1979 Peavey Bass 400 MarkIII

[Swampy]

Yo,

It's probably best that I let everyone know now that I'm not particulary savvy when it comes to this sort of thing, but I'm keen to learn etc. Having said that, don't be afraid to tell me if some of my questions aren't particulary on the
money.

I've just scored a Solid State 1979 Peavey Bass 400 MarkIII off one of my mates that's left the country on the cheap - I gave it a crank today but it seems that the power amp section of the circuit is having some problems - it sounds to me in fact that it's not functioning at all. I don't really know much about these amps but it's fairly obvious that something is going wrong somewhere along the circuit.

Can somebody provide any insight into where I should be looking for the problem? My troubleshooting adventure has at this point only strayed past the point of confirming that the cabinet and speaker cable are both in good working order.

Thanks,
Matt

[Enzo]

Which rear panel is on this amp? In other words which power amp? 400 Series, 400B/G, 400BH? something else? If it is not printed on the rear panel in bold letters, it will be printed on the solder side of the main powr board on the rear panel. Or another way - are there a row of black plastic transistor covers across the rear? Or are all the power transistors inside?

You say the power amp section is not working, so does that imply the preamp DOES work? Exactly what does and does not function. Power light come on? Is there any thump in the speakers at power up?

Inside:

Is ther appox 40v both + and - at the main filter caps? ANd so at the power transistors?

COnnect a speaker. Take the screws out of the rear panel and lay it down behind the amp. Power off, there is a cable from the front panel going to the power amp rear panel. It joins the rear panel in one corner. It is probably four wires in a 5 pin Molex connector. Unplug this and then power up the amp. The power amp is now operating alone. Is there any hint of sound? There are pins on the board where that connector was. One is ground, then there should be two pins with lower voltage, and one pin is the input from the preamp. Not on that order. Touch them each with a small screwdriver or something. If one of them makes hum out the speaker, then the power amp is working, if not it isn't.

If that works then we look into the preamp. But I want to know the power amp first so I can tel you about the power supply on it that the preamp needs.

[Swampy]

Thanks Enzo, heres the gist:

This is a 400B - the row of black plastic transistor covers are present on the back.

When I power this up, the light turns on and I get a pretty knarly pop coming through the speakers - I'm not to experienced in troubleshooting amps but the reason I believe it's a power amp issue is because I'm able to play and get sound back, but at a very low level, and the post gain controls don't seem to do anything on either channel. I can record the sound and link to an mp3 if need be (it's doesn't really sound like the preamp section is firing properly either, we're talking really fuzzy at low pre gain settings. I'm unsure whether this is related to the original problem specified or a completely seperate matter altogether).

Is ther appox 40v both + and - at the main filter caps? ANd so at the power transistors?

Sorry to noob out on you but are you able to give me a link or instructions on how to check this? I have a multimeter, I've just always been to scared to poke around the inside of anything being supplied power from a wall socket!!

Once the front panel was disconnected the I got an pop followed by a short hiss which faded out, then repeated over and over in a steady rhythm. I touched the pins as you suggested, and one (the top one) did give a slight hum, but not instantly - it took a about a second and a half to fade in. Definitely not the abrasive sort of hum I was expecting to hear.


Once again, thanks for your help.

Matt

[Enzo]

OK, I am conflicted. The MArk 3 Bass has integrated circuits, was a 1979 design, needed + and - 15v power for the preamp. In addition to saying Mark III on the front, it also had 5 input jacks - channel A, 1 and 2, then channel B 1 and 2, and Automix.

Does yours have just 5 input jacks and little 8-leg square plastic circuit chips on the front board?

The old 400B power amp panel did not produce + and -15v, it made only one +25v power supply for the preamp front panel. The preamps on the older ones were all transistor, no IC chips. They also had 6 input jacks - Normal A and B, Effects A and B, then Series, and Parallel. Does that sound like yours at all?

SO I am confused.

But plunging ahead, are there jacks on front labelled pre out and power amp in? Patch those two together with a spare cord and see if it helps. Reconnect the power panel cord first of course.

[Swampy]

Does yours have just 5 input jacks and little 8-leg square plastic circuit chips on the front board?

Thats the one, I've just noticed the 400B/G on the circuit board on the inside of the rear panel also. So we've identified this correctly now?

I tried patching the pre out into the power amp in, the only noticable difference is that the pop/hiss rhythm noted earlier oscillated at a faster frequency...strange one huh.

