DOD FX90 Works great...then effect fades after being on for a while.

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pedalladep
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Post by pedalladep »

Hey, thanks for looking at my post. Some help would be appreciated, if possible.
I've had this FX90 for a few years, just lying around and recently realized/discovered
that it is good for slap back, rockabilly-esque reverb or delay. Slap back... as I said.
It works great and sounds really good with rich tone and a nice slap back, like I was hoping,
but after about 10 or so minutes of use (time may be somewhat inaccurate) it begins
to lose it's effect. It still passes signal whether on or off and will produce a little bit of effect,
but the rich, full effect of the slap back will fade and it then doesn't do much.
After letting it sit, which usually means using again the next day, etc, it will do the same.
Work great for a bit, then fade. I don't think it's dirty pots or a bad input. I've cleaned those
and it works fine initially, every time, as I said. The faded effect syndrome is not intermittent,
so it's not like a bad connection that comes and goes. I see mention of the trim pots needing
moved in some cases. It's possible that could be a factor, but I don't think it makes sense
that it would be so consistent in working...then working less...eventually after a bit...have almost no effect.
I'm guessing a capacitor? I'm not much of a tech, so I don't know if that follows, logically.
Seem like a cap loading up over time might cause it. Or maybe it loads up, then leaks.
I'm not a tech, just guessing. So please let me know what you think. I can post a photo of the board,
though there doesn't appear to be anything obviously burnt, etc, upon looking.
It's an older version....well here...let me post some photos. (runs, gets camera...takes photos...)
Thanks, and any info would be great. I have some soldering skills, though not much on 'boards',
but I'd be willing to attempt a cap replacement or two if anyone's got some meaningful, and specific
instructions on that (smiley face). Thanks.
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ppluis0
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Post by ppluis0 »

Hi folk,

First recommendation: don't move the trim pots !
At the time you detect that the pedal stop work: the indicator led remains illuminated ?
Also the led goes on and off at each press of the stomp rubber pad when the effect is doing this fail ?
If this is the case, I suspect of the transistor that enable/disable the mixing of delayed signal, named TR7 in the following diagram.
At the same time another transistor named TR8 turn on and off the IC that process the delayed signal.
Carefully remove this transistor (TR8) to have an delayed signal uninterrupted and see if the trouble is produced solely by TR7

Image

Cheers,
Jose

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pedalladep
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Post by pedalladep »

Hi, Jose
Thanks for taking the time to reply and post that schematic !
The pedal stays on and acts normal except for the fading of the delay effect after using it for a while.
The light stays on when engaged and goes off when you click the footswitch, like normal.
When it has been on a while, and the effect is no longer working all the way, you can turn the footswitch
off and on and notice only a slight change (slight effect) whereas it was a full effect when I first started using it...in any given session
of playing. Would a transistor act this way? Take a while to start failing, even though it was working when I first start using it,
and then again tomorrow when I use it again? It will work tomorrow...for a while... and then fade. I'm not doubting your
ideas, just making sure that I'm getting the idea across of how it is failing.
Thanks and any other input would be greatly appreciated.

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ppluis0
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Post by ppluis0 »

Hi pedalladep,

Intermittent and thermal related malfunctions are always hard to identify.
According with your new description my advice is that you remove initially the transistor TR8
This way you will have continuously delayed signal available so if the fault persist then replace TR7
Suitable replacements can be J111 or J112 but perhaps any forum member can suggest other available transistors to do this function.

Cheers,
Jose

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mauman
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Post by mauman »

If TR7 is the faulty one, J113 are readily available and cheap in the US (less than $0.50 in single quantities) at Mouser and Digi-Key.

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pedalladep
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Post by pedalladep »

ppluis0 wrote: 03 Jan 2022, 01:28 Hi pedalladep,

Intermittent and thermal related malfunctions are always hard to identify.
According with your new description my advice is that you remove initially the transistor TR8
This way you will have continuously delayed signal available so if the fault persist then replace TR7
Suitable replacements can be J111 or J112 but perhaps any forum member can suggest other available transistors to do this function.

Cheers,
Jose
Thanks. Can you clarify a bit, please? Are you saying tr7 is a common failure, causing symptoms like my problem of fading effect over some use (15mins or so). And if I remove tr8, it will shed light on whether tr7 is the problem or not? I don't want to jump into
removing parts, etc, if I don't really know why I'm doing it or what I'm looking for. And when you say remove 8, do you mean just one leg, or take it out with the intent of replacing it back, or replacing it with another transistor? Sorry for questions about an answer you already gave, lol, but just trying to understand. I do appreciate your response. Thanks.

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ppluis0
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Post by ppluis0 »

pedalladep wrote: 08 Jan 2022, 04:04 Thanks. Can you clarify a bit, please? Are you saying tr7 is a common failure, causing symptoms like my problem of fading effect over some use (15mins or so).
Hi pedalladept,
I experience a similar fault with a Jfet from a TS9 of a friend of mine, that had the same behavior.
pedalladep wrote: 08 Jan 2022, 04:04 And if I remove tr8, it will shed light on whether tr7 is the problem or not?
I think that if TR8 is still on board can mask the search if the suspect really is TR7
My advice is to remove TR8 carefully, test the pedal without it, and preserve to be soldered again when the fault will pinpointing.
pedalladep wrote: 08 Jan 2022, 04:04 I don't want to jump into removing parts, etc, if I don't really know why I'm doing it or what I'm looking for.
As I say in previous post an intermittent or thermal related fault is hard to find and require several attempts until can be really solved.
pedalladep wrote: 08 Jan 2022, 04:04 Sorry for questions about an answer you already gave, lol, but just trying to understand. I do appreciate your response. Thanks.
Don't worry as I am disposed to try to help you since for me it is an opportunity to continue learning. 8)

Cheers,
Jose

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CheapPedalCollector
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Post by CheapPedalCollector »

I fixed one of these recently with help from that other forum.

The NE571 is misbiased, it's a design flaw of the FX90 as they were copying the functionality from the 15 and 18 volt older pedals.

The 22K hooked to pin 11 and 12 is not high enough to bias the op amp stage, I raised it to 39k and this fixed the issue for running the pedal on ~8v.

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