Vintage EHX Small Clone MINI-CHORUS SAD1024 Help please

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Fuzzlefty77
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Post by Fuzzlefty77 »

Hi guys,
Apologies for my first post being a bit of a long winded cry for help but I’ve picked up a non functioning EHX Small Clone.
To my delight it was a vintage one from the 80’s with the SAD1024 chip and all seems totally original. I still can’t seem to get it working after performing the following:

Checked:
Checking all resistors and polarized caps
Checking all the trannies
Checking the diodes.
All the off-board stuff is good.
I’ve tried different LM348, 4558 and the HEF4047 I/C’s to no avail.
Trim pot does changes voltages on pins 2 & 15 on the SAD1024 (it ranges from 0-6.75v, I dunno what I should be feeding into it?)

Symptoms:
Powers from adapter and battery fine. It passes signal fine, LED is good.
No chorus effect at all though which makes me wonder if the SAD1024 is kaput?!

I’ve taken readings from all the I/C pinouts and got the following (attached).
Capture.JPG
Pins 2&15vary with trim pot adjustments.

The LM358N readings seems “odd” to me compared to when I’ve checked these on other pedals?! The HEF4047 confused me a bit too :scratch:
I'm using the schematic attached.
EHX Small Clone 4100.pdf
(50.62 KiB) Downloaded 11 times
I've managed to troubleshoot and fix plenty of Chorus pedals previously (including more modern Small Clones) but this one is kinda kicking my ass.

Any pointers would be great (or even confirmation that it is the SAD1024 thats DOA).

Regards,

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lolbou
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Post by lolbou »

Seems like the wet signal is lost somewhere, so you need to scope or audioprobe the signal in the delay line (from pin 1 of the first opamp through the 1µF cap, and up to 2N4340 and 27k resistor.)

Could be a bad FET, bad transistors, opened resistor...... The SAD is not the likeliest of all then.
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.

Scruffie
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Post by Scruffie »

lolbou wrote:
09 Jul 2020, 22:55
Seems like the wet signal is lost somewhere, so you need to scope or audioprobe the signal in the delay line (from pin 1 of the first opamp through the 1µF cap, and up to 2N4340 and 27k resistor.)

Could be a bad FET, bad transistors, opened resistor...... The SAD is not the likeliest of all then.
I've been helping Fuzzlefty77 with this on the EHX forum and to avoid confusion from cross forum posting I'll point out the SAD clock voltages etc. are fine but its outputs are very low independent of input bias which isn't a good sign. But I've also asked him to scope the clock and LFO and check the solder joints and resistors around it.

Fuzzlefty77
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Post by Fuzzlefty77 »

Yeah sorry Scruffie, I only posted here and on the EHX forums with this issue. You are everywhere it seems lol. Should we just continue on here?

Just to clarify what I've just posted on the EHX forum (doh):

Output voltage on SAD changes a tiny bit to 0.3v when input bias is adjusted to max (6.75v).

Getting correct waves from all the pins you mentioned.

Guess I’ll try resoldering everything from the Rate pot to the LM358. Continuity seems okay but worth a shot. (the values from the LM358 are still bugging me)
Remove and try resoldering the SAD.
Remove all caps and test them.

Many thanks for all your help!

My Transistor reading are as attached. I thought these looked good (bit insure with the 2n4340 readings)?
IMG_20200708_091633119.jpg
IMG_20200708_092108964.jpg
IMG_20200708_093141825.jpg
IMG_20200708_092553870.jpg
Also attached both sides of the board for full clarity:
IMG_20200710_083215016.jpg
IMG_20200710_083150394.jpg

Scruffie
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Post by Scruffie »

Fuzzlefty77 wrote:
10 Jul 2020, 09:22
You are everywhere it seems lol. Should we just continue on here?
Like a bad itch.

Okay, if you're getting modulating square and triangle from the 358 that changes with rate, that's your LFO working fine, you have a square from the 4047 pins 10 & 11 which is your clock pulse at around IIRC 70-120kHz.

Which comes back to the SAD, bias voltage on its input, 0.3V above ground on its output. You can redo the solder joints, clean the socket, check the 39k output resistor coming from pins 6 & 11 isn't a short and lift one leg of the 10k coming from the same place to make sure nothing further down is dragging it down and if that doesn't change, it's dead.

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Post by Fuzzlefty77 »

Scruffie wrote:
10 Jul 2020, 11:07
Fuzzlefty77 wrote:
10 Jul 2020, 09:22
You are everywhere it seems lol. Should we just continue on here?
Which comes back to the SAD, bias voltage on its input, 0.3V above ground on its output. You can redo the solder joints, clean the socket, check the 39k output resistor coming from pins 6 & 11 isn't a short and lift one leg of the 10k coming from the same place to make sure nothing further down is dragging it down and if that doesn't change, it's dead.
Removed the socket and cleaned up all solder joints on board. Installed new sockets (didn't have 16dip so used 2x8 dips)......nothing changed with values from pins.
39k resistor checked on board and the removed from circuit and checked....its all good.
Lifted leg of the 10k resistor........nothing changed with the SAD pin values.

Weirdly when the effect is engaged now its almost like a treble booster?! Really trebley tone which it didn't do before :hmmm:

Guessing the SAD is a goner. Looks like I'll be converting it to use an MN3007.

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Post by Fuzzlefty77 »

So......interestingly (or maybe not) I converted this to an MN3007 schematic and it made zero change!!

