Repairing an Acoustic Control model 120 bass amp head

[jpcar]

Hi, I'm looking for some help repairing an amp!
I'm having problems with the power amp of an Acoustic model 120. The pre amp is working fine.
The amp powers up with quite a loud thud from the speaker. Also a thud on power down.
But then there is no signal from the speaker. I measured 65mV DC from the speaker terminals. I have checked all of the transistors and they seem fine. I also checked the electrolytic caps and they seem fine. I measured +44 VDC and -44 VDC coming from the bridge rectifier. And the same +/- 44 V on the collectors of the output transistors which are MJ 15024s.

When switching the power off, I hear the signal for a split second and a thud.

Any advice on what to do next??

Thanks!

Here's a link to a schematic of the amp.
http://music-electronics-forum.com/attachments/25116d1378895252-122.jpg

[Enzo]

That is only the schematic for the power amp section, do you have the rest of the schematic?  Not sure how you determined the preamp works, but assuming it does, we don't know what else is between it and the power amp.  Under play conditions and signal applied, is there signal present at C1 of the PA?  and at R1?  How about at the collector of the 40408?

[phatt]

If the Amplifier has an FX loop on the back panel make sure it's working before you venture too deep into the workings. 

With a spare guitar cord bridge the In and Out sockets of that loop.
they maybe labeled as *preamp out/PwerAmp in* but it's all the same thing.
The switch inside my not be connecting so bridging bypasses the internal switch, it's quite a common issue and at that age more likely.

I once replaced the whole radiator in my car only to find the $5 cap was faulty. :grr
Phil.

[jpcar]

Thanks guys! Sound advice.

So, I had checked the Pre amp Out and Power Amp In sockets and found at least the pre amp was working.
But! I just realised I made a foolish mistake. By not having the sockets (pre amp in & power amp out) screwed into the chassis they weren't getting ground.
So, next I need to trace for a signal. Heading to work first and then I'll get back to it!

[jpcar]

Using a square wave 1KHz signal from my scope I found signal at C1. It measured 4 Vpp and then at R1 it was 2 Vpp. At the collector of 40408 I measured a signal of 28 Vpp. Is that very high? Anyway it's there, an undistorted looking square wave.
There is a 480059 replacing the 40408 which I hope is a usable replacement. The amp has worked in the past so I think it is. Some of the other transistors are also replacements.
Where should a trace for a signal next?

Thanks for the help!

[Enzo]

40408 is an RCA transistor type, it is the number you would have ordered from Mouser or somewhere.  480059 is the Acoustic company's part number.

If signal is good on the collector of the 40408, then the 40408 must be working.  So now keep tracing through the amp.  The 40408 grabs that bias transistor through R13 and yanks it up and down with the music.  That in turn yanks the bases of the drivers and thus the outputs with the same music.   The 40408 is your voltage amplifier stage, everything to the right of it is all current amplification.  So the same signal ought to be present all over that part.  So is it?  Signal at the bases of the outputs?

Those fractional ohm resistors with the outputs, are they intact?  And is there signal on them?  R23,24.

Find the choke across R25.  R25 is 22 ohms, but the coil is just wire and wil have close to zero resistance.  SO measure R25 for resistance.   It should measuer essentially zero.  If it measures about 22 ohms, then the coil is open, or more likely broken free at one end or solder has cracked.  If it measures open, then the coil is open and the resistor burnt out as well.  Anyway, if signal got to R23,24, then is it on R25?

A sine wave is much friendlier to the amp and this effort than a square wave.  besides, how can you tell if a square wave is clipping?

[jpcar]

That's very useful advice. Thank you Enzo!

It's confusing when the transistor type and part number are similar like that but thanks for clearing that up.
Also for advising a sine wave instead of square wave.
Now, when I test for signal with a sine wave I notice that at 40408 collector I get a sine wave with the positive chopped off. In fact more than half the wave cut off. So, mostly flat line with some of the bottom part of the sine wave. I get the same signal at the bases of 40409 and 40410 and R23, 24. At the base of one of the MJ15024s I get the half sine wave. At the other I get -40VDC.
So I am actually getting output of the speaker but very weak and distorted. I didn't get anything before. This signal seems to be intermittent. Most of the time it's there but sometimes it just disappears.

What to do next?

I checked the 2N4248s and they are fine.

[Enzo]

A lopped off signal is clipping, but how large is your input signal?  It may be clipping from too large a signal, or it may clip from a problem.

Personally I use 100Hz rather than 1kHz, it is MUCH easier to listen to.

I'll come back later, I am on hold with insurance company at the moment.

[jpcar]

The signal at C1 and R1 is unclipped. Just when it gets to the 40408 collector that it's clipping and it's really severe and only missing the top of the wave.
I will check out using a lower frequency. Sounds like a good idea.

[Enzo]

Didn't ask if the signal coming in was clipped, just how large it was.  The schematic even says "0.7v RMS for full output.  If we put say 5v of signal in, it won't be clipped there, but it will be later in the circuit.   Is it clipped at the base of the 40408? 

Not saying that is the case, just want to rule it out.

What is the DC voltage on the collector of 40408?   What is the DC voltage on any terminal of the bias adjust pot or the bias transistor?  They ought to be within a volt or so of zero.

[jpcar]

There seems to be DC voltage all over the place.

With a signal of 200 Hz at the input. Scope measuring 400mV pp (which is 0.14V RMS)
At C1 - the sine wave is present at 400mV pp, with 280mV DC.
At R1 - sine wave at 48mV pp. with 304mV DC
At 40408 collector - a small portion of bottom of the sine wave is present, with 11.4v DC.
All three legs of the bias pot have measure 11.4VDC.

[phatt]

That's because it's a DC coupled Amplifier and there will always be some DC Voltage present.

The basic idea is to try (as best one can) to keep the 3 points (noted by Red Arrows) at close to Zero Volts DC. If the circuit reads excessive DCV at the speaker terminal then a fair chance something is wrong. 

In circuits like the old Valve stuff each stage was Decoupled via a Capacitor and hence no DC was present after the output Cap.

The hitch with DC coupled circuits (especially high current power amps) is that if something goes wrong it often takes out the whole circuit.

A limiting lamp is often used until you know the circuit is stable.
Hope that helps.

I'm sure Enzo can help explain anything I've missed.

Phil.

[jpcar]

Thanks for this info. I'm trying to learn about transistors amplifiers so this is useful information.

So, I found a bad transistor which I think is the problem. I took 40409 out of circuit and it's only working from base to collector, and not base to emitter.

Should I replace 40410 at the same time?

And does anyone know where I can get a good 40409, 40410 transistor or replacements?

[Enzo]

If you just google "RCA 40409 ebay" you will find many people selling them, either alone or in pairs.   I even have some left I sell at $5 each.

[jpcar]

Ok thanks!