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KB RT20/2 Tape Recorder

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Anonymous
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Hi
Happy new year to all you Vrats

I have just finished reconditioning a KB RT20/2 Tape Recorder, last of the "Christmas" pressies.It is all working both on record and playback but the audio on playback is extreamly "thin" there is enough volume but there is a distinct lack of base. Am i correct in thinking this may be caused by the bias going out of adjustment. I have attached the instructions for adjusting the bias. Please can someone confirm if a DMM will sufice to measure the 45v and is 45v DC or AC and if AC is this RMS or peak. What is the "diode probe" it refers to, will I need to make this. An Acos crystal microphone came with this recorder can you still get crystal inserts like the one in the picture.

rt20 3
rt20 2
rt20 1
 
Posted : 01/01/2013 12:06 pm
Anonymous
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I don't think the Bias makes the sound "thin". Too little and it's distorted, too much and you loose dynamic range,

A small ceramic beeper disc will work as a microphone, though not as sensitive (tested). A "Franken" combo of a record player cartridge insides and diaphragm off an earphone bud might do something (not tested).

Check for a dry joint / open circuit component /short circuit capacitor / dirty switch in the equalisation network.

Have you a test recording off something else to be sure it's a record fault?

 
Posted : 01/01/2013 12:26 pm
Anonymous
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Quoted by Anonymous

A small ceramic beeper disc will work as a microphone, though not as sensitive (tested). Check for a dry joint / open circuit component /short circuit capacitor / dirty switch in the equalisation network.

Have you a test recording off something else to be sure it's a record fault?

Hi 
Do you mean the small circular sounders you get in kids toys? I have changed all the capacitors except the clear polystyrene ones.

My other decent tapes are 7 inch and are slightly too big to fit on this machine and were recorded on a 1/4 track. The other 5 inch ones I have are very poor quality with lots of drop outs, I suspect the oxide is breaking down. I guess that C12 will make a difference to the base on record, I have not changed this.

rt20 4
 
Posted : 01/01/2013 12:38 pm
Anonymous
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the bias is AC may be 100 k/c hence the need for a diode probe. the bias is added to the record signal to stop saturation of the record head. if you have a scope use that. if you are useing tapes not recorded on this deck the heads may not be in alighnment.
use one of your tapes with dropouts and make a recording with no input. this should wipe the tape if it does it surgests that the bias ocs is working. dirty heads will cause muddy sound with a lack of bass so it sounds as if your heads are ok. to recap .bias is only used in the record mode .and will not effect playback of tapes made on another m/c. if the speaker is ok check by conecting another one,try cleaning the rec/playback switch.(some of these decks use a clever circuit that uses the audio output valve as the bias osc. last resort check the Azimuth adjustment.
rob t

 
Posted : 01/01/2013 1:06 pm
Refugee
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Provided your DMM can see 100Khz on its AC ranges you would not need to worry about AC probes. 

 
Posted : 01/01/2013 1:28 pm
Anonymous
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Do you mean the small circular sounders you get in kids toys?

Umm. They are in toys, some phones, greeting cards, alarms etc and available separately very cheaply. Piezo-ceramic disc bonded to a larger brass disc. I can probably post one in an envelope letter post if you can't find one.

They are also used in "Crystal" earpiece instead of original foil and "crystal" glued to centre.

Wide range of diameters. Some have a separate area for a 3rd feedback connection to make a "beeper" just by adding a transistor and resistor. Just ignore the smaller patch and connection. I've experimented with cutting a strip and using it as a mechanical sensor.

 
Posted : 01/01/2013 1:40 pm
Anonymous
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Hi
I checked the bias voltage with an oscilloscope and it was 42v I could not get it any higher. There was very little variation when I adjusted the core of the coil. I left it at the peak voltage, 42v. I changed the cathode bypass capacitors on V3A & B. The quality is much the same, I am wondering if there has been a deliberate attempt to boost the mid to high frequencies to compensate for the characteristics of the tape transport and heads. Maybe I am expecting too much from it. If you turn the tone control ¾ way round it is not too bad a balance of frequencies. Anyway it is back together now and put away in my collection, under the bench, run out of room for large items.

 
Posted : 02/01/2013 9:21 am
Anonymous
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Lack of bass on playback is not likely anything to do with the bias, as this only affects recording. have you tried playing a known good tape ?

Bear in mind these were bargain basement machines when new, so sound is going to be fairly ropey...try an external speaker , maybe the internal one has been damaged.

 
Posted : 17/01/2013 4:23 pm
Anonymous
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I'm trying to refurbish my dad’s old KB RT20 tape recorder. Does anyone have photos of the tape mechanism? There seems to be a spring missing on the right hand side that controls the tape tensioner arm.

 
Posted : 08/06/2019 3:06 pm
crustytv
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Hi and welcome to Vrat, here's an exploded plan of the deck, I've marked the four springs on it. A102123, A102493, A103011, A103025.

If you click on the image it will open in a viewer, then right click the image and select "open image in another tab" you will then able to click the mouse (+) to max size view the image.

rt20

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Posted : 08/06/2019 3:55 pm
Nuvistor
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Although very simple and not HiFi these decks gave very little trouble in the 60’s before Compact Cassettes took over.

The main failure was the spring on the left hand lever A102498 on the top left of the diagram, obviously not for the tape tension.

Frank

 
Posted : 08/06/2019 4:17 pm
Terrykc
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Yes, the toggle spring, as it is called. Of course, when the deck is viewed in the conventional manner, from the front, the spring is on the right hand side. When it breaks, the Stop/Play/Fast (re)wind knob just flops around.

No major dismantling required, just remove the knobs and the plastic cover - the spring sits on top of the deck.

Unless someone has got a stash hidden away somewhere, you'd probably have to make a replacement yourself, or get someone else to do it. Hopefully, if the deck hasn't been got at, the two parts needed for a pattern will still be under the cover.

We used to get all of our BSR spares from N. Myers in Theydon Bois but can't find a trace of them now. (I was wondering if they might still have a stash!)

 

When all else fails, read the instructions

 
Posted : 09/06/2019 11:48 am
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