Still no chorus effect. Maybe the SAD1024 wasn't dead after all and there is another issue?! :roll:

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Post by Scruffie »

What are the voltages on the MN3007?

Going from your previous voltages, you should expect to see something in the region of.
1. 8.5
2. 4.25
3. 4
4. 0
5. 0
6. 4.25
7. 4.5
8. 4.5

A BBD requires the correct power supply voltages, a correct input bias voltage, anti-phase clock signals and a source follower resistor to function. You tested all those on the SAD so I can't think what else it could have been, I don't generally like to declare them dead.

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Post by Fuzzlefty77 »

Scruffie wrote:
17 Jul 2020, 18:10
What are the voltages on the MN3007?

Going from your previous voltages, you should expect to see something in the region of.
1. 8.5
2. 4.25
3. 4
4. 0
5. 0
6. 4.25
7. 4.5
8. 4.5

A BBD requires the correct power supply voltages, a correct input bias voltage, anti-phase clock signals and a source follower resistor to function. You tested all those on the SAD so I can't think what else it could have been, I don't generally like to declare them dead.
Don't get me wrong mate, I wasn't insinuating I got duff advice. Far from it.... :thumbsup

I'll check all voltages again and report back.
This thing has got me totally perplexed!

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Post by Scruffie »

Didn't take it that way :) more speaking out loud for any others to check my thought processes in case I overlooked something, I'm not infallible!

If the new voltages check out as above, an audio probe might be the way to go next.

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Post by Fuzzlefty77 »

Sooooo.......first thing, I re-checked all pins previously on all Op Amps and I go a more positive result from the LM358. Previous voltages read in ()

1 - 6.8 (1.8)
2 - 6.8 (4.1)
3 - 4.9 (4.1)
4 - GRD
5 - 7.3 (2.2)
6 - 4.9 (2.2)
7 - 4.9 (4.1)
8 - 9.1 (9.1)

I noticed on the board there looked to have been a repair on a trace coming from the Rate Pot to pin 2 of the LM358, I jumpered that (green wire) in pic and that seems to have made a difference.
IMG_20200719_124715166.jpg
Right, to the MN3007 readings......these seem waaaaay out :shock:
Capture.JPG
Also, if I read them again a few minutes after power is sent to it, the readings on pins have dropped significantly (2v generally). The surface of the chip is also very hot after a while but I'm presuming thats because its being fed higher voltages then it needs?!
Bad MN3007?

Here's the front and back of the board as she stands now:
IMG_20200719_124742046.jpg
IMG_20200719_124703755.jpg

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Post by Scruffie »

You said your LM358 was producing a varying triangle wave on your scope before? It should have been fine.

Yes... those are... bad and hot chips generally aren't a good sign. Where did you source the chip from?

Take the MN3007 out and take the voltages on it again, also, I realise Pin 1 on the MN3007 is called 'Ground' but because the chip expects a negative supply relative to ground, that should actually be supply voltage and ground should be pin 5.

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Post by Fuzzlefty77 »

I did the schoolboy error of buying some off eBay.
Any good sources currently in the UK for MN3007?

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Post by Fuzzlefty77 »

Got hold of a "good" MN3007 and got these results to compare with Scruffies recommended results:

1. 8.5 - spot on
2. 4.25 - spot on
3. 2.3 - down on what is expected.
4. 0 - spot on
5. 0 - spot on
6. 4.25 - spot on
7. 2.6 - down on what is expected
8. 2.6 - down on what is expected

So my voltages IN at Pin 3 are not good and that obviously effects the OUT pins too :scratch:

(Just getting that nagging feeling that I am missing something so dumb with this now.......nothing seems to effect it)

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Post by Scruffie »

The bias :)

Pin 3 is the input, its voltage is adjusted by the bias trim, the output pins 7 & 8 track the input voltage.

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Post by Fuzzlefty77 »

Scruffie wrote:
24 Jul 2020, 14:19
The bias :)

Pin 3 is the input, its voltage is adjusted by the bias trim, the output pins 7 & 8 track the input voltage.
Unfortunately the 2.3v on Pin 3 is with the bias maxed. :shock:

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Post by Scruffie »

You must have made a mistake in your 3007 conversion, what voltage do you get on the 2N5087?

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Post by Fuzzlefty77 »

The saga continues.......
I've double checked my conversion and everything seems good to the schematic
The 2N5087 reads good, I get 0.725v across E(+) to B (-) and C(+) to B(-) and o/l in every other scenario. Do i need to check it in any other way?

I have now managed to get 4v coming in at Pin 3 on the MN3007 using trim adjustments (hooray.....) and the trim position seems to match every other pic I've managed to find online so thats good?!

BUT i've only managed this by totally bypassing the diode and 33k resistor in the circuit (boooo.....). So I'm going straight from the 2N5087 Emitter to Pin 3 on the MN3007. See the attached.
new Capture.JPG
I now have a slightly more effected signal when on but its more a faint buzz/white noise in the background. Basically no chorus still.

Maybe audio probe time?

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Post by Fuzzlefty77 »

Would help if I actually had said what the voltages at the 2N5087 were:

C = 9v
B = 8.5V
E = 4.25V

Any advice if these are what should be expected?

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Post by Scruffie »

Well seeing as the 2N5087 collector should be grounded, I'd say there's still a mistake :)

4V for the MN3007 input was just an average expected voltage, it wont be exactly that as the chip will have its own individual bias point.

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