Echorder EC444 Delay Troubleshooting

[sarakisof]

Hello everyone and a happy new year. I was given an old "B (like Better) TMC Echorder EC444" analog delay. These units were sold back then rebranded using many different names like Better 
/ Maplin / Gemini Echorder EC444 etc.

It uses a MN3005, a CD4001BE, three KA4558 opamps and some Si transistors like A733, C945.

The unit sounds nice, but i get stable intermittent sound. Signal comes and goes periodically stable like 3secs signal, 2secs no signal, 3secs signal, 2secs no signal and so on.
Any thoughts on this?
In general, when it comes for this kind of units and the fact that it was out of use and abandoned in a garage for so many years,  what things should i start looking for first?
For now, i have just started a quick recap with what i have available in hand.

Check each IC for voltages according to datasheets?

Unfortunately no schematic for this specific model available on net, but found many similar ones.

https://i.stack.imgur.com/24hbq.jpg

[iainpunk]

without voltage measurements i wouldn't know where to start, however, my guess would be an overly leaky cap in the power supply and the power supply cutting out periodically due to either current protection or something similar.

maybe stick a multimeter on the VCC bus and check for stability (changing DC voltage)

cheers

[sarakisof]

DC voltage on 78L15 output is stable at 14.75 Vdc.
Is there any schematics for those Echorder EC444 units out there? I've also replaced almost every power electrolytic, problem is still there. Periodically repeated intermittent signal.

[ElectricDruid]

Check for fine cracks in the tracks.

While those kind of "periodic" problems are often due to caps charging/discharging (and the time scales gives you an idea of the cap size) I once saw a similar thing with an old circuit board with a hairline fracture. When cold, it made a contact and started working. Then it'd warm up a bit, the fracture would move *very* slightly, and the whole thing would stop. Until it cooled down enough again...etcetc.

[sarakisof]

I have done so, no cracks, bad joints or anything weird there.
By "periodic" i mean the silent gaps between signal are equal.

Already did a full recap job and replaced CD4001 with new one. No change at all. I also have  all of the transistors and ICs in handy except for MN3005, but hell, if i am about to replace everything in there, I'd better build it from scratch. 

Tried to record some small tests so you can hear what i mean. 1khz and then some random guitar playing through the unit and straight into interface.

https://we.tl/t-pm73FDg336


EDIT: Is it normal that power trafo (16-0-16) secondaries voltage is always and constantly fluctuating  16.70-16.90V for the one tapping and 16.58-16.82V for the other tapping?
78L15 output is rock stable at 14.72V though.

[ElectricDruid]

Thanks for the sound files - that *is* really weird!

I notice it takes out *all* the sound, not just the wet signal. That means it's unlikely to be something directly related to the delay itself. I also notice that the sounds goes crunchy just before it cuts out.

The fact it removes everything means the problem has to be somewhere that's possible. To me, that means at the input buffer, or at the output mixer. A problem in any pre-delay filtering, in the delay, or in the post-delay filtering wouldn't cause the dry signal to die too.

Since there's no sort of FET-based in/out switching, the only way I can think of for the signal to totally die like that is if one of the op-amps goes crashing off into the rail and stays there. That might be caused by the op-amp bias varying, and that fits with the sound going crunchy before it dies. This thing runs on a 15V single supply, right? So there's a 7.5V Vref somewhere. But what could cause the Vref to oscillate like that? That's a new one on me.

[duck_arse]

no power supply also would take out all, with dirt. perhaps if you hack in some temporary DC supply to replace/bypass the rectifier/trafo, you could at least then rule them out. is the voltage regulator getting warm?

[ElectricDruid]

OP said the 78L15 was giving a stable output of 14.7V, but thermal shutdown of the regulator would explain the symptoms, it's true. Patching in an external power supply to see if that fixes things is certainly a good test to do.

[sarakisof]

78L15 doesn't get overheated by finger testing.

