Acoustic 370 slight distortion (at all levels) and bias?

[johnk_10]

Hi, i recently picked up another Acoustic 370 head (after selling my old ones years ago). it's as loud as i remember them to be, and sounds great, except for the fact that it has a very slight distortion going on in the background when playing at any level. when my current 320 head had the same problem, i adjusted its bias pot and it is now perfect, but the 370's bias is set with fixed resistors. i know that the factory specs stated that the 320 is supposed to draw 45-50 watts AC at idle (mine 320 sounds great drawing 42 watts) but my 370 is only drawing 26.8 watts at idle. i'm very well versed in vintage tube amps (for more than 30 years), but not so with SS amps like this. hopefully someone on this forum has run into this problem beofre and is familiar with the solution for it. any help would be greatly appreciated.
BTW,if it helps, this 370 is 100% original and i haven't seen any evidence of anything ever being repaired or any component ever replaced on it.
thanks in advance!

johnk

[gbono]

What does a sinewave look like at the output? Crossover distortion? Also see note on schematic: adj R304 for symetrical clipping with 2 ohm load and s301 closed (AGC defeat)..........

[johnk_10]

hi, i haven't put it on the scope yet, but i did try adjusting R304 slightly with no difference at all. should i not have done that?

[Jazz P Bass]

If the amp is only drawing 26 watts at idle, then "something" is not conducting.
You need to take some basic voltage measurements.

[johnk_10]

i have taken a few voltage readings...............
on the main 7800/100V cap C403 i'm getting 86.3V. on Q304's emitter/collector i'm getting 42.54V and 44.71. on the output cap C404, i'm getting 43.39V and on C405 i'm getting 60.0V. all of these voltages are pretty close to the spec on the schematic that i have for it.

[johnk_10]

Oh, and BTW, i've run the preamp out signal to a powered speaker cab and it's absolutely perfectly clean with no slight distortion in the background, which leads me to believe that what ever is causing it is in the power amp.

[Enzo]

Well, you really shouldn;t adjust R304 if you don;t know what it does. This amp has a single supply. That means the output sits at half the supply voltage and is fed to the speaker through a large cap to block the DC. R304 is the offset adjust, you turn it so the output is exactly half the power supply voltage. However, a volt or two either way will rarely matter.

You really ned to scope it to find out what the distortion looks like. We can talk all day about bias and crossover, and it will do no good if the problem is leaky diodes at the power amp input. Or if Q315 is hanging up. Scope across a load, scope on the amp side of the output cap. Scvope om eiother end of that resistor at the input

The bias is hard wired, but it would not be complex to replace R319 wuith something adjustable.

All those small e-caps throughout the amp should be replaced. Those 1uf, 2.2uf and such. They are in the signal path. Your preamp may sound OK, but I bet those are leaky anyway. Down in the power amp, ther is one right at the input.

[gbono]

Info on complete rebuild here http://music-electronics-forum.com/t22926/

[johnk_10]

Info on complete rebuild here http://music-electronics-forum.com/t22926/

yes, i saw that. i'm not sure that it pertains to me, since it appears that just about everything was replaced, but the pics of the PCB's was very helpful in making the part #'s easier/quicker to locate from the schematic.

i plan to put mine on a scope today as per Enzo's recommendation.
and many thanks to all that have replied thus far.

[johnk_10]

Well, you really shouldn;t adjust R304 if you don;t know what it does. This amp has a single supply. That means the output sits at half the supply voltage and is fed to the speaker through a large cap to block the DC. R304 is the offset adjust, you turn it so the output is exactly half the power supply voltage. However, a volt or two either way will rarely matter.

You really ned to scope it to find out what the distortion looks like. We can talk all day about bias and crossover, and it will do no good if the problem is leaky diodes at the power amp input. Or if Q315 is hanging up. Scope across a load, scope on the amp side of the output cap. Scvope om eiother end of that resistor at the input

The bias is hard wired, but it would not be complex to replace R319 wuith something adjustable.

All those small e-caps throughout the amp should be replaced. Those 1uf, 2.2uf and such. They are in the signal path. Your preamp may sound OK, but I bet those are leaky anyway. Down in the power amp, ther is one right at the input.

thanks! i really appreciate it. i'll put it on the scope today.

[olddawg]

I would venture a guess that when you crank the amp on a scope that you will find that your output is nonsymetrical with some output transistors not conducting. Probably because of an open resistor or bad connection. And...caps are cheap.

[johnk_10]

I would venture a guess that when you crank the amp on a scope that you will find that your output is nonsymetrical with some output transistors not conducting. Probably because of an open resistor or bad connection. And...caps are cheap.

i have measured just about every resistor on the power amp module and so far they all appear to be within spec. i also ohmed out all of the output transistor connections and they're all good too.
i may end up replacing all of the small electroytics as per Enzo's recommendation, but first i have to dig my scope out and test it. i'd like to first see if i can solve its distortion problem with replacing as few parts necessary.

[johnk_10]

Scope results

here's a few pics of the results that i got on the scope (all of them taken with a 2 ohm load)..........

preamp output @ 600Hz sine wave:



output before the output cap:



output at the speaker jack:



they all actually look pretty good to me(?)

[johnk_10]

........and if it helps, here's some pics of it when i crank it into distortion, both sides of the sine wave appear to start clipping at the same time and the same amount.

onset of clipping:



heavy clipping:


full clipping:

[Enzo]

Those pictures look pretty good, is that with or without a load?

If an amp makes a nice output without a load, but the waveform collapses with a load, then the output stage is not able to provide the current demanded.