Acoustic 370 grounding issue, ch2 buzz

[PRNDL]

I've got an old Acoustic 370 Solid State bass amp that has a minor issue with the low-gain channel, which buzzes when used. The 1st channel is fine.

The first issue seems to be the amp ground. Someone put in a 3-prong cord, but the green wire was cut and not connected anywhere. I took out the "death cap" and soldered the AC ground to the chassis where it was connected.

It still has issues with the 2nd channel buzzing. The scope shows a sine wave on (what I believe is) the board ground when the function generator is connected to channel 2, but not with channel 1, which makes me believe that the issue is the grounding scheme.

Both jacks are 3-terminal with two of the lugs tied together and wired to the board ground to form a voltage divider with different resistors in series (10K and 22K).

I'm not sure what to do next.

Could it be an improper connection to the chassis with one of the jacks?
Should put a ground wire from the AC ground to the jacks?

Any suggestions are welcome!

Here's a schematics page
http://acc.homeunix.net/schematics.html

[PRNDL]

Fixed. Volume is the next problem.

It turns out that someone wired the input jacks to the wrong places on the preamp board connector. It was a simple fix.

The next issue is the overall volume is low. At 300 watts, this amp should be LOUD.
I have it connected to a 16 watt Marshall speaker.

Any ideas?

[PRNDL]

I saw smoke ... some puffs between the power board and preamp, but I couldn't tell exactly where it was coming from. The chassis under the power board is very hot.

Perhaps one of the output transistors has shorted.

Since they are all in parallel, do you disconnect all of them to test them separately?

[cbarrow7625]

Do R322, R323, L301,R328,C404 look healthy? That would be the first place I would start. Those are all series elements on the output. If any one of them open up your volume will drop.

How do the rails look? Right voltage or sagging? Without seperate emitter/collector resistors on each transistor I worry about the health of all of the output transistors. They can't possibly share equal current for the life of the amp & not have one go down.

How do all of the reference bias voltages look? Looks like you are pretty much DC coupled from C301 through to the output transistors. There is one adjustment pot (R304) that could adjust the DC throught the output stage.

Let us know.

[cbarrow7625]

You can test with all of them connected just to determine if one of the 3 is shorted. If something is shorted then you need to pull them all individually to determine which one it is.

[PRNDL]

Do R322, R323, L301,R328,C404 look healthy?
Yes, and the output transistors tested fine with ohms.

The smoke came from the other side of the board, which might be the preamp board.

The difficulty is the hex screws for the pots is not one that I have in my set.
I'll have to go to Ace Hardware.

[cbarrow7625]

You can inject a test tone (sine wave) just ahead of C301 & isolate whether the volume problem is in the preamp or power amp. With around 0.5-1.5vac injected at that spot you should get something approximating full output if the power amp is fine.

[PRNDL]

R329 is sizzling and leaking.

It's a 22 ohm 2W connected to the speaker out (before the coupling cap C404) to the rectifier return (ground) via C313, another coupling cap.

I measured it as 72 ohms.

[Enzo]

SO is C313 shorted? Pull it and see.

The output on this single ended power amp sits at 45v, so this stability netwrok has to endure that. Not usually a problem, but if that cap fails, then the 22 ohm resistor finds itself across 45v. That is 92 watts. If C313 is only leaky, then a few less watts.

Replace them both, I'd say.

Then check the output of the amp for RF.

It is possible C313 is fine and the amp is oscillating at high frequency, like 50kHz or something. Scope the output and see. Such oscillation could burn up that resistor as it tries to shunt those HFs away.

[cbarrow7625]

PRNDL, I'd replace both the cap & the resistor just to be sure. Check the output with a scope like Enzo said. If C313 does not measure as leaky or shorted then you likely do have a high frequency oscillation. Even after replacing the parts they will just get cooked again.

How about the rest of the DC reference voltages? Have you had a chance to check & see if this thing is even running at the right voltages?

[PRNDL]

I replaced the resistor and found a cut trace on the preamp board that's for the ground (It's sort of odd that I missed that earlier).

The amp still doesn't work, although I believe it is a preamp issue because the signal at the monitor jack is in the mV range and looks hacked instead of a sine wave.