The fact it removes everything means the problem has to be somewhere that's possible. To me, that means at the input buffer, or at the output mixer. A problem in any pre-delay filtering, in the delay, or in the post-delay filtering wouldn't cause theSince there's no sort of FET-based in/out switching, the only way I can think of for the signal to totally die like that is if one of the op-amps goes crashing off into the rail and stays there. That might be caused by the op-amp bias varying, and that fits with the sound going crunchy before it dies. This thing runs on a 15V single supply, right? So there's a 7.5V Vref somewhere. But what could cause the Vref to oscillate like that? That's a new one on me.

I thought of that too and due to the fact i had seen black dark spots / discoloration (like those you see on overheated silicon resistors or ic's) on two of the three 4558 opamps, i decided to replace them with two TL072's i had in hand, but problem is still there. Will also replace the third one too just to be sure.

So far replaced all electrolytics, CD4001 and two 4548's.

Last components left intact are the MN3005, 78L15, C945 and A733. Which one could cause such an issue?

[ElectricDruid]

Can you post the voltages around the op-amps? That might show up an issue. Unhappy op-amps are usually fairly easy to spot.

[sarakisof]

Have replaced two of the three 4558's with TL072's because those two looked bad. But i guess it's time replace the third one too.

Voltage measurements:

---Two new TL072's---
Pins 1,2,3,5,6,7:  7.3 V stable
Pin 8 V+: 14.6 V stable

---Third opamp (the old 4558)---
Pin 1: FLUCTUATING  0 V - 5.4 V
Pins 2,3,5,6,7: FLUCTUATING 6 V - 7.3 V
Pin 8: 12.45 V - 13.95 V fluctuating

Straightforward replace the third 4558 or are those weird fluctuating voltages caused by smthng else nearby?

if i had scheme things would be easy.

[ElectricDruid]

Have replaced two of the three 4558's with TL072's because those two looked bad. But i guess it's time replace the third one too.

Voltage measurements:

---Two new TL072's---
Pins 1,2,3,5,6,7:  7.3 V stable
Pin 8 V+: 14.6 V stable

---Third opamp (the old 4558)---
Pin 1: FLUCTUATING  0 V - 5.4 V
Pins 2,3,5,6,7: FLUCTUATING 6 V - 7.3 V
Pin 8: 12.45 V - 13.95 V fluctuating

Straightforward replace the third 4558 or are those weird fluctuating voltages caused by smthng else nearby?

if i had scheme things would be easy.

As long as the pin 4 on the two TL072's is 0V, those look fine. That's good.

Yes, the 4558 looks weird. Pin 1 going from 0V to 5.4V explains where the symptoms are coming from. If the output is pinned at 0V, the sound will disappear, and it'll go all crunchy as it hits that limit - which is exactly what we heard.

So the question is...why's it doing that? The "weird inputs" is the obvious answer, but we don't really know why those are doing that either. And why would the +V power pin for that op-amp be varying? Perhaps all the op-amps got fried somehow or something? I don't know, but I think I agree that replacing the third op-amp too would be a good idea as a next step. If that doesn't fix it, then we move on.

PS: Totally agree, a schematic would make this a *lot* easier. I hunted online, but didn't find anything, even searching under the several other names of this box.

[sarakisof]

Replaced the third 4558 with a TL072 and fluctuating sound problem solved.  Mission accomplished!

Yeap pin4's are 0V indeed.
Me too, i think op amps got fried somehow in the past from previous owner.

**PS Edit: Hum and oscillation solved by alligator grounding the pcb. Its metal standoffs (supposed to get connected by screwedwith chassis) were left unconnected during previous recording upload due to repair job.

It sounds really nice, i get a little tiny distortion though. Can be controlled by playing with front panel dB switches, unit's volume pot and amp's pot but i think there's still there a bit whatever i do. I believe it has to do with the TL072's instead of 4548's (or 4548 would be even more dirtier  ), or with this thing below.

The "thing below" is that Peak Level led doesn't work at all. Never worked actually but i had bigger problems to solve 🤣.
Without schematic, but by tracing pcb, i guess we have a common circuitry TL072 - transistor - Peak Led.

So maybe A733 needs also be replaced? And maybe both Peak led and distortion problems would be solved?
What voltages should i expect on C945 npn?

EDIT: Tried to trace the pcb around two transistors and Peak Led.