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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 3rd May 2020, 11:10 am   #21
Paulus.d
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Default Re: Lockdown project - Sony TC-200

Thanks for that advice.... I dont have a capacitance meter just a little yellow plastic cheap DVM.... Not having the caps here is gonna be a pain..
One thing I will do is to reinstate the original C1...I can use the new ones to replace C3 unless you think this could be a bad call... Iwouldn't wanna find that one as being the issue and not do it..
Had a sudden thought.. as you go through the diagram, the voltages get lower and lower, at the preamp stage R101 is 42volts so that volt drop would be more crucial?
Also it's not a direct short or maybe it would kill the volts to nothing ?
Regards Paul
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Old 3rd May 2020, 12:03 pm   #22
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Default Re: Lockdown project - Sony TC-200

C3 idea sounds OK. Have you tried removing the valves to see what effect it has on the HT voltage.

Are you sure it is R101, my reading of it suggest that there should be little if any DC voltage at R101 ?

The mains voltage selection switch is correctly set at 240 ?
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Old 3rd May 2020, 12:42 pm   #23
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Default Re: Lockdown project - Sony TC-200

Yeah main voltage okay ....
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Old 3rd May 2020, 2:07 pm   #24
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Default Re: Lockdown project - Sony TC-200

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Originally Posted by DMcMahon View Post
For C108/C208 if you do not have any immediate replacements you could just measure the DC voltage at their connections to the control grid pin 5, should be very small DC voltage present if caps are good. They are not listed as paper but could still be leaky.
Theoretically there should be no +ve voltage at the control grid maybe with small negative biasing voltage present.

Do you know why the output transformer is missing, could the grid coupling capacitor (C108/C208) have previously shorted/passed DC and taken out the transformer.
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Old 3rd May 2020, 2:15 pm   #25
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Default Re: Lockdown project - Sony TC-200

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Originally Posted by Paulus.d View Post
Thanks for that advice.... I dont have a capacitance meter just a little yellow plastic cheap DVM.... Not having the caps here is gonna be a pain..
One thing I will do is to reinstate the original C1...I can use the new ones to replace C3 unless you think this could be a bad call... Iwouldn't wanna find that one as being the issue and not do it..
Had a sudden thought.. as you go through the diagram, the voltages get lower and lower, at the preamp stage R101 is 42volts so that volt drop would be more crucial?
Also it's not a direct short or maybe it would kill the volts to nothing ?
Regards Paul
Yes agree does not sound like a hard/dead short.
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Old 3rd May 2020, 2:21 pm   #26
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Default Re: Lockdown project - Sony TC-200

The way i's been removed, I'd say the transformer was taken for something else... For my peace of mind,I will switch it over to channel 2 just to see if that works in the same way as one. I've already swapped the valves over from 1 to 2 ...

I am a bit of a lay man so not quite sure what grid is lol ...
Reards Paul
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Old 3rd May 2020, 2:31 pm   #27
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Default Re: Lockdown project - Sony TC-200

What about R101 ?
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Old 3rd May 2020, 2:34 pm   #28
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Default Re: Lockdown project - Sony TC-200

R101 is on the aux in.... my bad .. I meant R5 33k ...the voltage there 42v (35v)
If that drops the head pre amp would die first ?
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Old 3rd May 2020, 2:49 pm   #29
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Default Re: Lockdown project - Sony TC-200

Very simplistically a grid is one of the electrodes in a valve, there being 3 different grids in a Pentode valve like your output valve. The Control Grid (pin 5 in your valve) is where the AC audio signal connects to (via C108/C208) and it controls the amount of electrons (current flow) from Cathode to Anode (the Anode being the output signal from the valve) to amplify the signal.
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Old 3rd May 2020, 3:15 pm   #30
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Default Re: Lockdown project - Sony TC-200

The design of the circuitry is such that the main HT (High Tension) voltage at the first reservoir capacitor C1-1 is progressively reduced/dropped by resistors, so that various parts of the circuit receive the correct HT voltage.

So the input to R5 is quite a bit lower than output at C1-1.

Difficult to know if the lower than normal/schematic voltage value at R5 would result in a very quiet output or how much effect it might have on the pre-amp gain.

