Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: smackoj on September 27, 2018, 12:19:45 pm
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This is a weird problem amigos. The amp is Acoustic 140 bass head circa 1972 possibly '82 and made in USA. The screws that hold the chassis in the box insert from the top (normal) but they screw into the bottom of the chassis. This results in the chassis sitting in the box upside down compared to most amps. Anyway here is the weird part. When the amp is sitting normal, i.e. with the face plate and controls in the upright position so all the lettering is readable, the amp distorts the notes. When I turn the amp on it's top so the controls are all upside down, it plays perfect with no distortion. I took the chassis out of the box and ran it to see if the box has something to do with the problem, but it still distorts on the one side and doesn't when turned over. Anyone have any ideas what could make such a problem?
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find it yet?
not your problem but the blue cap under the pwr board looks loose
pots, switches connectors, bad solder, n wires would be my guess, unless a substrate came loose in a tranny :icon_biggrin:
was the cap job done before you got it?
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Chopstick. (Get a helper to strum.)
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Sorry for the late check-in friends. I had to do several tests over the last couple days to try and isolate the problem. It must have just been a one time fluke that turning the head over made the distortion go away. It sounds the same now whether down side up or up side down. Those long blue filter caps are secured and don't move around Shooter. They were just put in quickly for testing purposes. I used those because the original fat can cap is toast, but I wonder now if these blue Segamo caps are bad? I put them in series because they needed to be able to handle more voltage than they are rated at (50 vdc) and the original fat can is rated at 75 vdc. So I better try one or two pwr filter caps that I know are good before I try anything else. Actually, in retrospect, it is possible that the in-rush dc hitting those blue caps could have damaged them. Or they might just be too old and dried out? When I first put them in the amp did sound like it should for a few first tries. This may take a while because I don't have any other big micro farad caps with low voltage around. Probably have to buy a few to test in it.
Thanks for the help amigos. :icon_biggrin:
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big micro farad caps with low voltage
there's no problem, except price, putting hi voltage caps in.
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Would you gents agree that trying different filter caps would be a reasonable thing to change first to see if the distortion goes away?
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If you don't have hum, or voltage readings that are way off, I'm thinking you're problem lays along the signal path...somewhere.
It sounds the same now whether down side up or up side down
by same you mean distorted ?
Is it "clean" at low volume, If you inject individual frequencies, does it distort more at one over the other, got a scope?
are there any line out, FX out, that you can split up the circuit?
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Hey Shooter; Yes, by 'sounds the same' I mean distorted. I don't have a scope but I will try the 'low vol' and 'diff bass notes' testing tomorrow and get back.I have a new 2500 uf filter cap on the way so that will make or break whether it is dried out caps?
Thanks again for the help :icon_biggrin:
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Hi amigos; I have replaced the power filter caps and the distortion continues. I put the meter on the output spkr jack and it registers .264 vdc. I think this could be the problem but not sure. Anyone want to comment on this new info??
Thanks as always,Jack
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I know the amp should not have dc on the output jack, and I think .264 is more than allowable, so could someone please tell me where to go from here?
thanks amigos
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any line out, FX out, that you can split up the circuit?
how bought schematic
I can't imagine getting to far without a scope, a listening amp might help
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Is this amp a single power rail or split +/- rail?
was the cap you replaced a PS cap, or a speaker cap?
look here, initial post;
https://el34world.com/Forum/index.php?topic=23232.0
you'll see C116 "feeding" the speakers, If yours has this kinda thing, I would suspect it should block DC from the speaker. I've rebuilt a couple of these and have just replaced them along with all the other Ecaps, which in my case is only a handful.
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Hi Shooter; No, there are no "line out" or FX so I can split it that way. And Yes it is a single rail for power not a plus and minus side. I replaced the PS cap and the speaker cap. I need to find the schematic because the way the spkr cap was wired when I got it, the line to the + spkr terminal comes out of the Neg - end of the capacitor. There was a small value Orange Drop cap in series with a 22 ohm resistor that went between the pos spkr terminal and the ground lug on the spkr jack. I figured that had been installed by someone to drain off DC from the output jack. I checked the dc on the spkr jack with the R and C connected to ground and I found .255 vdc. With the R and C removed I measured .264 vdc. So I feel that they were put there to try and control the dc getting into the spkr?
The way this amp is layed out it would not be hard to isolate the preamp from the powr amp. Each of the 2 channels has it's own preamp circuit board that sits alone on the side of the chassis. I could disconnect them one at a time if that would help isolate the noisy part?
