Author Topic: That Belton Brick oscillator LFO  (Read 595 times)

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anotherjim

That Belton Brick oscillator LFO
« on: July 10, 2016, 04:57:59 AM »
Been puzzling over the Schmitt-trigger oscillator in the Belton brick scheme, armed with the knowledge that the patent doc speaks of a Triangular wave at 1Hz rate being used. Now I've read several threads here and there about it, but nothing that pins the facts down.
Using the values published by the reverse engineers, I made the oscillator. No 74HC14 so I used 40106. That's with 3M3 feedback and 50p capacitor (OK, I used 47p).

The frequency is about 14Khz!
There seems to be doubt that the timing cap is 50pF, but I found it needed to be greater than 100nF to get into sub 10Hz LFO territory.
In the Brick, with is fed to an integrator/low-pass formed from op-amp pins 13,14. Rf=15k, Cf=560p.
I make the cut-off of that about 19Khz.
The clock of the effected delay is set with 3k24. If the PT2399 VCO chart was to be believed, the Clock VCO would be 10Mhz
The amount of modulation the oscillator places on the VCO control pin 6 is VERY weak.

What I can't understand, if all the above is so, how that translates to a useful modulation of the reverb.

Well, I don't think it can be so, the multilayer SMD ceramic caps in the Brick must be larger, probably both at or near 470nF. That 15k Rf for the integrator is 3M3 You can see it on this de-gooped image on the first PT2399 from the left...


Anybody else looked into this?
"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

Groovenut

Re: That Belton Brick oscillator LFO
« Reply #1 on: August 28, 2016, 05:36:21 AM »
Jim,

Sorry for the late bump, but just saw this. I have a version 3 that I pulled the case off to have a look inside (it was dead otherwise). I just measured the cap (marked C4 on the pcb) in question with my trusty Fluke and it's 1.2uF. I believe that to be a fairly accurate measurement as the cap goes to ground and is isolated from the circuit by a 3M3 resistor.

However, if I get a chance I will try to desolder and measure out of circuit to confirm.
You've got to love obsolete technology.....
LPD Custom Effect Pedals*iTunes Music*

anotherjim

Re: That Belton Brick oscillator LFO
« Reply #2 on: August 28, 2016, 10:25:41 AM »
If you can get it going Lawrence, it would be interesting to know the LFO frequency, and also the pin5 clock frequency of all 3 delays.
"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

Groovenut

Re: That Belton Brick oscillator LFO
« Reply #3 on: August 28, 2016, 01:35:27 PM »
Jim,

According to my calculations (1.2*106/3,300,000*1.2 = 0.303Hz), those values put the oscillator frequency at 0.303Hz. Also, it looks as though only the center PT2399 in the schematic you posted is connected to the oscillator.

I dont have a functioning brick that is unmasked, so I cannot do a physical test of the pin 5 frequency.
« Last Edit: August 28, 2016, 01:39:56 PM by Groovenut »
You've got to love obsolete technology.....
LPD Custom Effect Pedals*iTunes Music*