Acoustic B800H Bass Head Finally Dies

[themusic]

I recently acquired a Acoustic B800H head. Worked great for 3 mos. and then slowly but surely died.
When first turned on, it would distort but about 30sec. later would clear up and play fine for the entire night.
A couple of days later, same thing but about 5min. to clear. This pattern continued , "up to 30mins to clear.
But now she's finally died , just a low volume distortion when played. Checked the preamp section and it works great.
Inside shows no apparent burns or fried components. I tried chopsticking for loose connections or solder joints again nothing gained.

I can't seem to find a schematic so I could did a little deeper ...Thanks

[The Dude]

Certainly not always, but often the "warmup period" is caused by an e-cap that is drying out. If you have an ESR meter, look for an electrolytic that has high ESR.

[themusic]

I just have my Fluke 83, would that work ? thanks

[The Dude]

I'm not horribly familiar with that meter. From looking online, I believe it will measure capacitance, but not ESR. You would have to remove the caps to test, which could be time consuming. If you decide to go that method, pull first caps that are near a high heat source (regulator heat sinks, etc.). If you can get ahold of an ESR meter, you can check in circuit and save some time. You could also start by checking for ripple on any power supply lines and/or seeing if any supply voltages are low. That may lead you to a dried out cap.

[themusic]

I'll get on it ... thanks

[J M Fahey]

Dear Dude, this is not a real Acoustic from mid 80's but a modern Guitar Center fabrication, made by some Asian OEM supplier, with Acoustic looks and brand which they probably bought at auction.
So wouldn't suspect leaky or high ESR in 3 month old caps.
In fact, I would not touch it with a 10 foot pole and just send it straight for servicing under warranty.

IF and ONLY IF that is absolutely impossible, I'd start troubleshooting it.
I'd start by injecting 100mV 400Hz tone in the input,
http://www.mediacollege.com/audio/to...6bit_30sec.mp3
set MP3 layer to loop or "repeat one" so you have continuous tone.
set all controls to 5, measure AC volts at effects send, Link out and Tuner out, setting the last one to Pre and Post settings to see which, if any, works, and play with volume and gain controls to see whether you can get 500mV to 1V there.
Tuner out might be same level as input signal, at least in the Pre setting.
At least to know whether it's a preamp or power amp problem, then we'll look with finer detail.
also connect a known good cable from effects send to return, then same from line out to link in ... does it restore sound?

But in any case the main course of action is: SEND IT BACK.

[themusic]

Dear Dude, this is not a real Acoustic from mid 80's but a modern Guitar Center fabrication, made by some Asian OEM supplier, with Acoustic looks and brand which they probably bought at auction.
So wouldn't suspect leaky or high ESR in 3 month old caps.
In fact, I would not touch it with a 10 foot pole and just send it straight for servicing under warranty.

IF and ONLY IF that is absolutely impossible, I'd start troubleshooting it.
I'd start by injecting 100mV 400Hz tone in the input,
http://www.mediacollege.com/audio/to...6bit_30sec.mp3
set MP3 layer to loop or "repeat one" so you have continuous tone.
set all controls to 5, measure AC volts at effects send, Link out and Tuner out, setting the last one to Pre and Post settings to see which, if any, works, and play with volume and gain controls to see whether you can get 500mV to 1V there.
Tuner out might be same level as input signal, at least in the Pre setting.
At least to know whether it's a preamp or power amp problem, then we'll look with finer detail.
also connect a known good cable from effects send to return, then same from line out to link in ... does it restore sound?

But in any case the main course of action is: SEND IT BACK.

The head is about 3years old "over the warranty I'll go ahead start with what you recommend... still wish i could get a schematic...thanks

[The Dude]

Have you tried contacting Acoustic to see if they will send it to you?

[Jazz P Bass]

Here is the 450-600.

Maybe it's close enough.

B450-600H.zip

Do give them a shout, as they have always helped me in the past.

[J M Fahey]

Yes, ask them for an 800 schematic, will make things way simpler.

That said, it should be close enough, designers often have their own pet ideas and tend to repeat them in different products, so ...
The 600 already is a beast of an amp: +/-75V rails ,driving 2 ohm loads . Hairy.
The 800 might be about the same, with , say, extra 10V rails and maybe an extra pair of power transistors.

So 100mV should be injected and we can suggest what's to be expected at different test points ... if one fails then we are close.

Notice that there are *two* switchable ICs used, one for plain muting (hint hint) and the other to select between notch/flat .
Any of them misbehaving might kill signal for good.

There is also a 3080 IC working as compressor in the power amp; if it decides for some reason that the amp should mute ... it will m ute.

The point is that we have multiple suspects, so we'd better at least reduce the search area.

IF you could get a scope or install a software one in a notebook, that would help shorten time.

[themusic]

Yes, ask them for an 800 schematic, will make things way simpler.

That said, it should be close enough, designers often have their own pet ideas and tend to repeat them in different products, so ...
The 600 already is a beast of an amp: +/-75V rails ,driving 2 ohm loads . Hairy.
The 800 might be about the same, with , say, extra 10V rails and maybe an extra pair of power transistors.

So 100mV should be injected and we can suggest what's to be expected at different test points ... if one fails then we are close.

Notice that there are *two* switchable ICs used, one for plain muting (hint hint) and the other to select between notch/flat .
Any of them misbehaving might kill signal for good.

There is also a 3080 IC working as compressor in the power amp; if it decides for some reason that the amp should mute ... it will m ute.

The point is that we have multiple suspects, so we'd better at least reduce the search area.

IF you could get a scope or install a software one in a notebook, that would help shorten time.

Great all set up for my testing , inject my 100mv ...switch on... and a short pop and everything back to normal !!!!
I probe , wiggle, smack it , and still works fine... I really need this amp to fail... gotta be something loose, solder, connection, thanks

[themusic]

Just and update...Went back thru and check , tightin and reseated all cable and wire connections and it been working with no problems.
Gotta believe one of those connections was the problem...thanks

[Mars Amp Repair]

I know this post is old, however does anyone know about the newer B600H that is made in China? It has a cooling fan so I'm supposing it is class D. Does anyone know for sure? Thinking about buying one for testing speaker & such for repair bench but don't want a D class esp is there is no ground reference on the speakers...thanx, glen

[nosaj]

I know this post is old, however does anyone know about the newer B600H that is made in China? It has a cooling fan so I'm supposing it is class D. Does anyone know for sure? Thinking about buying one for testing speaker & such for repair bench but don't want a D class esp is there is no ground reference on the speakers...thanx, glen

Looks like it has a regular transformer https://elektrotanya.com/acoustic_b4...wnload.html#dl
nosaj

[Mars Amp Repair]

Thanx all. Fortunately, i found a local good ole privately owned long standing music store that had a few of the Peavey Standard heads from the 70's-80's to choose from.
As I'm really only using this for repair purposes i went for the PA400 head that had an effects loop and power amp-in type features. I was able to get them down to $100 out the door which was less than i had planned on spending in the first place...AND does anyone really go after a 70's PA head anymore?!
Thanx again... glen.