Old Acoustic Control 370 issues

[370Fan]

Hello everyone.

I have an old Acoustic 370 that is developing "issues". Last week at a gig I plugged 2 cabs into the output and it sounded very distorted. When I unplugged one of the cabs the signal came back nice and strong so I left it that way for the rest of the gig.

Some things I noticed: When I had both cabs plugged in (around 2 ohms) the output became "dirty". It also became extremely bright. It was almost impossible to cut enough of the treble out of the signal - I had to turn ALL of the treble knobs to 0. When I went back to one cab (4 ohm) everything returned to normal.

I've replaced a couple of things over the last few years, namely the big power cap and the 3900uf cap where it's coupled to the output. I also replaced the bridge, which was shorted, but I don't think that would have any affect on the output, but it did quit blowing fuses.

The output cap was new old stock. Do these things degrade with age? Should I start looking at the high power output transistors (2n3055)? After all, they've been in there since the early '70s.

I'm not a tech and I don't have anything more sophisticated than a Fluke multimeter. But if it's something simple like replacing the output transistors, I can probably handle that. I do have a schematic and I've got good at stopping before I let the smoke out of anything. Like Abe Lincoln said: "Good judgement comes from experience, and experience comes from bad judgement".

I appreciate the help.

[52 Bill]

When I unplugged one of the cabs the signal came back nice and strong so I left it that way for the rest of the gig.

Did you test the cabinet that you unplugged to see if the problem was speaker related?

[370Fan]

I did. There's two 1/4" jacks in the back wired parallel so it was easy to swap since I had two lines running to two cabs. I just pushed one back in and pulled the other out. It sounded the same as the other single cab. They're identical cabs with EVM-18Bs in them.

[nsubulysses]

Just to chime in, since you asked -- don't buy NOS caps. Manufacturers seem to say caps last anywhere from 7-20 years. They will degrade even faster if they are on a shelf and not in use. Its' like buying a 30 year old raincoat. Looks nice (NOS!) but then when you put it on it crackles and crumbles because the rubber is all dry rotted.

If the amp works the 2N3055s are fine. I can't think of a reason off the top of my head why it would work at 4 ohms and not 2, but I'm sure some other tech enthusiast will chime in.

[gbono]

So did the amp ever work with two cabinets (assume 2 ohm total load)? Just for grins take your DMM and measure the DC resistance of both loudspeaker cabinets (I assume you will measure the same resistance but let's rule it out as an unknown)?

I have to agree that "NOS" aluminum electrolytic capacitors are a great deal for the seller but not so much for the buyer.

[Enzo]

30 yer old caps? 30 year old cans of tuna? I wouldn;t buy either.



One test of a solid state amp is loaded versus unloaded. If an amp puts out a nice clean wave unloaded, but distorts under load, we then think the output stage cannot provide all the current the load demands. May or may not be the case, but I have no trouble believing the output tansistors are OK at 4 ohms but the doubled current at 2 ohms is more than one of them can take.

Scope the output for a clue as to the nature of the distortion.

[370Fan]

I checked the cabs and found one to be 5.1 ohms and the other to be 6.8 and combined I got 2.8 ohms. I was assuming both speakers to be 4 ohms. My bad...

I did have both of them plugged in at a gig last summer though and it sounded quite spectacular.

For now I'm taking everyone's advice and ordering a new output cap. It'll be here Monday. Emphasis on the "new".