I'm having some intermittent problems with a guitar amp (but much of the time it works fine). On inspection I find a lot of the capacitors with a brown-yellow goo. Is this indicative of a problem, or is the goo some kind of stuff used to hold components in place during assembly? What is the goo, and where does it come from? Do these capacitors degrade gradually over time? (None of the tops of the caps seem bulged.)

Thanks
Peter

 

That goo is glue used to support the bigger capacitors. The capacitors look OK to me.
Intermittent problems are more likely to be a bad joint/connector/switch/control. Does it come and go if you tap it?

 

Looks like glue to me.
Yes, capacitors degrade over time. Some degrade imperceptibly, some with physical changes, some explosively.

1) Imperceptibly - slow reduction in capacitance values and increase in effective series resistance (ESR).
Symptoms - more AC hum in audio, motor boating effect (low frequency oscillation),

2) Physical changes - bulging at the top, brown effluent at the centre at Y at the top, brown leakage at the base.
(Y is stress relief in case of excessive internal pressure.)

3) Explosive destruction - sometimes these happen for no reason except end of life or voltage breakdown, often resulting in a short across the capacitor. You will know this when it happens.

Some people as a matter of practice replace large electrolytic capacitors when they have nothing else to go by.
Some people pull capacitors and measure capacitance and ESR.
I prefer to determine that the capacitor needs replacing before doing that.
Depending on the circuit application it may be acceptable to replace with slightly higher value capacitance. New electrolytic capacitors are often 20% higher than nominal value. It is ok to replace capacitors with higher voltage ratings. It is a good idea to go 50%-100% higher than actual circuit voltage assuming that the physical size fits.

 

That goo is glue used to support the bigger capacitors. The capacitors look OK to me.
Intermittent problems are more likely to be a bad joint/connector/switch/control. Does it come and go if you tap it?

Thanks. I subsequently read elsewhere that capacitor legs and mass can sometimes act as a vibrating spring-mass system. In audio settings this can be nasty at some frequencies. So the glue also dampens mechanical movement.

The specific problem I've had is that reverb / tone change only kicks in sometimes (with a plop noise) when the amp is hot / has been on for 30 minutes. I found a video online suggesting a common problem was some PCB current leakage between gate and source of the MOSFETs (which overheated and burned some resistors), but I appear to have a slightly later version of the board. I jiggled stuff, tweaked the tube biasing, didn't spot anything, put it all together again before I tap-tested anything, it worked, but if the problem persists I'll open it again and try a tap-test.

Its a valve amp - a Blackstar HT5R combo. I see it uses a SpinSemiconductor FV1 DSP - (http://www.spinsemi.com/products.html) - apparently popular in some pedals for signal processing for reverb and I suspect for their so-called ISF tone shaping and speaker output emulation via headphones. Output waveforms on speaker emulation looked very "digital", even before I knew it had a DSP chip! Top trace on this pic is 440Hz sine input, bottom is output from 4x12 speaker cabinet emulation.