that can be obtained by calculation.

Remember that most capacitors have quite poor value regulation, so tolerances of +/-20% is normal, with variances that can be even greater depending on frequency and temperature.

That plan you attached is very small so it's hard to see it. Try exporting the file rather than taking a screenshot, or if the forum compression makes it too small I've had success linking files from Dropbox or similar.
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Old 4th Nov 2019, 12:34 pm   #5
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Default Re: Different capacitors in the sassi and different values in the schematic !

Errr.....


What's a "sassi"?
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Old 4th Nov 2019, 12:46 pm   #6
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Default Re: Different capacitors in the sassi and different values in the schematic !

I am guessing its a Chassis.

Cheers

Mike T
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Old 4th Nov 2019, 12:47 pm   #7
GRID LEAK
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Default Re: Different capacitors in the sassi and different values in the schematic !

I think he means chassis.
John
Oops. Crossed with Mike!!

Last edited by GRID LEAK; 4th Nov 2019 at 12:53 pm. Reason: crossed post
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Old 4th Nov 2019, 12:49 pm   #8
Uncle Bulgaria
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Default Re: Different capacitors in the sassi and different values in the schematic !

There is no 'sh' in Greek, though sigma sounds more like a sh to English ears which is where the confusion has probably arisen.
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Old 6th Nov 2019, 1:10 am   #9
Lef de art
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Default Re: Different capacitors in the sassi and different values in the schematic !

Sorry for my bad English guys
I mean chassis.
Vary bad spelling by me but you guess correctly at the end.
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Old 6th Nov 2019, 11:39 am   #10
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Default Re: Different capacitors in the chassis and different values in the schematic !

Thanks everyone
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Old 6th Nov 2019, 9:24 pm   #11
Lef de art
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Default Re: Different capacitors in the chassis and different values in the schematic !

Comparing the sound i have now with the sound i had before changing the capacitors i have the filing that i have lost enough bass, low frequencys.
As it is normal in the replacement capacitors process it wasn't easy to find the exactly value in capacitors. So some changes was made...
There is any chance the following replacements to be wrong?
What is your opinion?

0,005uf / 150v replaced with 0.0056 250v

0,002uf / 400v replaced with 0,0033 400v

0,025uf / 300v replaced with 0,027/ 400v

0,5uf / 150v replaced with 0,56/ 250v


Or this differences are not important?
Thanks in advance guys
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Old 6th Nov 2019, 10:10 pm   #12
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Default Re: Different capacitors in the chassis and different values in the schematic !

Those replacements look OK. In practice .005 can be replaced by .0047 or .0056 and .002 by .0022, .025 by .022 or .027 and 0.5 by 0.47 or 0.56. In each case you have gone slightly higher which won't make any difference.
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Old 7th Nov 2019, 11:35 am   #13
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Default Re: Different capacitors in the chassis and different values in the schematic !

I don't understand then why the recorder sound has lost so many high frequencies
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Old 7th Nov 2019, 11:56 am   #14
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Default Re: Different capacitors in the chassis and different values in the schematic !

Have you checked the resistor values?
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Old 7th Nov 2019, 12:55 pm   #15
Lef de art
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Default Re: Different capacitors in the chassis and different values in the schematic !

The chance in high frequencies started after capacitor change. Can't be from resistance
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Old 7th Nov 2019, 1:35 pm   #16
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Default Re: Different capacitors in the chassis and different values in the schematic !

It is likely that some of the resistors have become unstable after such a long time. In the amplifier, I expect some are used in RC filters. If you've got used to the sound before you changed the capacitors, and the resistors are unchanged, it is a possibility that the frequency filters made from resistors and capacitors have now changed value. The result would be as if a tone control were continually in operation. I think it would be worth measuring some of the likely components, as changing old parts for new can remove the mask that concealed other problems!
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Old 7th Nov 2019, 4:57 pm   #17
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Default Re: Different capacitors in the chassis and different values in the schematic !

Another possibility however...are you ABSOLUTELY CERTAIN that the replacement capacitors have been connected correctly? However careful you may have been, it's possible that one capacitor has been connected to a wrong place....worth going over everything to make sure....we've all done it!
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Old 7th Nov 2019, 8:53 pm   #18
Lef de art
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Default Re: Different capacitors in the chassis and different values in the schematic !

Yes i have double checked it. Maybe im was unlucky and one of capacitors does not work correctly
All the problem is in recording high frequencies.
Too much treble and no bass
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Old 8th Nov 2019, 11:10 pm   #19
Lef de art
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Default Re: Different capacitors in the chassis and different values in the schematic !

Ooooooo my bad English

Low frequencys i have lost not high !!!

The new recordings they don't have bass

Sorry for the confusion.... I really try with my English
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Old 9th Nov 2019, 5:54 pm   #20
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Default Re: Different capacitors in the chassis and different values in the schematic !

Your English is perfectly comprehensible - you should hear our Greek!

Have you checked the resistors for any out of tolerance yet?
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