Cannot get Sinewave on my TD125 MKII speed control board

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#1
Hi

My TD125 MKII stop running and I found one of the op amp was not working. I replaced the op amps with two new ones (Old: FU6A7709393 New:LM709CM), I can adjust the voltage to the correct values for all 3 speeds but the motor only vibrates but not turning. So I borrowed a scope from my friend to check out the wave form and find the the wave is some what like the saw teeth on all speed.

I have disconnected the motor and disable the 2nd stage for easy troubleshooting and tried the followings. I still could not get the board to produce a good sinewave! .

1. Replaced the caps at the Wein Bridge.
2. Replaced the transitors tn the first stage.
3. Used one of the old op amp (working one) in the first stage.

I still cannot get a good sine wave produce from the 1st stage with the Wein Bridge. I am running out of ideas. Can anyone help? :bawling:

By the way, the lamp comes on ok!

Regards,

Alfred
 
#3
Hi Ashok,

Yes, the oscillator voltage stabilisation lamp ( La1) come on ok. I do have all the services manuals that are available on the internet already.

I need to find out why I am getting the saw tooth type of waveform instead of sine wave. I get the same waveform even when I put the orginal Op Amp (the one still working) back into the circuit. I have run out of idea! 😕

Best regards,

Alfred
 
#5
Hi Hugo,

I have replaced C2/C4 with two new ones (1000uf 63V) and I am getting good voltage reading from both multimeter and osciliscope!

For troubleshooting sake, I focus on the 33rpm portion and I have replaced C12 and C14 with two new 0.1uf already!

Alfred
 
#6
I whish I could be of more help.
In case like this I try to start from scratch. Confirm every transistor is good and in its proper position, check if the opamps are not reversed in the sockets, everything soldered well, trace the different voltages, measure every resistor, suspect the motor.
I'm pretty sure it's something stupid, constantly overlooked.
Your main question, why the sine wave is a saw-tooth, I can not answer.
I have a TD125 in front of me and I’m willing to measure some voltages but the circuit is not the same as the MKII

Keep us posted.

/Hugo
 
#8
Cannot get sinewave on TD125 MKII

Alfred - did you get it working? I have a similar problem but cannot source the LM709CMs over the net here in the UK - where did you find them and did you also find the output transistors?

By the way I used to live in Hong Kong - near Sai Kung until 10 years ago. How is the old place?

Brgds,

Quadrico
 
#9
Hi quadrico,

So you have a TD125 MKII as well! By the way, I have changed all the caps on the board but I still can get it to work! I do get a sinewave if keep the magnification low. Once turn up the magnification, it sinwave becomes a saw tooth!

Let me know if you need any help to source the op-amp!
 
#11
IC's and transistors

This could be really helpful - how much would they cost and how would we do this? I have some old HK$ in a drawer!

Thanks,

Quadrico

As regards the sawtooth - I have no ideas except that with the Wien bridge oscillator with minimal feedback control apparently you have to keep the oscillator just running to avoid harmonic distortion. What this means in practice with this design I don;t know but there are some useful ideas on www.lenco-lovers.com - follow TD 125 threads.
 
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