Acoustic 370 turn on thump

[Randall]

I have an Acoustic 370 head that the customer loves and has had for decades. He says it works just fine, except it has developed a thud and a kind of a short squawk on power up. The three filter caps in there are enormous, and expensive. Would you think replacing these 45 year old dogs would do the trick? I'd hate to sell him a pricey cap job and then not have it solve the issue.

And while I am in there I figure I should also do the other 25 or so low voltage lytics, can't hurt right?

https://music-electronics-forum.com/...8&d=1549502376

[Enzo]

I don't assume it is a big cap. Of course could be, but I don't remember ever having to replace the main power cap. yeah, I see Mouser getting $44 for an 8100uf at 100v.

But isolate the problem. Power amp or preamp?

There is a power up mute, the mute circuits are lower left in power amp, Q315 is the muting element. C315 has to charge up to turn the mute off. If that cap wears out, it unmutes instantly. Lets your thump through.

Look at the line out jacks while it is running. Is there any DC sitting there? (as if C127 were leaky) In fact you could lift C127 in the preamp there and see if the thump still happens. C127 is the preamp output cap.

Frankly, C127 and a number of other caps are little 1uf electrolytics. I suspect them way more than big caps.

Disconnect the speaker, and connect a line out jack to some other amp. Is the thump noise present there?

We don't toss a bunch of big caps at it hoping it was the problem, we find out where the problem is, then fix THAT.

[Leo_Gnardo]

I have an Acoustic 370 head that the customer loves and has had for decades. He says it works just fine, except it has developed a thud and a kind of a short squawk on power up. The three filter caps in there are enormous, and expensive. Would you think replacing these 45 year old dogs would do the trick? I'd hate to sell him a pricey cap job and then not have it solve the issue.

And while I am in there I figure I should also do the other 25 or so low voltage lytics, can't hurt right?

https://music-electronics-forum.com/...8&d=1549502376

Yep, there was a link just a few days ago to a MEFster who laid out the whole procedure from soup to nuts in about 2009. I copied it over to my files, but woops didn't make note of the thread ID number. Poke thru the last 2 - 3 days entries, you'll find it in a couple minutes. A very handy guide to have on file!

If nothing else, it's always a good idea to replace the DC blocking cap on the speaker output line. If that ever failed short, your amp would be in deep doo doo - and worse for your speaker. I had one fail a few years ago but it lost capacitance without passing DC thank hevvins. Made for a thin rasty nasty tone.

Since it's a single ended power supply, you get thumps on switch-on and switch-off while that output cap charges/discharges. To keep some customers satisfied I've added a standby switch that simply keeps the speaker disconnected while the power supply charges up. It's not a bad idea to let the amp drive an internal resistor while charging, to keep the DC level at zero when you do switch your new standby into operate mode. the resistor can be say 1000 ohm 5 watts, it just has to discharge the DC, doesn't have to simulate an actual speaker load. You can steal the "ground" rocker switch for this purpose, or mount a toggle switch on the back panel. Or any other kind of switch you want, as long as the contacts can handle the output current this amp can deliver.

[g1]

Yep, there was a link just a few days ago to a MEFster who laid out the whole procedure from soup to nuts in about 2009. I copied it over to my files, but woops didn't make note of the thread ID number. Poke thru the last 2 - 3 days entries, you'll find it in a couple minutes. A very handy guide to have on file!

I think he could have shortened it up a bit if he'd said 'just go ahead and replace everything already'.
https://music-electronics-forum.com/...ad.php?t=22926

[Leo_Gnardo]

I think he could have shortened it up a bit if he'd said 'just go ahead and replace everything already'.
https://music-electronics-forum.com/...ad.php?t=22926

Thar y'go... thankee g1 ! Randall this is it. Also good advice from Enzo. See, I never knew about that transistorized mute. Although I did know about the power limiter circuit, which in old Acoustics is always out of whack.

I have a very demised 370 sitting here way too long, and with covid-sheltering time on my hands I'm sure its owner will be pleased to have it working again. With the handy guide, I needn't flail around as I usually do with transistor amps. For this one, a brace of 15015's to start...

[Randall]

I have no dc on the line outs, labled monitor out on this amp. I do however have a slowly rising dc on the speaker out if it's unloaded. It goes from 0 to 3.5 - 10v, depending on which time I measured. 0v with a speaker connected.

I get no thump thru the monitor out jacks, there is a slight little quick rising squawk, but it's so small as to be not an issue. It may be what I described earlier along with the thump, having been amplified by the power amp.

Sadly, the components are not marked on the boards, so rather than spend hours chasing tails, I will advocate changing all the board lytics at once and see where that puts me. Would a 10uF sub make for a slightly longer mute state for the 6.8uF C315?

Leo, by output blocking cap, you mean the big 3900uF, correct?

[Leo_Gnardo]

Leo, by output blocking cap, you mean the big 3900uF, correct?

Yes, that's the one. In fact I subbed in a 2200 uF radial lead cap because that's what I had on hand. Didn't notice any audible reduction in low bass response, although I'm sure measuring meters/scope would reveal a bit of loss there.

[Randall]

Enzo,

I still get the thump after replacing the mute circuit cap C315 (it is 6.8uF, I subbed a 10uF), and lifting the preamp output cap C127. Also did it with the 3990uF output blocking cap subbed to a 2200uF like what Leo did.

Any more ideas? I am still leaning towards a full board re-cap to see where that gets me.

[Enzo]

Ome more before I give up on th mute, with power off measure resistance of JFET S-D. Should read low resistance.

[dmeek]

The FET mute circuit just mutes the audio at the input of the power amp. Isn't the thump caused by the output of the power amp rising to +45V at turn-on?
The only solutions would be to slow the power supply rise or add a speaker relay with delayed switching. Or as Leo said, a standby toggle switch with a resistor
from the output cap neg to ground

[g1]

I didn't think there was any verification that there was no small thump on the power amp input being amplified.
The mention of rising DC on the output was with no load, I think. So just the output cap charging.
Maybe I mis-read something.

[Enzo]

The thump appears at the output, we don't yet know just WHERE it is coming from. I first looked at the mute to eliminate it or accuse it. We need to narrow it all down. We just don't want to think up reasons not to check something.

This model has always had to ramp up the power supply, but the OP reports the complaint was that it had recently developed a loud thump that it didn't have before.

If all of them had always done this since new, then we could consider reengineering it. Right now I for one still focus on fixing it.

[Randall]

"power off measure resistance of JFET S-D. Should read low resistance."

It reads 300 ohms.

g1, there was no thump on the line out send to another amp, and the power amp still thumps with the preamp output cap C127 lifted.

Also, something else I failed to mention, the speaker makes a small pop when first plugged in, even with the amp off, is this the charged 3900uF cap?

[Enzo]

Yes, the cap has a bot of charge, and it discharges through teh speaker.

OK 300 ohms sounds like a good JFET.

[Randall]

OK, this is interesting, I grounded the top of Q315, the mute JFET, and switched it on. Still has the thump. And this is with the preamp output cap still lifted.