Sovtek green small stone not phasing

[Suburbanite]

Hi, Ive acquired an old small stone which has a few issues, namely, it won't phase, opened it up and couldn't see much wrong, a broken lead to the led and a dirty stompswitch so Ive bypassed the switch and re soldered the led, when I put a sound source through theres a small volume drop and when I switch the color switch it takes the treble out,

also looks possibly like someone has changed the rate pot, not sure but it looks a bit new, its a "a 1m" thought it might need to be a linear rather than an audio

not sure where to start next and I can't seem to find any schem's for this model

any help would be gratefully recieved

[Dave Curtis, dB AudioTech]

Here's a stab in the dark: I have an old Bad Stone that didn't work. I tried a bunch of stuff before changing the power supply cap, which fixed it.

[Suburbanite]

Would a power cap actually stop the phasing? theres still a signal getting through although there is a volume drop, thought it might of been one of the chips as they're whats responsible for the timing of the sweep?? am I barking up the wrong tree?

What I'm nearly decided on is changing all the components for new (and better quality) but I'm struggling to identify all the components, I know the pedal isn't worth that much but I quite like the ugly thing

[52 Bill]

...I know the pedal isn't worth that much but I quite like the ugly thing

They are cool in an ugly sort of way.

I would first double check whatever the earlier repair person did, to be be sure that it is not the cause of the problem. I have fixed a couple of these through the years, and the problems are often mechanical and usually related to the switch or the pot.

I think that it's a very basic 4-stage phase shifter, still based on transconductance op-amps. First check to see if the oscillator is working. Either scope the circuit or look for fluctuating voltages on the osc. chip.

[oc disorder]

Here's some links
http://www.lynx.bc.ca/~jc/moddedSmlStnSchm.gif
http://www.lynx.bc.ca/~jc/SmallStoneLyt.gif
http://www.fortunecity.com/tinpan/hu...smallstone.jpg

The first one has..."mods"
The layout may be similar in the 2nd link
The 3rd link the diagram was drawn in '75

[vincediamond]

look for fluctuating voltages on the osc. chip

Yes if you see fluctuating voltages or a Sinewaveform on the output (pin 6) of the LFO or Pin 5 to any of the CA3094 chips phase shift network stages

Pin 5 should be the transconductance INPUT to the CA3094 chip

either than are EH1084 chips or CA3094 datasheets will tell the pinouts also

[52 Bill]

Pin 5 should be the transconductance INPUT to the CA3094 chip

either than are EH1084 chips or CA3094 datasheets will tell the pinouts also

If this is a green Sovtek version, the chips are not EH1048/CA3094's. Maybe LM13600's?

[Dave Curtis, dB AudioTech]

Would a power cap actually stop the phasing? theres still a signal getting through although there is a volume drop, thought it might of been one of the chips as they're whats responsible for the timing of the sweep?? am I barking up the wrong tree?

What I'm nearly decided on is changing all the components for new (and better quality) but I'm struggling to identify all the components, I know the pedal isn't worth that much but I quite like the ugly thing

My Bad Stone was doing the same thing; passing signal, slight difference in volume, etc. I checked the switch, pots and swapped chips to no avail. Changed the 470uF cap (IIRC) and it worked.

[52 Bill]

If this is a green Sovtek version, the chips are not EH1048/CA3094's. Maybe LM13600's?

I stand corrected, the green ones did use CA3094's. But I did remember correctly, as there was a version that did use the LM13600's.

[vincediamond]

The LM13600's IC chip has a different pin out than the CA3094

CA3094 is a 8 pin chip
LM13600 is a 16 pin chip

The LM13600 is a dual inline chip so maybe to save room and space on the PCB

[Suburbanite]

these are the ca3094e chips,

Newb question would I test earth to pin to check for voltage fluxuations or pin to pin (no scope)

[Suburbanite]

heres a pic if it helps

[52 Bill]

Yes, earth the black lead of your meter and read the voltages on the pins of the IC's. If you read the voltages on one of the chips in the phase shift circuit, you can compare them with the voltage readings you get on the other three chips. Look for one of the chips to have odd readings from the others. My guess would be that one of your four phase shift circuit chips is bad.

In the photo, there seems to be a burned resistor on the lower left side (just above the jack). Is it ok and does it read a correct value?

[Suburbanite]

resister reads 47 so ok, but good eyes there, I hadn't seen that heat mark before, might swap it out any way

[mollotof]

i have exactly the same problem with a old small stone (around 1980).

No phasing, just a loose of volume. Color switch and rate pot do nothing.

All the traces are ok (checked with multimeter)

All the resistors are ok (checked on board with multimeter). The 47 ohms on the +9v input was burned but sill fonctionnal. I changed it anyway.

The 4 transistors are OK (unsoldered and test with the diode test of my multimeter).

All the condensators seem OK :tested ONBOARD with an softare oscilloscope : I injetct an audio signal (sine 440 hz) in input and i "see" it in output.

I think that the UC5 (wich "drives" the LFO isn'it ?) is dead.
How can i test it ?