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Author Topic: Polytone 102 Tremolo  (Read 251 times)

ilyaa

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Polytone 102 Tremolo
« on: January 24, 2019, 02:02:30 PM »
finally got this amp almost back to life -

the power amp was busted and, because i could not find a schematic that seemed to match the amp i was looking at, i figured id just replace it with a simple class D thing. got a kit online and wired it up - it even fit the heatsink with no modifications necessary! - and the amp is working fine.

last step: the effects. the reverb is a bit weak (i think i can handle that), but the tremolo is not working at all. the good news is i found the right schematic! the bad news is, i really cant understand how the tremolo circuit is supposed to work - which obviously makes it hard to troubleshoot.

the right schematic is attached - can anyone with a bigger brain than mine explain how this tremolo is supposed to work? and whats that symbol by the LDR?


tonyharker

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Re: Polytone 102 Tremolo
« Reply #1 on: January 24, 2019, 02:37:54 PM »
The symbol is the Greek letter Lamda which indicates the resistor is light dependent. The resistor in a circle above R53 must be a light source (48v lamp?).  This is coupled optically to R16 which is a Light dependant resistor. 
« Last Edit: January 24, 2019, 02:44:00 PM by tonyharker »

Jazz P Bass

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Re: Polytone 102 Tremolo
« Reply #2 on: January 25, 2019, 12:44:21 AM »
The IC U2B is set up as a self running low frequency oscillator.

Look above it at IC U3B, pin #9/ R20 also has the Lambda symbol.
I take it that R20 is the LDR.

U2B's oscillating output will allow a change in the resistance of R20.
There is your Tremelo.
« Last Edit: January 25, 2019, 11:56:34 AM by Jazz P Bass »

lapsteelman

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Re: Polytone 102 Tremolo
« Reply #3 on: February 01, 2019, 12:04:00 AM »
The circuit with the photo resistors  (U3A and B)  looks more like a simple phase shifter...   The first thing I would check is to make sure the lamp is flashing.. perhaps it's as simple as a burnt out lamp?
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ilyaa

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Re: Polytone 102 Tremolo
« Reply #4 on: February 01, 2019, 01:29:09 PM »
thanks, guys!

lamp was on, but no oscillation -

replaced C45 and C46 - the phase shifting caps - and were back in business!

those were weird old black boxy caps - prob dried out just enough to not do their job in this application -

vibrato sounding great!

had to reduce the gain on the reverb a bit - it was feeding back - but amps put together and back in action!

thanks again!

 

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