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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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20th Nov 2019, 3:54 pm | #1 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
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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. |
20th Nov 2019, 8:43 pm | #2 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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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 |
20th Nov 2019, 9:09 pm | #3 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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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 |
9th Dec 2019, 1:56 pm | #4 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
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Re: K.B discomatic white noise
Thanks guys.
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9th Dec 2019, 2:40 pm | #5 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
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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. |
8th Jan 2020, 1:27 pm | #6 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
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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. |
11th Jan 2020, 9:51 pm | #7 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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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 |
24th Jan 2020, 7:09 pm | #8 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
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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. |
24th Jan 2020, 9:41 pm | #9 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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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25th Jan 2020, 1:14 pm | #10 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
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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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6th Feb 2020, 2:24 pm | #11 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
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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. |
7th Feb 2020, 6:10 pm | #12 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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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. |