Acoustic 270 - trouble in preamp

[Drummer4gc]

Hi all,

I'm working on fixing up an Acoustic 270, and I'm getting that fuzzy, farty, misbiased transistor sound at the emitter of Q101, the very first transistor in the preamp. Here's the schematic/service manual:

http://music-electronics-forum.com/a...ce-manual1.pdf

Voltages for Q101 are:

C 34v
B 1.4V
E 0.9V

Any idea what could be causing this? I'll likely go through and replace all the old electrolytics just to be thorough, but I'm not sure that would be the cause for this.

On a semi-related note, is it also recommended to replace the 1uf tantalums that are found throughout the preamp section on an amp of this age?

Thanks!

[Dave H]

Hi all,

I'm working on fixing up an Acoustic 270, and I'm getting that fuzzy, farty, misbiased transistor sound at the emitter of Q101, the very first transistor in the preamp. Here's the schematic/service manual:

http://music-electronics-forum.com/a...ce-manual1.pdf

Voltages for Q101 are:

C 34v
B 1.4V
E 0.9V

Any idea what could be causing this?

Do you have 16V at the collector of Q102?
Q101, Q102 are a DC coupled feedback pair. The voltage on Q101 C will be one Vbe (0.6V) down from the positive rail so 34V could be OK. If Q102 C is at 16V then the bias is correct.

[J M Fahey]

Hi all,

I'm working on fixing up an Acoustic 270, and I'm getting that fuzzy, farty, misbiased transistor sound at the emitter of Q101, the very first transistor in the preamp.

How do you know that?
Are you scoping it?

Voltages for Q101 are:

C 34v
B 1.4V
E 0.9V

Look very reasonable.

Any idea what could be causing this?

Not sure yet you have a problem, please answer above question.
What signal are you feeding at the input?

More important: what is the actual complaint about that amp?

I'll likely go through and replace all the old electrolytics just to be thorough, but I'm not sure that would be the cause for this.

On a semi-related note, is it also recommended to replace the 1uf tantalums that are found throughout the preamp section on an amp of this age?

Like Enzo often says: "First you solve the problem, then upgrade/improve/updatethe amp."

[Drummer4gc]

Thanks for the thoughts - let me give some context for this.

I picked up the amp and it was reported to be blowing fuses. I found two shorted 2n3055s in the power section, which would explain that. I removed them, and now I'm able to get the amp turned on and stable (no signs of overheating) with only the Q406/411 and Q407/412 pairs of power transistors inserted.

With this arrangement, I get fuzzy blatty sounding signal at both the monitor outs (so preamp only) and speaker outs when I play guitar. It's clear with a strong open low E string plucked or when a chord is played, so with hotter signals, but not using unreasonably hot pickups. This happens with both hi and lo inputs. To track this down, I've been audio probing the preamp, and it originates immediately after Q101 as I mentioned above.

I realize this isn't a particularly methodical approach for troubleshooting. I do have a scope that I can use to get a better sense of the distortion, but I felt like my ears told me enough to know something isn't right.

I'll look for 16V on Q102 this evening and get back to you all. Thank you!

[J M Fahey]

To track this down, I've been audio probing the preamp, and it originates immediately after Q101 as I mentioned above.

Ok, I see where you´re coming from.
"immediately after Q101" is an internal point of the Q101/102 gain block , and it being "a diode" (Q102 BE junction) I almost expect distorted voltage there, what matters is the signal at Q102 collector which is the true stage output: is it distorted?
I would be surprised if it is, considering DC voltage there is normal (around 17V or 1/2Vcc as pointed by Dave H).

I picked up the amp and it was reported to be blowing fuses. I found two shorted 2n3055s in the power section, which would explain that. I removed them, and now I'm able to get the amp turned on and stable (no signs of overheating) with only the Q406/411 and Q407/412 pairs of power transistors inserted.

Cool but please whenever possible replace missing transistors .

With this arrangement, I get fuzzy blatty sounding signal at both the monitor outs (so preamp only) and speaker outs when I play guitar.

