Frog channel won't go to 00 or FF [FIXED]

[kgallen]

[NO ACTION REQURIED]

Documenting here a problem I had on my Fat Frog and the repair required. This can equally apply to other consoles in the Frog range - and in time possibly other consoles with faders.

Thanks to @Edward Z88 who provided support despite having moved on from Zero88 and @RJP for providing schematics.

The background:

I bought my Fat Frog new in 2006, so at this point it's 19 years old. It's largely in "as new" condition as I look after my kit. I've not used it regularly since 2015 when I bought my FLX, but it has had the odd outing, most recently in 2021. I've stayed on top of the CR2032 battery replacement and I'm blessed that the original floppy drive still works perfectly. All faders are still in pristine condition.

The issue:

This was an intermittent problem, so hard to narrow down. The usual range of channel intensity values shown on the Output Screen is 00-FF (FF in this case means "full" or 100%, not hexadecimal 0xFF). On occasion my console would only move from a low non-zero value, say 02 to 05 (percent) up to a maximum of around 94-98 (percent). I first saw this actually on the fixtures section with the thumb wheel (so not a linear fader), where the fixture intensity would only go up to 96%. Subsequent testing showed the same issue on the generics faders (including masters). Strangely this issue also showed when playing back a memory stack (where the memories had successfully been programmed with 100% intensity values).

You can enter test mode by holding the F1 key whilst powering up. In this mode, I could see the 8-bit values of the faders (all of them on the console) would only travel between about 0x05 and 0xFA rather than 0x00 to 0xFF.

OK, cut to the chase, what was the issue...

After some testing internally, it was identified that the reference voltage rail for the fader banks was sometimes low. This VREF should be around +5V with respect to its AGND. Practically it will be around +4.8V (and I determined that it should not go below +4V for correct operation). On occasion this reference voltage was measured as low as +2.8V and this is when the issue presented.

After quite some time this was tracked down to a failed 4.7uF/16V tantalum capacitor on the Submasters PCB that is used to decouple the VREF locally to this board. There is a similar capacitor on all of the boards with faders (or potentiometers) and they are all effectively in parallel across VREF/AGND, so a failure of any one of them would result in similar failure symptoms.

After removing the Submasters board it only took a couple of minutes to replace the capacitor. In this case I used a 4.7uF electrolytic, which is good enough in this application - and had to lay it down as it was slightly taller and would have fouled the front panel).

So far the issue seems resolved. Hope this might help others who may have a similar issue as our Froggy friends age more.

Note: Essentially the same circuit is used everywhere in Zero88 (and likely other brand) consoles for reading the faders. The faders, usually 10K, have a VREF at the top, a GND (0V) at the bottom and the wiper goes via some 4051 analogue muxes and finally feed into an ADC channel on an embedded micro.

Here are a couple of photos of the Submasters board and of the failed tantalum capacitor which is clearly discoloured brown when compared to similar components.