Technics Sp10 Woes

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Wolfgang Felber
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#46 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Thanks for that, nice to know other people are interested in my quest to get the SP10 going. I have to check if I am getting a 50K signal from the oscillator.
Wolfgang Felber
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#47 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick, OK checked the 50K which is supposed to be coming from C2, I am getting 29.23 v dc between C1 & C2, no reading as far as AC is concerned. this is with the lead from the motor disconnected, I have done this because, if I leave the motor leads connected, the driver transistors get very hot. I have removed the earth lead from the scope power cable to ensure there is no feedback through the earth on the probe. I have also ordered a isolation transformer so there is no more chance this happening. OK, between C2 and the ground I am getting 3.36 v DC but no 50K. I have attached a photo of the scope read out. One photo is the 3.5mhz coming from the crystal and the other photo is what is coming from the C2 and ground as well as a photo of the position of the test probe leads. Looking at the circuit diagram at C@, I should be getting 32 volts with a 10 volt peak to peak with180us. I dont appear to be getting that but this is with the motor disconnected, is that going to make a difference?
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#48 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

sorry forgot the photos.
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Nick
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#49 Re: Technics Sp10 Woes

Post by Nick »

Without the motor connected TR28 has no supply to its collector so you will need to have it connected and then power up, check, power down as quick as possible to avoid hurting the driver transistors.
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#50 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick, reconnected motor, still no 50k waveform but the scope reads 32.4 volts DC. I built a cheapie LC meter to measure the windings, they read as follows A3 = 34.24mh, A2 = 34.20mh A1 = 33.91mh. pretty close I thought. That was connecting AC to A1, A2 & A3.
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#51 Re: Technics Sp10 Woes

Post by Nick »

Is that at C2?

If it is, then I guess we need to look at T1 and TR28 as the 50k oscillator is self contained with that part and the motor winding between C1 and C2
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#52 Re: Technics Sp10 Woes

Post by Nick »

Checking, there is the test point connected to C2 specifically for checking the 50k Oscillator on the drive board.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#53 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick , I used the probe on the C2 and the ground on the metal part of the turntable base. This is the bit that I am not sure of, to make an oscillator you need a capacitor and a inductor, does this mean the T28 amplifies the signal produced by the oscillator? I guess what I am saying is should I check the capacitor C10 and C9? The other thing is, what do you do when you adjust the ferrite slug in T1? To make sure I don't short circuit anything anymore, I have removed the earth pin from the Scope power cable, I hope have done the right thing.
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#54 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick, tested C9 and C10, C10 was reading 8529pF and C9 was reading 9795pF. C18 was reading leaky in circuit with an ESR reading of 1.17ohms which looking at the book is OK. I will take it out of circuit.
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#55 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick, trying to work out what is going wrong, I tested T1 for resistance, one side is 0.3 Ohms the other side is 2.1O ohms the coil was reading 62uh on one side and 622uH on the other side.
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#56 Re: Technics Sp10 Woes

Post by Nick »

T28 provides the gain, the transformer provides the feedback and phase shift to create the oscillator. Looking at it, if you connected C2 to ground, its possible you damaged the set of coils between C1 and C2. That would stop the oscillator. I would check resistance between C1 and C2 (disconnected), but also make sure that its not shorted to any of the other motor coils.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#57 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick , are you taking about the coils in T1 or the coils in the motor, not the main coils, the 3 small coils at 120 degrees? If so, how do I test the tiny coils in the motor, just with DC resistance?
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#58 Re: Technics Sp10 Woes

Post by Nick »

Yes the motor coils between c1 and c2 check resistance and any connection between that and the metalwork or other coils.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#59 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick,C1 to C2 = 1.2 ohms with 2.08uh
PC to P1 = 9.1 ohms
PC to P2 = 9.3 ohms
PC to P3 = 9.4 ohms
T1 that forms part of the oscillator circuit, does that have a 1 to 1 ratio or a 10 to ratio? the reason I ask is I looked at other oscillator coils for radios and they dont have a 1 to 1 ratio, they vary from 1 to 10 or 13 or 35.
That DC reading between C1 and C2 seems low!
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#60 Re: Technics Sp10 Woes

Post by Wolfgang Felber »

Hi Nick, just a question, If I put the output of a signal generator at 50k across C1 and C2, I should get some voltage out of PC to P1 or 3. This will test the coils on the input side and tell me if I get something on the output. Correct? I am concerned that I may have damaged the very fine winding in the oscillator input coils when I connected C1 to C2 with the scope. What are your thoughts?
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