It's somewhat of a difficult chore to trace out the preamp since it is mounted upside down. There seems to be enough wire to flip it around.

[PRNDL]

I'm still working on the amp.

I replaced the 22 ohm resistor with two 10 watt 10 ohm resistors (Yes, it's overkill, but the local shack doesn't seem to care about electronic parts, and this was the only thing close).

The ground trace on the preamp board burned out about 1/2 an inch with a lot of scorching on the rest of it. So there's a ton of current going through there.

This is because the entire amp is grounded through the preamp trace to the chassis at the input jacks.

I've cut the jumper between the power board and preamp board grounds and plan on adding a ground wire from the power amp board ground to the chassis. That will protect the preamp trace with a risk of adding a ground loop, but will allow me to test the amp without risk of burning the preamp board trace even more.

I did see some hair (high frequency) on the power supply rails, but it wasn't very high in amplitude.

[PRNDL]

It's HF oscillation in the power amp section.

The preamp signal before R301 (27K) is a sine wave (as expected), but it has high frequency oscillations afterward. It's bursts of HF with no signal in between, so there's some kind of switching with it.

The power rails look OK -- +90 volts DC from the big cap and +45 DC before the speaker cap. I also checked the DC test voltages and they seemed fine too.

I'm not sure what too look for ... any suggestions are welcome!

[cbarrow7625]

What about the other DC reference voltages on the schematic? Is there 11VDC at the base of Q301 & Q302? 11.6VDC at the emitter junction of those transistors?

Is there 43.7VDC on the collector of Q304 & 41.7VDC on its emmitter?

How about the voltages around Q310 & 311?

How about the voltages around Q316 (bottom of the schematic)? .3VDC at the base & 10.4VDC at the collector with 9.8VDC at the junction of R333 & R334? That is a protection circuit. If more than about .6VDC is present at the base of Q316 it will "close" that transistor, pull down the 10.4VDC & cause Q315 to shunt the input signal to ground.

What is S301 in the amplifer? Is that some kind of reset switch for the protection circuit?

Find out some of the voltages & let us know. Seems like you are closing in on the problem slowly but surely.

[PRNDL]

I did measure all those voltages this morning and they seemed fine (it took a while to trace out the board).

I'll measure again with the DMM and write them down this time.

S101 is an AGC defeat button that is used when setting the trim pot R304.

[cbarrow7625]

That makes sense for S101. AGC, huh? I just assumed it was protection. I guess, in a way, it is.

If the DC voltages check out then the next thing to do is figure out what frequency the oscillation is at & where it appears in the circuit. Everywhere? Just at the input? Just affecting the AGC? Does the oscillating frequency appear on the output?

I know you said that there was some "hair" on the rails. HAve you tried parallelling another PS cap to try to eliminate the "hairs"? Are the filter caps in good shape? Have you looked at the DC rails using the AC input on the O'scope? Whadaya see?

[PRNDL]

BAD NEWS

I slipped when measuring 41 volts at the collector of Q303 and hit the ground bar (it's right next to it). Smoke and a burnt fuse. It smokes out the bottom of the board near Q303 with a new fuse, but I can shut it off before the fuse blows.

Now I'll have to figure out what's blown and replace it before I can get back to diagnosing.

BTW, Q316 had incorrect DC voltages. 5 at the collector instead of 10.4 and .24 at the base instead of 0.3. C315 had 5 instead of 9.8 Q316 is the transistor for the AGC.

[cbarrow7625]

Yikes! I hate it when that happens.

Sounds like either Q316 is partially turned on (i.e. bad) or one of the associated passive components is bad. Either R330 has drifted way up in value or R334 or C315 are partially shorting to ground. Makes me suspect C315 or R330 as the most likely culprits. Check them while you deal with the other blowed up stuff.

0.24 at the base sound close enough to me to be OK.

[PRNDL]

R327 is smoking, which means the output transistors are drawing too much current.

I'm wondering what to do ... this amp has already been to several techs without success and they're not that expensive used. The problem is that I can't turn the amp on for more than a second or two before it smokes and blows a fuse.

I tested Q303, 304 and 316 and they seem fine.