It's clear with a strong open low E string plucked or when a chord is played, so with hotter signals, but not using unreasonably hot pickups. This happens with both hi and lo inputs.

I realize this isn't a particularly methodical approach for troubleshooting. I do have a scope that I can use to get a better sense of the distortion, but I felt like my ears told me enough to know something isn't right.

I'll look for 16V on Q102 this evening and get back to you all. Thank you!

Ok, the distorted sound seems to be coming from the preamp.
Check for clean signal at a few points along the path: Q102 collector, volume pot "hot" terminal and, then rising some volume to, say, 2 or 3: Q 106 emitter, Q108 emitter, Q112 emitter, Q115 collector, Q116 emitter (which is Line out).

This is a complex amp, with sound branching into many alternate paths, then rejoining again, so it´s hard to find *where¨* is the distorted sound appearing.

A scope here would be really valuable both because it saves a lot of time and looking at a waveform is way more straight and useful than hearing a sound and trying to imagine what´s happening.

Essentially not different than "listening" at every point, only you immediately see whether signal has top chopped, bottom chopped, weird DC voltage, oscillation, "gating", etc. which is not detected through headphones.

[Drummer4gc]

I'm getting 18.5V at Q102 collector, so higher than expected 16V. This is with 34.5V at the emitter, so not too much higher than expected there. Audio probe gives a clearly distorted/incorrect signal at Q102 C with moderate to heavy guitar input - same as I described before, but louder (obviously).

Happy to pull out the scope if that will help. Thanks for your thoughts!

[Dave H]

I'm getting 18.5V at Q102 collector, so higher than expected 16V. This is with 34.5V at the emitter

I think 18.5V is reasonable given that the Q101, Q102 circuit has high DC gain (x19, same as the AC gain). It's possible that your high output pickups are overdriving the input. It would only take about 0.8V peak at the input of Q101 to overdrive it. I'd check it with a scope.

[Drummer4gc]

Here's the scoped signal at Q102 C with a 0.8V p-p input signal at 1000hz:



I have to bring the input signal down to about 0.35V to get a clean signal at that point. Doesn't seem right to me.

Any other thoughts? Appreciate you all sticking with me on this.

[J M Fahey]

Here's the scoped signal at Q102 C with a 0.8V p-p input signal at 1000hz:

[ATTACH=CONFIG]49020[/ATTACH]

I have to bring the input signal down to about 0.35V to get a clean signal at that point. Doesn't seem right to me.

Any other thoughts? Appreciate you all sticking with me on this.

Please consider it is a VERY OLD design ..... 1971!!!!!!
Incredibly good for such an early age, very high power (average amp on stage was 100W) and with a first class speaker complement: 2 x Altec speakers (plus a very annoying horn).
I very well remember when one of our best Musicians showed up with a 270 for Guitar and a 370 with 2 cabinets for Bass: the Guitar would shred you to pieces and then the Bass would smash them into pulp.

But ... they did not have a time machine, they could only design for the instruments of the day .... the Di Marzio overwound high output pickups were well into the future, so they designed a very sensitive preamp (only 18mV necessary for full output ) and of course such a sensitive preamp, coupled to low rail voltage used by transistors makes for an easy to overdrive combination... by modern standards that is.
Look at official specs yourself:



It should be easy to lower input stage gain to modern standards (10X instead of 20X should be fine) BUT as Dave H mentioned above, Audio gain and DC gain are the same here, and modding one will change the other and make gain stage misbiased, distort, etc, so only option is to use the low gain input.

One problem is that input mixing resistors are very low value, so very low input impedance (only 30k), I suggest replacing R101 by 47k and R102 by 100/150k, your choice, whatever cleans Q102 collector output when strumming hard.
Might even need 220k there if you have active pickups or go through a couple pedals.

And reserve high gain input only for classic wound single coil pickups.

[Dave H]

It should be easy to lower input stage gain to modern standards (10X instead of 20X should be fine) BUT as Dave H mentioned above, Audio gain and DC gain are the same here, and modding one will change the other and make gain stage misbiased, distort, etc

Change R104 to 6M2 and R108 to 1k2 perhaps?

