Acoustic 136 bass amp possesed!

[carldacorte]

Ok... Maybe it's not possesed, but it sure is acting wired. When I turn the amp on, it crackles and pops, then it'll settle down and just hum like crazy. The amp still passes a signal, but this hum is LOUD! The preamp seems to be fine as none of the controls effect the volume or tone of the hum.

Any idea of what I should be looking for when I open this thing up?

[Dave Curtis, dB AudioTech]

Power supply caps is the first thing that comes to mind. I'd probably replace all the electrolytics and go from there.

[jrfrond]

Vintage Acoustic amps are known for crispy solder joints on the driver PCB near the power transistors. The preamp PCB's too, particularly the inductor used in the tone control circuit. Oh, and don't be fooled by joints that LOOK good. You may find components whose leads JUST made it through to the solder side of the PCB. Tug on the leads of suspected components with needle-nose pliers just in case.

[gbono]

Bypass caps on the pre-amp boards, all the potentiometers are soldered to the PCB so check these and resolder. very rarely I have seen a preamp transistor go bad (2N2484/2N4248/etc).

[TomCarlos]

Hey Friends....

I just picked up an Acoustic Control 140 Bass Amp- similar to the 136.

In the power supply, it generates 70 something volts (give or take). Why on earth would Acoustic design the amp around an 80v Electrolytic Cap? Isn't the rule of thumb you take the actual voltage then use a cap with a voltage rating that is 50% - 100% above the actual voltage?

Next Question- Anyone know of a good source for parts? It's not easy to find a 3900uf Screw Type Cap or Thermistors that do not have specs. I have the parts list and schematic. But I do not see much in the way of parts specs.

Thanks in advance... Tom

[gbono]

Derating a capacitor DCWV is a good practice in terms of extending the components reliability but cost and component size are increased. BTW I have over 20 of these caps from various 140, 150, 150b, 136, etc repairs and they all have acceptable measured ESR/DF and capacitance values. I replaced them because most of the customers wanted to use their amps for gigs and I couldn't predict the remaining life of the capacitor. Temperature is another contributor to the reduced life of one of these components. I believe I found the Al electrolytics at Mouser.

Why do you think the thermistors are bad? The only time I've replace them is when some previous owner/tech had removed them from the circuit? I have sources or alternatives for almost everything on these ACC amp designs except for the inter stage transformer – though I have never seen a bad one.

[TomCarlos]

GB:

Actually, the P/S Cap measures at 4000uf. But it looks real old. So this might be a good time to replace it.

All I know is that this is a "Project Amp." There are components on the Power Board that are fried. So I will slowly but surely go through this entire amp, piece by piece, and check every component.

When I mentioned the "Thermistors", the parts list simply says "Thermistor"... it does not give any other type of description. Same with the Transformer on the Output Board.

So part of the fun will be to figure out what these components are and their replacement!!

Tom

[J M Fahey]

Sorry by repeating the question, but, why would you replace them? ... unless they are only carbonized remnants of the original parts.
And not knowing their value, being an obscure and obsolete part, with not known suppliers ... I would most respectfully leave them alone.
As for the driver transformer, I very much doubt it's burnt.
Start by placing your multimeter red probe, on an AC scale and through a .047x400V capacitor, on the positive side of the power supply (those 70VDC).
How much AC (ripple) do you measure there?.
Less than 2 volts is normal; 4 or more beg for a new Capacitor.
Modern ones will be 1/4 the size, radial, and with solder legs, not screw ones.
You can easily get a 4700uF, 100V one for a fraction of the cost of an exact replacement.
Measure DC voltages across output transistor legs and post them here, as "Qxx: VCE=xx V (around 35VDC); VBe=yy V (around 500 mVDC)" <-- all measurements no audio signal.
Also test the output capacitor for leakage, you should have 0mV across the speaker legs; at most less than 100 mV DC, and that's already too much.
Post the schematic so we all talk about the same.
Good luck.

[TomCarlos]

Thank you JM...

I have a schematic for the Acoustic 134. It is a similar amp. The bass amp uses two 17-10 preamp modules. Everything eles should be the same.

Right now, both preamp modules and the power amp module are disconnected. There are several components on the power amp board that are fried. So I will need to drop a small load resistor across the P/S cap to test it under a load. Unloaded, there is 73.5 volts dc (close enough). So if I draw 1.5 amp, that should give me a good indication of the cap performance and ripple? Otherwise, there is virtually no ripple on the cap (with no load).