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if that would help
it would,
when I 'shoot a circuit, I get all the "data" about what's wrong, verify/fix the DC, then move on to the signal path, usually breaking it in 1/2, then 1/2 again
a schematic, meter, o'scope, and sig-gen gets you 90% pretty quick, going back when you go down a rabbit hole gets you the other 10% :icon_biggrin:
your RC circuit is the same config as mine, so it's probably a "standard" thing, my guess it's there to short out >20k stuff
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I finally found a usable schematic. I hope someone knows how to load this into Doug's schematics library? This is not an easy one to find. One thing that is important to note about this Acoustic Control Corp. model 140, is that the model 136, 140 and 150b all use the same schematic. The model 150 is the same amp except it has reverb and tremolo while the 150b does not. I am starting to examine all the parts and wiring everything according to the schemo.
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should be on your way now,
inject a 1Khz ~ 100mV rms signal at the input jack, (specs are 21mV to 1.16vac at input). make that scope ch1, once the scope is synced and displaying a good signal, use ch2 and probe "sig out" from the preamp connector (C). walla you're either moving right, or left.
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This is a very old power-amp plan and will take time to remember.
Drawing-note: there is one three-winding audio transformer from driver to output but it is drawn in *three* places. The Gr/Yel windings are drawn right AT the power device Bases, coupled to the winding over at Q302 Collector.
Output DC is NOT a problem. The speaker is capacitor-coupled. Ideally the output DC fades to zero after infinite time, assuming some DC load on the output (speaker, or say 100r test resistor). The e-cap will leak. New, not enuff to matter. At this age, C311 should be suspected and replaced. IAC, 0.25V in a stage-speaker suitable for this power level will do NO harm.
There's like 5 voltage-notes on the plan, report them.
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three-winding audio transformer
I printed out the schematic to "study" between beerbaitngas sales, you have made that way easier!
thank you
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Thank you PRR and Shooter; I will replace C311 and get some voltages at the 5 points. I am also going to replace Q301 which is shown as MPSA09 Si npn transistor. It looks like it has been replaced and could be a mismatch (I can't read the little numbers but I can see it is not an MPSA09 tranny.
:icon_biggrin:
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I replaced the PS cap and the speaker cap.
C311 is what I call the speaker cap.
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Yeah, that's what I mean too. I replaced C311 polarized cap again. The amp seemed to show more balls than before, and by my ear the distortion clipping changed somewhat but the problem still remains. I need to put the chop stix to her but I need a competent instrument strummer first. In the meantime I am letting the caps simmer on my Variac to see if that produces any change?
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Not to nag, but.... voltages?
Electricity is invisible. Random poking and parts-swops is working blind. You could get lucky. But voltages has a 50:50 chance of leading to the problem.
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I don't mind you nagging me PRR :) I actually stumbled onto what I think was the culprit. As I said I replaced C311 again and let the whole amp simmer on lower voltage for several hours. That did the DC trick and it all but disappeared. So after that I plugged in a guitar and it was a little better but still distorted. It just must be dumb luck, but I knew that someone had been working pretty extensively on the Power Sup PCB. There were several places on the board where the solder pads were loose and pulling away from the board. I was inspecting the solder joints again and found one of the large 400 ohm 10 watt resistors was loose and the solder pads were broken. I fixed that and another spot on the board where the previous tech had soldered a jumper wire and Wallah, she's running strong and quiet with nice clean notes! There are 2 more jumpers someone put on this board but the connections look secure and the amp is working good so I will leave those alone for now. Here is a picture of the Power PCB. You can see all the places that look like they have been re-soldered many a time.Thanks so much PRR and Shooter! :icon_biggrin:
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"the last thing you did........."
you can fix a lot of stuff by starting there :icon_biggrin:
For your future and the next guys, noting up a paper schematic including voltages come in real handy
glad you're running
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Me too! This amp is just the perfect size for practicing.
Muchas Gracias Amigos
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necro post but I've been through 4 or 5 of these so far (the first one having similar symptoms as op) and r311 (560 ohm 5w resistor) on the power board has always been super toasty, sometimes to the point of no longer being connected to the trace or already replaced (poorly, with the trace lifted). that resistor and the two 400 ohm 10w resistors (r312 and r313) are best placed further away from the board because they get HOT
these amps are lovely for bass and guitar!