Ampeg ba115hpt distortion after the note

[Lucaslz]

Hello.
The bass player in our band has asked me to have a look at his Ampeg ba115hpt because of some slight distortion that lingers after a note is played. It sounds a bit like a bad speaker or some rattling hardware in the amp. On the internet and in this forum I have found that a multitude of people have had a similar problem and it is usually because of a cold bias which can be adjusted, or a faulty headphone jack connection or opamp. At this point I have opened the amp up and have made a visual inspection. The only thing that looks suspicious is some dried yellow smear on top of one of the 5600uf capacitors. It seems to test okay in circuit but I will remove it and test it again. Here is a pic of the capacitor. What do you think? Electrolytic leak or somebodies breakfast?


Also, what is the proper procedure for resetting the bias??
Many thanks,
Lucas

[Enzo]

You found on the internet several causes of "distortion" all of which are unrelated to each other. In other words it could be most anything. So rather than setting out to adjust your bias, why not test the speaker? Does the distortion exist in the headphones? Can we connect the amp to some other speaker?

[DrGonz78]

Plug in some headphones and check if the output there is distorting. This will help us figure out if the distortion is coming from the preamp. No distortion on the headphones then it is on towards the power amp side of things. Also, I had one of these that rattled really bad since the plywood cab was vibrating badly. If it is the cabinet making noises that should be very easy to figure out by removing speaker and testing the amp that way too.

Here is the schematic... Page 1 has notes about how to set the bias. You will measure voltage drop across specified resistors and adjust via AP1 bias adjust trim pot. You will want to run the amp with no load or speaker attached for at least 10 minutes to get accurate bias reading. Check it out and see what it is reading.
http://music-electronics-forum.com/a...a115hptnew.pdf

Edit: Simulpost!! Enzo you are quick! lol

[Lucaslz]

Thanks for those replies and the link to the schematic, i will test the headphone output and speaker and cabinet this afternoon. Those 5600uf caps are kind of difficult to get over her in Vienna Austria, and pricey as well. Conrad which is the main supplier for consumer electronic components only sells 63v or 200 volt and the 200v version costs 30 euros per piece! I can order them from Farnell for about 9 euros but then 10 euros shipping costs are added. oh well, maybe they are okay anyway, will find out soon.
Thanks again and will get back with some answers tomorrow.

[J M Fahey]

Measurements might prove the contrary, but unless confirmed by something else, those expensive caps should be fine; that yellow smear does not mean much, might be some kind of glue and in any case what you hear does not imply they are bad anyway.

A scope might confirm whether you have a clean output or not, but lacking that, you can still make a couple tests with just a multimeter and ears

As Dr Gonz said (thanks for the schematic ) , there is a bias procedure there.
1) Since the possibility of damage is great, for now do not touch the bias trimmer/preset potentiometer called AP1 ( 500 ohm) but as indicated measure voltage drop across (end to end) R72/73/74/75 in turn, itīs unlikely they are all the same, but should be around the values suggested (15-20 mV DC) , measure and post here

2) it might be crossover distortion, which is a power amp problem rather than a preamp caused one OR a rubbing/dirty speaker voice coil, both sound quite similar (distortion trailing a dying musical note or chord) so to separate them youīll need 2 extra tests to make sure, in both cases youīll replace suspects with "known good" stuff:

a) with amp off disconnect internal speaker and connect power amp out to a known good speaker , it may even be a Guitar or PA speaker, we donīt want to play loud at all, quite the contrary, play a bass note or even better a chord (which is complex and makes distortion stand out) and let it die.
If you still hear it, thepower amp is a strong suspect; if not, itīs fine.

b) now the contrary: plug the bass into a known good amp, may even be a PA/keyboard one, not so sure about a guitar one because some are "always dirty" , again play a note or chord and let it die, keep your ear close to the speaker

As you see, I try to avoid messing with the Bias pot unless confirmed necessary, but first do these tests and postvresults.

Problem is that different to tube amps, where worst case tubes may get red hot if misadjusted, that takes quite a few seconds and in general you see it and back up, in transistors you see nothing and can easily blow them; also the original trimmer is quite sensitive, may get dirty, etc.

Just in case build a lamp bulb limiter (search it) and use it when you adjust bias, is easy to make and might save you a headache .

3) yes, it might also be a bad headphone jack contact, a distorting/unstable preamp or power amp, dirty/bad connectors, etc. , but letīs discard the standard problems first.
If not, you can go on checking other possibilities.

[eschertron]

b) now the contrary: plug the bass into a known good amp, may even be a PA/keyboard one, not so sure about a guitar one because some are "always dirty" , again play a note or chord and let it die, keep your ear close to the speaker

Have we tried other instruments in this amp? Some instruments (basses especially!) have active electronics and folks forget to check the batteries? Could that be the case here?

[Jazz P Bass]

I am with Enzo.

Try another speaker before attempting anything else.

