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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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15th Jun 2018, 4:33 pm | #1 |
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KB ORP10 "Tunetime"
On the conveyor belt tonight Bruce we have a nice little 1957 Kolster Brandes ORP10 "Tunetime" record player.
This record player is due a service and capacitor change. I have seen it working and I have already spruced up the case a little to take photos. I think it looks very cute in its red and cream polka-dot finish. More photos http://www.kbmuseum.org.uk/kb_images/orp10/orp10.htm This must have been another budget record player from KB, the record changer plinth is made from hardboard and is showing some signs of warping where the mounting springs are pushing down on it, I may need to look at strengthening it under the plinth. The rubber turntable mat is in reasonable condition, so often with this model of deck the rubber mat has gone all floppy and cracked or turned to stone and cracked. It has a mains isolating transformer and a single ECL82 valve with a selenium bridge rectifier. Mike Last edited by crackle; 15th Jun 2018 at 4:45 pm. |
15th Jun 2018, 4:49 pm | #2 |
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Re: KB ORP10 "Tunetime"
In terms of being "budget", well this one was a little higher up the scale. This was mainly due to a well designed 2 stage amplifier with NFB.
I had one and the sound was surprisingly good and loud from the 7" x 4" speaker. Of course, it's essential to play it with the lid closed! I would use some 5 ply reinforcement under the 3 springs.
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16th Jun 2018, 9:35 am | #3 |
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Re: KB ORP10 "Tunetime"
I have taken the amp and speaker chassis out and removed the PCB from the chassis board.
There were some capacitor types I have not come across before. They look like paper/wax types, but seem to be sealed with a solid "gel". But this 0.01uf one in the feedback circuit still showed excessive leakage at 350v, so it was changed. and the 0.05uF audio coupling cap in the pentode grid circuit also showed significant leakage, so it was changed. This little 0.01uf capacitor also showed leakage, probably not too critical in its position in series with the grid of the triode, but it was changed anyway. One other critical component to check is the cathode resistor, this is the bias resistor for the OP pentode. Sometimes these can go low in value and can reduce the bias voltage on the grid causing excessive current to flow in the valve. This one measured 469 ohms so still very good. The triple electrolytic is reforming nicely on a slow trickle charge via a 15k resistor. Total leakage for all sections is now down to less than 1mA. Another first for me is seeing a "foreign" made Brimar valve. The record deck plinth needs some reinforcing. It has already been repaired with Araldite at some time in the past. Not sure yet how I am going to tackle this, I will have to see what I have in the way of spare wood. I have marked with felt pen the batons that the plinth rests on. Mike |
17th Jun 2018, 7:45 pm | #4 |
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Re: KB ORP10 "Tunetime"
I have hit a few snags with the record player amplifier.
I got to the stage of testing the player, the cartridge looks like one of those Chinese ones, it only had one channel connected.. But that was not the main reason for the player sounding so rubbish. It was very quiet and the tone control was behaving very strangely. The cartridge, I installed a second pair of wires for the other channel. It was distorted when turned to treble and it basically the tone control acted like a second volume when turned to treble cut, practically all the audio disappeared to a muffled whisper. Voltage checks showed an HT of only 70 volts, so there was a problem with the metal rectifier. I have changed this for 4 1N4007 diodes and a 660 ohm resistor to get the HT back down to near 200 volts. There is a lot more volume there now using my DAB radio as a signal source. The on/off tone control pot is rated as a 500k inverse log. However the pot was measuring over 3.5M ohms. I dont think this is agreeing with the tone circuit. I have a 470k log switched pot I will give it a try. What is the difference between inverse log and log. The speaker had a few little punctures in the diaphragm, probably caused by a screwdriver being pushed through the grille. They were easily dealt with, no need for any tissue paper, just dab on some diluted PVA glue and push the fibres back together. I have made and fitted a reinforcing plinth out of 3 ply and glued it to the original plinth. I made recesses in the new ply plinth to fit the original supporting batons along the inside edges of the cabinet, so that it would sit flush at the same height as original. Mike Last edited by crackle; 17th Jun 2018 at 8:03 pm. |
17th Jun 2018, 8:22 pm | #5 |
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Re: KB ORP10 "Tunetime"
As I hinted at earlier, as good as a design for an ECL82 single-ended amplifier as you are liable to get anywhere. Even the original (optimal) Mullard circuit is little better, apart from the power supply. Note the large value coupling cap, the tone control arrangement and the NFB. No wonder these sounded good.
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17th Jun 2018, 8:47 pm | #6 |
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Re: KB ORP10 "Tunetime"
Log and Inverse Log are as they say, just as though the tracks were reversed. Tone controls can usually be Linear.
Log pots are for volume to match our hearing response. |
17th Jun 2018, 8:48 pm | #7 |
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Re: KB ORP10 "Tunetime"
I believe inverse log is the same as anti-log. Log pots change resistance quickly at the beginning of the track and then 'spread out' towards the end. Antilog (or inverse log) work the opposite way by changing resistance slowly at the beginning and then quickly at the end.
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17th Jun 2018, 9:36 pm | #8 |
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Re: KB ORP10 "Tunetime"
Would it matter if it was antilog for a tone control in this circuit, could you not just reverse the connections?
I expect the very high value of the pot would account for the strange behaviour of the tone control. I know on a radio when you turn the tone down you normally expect a little drop in volume, but this one went really quiet. I will fit my modern 470k pot in tomorrow and see how it goes. Thanks Mike |
17th Jun 2018, 10:02 pm | #9 |
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Re: KB ORP10 "Tunetime"
To me that's an odd circuit. Looking at the circuit I'm not surprised that it turns the volume down as well as alter the tone. But that's the way it's shown, so obviously original - you could modify the circuit.
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17th Jun 2018, 10:10 pm | #10 |
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Re: KB ORP10 "Tunetime"
I dont understand the reason why it is wired as it is. You would normally expect to see the 680k resistor connected to the grid.
Would I be right in thinking that this tone control relies to an extent on variable bias, so that as the slider gets near to the capacitor the bias is reduced and the valve amplifies more compensating for the drop in volume caused by the loss of the higher frequencies through the capacitor.. Mike |
17th Jun 2018, 10:20 pm | #11 |
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Re: KB ORP10 "Tunetime"
The bias is cathode/auto bias, the bias voltage on the grid with respect to cathode is the same as the voltage between cathode and chassis, no DC current is drawn by the grid circuit, the tone pot and the bottom capacitor just form a voltage divider for the signal, the signal voltage at the junction will be frequency dependant.
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17th Jun 2018, 10:32 pm | #12 |
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Re: KB ORP10 "Tunetime"
If the original tone control is anti-log (inverse log) it might be better to use a linear pot rather than a log pot.
The linear pot is a kind of 'half way house' between the two extremes. |
18th Jun 2018, 8:32 am | #13 |
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Re: KB ORP10 "Tunetime"
I have fitted a replacement tone control, and it works much better now, just a slight drop in volume as the treble is cut. Only one slight problem, I have obviously used this pot before and filed a flat on the shaft which does not align with the flat in the KB knob, resulting in the pointer being about 120 degrees out.
I refitted the little Chinese cartridge, and it actually sounds quite good now, both channels are now wired up together so I am getting a fuller sound. Here is a link to a demonstration of the player working. https://youtu.be/I5GYKAXlWiI Thanks for all you help and advice. Mike |