DOD FX90 rev j

[bagudan]

Hi. Someone gave me an old fx90. Signal is coming through, but no delay. I´m having a hard time finding a schematic for this specific layout. Makes audioprobing a bit difficult, to me at least
Maybe someone could point me in the right direction.
I have already replaced some of the electros, on the power supply, the ones feeding the clock chip...and so on...
The mn3005 has been replaced by the guy before me....I still have the original....they are supposed to be expensive chips?
The mn3101 has also been replaced
but the ne570,mc1458,and the mc14007 are the original ones. Should I look for replacements for these ic´s?

Thanks

[anchovie]

You could audio-probe the ICs based on pinouts from datasheets. Each section of the compander will have an input and output, as will the BBD and the two halves of the opamp.

[George Giblet]

PM me your email and I'll send you the rev A schem.

I'd start with DC voltages on opamps and compander first.

It could be something simple like the clock circuitry has stopped or the BIAS setting on the delay chip is off (or the trimpot is damaged).

[bagudan]

Thanks for the schem

voltages on the mc1458

1     4,28
2     4,28
3     4,28
4     0
5     4,26
6     4,26
7     4,26
8     8,5

Voltages on NE570
1      0,9
2      1,8
3      1,8
4        0
5      1,8
6      1,8
7      4,5
8       1,8
9       1,8
10     6,5
11     6,5
12     1,8
13     8,5
14     1,8
15     1,8
16     0

Voltages on 4007
1      8,5
1      8,5
3      0
4      0
5      8,5
6      8,5
7      0
8      0
9      3,9 
10    7,0
11    0
12    7,0
13    0
14    8,5

Will try audioprobingprobing on the ins an out on the chips later tonight

[George Giblet]

The voltages look good.

One thing though, it appears the NE570 halves are reversed compared to the Rev A schematic.
I can tell this because the schematic predicts pin 11 at 1.8V but in reality pin 6 is at 1.8V, there
a few other voltages that tell the same story.

Checkout figure 6 on this page to swap the NE570 pins on the schematic,
http://www.synthdiy.com/files/2006/NE570_3.pdf

When you inject a loud signal *and* the delay is enabled, you should the voltage on pins 1 and 16 increasing.
If pin 16 is stuck on 0V then there's something wrong around TR8, the 100k to the base or the 0.56uF cap.
If pin 16 is stuck on 0V the gain of the NE570 half that follows the delay will be forced to nearly 0 and there will be no output.
You could lift the collector pin of TR8, which goes to pin 16 to force the effect on - assuming of course that's the problem.
If the voltage on pin 16 still doesn't increase with signal the 0.56uF cap might be dead.

[bagudan]

Hi. Been at it again, and thanks for all the help:)
Tried lifting the collector on tr8(ended up replacing it), also replaced the 0,56ufcap with two 1uf´s in series. No luck, unnfortunately. and the voltage on pin 16 og the ic is still around 0volts

Then I did some audioprobing on the IC´s:

MN3005
Audio on pin 7, which should be the input
A loud whine on pins 3 & 4

NE570
Audio on pins 1,2,3,6,7
and an extremely weak signal on pin 10, but its there!


MN3101

Whining noises an all pins

4007

No audio

Hmmm? am I getting any closer.

Thanks

[George Giblet]

Whining noises an all pins

Audio will only appear on pins 7 and  3/4.  The problem with delay devices is there's no filtering to turn the sampled audio back to audio.  There's an unconventional filter on the output of the delay, the 470pF cap - I'm not convinced of the filter's effectiveness

You could try probing pins 3/4 via a 10k resistor, then on the audio probe side of the resistor put a capacitor to ground, value say 4n7 to 10n.

There might be a problem with TR3.  It could turn out the 5k trimmer on TR3 is stuffed.

Assuming the schematic is correct, it's a weird circuit because the output the MN3005 directly connects to the compander instead of going through the filters first (TR4 and TR5).

There's comes a point with these unconventional circuits where I suspect there's something not quite right with the design.  I would put the 10k + 4.7n filter between the delay and the base of TR5 and take it from there.


-------------------------------------------------------------
Edit:
Actually before you start playing around with the filtering, how is the audio on the emitters TR4 and TR5.  If there's none check the DC voltages on the emitters.  Near TR5, check for audio both sides of the 0.33uF cap.

[bagudan]

Hmm the layout apparently is somewhat different than the schem you gave me. There is no 5k trimmer (Only a 1M,2k2, and a 50k)
There is a 2n5088 transistor between the mn3005 and the ne570, with no audio on the emitter and 0 volts as well

The transistors are not placed as in the drawing next to the schem, so its hard to tell what transistor I´m talking about. The emitter connects to pin 5 on the mn3005. The collector connects to pin 16 ont the ne570

By the way. On the 1m trimmer, there is 821 pf cap soldered between the lugs 2 and 3, on the solder side of the bord?

[George Giblet]

I've sent you the 585 schematic.  The filter opamps (after the delay) may be transistor buffers.

I still recommend using the audio probe, and checking at the DC levels, on the outputs (emitters).