[Lucaslz]

Okay, the current situation is as follows;
I have tested with different instruments.
I have tested the headphone output - result is no distortion.
I have tested amp with other speaker - result distortion
I have tested another amp with ampeg speaker - result no distortion
I have attempted to measure the voltage drop across resistors r72 through 75,
with no load and no signal, but this is tricky since the silicon glue covers the labels and there is no layout
In the schematic. However from the four big white ones in a row I am getting
35 volts on the one farthest from the output transistors. The other 3 are reading 0 to 0.1 mv
The other 2 big white resistors are reading 39v and 1,5 to 6mv, it seems to not stay constant.
Obviously the two resistors with the high voltage are r66 and 68 so that is probably normal.
It looks like the bias needs adjusting... Or?

[J M Fahey]

I have attempted to measure the voltage drop across resistors r72 through 75,
with no load and no signal, but this is tricky since the silicon glue covers the labels and there is no layout

One of us needs glasses because on your picture I clearly see 4 large white ceramic labelled 10 Watts , 0.33 ohms
Those must be R72>75.
Ends seem to be surrounded by short sleeves but enough metal is visible to measure.
Measure voltage end to end on the 4 staggered ones you show.
I can also see the end of nother to the right, but yu chose not to show it, so we are in the dark.

Before measuring voltage, with the amp off, measure resistance across the 4 x 0.33 ohm ones, they should read almost as shorts;then turn amp on and measure voltage across them.

In the schematic. However from the four big white ones in a row

I see no "row" in the picture, they are neither aligned vertically nor horizontally but staggered, almost in a diagonal line, please confirm; they also show labels, mention them, four large white ones is confusing if there are more than 4 .

I am getting
35 volts on the one farthest from the output transistors.

Please confirm.
I suggest a new picture, somewhat further away, showing all large white resistors.
If some label is invisible, either because itīs covered or points the other way, please edit picture and label them or write on white body with something.

The other 3 are reading 0 to 0.1 mv
The other 2 big white resistors are reading 39v and 1,5 to 6mv, it seems to not stay constant.

Please show a wider angle picture showing all, and label them if possible, seems we are talking about 6 resistors so far.

Obviously the two resistors with the high voltage are r66 and 68 so that is probably normal.

We expect some 35V across the 330 ohm ones, so please both show them in a picture and measure (with power off) that they actually are around 330 ohms.

It looks like the bias needs adjusting... Or?

So far it looks like that ... but we also might have a shorted power transistor and an open emitter ballast because we have not yet clearly separated large white emitter ballast resistors (0.33 ohms) from unseen large white preamp supply voltage dropping resistors.

So please confirm before going any further.

[Lucaslz]

Thank you Jaun for your help.
There are 6 big white resistors, 4 x 0,33ohm and 2 x 330ohm. Yes, you are correct, the 4 are staggered and not 'in a row', I was getting lazy with my terminology, and directly next to those are two more staggered. It turns out that the top one of 'the four', farthest away from the output transistors, is a 330 ohm as is the top one of the two staggered ones next to those, so the ones needed to measure the bias are the 3 closest to the OT's and the 1 closest to the OT's. I hope that makes sense. Anyway, I unplugged the amp and measured the resistance of the bias potentiometer. It read around 186 ohms. I sprayed it with some deoxit and gave it several turns back and forth, and then tried to set the resistance a little lower to about 170 ohms to increase the current. I then turned the amp back on and measured across one of the resistors(r72) and got a reading of 26ma. This was too high so I unplugged it again and set the potentiometer to around 177 ohms ( I wanted 179 but it wouldn't let me even with the tiniest adjustments it jumped between 177 and 182). So I turned it back on and measured the voltage drop again and it read about 16ma. I then tested it with speaker and instrument and the distortion was gone and the amp sounded good. With no time to loose since the band was already standing around waiting to get started, I reassembled everything, fitted the amp back in the cabinet, and we had a successful practice. Thanks to everyone's help here and previous posts I was able to get this done efficiently and quickly.
Thanks again!!

[J M Fahey]

Cool, and glad you found it.
Hey, you were somewhat impatient and lucky
You might as well blown the amp.

The reason is that those trimmers are somewhat flimsy, work reasonably well if set up at factory and not messed with but many comment on "just touching them and bias jumping wildly" .
So much so that some replace them with a way more precise and expensive "10 turn trimmer pot".

Yours already had some problem on the track, be it a burnt spot, a drop of glue, rat or cockroach pee, anything, thatīs why:

( I wanted 179 but it wouldn't let me even with the tiniest adjustments it jumped between 177 and 182)

Thanks God both settings you could achieve, either 16mA and even 25 mA were acceptable, but it might have easily jumped from crossover world to, say, 100mA oe more, which usually is followed by 2 posts:
First one: "hey guys, I solved it"
Second one (3 days later) "donīt know what happened, was working fine, and in the middle of rehearsal/show it burnt the fuse".

Thatīs why we suggested going slow, using a bulb limiter "just in case", etc.

But again, glad you solved it