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DISTORTED OUTPUT

Started by DrewV, March 23, 2011, 05:59:22 PM

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DrewV

Hello folks,
I have a Beta Bass amp that is distorting at the speaker output. Supply voltage is good. Can't find any bad transistors. Preamp out from the back panel is clean, but power amp in is distorted like the speaker output.
The amp sputters and cuts out. The power amp has plenty of output level but I have to crank the "Drive" control ( first gain stage) At this point it easily goes into self-sustaining mode. At low "drive" level the amp acts like the signal is being compressed and distorted at the same time. Has anyone seen these symptoms before? Thanks in advance DrewV

Enzo

Try a patch cord across each FX loop.

DrewV

Enzo, I rechecked the fx loops, no difference. Diodes CR4 & CR10 read shorted in circuit but good with one leg pulled. They arejust before  L1 going to speaker output.  I powered the amp up with those diodes still disconnected on one leg and the amp sounded the same!

J M Fahey

Google the schematic and post or link to it, , so we all talk the same.

DrewV

So far these are the best schematics I could find.

J M Fahey

Fine. Definitely better than nothing.
I've cleaned it a little, check the attachments.

Start by unhooking the speaker and testing it with another amplifier, does it work properly?
Now the opposite: try your Sunn with another, known good speaker.
Then, with no speaker attached, do you have any DC at the speaker out?
There seems to be a connector between the board output  (TP6/L1/R52/C15) and the actual speaker jacks.
Drop a tiny amount of WD40 or similar on its legs and push/pull it a couple times, it may be rusty/grimey.
Wipe any excess.
Do the same on output jacks, work a plug in/out to self-clean it, always aided by a little WD40; wipe any excess afterwards, we don't want dust clinging there.
Do the same on the input side, on the left side of C1.
Lift its left leg, inject clean sound there (a CD/MP3 will do).
Any improvement?

DrewV



Start by unhooking the speaker and testing it with another amplifier, does it work properly? YES
Now the opposite: try your Sunn with another, known good speaker. OUTPUT DISTORTEDThen, with no speaker attached, do you have any DC at the speaker out? NO DC
There seems to be a connector between the board output (TP6/L1/R52/C15) and the actual speaker jacks.
Drop a tiny amount of WD40 or similar on its legs and push/pull it a couple times, it may be rusty/grimey. DEOXIT, not the cure
Wipe any excess.
Do the same on output jacks, work a plug in/out to self-clean it, always aided by a little WD40; wipe any excess afterwards, we don't want dust clinging there.
Do the same on the input side, on the left side of C1. DONE PREVIOUSLY
Lift its left leg, inject clean sound there (a CD/MP3 will do).Injected sound at MASTER ACCESSORY POWER AMP LINE IN. Same result.
By the same token, the preamp out from the same place yields a normal output.
Any improvement? No

here's what it sounds like:  http://www.drewswebsite.com/beta_bass_output_distortion.htm

DrewV

UPDATE TO LAST POST: Q7 on the power board is bad. All other transistors are good. My Question, Is this the cause of my distorted output. The collector of Q7 is tied to the Base of Q1 via R1 and also to CR11 and also to the collector of Q1 via R3-150u/5w. 

J M Fahey

#8
Well, I wished with all my hart it was some "mechanical" problem, unfortunately no we'll have to get into real troubleshooting.
Now I'm in a middle of a busy Saturday (*everybody* has an "important show" tonight), but tomorrow or Monday I'll suggest some voltge readings.
A scope would be great, but ....  :(

EDIT:
Saw your last post:
QuoteQ7 on the power board is bad
What does it mean?
Is it open ... shorted ...smoking ... lacks a leg ... ?
Anyway it must be replaced.
EDIT2:
Why did it go bad?
Maybe Q8 , 9 , or 10 is bad and killed it.
Most spectacular bad is short, but open is just as bad, and often goes unnoticed.

DrewV

 Q7 Base to emitter open
All other transistors check good with tester thanks

J M Fahey

OK, so it was killing the upper half of your waveforms.
That's why you *had* audio (the lower half) but real kinky.
Hope that's the only problem, but *maybe* some other transistor downstream is open, *or* the ballast resistor(s).
In a cascaded current gain arrangement like that, when one transistor opens, the one upstream goes from driving it (a relatively light task) to driving the load impedance on its own, about 20X the work it's supposed to do.

DrewV

JM
I use  a dynamic transistor tester, so I I'm confident none of the other transistors are bad. the transistor upstream from Q7 is Q1 TIP 29, (I think that's what you mean), which did get very hot once or twice, but it's a stout looking thing with a huge heat sink on it. It likewise tests good. R 25 (125ohm/5W- collector Q11) is 10% out of spec as is R52 ( 2.0ohm/5W- just prior to spkr output) is 25% out of spec. The only other thing I see right now is C1 (2.2uf/25V NP). It builds to OL rather quickly.
So, including Q7, that's what I'm going to replace. I've got an order into Mouser. I should be done by Wed. I'll get back with the results. Thank you for your insight. Drew

DrewV

 Installed new parts. Amp is working. Thanks again DrewV

joecool85

Quote from: DrewV on April 03, 2011, 02:41:19 PM
Installed new parts. Amp is working. Thanks again DrewV

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