Vintage amp crackles/fizzes on startup then has reduced output volume/power?

[vipor3D]

Apologies in advance if this isn't the correct place to post this. I have a vintage Lab Series L5 (solid state) that I've had for about a year and a half. When I got it, everything worked fine and the volume was so loud I could barely turn it up to 2-3.

However, a couple of months ago while I was playing I noticed a brief moment where the output power/volume seemed to fade a bit but then came back. When I turned the amp off, there was a very loud pop that came through the speakers. Not your typical turning-the-amp-off pop, but a much louder unpleasant pop that almost hurt my ears.

I decided not to turn it on again and was going to take it to my local amp tech but found out he unfortunately passed away unexpectedly, so that's no longer an option. I had some time to kill today and decided I'd try turning the amp on again just to see what it would do, and I definitely noticed an issue.

As soon as I turned it on, the channel volume and master volume were both turned to zero but a decently loud crackling/fizzy/radio-static type sound came through the speakers for a good 5 or so seconds then faded out to silence. After that, I could play through the amp and all of the functions were fine, but the overall volume/power output was noticeably lower than I'm used to with the amp. Previously 2-3 would be way too loud, but today I had it on around 3.5 and it wasn't loud at all.

Just wondering if anyone with more experience recognizes the possible issues that could be related to the symptoms I've encountered? Thanks in advance

[Stevem]

It’s high time to replace the two main power supply rail electrolytic filters and every other electrolytic type cap in the amp.

If that does not cure the issue all on its own then at least you have then narrowed it down to a semiconductor problem and the cap/ filter replacement you would have done away just due to age.

[LOUDthud]

On the back panel there is a "Power Amp Input" jack. This jack has contacts that connect the Preamp output to the Power amp input when no plug is inserted. These contacts can become corroded with non-use. A squirt of contact cleaner or just inserting and removing a plug several times will renew the contact and bring the amp back to normal operation. If that doesn't do it, simply plug your guitar in there. The level will be low but see of the crackles/fizzes are gone.

[thetragichero]

for what it's worth, a sick solid state amp will continue to get sicker the more it's turned on. a current-limiting device (like a lightbulb limiter) is absolutely mandatory for troubleshooting. i also wouldn't plug into a load until it's sorted