Learning by experience can be difficult!!

[cbarrow7625]

If that's the case, I would guess you don't have just 160mVat the junction of R325/326. It's probably more like 40 Volts!

One of those transistors must be bad. Definitely pull & check Q312-314. If you don't find it there, check Q310 & 311as well.

[PRNDL]

I think it's Q310, although it's pretty hard to tell when it's in the circuit, and it's mate (Q305) is NPN instead of PNP. (The power transistors Q312-314 and their driver Q311 tested fine).

Would a test be to take it out off the board and power up to see if there's no smoke?

I could also take out the Q311 to 314 pretty easily.

BTW Are those transistors glued to the square metal case / heat sink?
I'm having difficulty getting Q310 out, although sometimes it's hard to get all 3 pins hot at the same time.

[PRNDL]

Now we wait for UPS delivery - I ordered fuses and two transistors.

It's nice to work on cool amps like this.

[Enzo]

Get all three pins hot at the same time? The way to remove a transistor is to remove the solder, then lift it out. I use a desoldering station, but I have for years used other methods in the field.

Many guys like solder wick or solder braid - same thing - lay it on the spot of solder and then the iron top of it. It absorbs the solder leaving a clean spot.
http://www.mcmelectronics.com/content/productimages/s3/3857654.jpg

I don't like braid, but I do like a sucker. These are cheap, but I hate them more than braid.
http://www.mcmelectronics.com/content/productimages/s3/3857043.jpg

I like the kind you cock then hit the trigger button.
http://www.mcmelectronics.com/content/productimages/s3/21-8235.jpg

Trying to melt all the legs of a transistor at once is doomed to fail. Without the heat sink, one can bend back the transostor extracting one leg at time, but not on the heat sink like that.


Q305, Q310 are RCA40409, 40410. The heat sink is PART OF THE TRANSISTOR. If you screw around with the NTE cross, it will not have the heat sink and will burn up pretty soon. MAgi parts sells real 40409s, and I am sure others do too. it is conceivable a TO220 with a finned heat sink screwed on might sub for it, but I use the real part.


Do we know C313 is OK? If it is open, the amp will be unstable.

If the outputs draw too much current sitting there, then I suspect the bias circuits, so look into Q303.304 and all those resistors around them.

[cbarrow7625]

Would a test be to take it out off the board and power up to see if there's no smoke?

NO! Don't do that. You would wreck the biasing & take out a whole bunch of other parts in the meantime.

If you have a meter with a transistor tester on it you could use that to check it. To check it long-hand you need to basically check it for internal shorts. From the emitter to the base, check it like you would a diode (for Q310, theemitter is the anode of the diode & the base is the cathode). You should get about a .6V drop across it in one direction & nothing across it in the other direction. Just like a dioide, if you get a reading in both direction it is bad. Then measure for any leakage between the base-collector (you should measure "overscale", or extremely high resistance) and between the emmitter-collector (same deal). If you measure any resistance between these points, there is a problem with the transistor.

[PRNDL]

The way to remove a transistor is to remove the solder, then lift it out.
I was using solder braid, but didn't know the heat sink was part of the transistor.
MAgi parts sells real 40409s, and I am sure others do too.
Can you tell me the web site (I couldn't find it).
Thanks!!

Do we know C313 is OK? If it is open, the amp will be unstable.
I'm not certain how to test a coupling cap with a DMM.

[Enzo]

Pull the cap. Now set your meter to diode test and hold the probes on the cap leads. After a second or two at most, the cap wil have charged up - you see the ohms display sweep up them max out. Now remove the probes. The cap chould now be charged up to a volt or so by the meter diode test. flip the meter to DC volts and see how long the cap remains charged. The meter will discharge it slowly. COMpare to a good 0.1uf cap from your drawer. if it won't charge up, then it might be open.

Sorry, typo, Magic Parts sells 40409 and 40410. www.magicparts.com wholesale only though
A quick google shows www.vibroworld.com has them for $15.

These guys ask $11 for them http://www.soundchamberrepairs.com

In fact just google RCA 40409 and a whole world of places come up

Google is usually my first place to look when my notmal suppliers don';t have something.

