Bassman 250 210 Combo Distortion

[rocksmoot]

It's been a tough week in the Smoot household for gear. Aside from the blown GK NEO speaker (there's a thread on that too) my trusty little Bassman 250 210 has gotten ill as well.

The amp has developed some nasty distortion that seems to get worse the harder I dig in. I disconnected the internal speakers and ran it through another cab that I know is good, wired it up with a good Neutrik speaker cable and alas, there is still distortion in the signal. It appears that the problem is in the amplifier itself.

Before I haul this off to a tech, I'd like to know if there are any common problems with this amp that I might fix myself. Are these amps known for any particular kind of failure that produces these symptoms?For example, I know that some distortion can be introduced by bad power supply caps, and I'm handy with a soldering iron and have some electronics knowledge, but I don't have an o'scope to trace the distortion and I don't want to go off on a crap shoot if the Bassman 250 isn't known for chronic power supply cap failure.

An exhausting (if not exhaustive) search of the web turned up nothing really useful, so I'm turning to you folks here.

 

[will33]

Start with the easy/cheap stuff first. That means if you have an active bass, put a new battery in it.

Otherwise De-Oxit cleaning in all the pots, jacks, and switches.

If it has a preamp tube, cleaner in that socket as well, make sure the current tube is seated good, and try a different, known-good tube too.

 

[will33]

Also, using the effects loop to patch it with another known-good amp can isolate the problem to either the pre or power section.

 

[rocksmoot]

Thanks for the tips, Will. Such good common sense that is all too uncommon in me, most appreciated. It's an all solid-state amp, so no tubes. I'll give the other steps a try.

 

[will33]

Other than that stuff, there isn't a whole lot a player can do.

Maybe see if you can spot a bad solder joint somewhere, broken jack, maybe a resistor that looks obviouly burned or a cap that is obviously bulged up or leaking oil, though you still wouldn't know what caused them to fail. That stuff needs schematics, diagnostic equipment, and the kmowledge of what to do with it.

 

[BassmanPaul]

Try looping from the FX send and return with a signal cable and see if that helps. By doing so you have eliminated the normalizing switches on the jacks.

 

[rocksmoot]

Okay, I've done the following:

Looped the Bassman fx send & return - no change, still distorts
Connected an Ampeg PF800 fx send to the Bassman fx return, no change
Connected the Bassman speaker out directly to the GK cab, no change
Connected the Ampeg fx send to the Bassman fx return, no change
Connected the Bassman fx send to the Ampeg fx return driving a GK NEO 410 - no distortion

So it would appear by the steps taken above that the power amp is producing the distortion. I'll tear it open tomorrow to poke around inside to see if there's any obvious ickiness happening in there.

By the way, Fender has the schematic for this amp on its support website.

I appreciate your input, folks!

 

[rocksmoot]

By the way, doing this has given me an appreciation of how good the PF800 sounds. Of course it ain't a fair comparison since the Bassman is sick, but still.

 

[will33]

Can see if there is a bad solder joint or bad jack on the speaker outs. Also clean/reseat any ribbon cables or other type of plug-in type connectors in there. If that doesn't help, probably tech time.

Yes, the PF's sound nice and fat.