Acoustic Control 450 Base Amp - Check Up / Tune Up Low Output

[TomCarlos]

MEF Members….

A friend asked me to perform a checkup of an Acoustic Control 450 Amp Head. When first turned on, I could tell the pots were dirty and needed cleaning. After doing that and cleaning the jacks, I have output. But I do hear a slight distortion and for an amp that is supposed to produce 170 watts at 4 ohms, I am no where near that. And no, I am not talking about the Distortion circuit. I am talking clean signal.

This amp uses the 170045 Power Board.
The Power Supply measures at 90.3 vdc.
It appears that all of the Electrolytic caps are original.
At the Output Blocking Cap (2000uf), I am measuring 47.9 vdc on the plus side and 5 mvdc on the minus side.

To check the output, I inserted a 900hz signal on the input. Tone controls are flat. With my scope (and HP 400E voltmeter) I am measuring 28 vrms with no load. My ground probe in on the main filter cap (-) lead. The signal looks relatively clean. I do not see crossover distortion (see below for discussion on cross over distortion). But the minute I attach a 4 ohm load, I measure (a paltry) 4.0 vrms before the wave form starts looking non symmetrical. This is just see a fraction of what this amp should be doing. I can also hear the output transistors singing. The power supply drops to 88 vdc.

I pulled the covers on each of the output transistors They appear to be the same type – STC 480037. But perhaps some were replaced at a point in time because the logo looks slightly different. I’d think if they were all original that they would look the same. I have not checked them yet.

I found a thread from 2014 – Thread 37239. In that thread, “vertamps” and “Enzo” are discussing the bias control and crossover distortion. So I will tackle that once I resolve the low output problem.

Before investing too much time and effort into this, I guess I am trying to get a baseline of what I should be looking at first and make a recommendation to the amp owner.

Suggestions would be appreciated. In the meantime, I am going to pull the output transistors and do a quick check with my Peak Atlas Transistor Tester.

Thanks!

[Leo_Gnardo]

That output DC blocking cap may have lost its microfarads. Had one do that a couple years ago in an Acoustic 370. Easy test, simply parallel the one you have there with 1000 + uF cap, it doesn't have to match the existing cap's uF rating BUT it must have at least the same voltage rating. If that brings back your signal then your decision whether to order the same cap or punt with what you have on hand.

In another recent thread Randall was puzzling out a failed 370. I mentioned I'd replaced the original 3900 uF cap with 2200, and it still had plenty of bandwidth down in the boomity-boom region. It was a matter of time - of course I could have ordered up a similar value cap, probably 4700, but the crustomer was in a typical "gotta have it right now" hurry. I figured not much to worry about trimming a bit of low end fluff. He got his amp done fast & was happy as a clam.

Of course the 450's problem may turn out to be something else entirely. I had an idea there's a limiter involved - many of the Acoustic amps of this period had one built into the power amp - and the resistor that sets the limiter's threshold had a tendency to drift over time often clamping the output at some low level. If the output cap fast check & simple fix bears no fruit then let's see what about that limiter next.

[TomCarlos]

Thanks Leo....

As per your suggestion, I tacked on a 1000uf across the existing 2000uf cap - no change. I will pull the 2000uf to check it outside the circuit.

[Leo_Gnardo]

Thanks Leo....

As per your suggestion, I tacked on a 1000uf across the existing 2000uf cap - no change. I will pull the 2000uf to check it outside the circuit.

Then we're onto the rest of the power amp. There are some quirks in that circuit, for now I'll have to leave it to those better equipped with solid state knowledge to point out what may be amiss. FWIW one item that sometimes catches me up is the pair of current limiting transistors, sometimes one or both are fouled up and that will clamp the output power to some very low level as you are seeing. Q414 & 415 plus associated diodes, resistors and possibly that MJ431 - Q403. But, I'll admit I'm outta my comfort zone so I'll leave it to the transistor boffins to carry on from here.

[TomCarlos]

Thanks again Leo...

I will look at those components.

The DC Blocking Cap measured 2190uf and ESR = .09 ohms.

I pulled each of the power transistors. They appear to be ok. The HFE ranged from 10 to 72.

[Leo_Gnardo]

Thanks again Leo...

I will look at those components.

The DC Blocking Cap measured 2190uf and ESR = .09 ohms.

