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Old 20th Nov 2019, 3:54 pm   #1
vampyretim
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Default K.B discomatic white noise

Hi all,

I'm trying to reduce the hiss on a K.B discomatic and I read somewhere on this website that the hiss was reduced by replacing the first two OC75's for another type of transistor. Would anyone recommend a suitable replacement?
Has anyone any experience with this type of amp?

Many thanks.
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Old 20th Nov 2019, 8:43 pm   #2
crackle
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Default Re: K.B discomatic white noise

Here is a list of OC75 equivalent transistors, https://alltransistors.com/crsearch....=0&r2=0&r1r2=0
But have you tried swapping the transistors around, if the noisy one is in TX1 position then move it to TX2.
Mike
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Old 20th Nov 2019, 9:09 pm   #3
Ed_Dinning
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Default Re: K.B discomatic white noise

Hi Tim, some of the resistors can get noisy, a quick squirt of freezer spray or a blast with a hot airgun can identify the culprits

Ed
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Old 9th Dec 2019, 1:56 pm   #4
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Default Re: K.B discomatic white noise

Thanks guys.
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Old 9th Dec 2019, 2:40 pm   #5
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Default Re: K.B discomatic white noise

Try whatever you have to hand. You may well find even silicon types will work.

An AC126 should be a perfect sub if you have any.
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Old 8th Jan 2020, 1:27 pm   #6
vampyretim
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Default Re: K.B discomatic white noise

Hi all,

I've replaced the two front end transistors for silicon types yet am still getting really bad hiss. I also need to replace the 50 ohm lin bias pots but am having real difficulty finding a replacement.

Any help much appreciated.
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Old 11th Jan 2020, 9:51 pm   #7
crackle
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Default Re: K.B discomatic white noise

See Post #30 of this thread, which shows my solution to the trimmer pots.
https://www.vintage-radio.net/forum/...d.php?t=131587

Mike
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Old 24th Jan 2020, 7:09 pm   #8
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Default Re: K.B discomatic white noise

Hello all,

Just a little update. I'm working on three of these at the moment and they all hiss terribly!

I have managed to bring the hiss down in one of them to very tolerable levels.

I decided after trying several different types of transistors including silicon ones to keep the original oc75's in place, the silicon ones made no difference to the hiss. I then decided out of frustration to replace all of the carbon composite resistors in the preamp for 1/4 W modern types (I think they're carbon film). I still had an annoying hiss until I replaced the negative feedback r21 2.7k resistor.

It now sounds great!

All electrolytic caps have been replaced and I'm quite impressed how punchy these little amps are. I wouldn't advise replacing the oc75's and I'm going to test the other two that I'm working on and see how little I can replace to reduce the hiss.

Thanks for everyones help.
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Old 24th Jan 2020, 9:41 pm   #9
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Default Re: K.B discomatic white noise

Congratulations, sounds like your persistence has paid off. A good example of why new components are better than old no matter what some say!
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Old 25th Jan 2020, 1:14 pm   #10
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Default Re: K.B discomatic white noise

The OC75 is just a low noise OC71. I agree there's no point in changing them if they're not faulty.
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Old 6th Feb 2020, 2:24 pm   #11
vampyretim
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Default Re: K.B discomatic white noise

Hi guys,

I'm having trouble with the mechanics now. The thing seems to be stuck in record playing mode and I can't get it to go along its run to pick up a different record.

Has anyone any experience on the mechanical side?

Many thanks.
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Old 7th Feb 2020, 6:10 pm   #12
crackle
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Default Re: K.B discomatic white noise

I dont consider this item as very suitable for anyone inexperienced to restore, I know this because I have stripped one and restored it to working having not ever seen one before and had to ask for advice on many occasions. But I had restored quite a few record players and other mechanical music players.
There are many mechanical actions that need to take place in sequence in order for it to move along, select a record, load it, position the pickup, play the record, lift the pickup off the record, return it to the rack, and move on to the next selection.

Please can you be more specific as to what the player is doing and how far it is getting in its sequence of events whilst playing and at the end of playing a record.
If you dont have the service manual it is available on my web site. http://www.kbmuseum.org.uk/kb_servic...comatic_kb.pdf
It includes exploded diagrams of the mechanical parts so you can work out how the various trips work.

Have you read through my thread, see link in post #7 of this thread, there may be some clues in there.
It really is getting the mechanism on the bench and getting down close to peer at and see every little action at the end of a record.

Mike

Last edited by crackle; 7th Feb 2020 at 6:20 pm.
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