[Drummer4gc]

Thanks so much for all your answers. Seeing the max input in the specs sort of clinches it - I suppose it's working as designed, which is what I'm looking for. I guess I could always turn down the guitar....

Much appreciated everyone!

[J M Fahey]

Change R104 to 6M2 and R108 to 1k2 perhaps?

Maybe, whatever it takes to reduce DC and AC gain to, say, 10X and still keep output rail somewhat centered .

But, really the short and practical path is just to mod all of 2 input resistors, which to boot is easily readjustable if not enough.

Now if it were an Acoustic Reissue made by the thousands (or at least hundreds), yes, it pays to reengineer.
In fact, in that case and if I wanted to keep the single supply preamp for any reason, I would replace frst gain stage by 1/2 TL072, and call it a day, go figure.

[Drummer4gc]

Hey all, I'm back unfortunately with more/different issues.

I replaced all of the output transistors with mj15003s - two of the original ones were shorted, so I went ahead and replaced all 8. Fired up with a lightbulb limiter, bulb was dim and sound came through, so I fired it up. It seems to work, sort of. From what I understand, this thing should blow my eardrums before even approaching halfway on the volume. Instead, I can crank it to ten, and it's loud, but not that loud, and the sound is very distorted.

Things I've checked so far:

-DC voltages with no input - they are a little low but seem to be within the ballpark (I think closer to 80V/40V for the positive and half voltages, instead of 87V/45V)
-There is no DC on the speaker out
-Power Amp outputs a ~12V sine wave at full volume when I'm expecting 20V per the schematic. This is with no load.
-1000 Hz sine wave on the input with a scope: If I scope the input (before it even hits anything), after I turn the power on, the sine wave turns "fuzzy" after a few seconds. I'm not super versed in using my scope, but I can zoom in real close and see that the original sine wave now appears to be made up of a very fine sine wave. I'm wondering if this is the ripple that should be smoothed out by the filter cap, and if this is an indication that the filter cap needs to be replaced. Turn the power off, sine wave goes back to looking normal and clean.

Any ideas? I think the last issue I described seems not normal, and frankly, it wasn't happening when I was scoping the preamp in the image above. I'm just not sure if it's also related to the low volume I'm facing. It's hard to keep scoping through the whole circuit for AC values as listed in the schematic as the sine wave basically turns to mush the further I go.

Thanks for your ideas and help!


EDIT: Sorry, not trying to keep adding variables and questions. I just realized that I see the same "fuzziness" on the signal when I turn the amp on and scope the signal generator, even when it's not connected to the amp. I'm wondering now if this is due to the scope and amp being run on the same circuit.

[Drummer4gc]

Sorry, you can disregard the last message, as I've answered a few of my own questions and made some progress on the repair. I would edit the post, but that option seems to have disappeared for me...?

Anyways, I do have a couple more simple and concrete questions:

1. There appears to be about 500-600mv of noise on the preamp output, per the scope with no input signal. Zooming in, it's a wave that measures at 5MHz. What's this about?

2. I'd like to replace the electrolytics in the amp, since they're old. All of the 1ufs are tantalums. Is it recommended to replace these in an old amp too, or just the aluminum electros? Should they be replaced with tants or are aluminums ok?

3. What's the best way to calculate idle current on this amp? I've read to take a voltage measurement across the resistors on the output transistors, so R422 or R423. I see a ~9.9mv drop across this resistor. Can someone check my math? .0099V / .1 ohm = 99ma idle current. Is this accurate and/or reasonable? And how can I check that against the 50W at idle specification given in the service manual?

Thanks!

[Enzo]

I suspect the 50w at idle spec is for mains draw, ie roughly half an amp from the mains at idle.

99ma seems high to me, I think you are going to need to bias it cooler. I suspect the MJ15003s are running hotter than the original transistors.

Is that 5MHz noise signal present in the power amp too? on the power supply? On the preamp power supply? All through the preamp or just at the output? Half way along the preamp?

You have DC voltages on the schematic at various points in the preamp, how do yours compare?