I pulled each of the power transistors. They appear to be ok. The HFE ranged from 10 to 72.

Wow, that's kind of a wide range of Hfe isn't it? I'd expect the ones with higher Hfe to be turning on whilst the lower ones sorta remain asleep. Where's our crew of solid state experts? Hope they'll show up soon.

[Enzo]

You are measuring hfe at meter voltages. And tiny currents. WHen these things are cranking it changes. And besides that is what ballast resistors are for. Not worried.

The output looks good until it is loaded. That tells me the output stage cannot provide current. Doesn't mean the outputs themselves are bad. COuld be an open resistor or a cracked trace or a broken solder joint.

[TomCarlos]

Thanks Enzo.... I am looking at the physical connections carefully. Will report back.

[TomCarlos]

ACC_450_Output.JPG

Hey friends...

I was laid out with a back injury for awhile. I return to MEF and everything looks different!! In any event, I want to solve the mystery of this amp.

Where are we? I removed the power transistors and cleaned the leads going into the sockets. I checked the emitter resistors of the same transistors and they measured fine. I reflowed the solder joints on the board. The amp is in the same condition. I can get a 28 vrms output swing with no load. But the minute a load is on the output, I get a fraction of that. I can only get to 8 vrms with an 8 ohm load.

I measured the dc on the output transistors.

Q407
E = 47.2
B= 47.8
C= 89.7

Q409 and Q411
E=47.1
B=47.2
C=89.5

Q406
E=0
B=.7
C=47.3

Q408 and Q410
E=0
B=.074
C=47.2

It is as if the combinations of Q409/411 and 408/410 do not have the proper forward bias voltage between the base and emitter leads. According to the schematic, the base of Q408 and 410 are set at .2 volts? Is that right?

And referring to Thread 37239, where Enzo talks about the voltage difference at the bases of Q404 and Q405... I am measuring 49 vdc at the base of Q405 and 47.9 vdc at the base of Q404.

Does all this look ok so far? Maybe Leo is onto something in his reply. But before I go there, I wanted to get a check on the above.

Thanks...

[Enzo]

So all the transistors can put out full voltage under NO load, but not loaded. SO look on the amp side of the speaker cap. Does the signal level drop there too?

Look lower right corner, the speaker does not connect to ground, it connects to that resistor. R433? or maybe 455? Can't read it. Is that open or gone way high?

[TomCarlos]

Enzo... Yes, the signal drops on the (+) side of the 2000uf cap.

R433, .03 ohms, it is ok. Too low to accurately measure that value. So I am reading the resistance of the test leads.

I also tested R505 (680 ohms) and Rr04 (33K ohms), the resistors that come off the Booster Out.

[TomCarlos]

Ok... some progress.

I pulled the power amp PC board which enabled me to work on some connections. I changed out a couple mica insulators and now, I get 100 watts on a 4 ohm load, clean signal. The amp is rated at 170 watts, 4 ohm, less than 5% distortion. The image below shows the bottom edge flattening before the top. The 21.75 vrms with the waveform shown is 118 watts. I have no idea what 5% distortion looks like.

So is this as good as it gets?

[Enzo]

My concern on the 0.03 ohm resistor was not that it might read 0.06 or something. I was concerned it might be open or sitting at 10k or something. In other words a gross failure.

WHich board do you have, the 170045 or the 170051?

What is your main power supply? Schematic says +92v. I don't care if it is 90 or 96, just want the number. THEN what DC voltage does your output sit at. Not the speaker lead, but the amp side of the speaker cap. It should be close to one half the main supply, on the drawing 46v. Look farther left and see "Detail B", that is the centering circuit. That is adjusted to center the output voltage. The farther OFF center it is, the sooner you start clipping on one side. Just a possible detail.

[TomCarlos]

Thanks...

The board is the 170045.

I found the symmetry pot (blue pot in the photo) and made the adjustment. The supply is 90 vdc. The dc output is now 45 vdc. With no load, the signal is now symmetrical.

Unfortunately, the amp is once again doing what it did before, partial power when I apply a load. As I increase the signal, I can hear the output transistors whistling. So something must be intermittent. I wonder if I have a bad output transistor or the whole bunch needs replacing....

[g1]

Is your load floating? If I hear something singing (like you said the output transistors), I often find something working hard. If your load is not floating, and the speaker (-) gets grounded, it shorts out R433. This can cause mayhem.