If you didn't realize the heat sink was a part of things, then I guess it WAS hard to get them out. Unsolder the two pins onthe heat sink.

[PRNDL]

It's not smoking!!

I replaced Q310, Q304 and Q316 and pulled two bad power transistors.
I plan on buying a real 40410 and C313 (I used a .047? 600V cap I had on hand).

I think the original problem was Q316, which is part of the AGC circuit. It may have failed while I was measuring voltages and took out some other transistors in the process.

The next step is to see what the scope shows for output, but I need to take a break.
That was hard work!!

Is it best to replace all of the output transistors, or just the bad ones?

[PRNDL]

Something is still wrong - there's no output.

OTOH, the input signal reaches the power board just fine, and there's no smoke, which is nice.

Here's some DC measurements.

DC+ = 85
DC2 (output) = 85 (should be 45)

Q301/2 C =13
Q301 B = 12
Q302 B = 23 (should be 11)

Q316 C = 6.4 (10.4)
Q316 B = .49 (.3)
C315 = 12 (9.8)

Q303 C = 85 (41.7)
L301 = 85 (43.7)

Q311 B = 85 (120 mV)
Q312-14 B = 85 (160 mV)

My first guess is one or more of the upper output transistors has shorted.
Since one upper and one lower already tested bad, it may be best to replace all.

Which replacement is best? 2N3055
2N3055AG (ON Semi) = $3 2N3055G (On Semi)=$1.75 2N3055 (ON)=$2.10
2N3055 (STMicro)=$1.40

There's a tech note to use only matched RCA 2N3055's as other brands fail prematurely.
Vibroworld is the only place I've found them, but they're closed.
I also found some NOS RCA 2N3055HOM, which are supposedly better performers.

Q315 is failing the resistance tests .34 from the collector to emitter either polarity.

Is this correct, or should I be looking for something else?
(Also, would it be wise to remove all the output transistors until the DC voltages are correct during test?)

Update: Q302 is bad. I missed it by testing only BE and BC.

I'm worried about multiple transistor failures on this board.
Would it be wise to replace all the transistors?
Also, R322 and 323 have brown marks but test fine. Should I order replacements?

[PRNDL]

Well, it's fixed.

I ended up replacing 20 components - all the output and a few driver transistors and a bunch on the preamp board, a few diodes and a couple of resistors. It turns out that when one bad transistor burns out it takes out its neighbors and works its way through everything.

I've learned a lot about solid state amps in the process.

This amp sounds incredible ... it sounds crystal clear with some kind of emphasis on the high end that's hard to describe. I'm using a crappy old Kustom PA speaker and turning down the treble a lot -- it must be real nice with a good 4-ohm bass cab.

Thanks for the help!!

[cbarrow7625]

PRNDL,

That's great to hear. Looked like that had to be the case pretty early on. Glad you had the chance to poke around and figure it out yourself. That's always the best kind of learning.

In discreet solid state amps where everything is DC coupled it's pretty hard not to take out everything downstream when one small transistor goes. It throws of all of the bias voltages in the entire power amp. There are complex protection circuits that can be designed in to handle such things but most folks think they never sound as good once all of the additional circuitry is hanging off the "working parts".

Good luck with it. I hope it stays together for ya.

[PRNDL]

Thanks!

I think there were several things going on at the same time, which made it confusing.

A previous tech had wired the input jacks incorrectly to the preamp board. Another previous tech added a 3-prong cord but didn't connect the ground wire. The amp was oscillating because that .1µ cap was bad and it took out the 10 ohm resistor. The power amp transistors must have been weak and blew during testing.

The owner is balking at $200 for the repairs, which I thought was very reasonable given the cost of parts and time spent. It took 3 different vendors to get all the parts, plus a huge amount of testing.

His main concern, however, is whether the amp is really fixed. Apparently the last tech that worked on it wanted more money to repair it, even though it didn't work properly.

Fortunately, I began the conversation by asking him to bring his guitar or bass to try it out when he picked it up. I also suggested bringing a speaker, but those things are huge.

We'll see.

[Enzo]

Ask the customer what he thinks the local car dealership would charge to replace an alternator. What's that, three bolts and a couple connections? How close to $200 will that come?