Is your 42V figure the input to R5 (RHS on schematic) and 35V the output side (LHS on schematic) ?
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Old 3rd May 2020, 3:37 pm   #31
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Default Re: Lockdown project - Sony TC-200

The Control grid on the output valve may be pin 2, pin 5 may be the cathode, pins numbers 2 and 5 are shown next to each other on the schematic.
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Old 3rd May 2020, 3:50 pm   #32
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Default Re: Lockdown project - Sony TC-200

Looking at pin out of a similar valve looks like the Control Grid is both pins 2 and 5 (internally connected in the valve) and the cathode is pin 1.
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Old 3rd May 2020, 3:58 pm   #33
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Default Re: Lockdown project - Sony TC-200

The 42v is on the left side, output ....as the imput to the transistors...

Looking over the circuit board, found R115 quite discoloured ...so tested C109..i pulled one end out of the board, It's not shorted and R115 is still reading 150... the channel 2 version R215 is normal ....
Thought I'd found the culprit then....
Regards Paul
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Old 3rd May 2020, 4:58 pm   #34
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Default Re: Lockdown project - Sony TC-200

If the HT's low on playback it might also be worth checking the main HT filter resistors shunt resistor (R3) and the relevant contacts on the Rec/PB switch.

Lawrence.
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Old 3rd May 2020, 6:26 pm   #35
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Default Re: Lockdown project - Sony TC-200

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If the HT's low on playback it might also be worth checking the main HT filter resistors shunt resistor (R3) and the relevant contacts on the Rec/PB switch.

Lawrence.
It's not low from the start but goes low after 30-60 seconds ... I've checked various points on the PCB , the low happens at all,except for output valve pin 7
Paul
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Old 3rd May 2020, 6:41 pm   #36
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Default Re: Lockdown project - Sony TC-200

Okay , game changer..... the loss of output and lowering of HT are NOT related...
I was reading on pin 7 of the output valve... it was 182v ..playing started then the volts went down to 113v ..it was STILL playing....then it slowly died.....
That means caps and HT are probably okay ...
Paul
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Old 3rd May 2020, 6:46 pm   #37
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Default Re: Lockdown project - Sony TC-200

Also ..even in low volume state...the volume control is still active ...
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Old 3rd May 2020, 7:03 pm   #38
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Default Re: Lockdown project - Sony TC-200

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Quote:
Originally Posted by ms660 View Post
If the HT's low on playback it might also be worth checking the main HT filter resistors shunt resistor (R3) and the relevant contacts on the Rec/PB switch.

Lawrence.
It's not low from the start but goes low after 30-60 seconds ... I've checked various points on the PCB , the low happens at all,except for output valve pin 7
Paul
That seems to contradict what you measured in Post#12 so far as the anode voltage goes.

Best to measure the relevant voltages in playback and post the results....

...V102 anode (pin 7) screen grid (pin 6) control grid (pin 5) and cathode (pin1)

Also the voltages at points labeled 3 & 4 on the schematic.

Fault aside, just so you know, the HT voltage at switch on will be very high compared to the voltages given in the schematic, that's due to the fact that a solid state rectifier is used, at switch on the reservoir capacitor C1-1 will charge up almost to the peak voltage of the rectifiers AC input voltage, the peak voltage is the AC voltage (RMS) multiplied by 1.414, you can see on the schematic that the AC voltage into the rectifier is given as 130 Volts, that multiplied by 1.414 equals approx. 184 Volts, that voltage will reduce over time as the valves warm up and start to conduct.

Without the other output transformer fitted the screen grid of V202 should be isolated from its supply.

EDIT: Post crossed.

Lawrence.

Last edited by ms660; 3rd May 2020 at 7:16 pm. Reason: addition
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Old 3rd May 2020, 7:24 pm   #39
Paulus.d
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Default Re: Lockdown project - Sony TC-200

okay here's what I measured...

Pin 7 ....154v
Pin6 ....stablised at 116v
Pin 5 ..... 0 but hums when touched by the probe
Pin 1 ....7.2v
point 3 ....9.7v
point 4 ...... 77v

Paul

Last edited by Paulus.d; 3rd May 2020 at 7:34 pm.
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Old 3rd May 2020, 7:33 pm   #40
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Default Re: Lockdown project - Sony TC-200

Either side of R5.

Lawrence.
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