Rule #1 in guitar amp repair - Check the obvious!

[Axtman]

I spent WAY too much time fooling around trying to figure out why a friend's amplifier did not work only to realize that one of the speakers was blown. The amp was a Peavey chorus amp with a nonworking chorus. The way the chorus works is that only only of the speakers gets the effect (the blown one). I could get the chorus effect in headphones but not through the speakers. I checked about every part in the power amp section to no avail. I checked every solder joint and even installed a new headphone switching jack. Finally I get the bright idea to switch speaker leads and not connect the "good" channel. The "bad" channel worked on the other speaker. Now I want all that wasted time back!

[Jazz P Bass]

I just fixed a Yorkville powered speaker amp.

Nothing major.
An aweful lot of cheezy solder joints.

Ha Ha!

Both speakers are blown.

[Justin Thomas]

Well, how many posts do we get that go for 50 posts and the issue is never resolved, then some wise Old-Timer will say, "tubes are in sockets for a reason..." And so after two PTs, 40 resistors, a thorough cap job, 5 speaker swaps, every joint resoldered, and right before the OP buys a new MM $$$OT$$$, we find out it was a bad 12AX7 in V1...

There's a list in Jack Darr' s book, and on RG Keen's Geofex page. USE IT! I'd also throw in, check the easy stuff first...

Justin

[Axtman]

Thanks Justin,

Since there is no blackboard nearby and I don't have any chalk, I will write Jack Darr's list here....but not 100 times.

From Page 51 of "Electric Guitar Amplifier Handbook" by Jack Darr

"Likelihood of failure come in this order:

1. Tubes
2. Power supply
3. Components - Resistors, capacitors, and controls
4. Cables - plugs and wiring between the guitar and the amplifier
5. Transformers - output transformers, speakers and power transformers"

[Justin Thomas]

Enzoism #34,318: "never look for an excuse NOT to check something!"

Justin

[Enzo]

Enzoism #1, isolate the problem. First thing I do is determine if the problem is the amp or the speaker. If the speaker, is it the speaker or the wires. If the amp, is it preamp or power amp. Basically what I call the front counter checks - things you can do while the customer stands there with his amp on my counter.

As long as transformers are at the bottom of your list, I;d say it is a good start on a list.

[Axtman]

How about:

Don't assume anything. We all know that when you assume you make an "a__" out of "u" and "me"!

I assumed that the speakers were good. I mean who has ever heard of speakers going bad?!

[nickb]

Thanks Justin,

Since there is no blackboard nearby and I don't have any chalk, I will write Jack Darr's list here....but not 100 times.

From Page 51 of "Electric Guitar Amplifier Handbook" by Jack Darr

"Likelihood of failure come in this order:

1. Tubes
2. Power supply
3. Components - Resistors, capacitors, and controls
4. Cables - plugs and wiring between the guitar and the amplifier
5. Transformers - output transformers, speakers and power transformers"

In my experience #1 is connectors
These are so common between PCBs . Don't forget the jacks and their switch contacts. Often these are nasty, hard to pin down intermittent faults.

[mhuss]

Back in Darr's day, most amps were point-to-point, with some tag/turret boards. No connectors to speak of, everything was soldered.

Unfortunately, nowadays, I think you may be right. For example, take this DSL... (please.)

[nickb]

Back in Darr's day, most amps were point-to-point, with some tag/turret boards. No connectors to speak of, everything was soldered.

Unfortunately, nowadays, I think you may be right. For example, take this DSL... (please.)

Yes, indeed - it's called 'progress'.

[Mick Bailey]

Here in the UK we have plenty of mains plugs with screw terminals. These come loose or can be badly fitted. I had a Roland SS amp in that would break up at high volume. OK with quiet playing, but hit the strings and the thing would buzz loudly. Just loose plug terminals, but sounded like the amp had a serious fault.

[christarak]

No. Takes this TSL first, then the DSL I'm currently working on, then take mhuss' DSL.
Really? So many PCB's in a guitar amp which sounds like angry bees stuck inside a shoe box? Really? How could Marshall have ventured so far from their once noble sounds. Its sad.

[christarak]

Same in Australia, Mick. I had a Tech 21 amp with an intermittent problem. The only way I eventually found the problem was with a ever-so-slight rattle in the power plug.

[Tone Meister]

...I mean who has ever heard of speakers going bad?!

Funny you should ask. About 5 years ago I "repaired" an Ampeg that would drop all volume after 20 minutes of steady playing. The owner had previously taken it to three (3) reputable shops and all had returned it "fixed" along with the appropriate repair charge, one of these being a self-proclaimed "Ampeg Expert." By then, with two failed repairs under his belt, the owner asked the Ampeg Expert to please play test the amp for an extended period before returning it. That didn't happen because the tech was of course an Ampeg Expert and didn't need to play test it. So the owner paid the bill, took the amp home, and after a few minutes of play time the amp lost volume again, making for a third failed repair. With the amp owner frustrated, the amp sat in the corner of his studio for 10+ years until 2013 or so.

So fast forward, I was commissioned to do some recording at that studio and asked about using the amp on some tracks. Got the full, frustrating story then offered to check it out.

So short version, took the amp home with me on night #1 and checked it out. Worked perfectly for 20 minutes, then it cut out on me. Pulled an Enzo and checked the hot amp into another speaker cabinet - the amp worked fine for an hour or so with no problems. Then did a 9v pop test on the speaker and the cone jumped as if the speaker was good. Then did the reverse, tried another amp into that speaker and after 15-20 minutes - you guessed it - that amp dropped volume! So I broke out my trusty Z meter and checked the impedance and that 8-ohm speaker measured 18 ohms. Voice coil was opening up once it got hot and would drop all volume.

Installed a used speaker from a HRD and that amp is still kicking 5 years later after lots of studio and live use. So yes, speakers do go bad and not all of them will die a complete death.

[bsco]

Thanks Justin,

Since there is no blackboard nearby and I don't have any chalk, I will write Jack Darr's list here....but not 100 times.

From Page 51 of "Electric Guitar Amplifier Handbook" by Jack Darr

"Likelihood of failure come in this order:

1. Tubes
2. Power supply
3. Components - Resistors, capacitors, and controls
4. Cables - plugs and wiring between the guitar and the amplifier
5. Transformers - output transformers, speakers and power transformers"

Right on.....Actually I just purchased that book on Amazon....Excellent book.....