73.5V with a 50 ohm load draws about 1.5 amp. So looks like I will need a 100 watt resistor? Ouch... But I think I can come up with a resisor like that.

The next step will be to pull the output transistors and surrounding resistors. From there, I will begin to pick apart the power amp board. Eventually, I will take the measurements you suggested.

Thanks, Tom

[J M Fahey]

A cheap and reliable load is made by using 6 12V/15W car lamps in series. They should cost around 1$ each.
They will "eat" somewhat over 1A, and almost 100W from your supply, so ripple and voltage under load measurements are reasonably realistic.
Scratch all carbonized board material, even if that means ending with a copuple ugly holes.
Then you will try to reconstruct tracks.
OPTION: if the power amp board is in *real* bad shape, you can build another , new, different, power board, and use the preamps and PSU.
A couple pictures from the inside of your chassis, specially some showing the output transistors mounting, will help a lot.

[gbono]

I bet the 5W?/560 resistor got very hot on the OP board also the driver transistor with the integrated heatsink is probably toast too. Check all four output transistors (2N3055) and JM's comment about the output 1000uF cap is spot on too.

The termistors are NTC 100 ohms at 25C (room temp).

Interstage transformer provides phase inversion and coupling to quasi complementary output stage (errr.... push pull emitter followers). Doubt this component is damaged.

I believe most of these amps were abused by folks who ran them into sub 4 ohm loads and/or poor/cold solder joints causing high resistance ohmic contacts. Not to mention 37years of excessive heat building up in poorly ventilated enclosure design, temperature cycling, vibration, spilled beer, etc.

Is it just me or have these amps (140,150's, etc) taken a hugh nose dive in resale value? Must be the new Acoustic "brand"/product eating up market share.

[TomCarlos]

JM:

Excellent idea for the P/S load. Thank you.

I attached 2 photos. The first is of the Power Amp Output PC Board. As you can see, one of the caps and a resister blew. There could be more bad components on that board. I will check them one-by-one. The cut wires will need to be replaced. Someone (before me) cut the wires to the Output Driver Xfrm and Output transistors and did a horrific job of this.

Pic 2 is of the output transistors. I'm not sure what type of transistors are being used. The parts list calls for 60085. The cans say RCA 48 15. But I cannot find those parts on the net. All 4 transistors are bad. The transistors also have a .5 ohm, 5 watt resistor hanging off the emitters. All 5 resistors are bad. So you know there was some serious current flowing threw the output section.

Do you or does anyone know what type of output transistors are used in this amp?

Oh... one other thing... Never ever solder anything directly to the lead of a transistor!! I need to replace all sockets and insulators for the output transistors. They are in horrific shape.

[J M Fahey]

Hi Tom.
Too sleepy to answer in full, but one little detail: our friend GBono already told you that those output transistors are actually 2N3055 on disguise, so 4 regular ones should do fine.
As a side note, Enzo usually says that for about the same price he prefers to use modern, robust ones; I think he suggests one of the MJ150xx series.
Replace all 4. Cheap investment, huge improvement.
The beauty of transformer driven designs is that there is no DC coupling (duh !!), so you can concentrate on repairing the output transistors and couple associated components first, and only then see how you drive them.
The PCB does not look that bad.
Blue Book values aside, it's a fine amplifier.
Agree that hooking every speaker on sight is a great amp killer.

[gbono]

Ouch someone figured a way to reverse bias that capacitor - must have been loud too. Yup going to need those thermistors - here's the link you need - it's a gift it took hours to find these guysDisc & Chip Style Thermistors - Temperature Probes - NTC Thermistors

BTW if you haven't done so already down load the service manual for the 150 here http://acc.homeunix.net/manuals.html

[TomCarlos]

Thanks again friends...

I already downloaded the service manual and reviewing.

Today, I needed to replace two TO-3 sockets. They had been damaged and I could not see myself leaving them in the amp. Hey, that was hard work. I also replaced all .5 ohm 1/2 wirewounds. Those two were toast.

What else... Oh, I put a load on the P/S- 90 ohms, 200w resistor (just for a short time period). So I'm pulling less than an amp and the power supply dropped to 66 volts. I guess that is telling me something!!

I also started to replace the Electrolytic caps on the Power Amp Board- waiting for my shipment from Mouser and Jameco to arrive. I checked the MPSA09 and 40410 transistors. They seem to be ok (tested with a heatkit transistor checker). The 6.8 ohm resistors need to be replaced too. I was able to find info on Thermistors and ordered a few from Mouser. I also ordered the 3900uf cap.

I'll keep you all posted on my progress as I move further along.

Tom