More Acoustic 470 issues

[gbono]

Hello,

I have a problem with a 470 which is driving me crazy trying to diagnose. The amplifier plays fine until the output transistor warm up. After the OP transistors heat up the amplifier starts to clip (ie.distort) the signal.

I have the exact same problem with a 150. I have replaced all the electrolytics on the output board, replaced OP devices and check all the resistors. I assume that both amps have similar problems but we all know about assumptions.
THX

[R.G.]

Hello,

I have a problem with a 470 which is driving me crazy trying to diagnose. The amplifier plays fine until the output transistor warm up. After the OP transistors heat up the amplifier starts to clip (ie.distort) the signal.

I have the exact same problem with a 150. I have replaced all the electrolytics on the output board, replaced OP devices and check all the resistors. I assume that both amps have similar problems but we all know about assumptions.
THX

This is an issue with the Class AB biasing not thermally compensating the output transistors. I'm not familiar with the 470, but it's usually a string of diodes plus a resistor or two or alternatively a smaller-signal transistor connected between the bases of the two complementary driver output devices.

[gbono]

Thanks for the reply. The Acoustic Control amplifiers that I have seen all seem to be quasi-complementary output stages (these are 1970's technology). The 150 does employ several varistors in the power output stage circuit. I have tried to test these but do not have a data sheet. They seem to function and have the same behavior as "good" devices.

I have even replaced the heat sink compound on the output devices. The bias resistors on the output devices also check out as well.
THX

[R.G.]

If you can post or email the schematics, I'll point you to a solution, even if it's replacing the bias network with a newer and better one.

[gbono]

R.G.

The 150 schematic is 350KB and I'll email it to you if you send me your email address. The 470 file is almost 2MB.

THX

gbonosurf@yahoo.com

[R.G.]

You have mail.

[gbono]

Thanks R.G.,

I've sent the schematics. Just wondering if the problem is the supply sagging during hard program/input. I did a very simple inital check of the supply just to make sure it was putting out at least 70VDC at rest.
Also remember checking the supply line with a scope and during hard program the line would drop >5V (this with the scope DC coupled).
THX

[R.G.]

Got the schems.

ON THE 470:

It appears that there were two models, one presumably earlier with diode-string bias and a possibly later one with an active Vbe multiplier bias.

For your testing purposes, find two junctions: First, R420 and CR411, second
R418 and CR410. Short those two points together with a wire. Then try the amp again.

The amp will sound terrible, as this will induce gross crossover distortion, but let the outputs warm up and see if you get the later distortion you were experiencing. If so, you have a bias issue and that's easy to fix.

If the same problem still exists, the bias is not the issue.

ON THE 150:
ACK!! That's a transformer-coupled totem pole output!

But I still think it's a bias issue.

I need two sets of measurements to tell what's going on. Can you measure the voltage on the collectors of Q304 and Q306 both when the amp is just turned on and cool, and also when it has gotten hot and is distorting?

Do the first set with no signal and the amp cool. Do the second set after running a signal for a while, having it start distorting and then removing the input signal.

[gbono]

R.G.

I'll look at the 470 tonight. Do you want a resistive load on the output when measuring Q304/306? I think if have that data somewhere or will re-test.

THX

[gbono]

R.G.,

Okay I retested the 150 with the following results:
Collector voltage on Q304 Vce= 74.1 V cold Q306 Vce=37.6 V cold

I played through the amp until the heatsink was hot and monitored Vce and output waver form - NO CLIPPING!!

Collector voltage Q304 = 71.2 V hot and Q306 = 35.7 V hot

Here's what changed - switched line voltage source from old Variac to dedicated 120V/20A socket.

Any comments?

THX

[R.G.]

Yeah - hook it back up to the variac and run it until it distorts. Then measure the AC voltage getting to the input of the amp from the variac.

You may have discovered that you have a failing variac.

[acousticcontrolcorporation]

www.acousticcontrolcorporation.com

[Bruce / Mission Amps]

www.acousticcontrolcorporation.com

If you are going to take the time to type... why don't you participate instead of leaving your website URL?
How about just sharing some info like the rest of us.
Otherwise your just another annoying free commercial here and that is not in the true spirit of AMPAGE.