Other then that, same fuzzy tone, same minimal output level. ..

Matt

[Enzo]

OK, 400B/G males sense. Plain old 400 doesn't.

FIlter caps are the two large cylinders at one end of the power board. The cable to the front is at the opposite end. We pulled it off before. The pin nearest the corner is the input to the power amp on that cable connector. Touching it should make a reasonable little hum, nothing ear shattering. That tells us the power amp is at least trying to work.

Since you tried the patch cord trick and it didn't help. that means it is not a jack problem. SO any problem ther is will be in the circuitry.

So with that cable off, you will now have to take voltage readings while the amp is live. If you are not comfortable doing this, now is the time to take the amp to a tech. I can about guarantee some part will have to be replaced. Probably nothing expensive, but we need to find out what. In my shop. this repair would likely cost about $60-70. Check with your local shop.

There are three pins, a space, then one more pin sticking up for that cable. The one in the corner is the input. The third one is ground, and is electrically connected to the chasis. The other two pins carry 15VDC. One is positive and the other negative - I forget which is which. Set your meter to DC volts. Hold the black probe against the chassis, that hex headed screw holding the board right in the corner is a handy spot, the screw is part of the chassis. Now with the red probe, touch it to the second and third pins. See if each pin has about 15v of DC there. One will be negative. Close is good enough, so don't sweat 14.8v or 16.2 volts. If they are not OK, then we will replace a voltage regulator IC on the power amp.

If they are OK, we move on. REconnect the cable to the front. Power off while you do that. Then back on. Now ground the black probe again, and with the red, remaesure those two 15v pins. Now the connector will be in the way, so stick the probe down into the plastic connector hole to touch the pin sitting inside the hole. Are both 15v voltages still there? I first wanted to see if they were there at all, now we checked to see if they stay there under load. If they are, we move to the front board.

If one or the other collapses when loaded by the front panel, we either have a voltage reglulator bad - most likely - or there is a bad IC up front.

To work on the front requires demounting the front panel from its chassis. Yes, removing all the knobs, all the nuts under the knobs, etc. Once free, set it flat on the table in front of the amp. After it is on a little while, touch the top of each little 8-leg thing. ANy getting hot? Hot ones are likely bad.

If power voltages are OK, chances are only one small part is bad, and you are out cheap. If the power amp has issues, the large transistors can cost you about $7-8 each. But I don't get the impression that is your trouble.

[Swampy]

OK, so we have 15v + and - when the Motex connector is un/plugged, so I moved onto the front board.

I let it do it's thing for five or so minutes, then checked out each of the 8 legged suckers (are they the IC's you were referring to earlier - " there is a bad IC up front."?). Sure enough, the four closest to the input jacks (there are 6 in total on the board) were warm, with the two closest to the top and bottom, respectively, more so then the other two.

So, where to from here?

Thanks,
Matt

[Enzo]

Keep track of which way they go in the sockets. I am not sure they all face the same direction. They are in sockets, aren't they? For example the two near the Automix jack face opposite ways. Verify yours all face correctly - any backwards, please replace.

Note on the layout each IC has a little 1 next to one corner. That is pin 1. ICs all have something to indicate pin 1. Many have a notch out of the pin 1 end, while others have a round dot or hole in one corner - the pin 1 corner. In either case, if you look at teh IC with the notch at top or the dot upper left, then pin 1 end is at top. Pin 1 will be uper left. Pins count down the left side, then from the bottom up on the right side. So pin 8 would be upper right. Or pin 14 would be upper right.



I just noticed, I forgot there are also come 14 leg ICs. TL074s. They are simply two of the 8-leg type in one IC. See if they are hot too.

I just posted the schematic and layout at www.ampix.org in the Enzo gallery.

Do this: with everything all connected, power up and measure for DC voltage at each jack. There should not be any. You can either plug a cord into each jack and measure at the other end of the cord, OR flip the board over and measure at the solder for each jack. DC on any jack is wrong. Look at the schematic, see the pair of diodes at each jack? A shorted one puts DC on the jack.

Look to the right on the drawing. The pre out and line out are driven by sections of U9. A bad U9 could also put DC on those jacks. SO we now want to take voltages at these ICs. All readings taken with meter refernced to chassis. The larger ICs are all TL074 - quad op amp ICs. The four corner pins are the outputs. So check the four 14-pin TL074 ICs for DC on each of the four corner pins. SHouldn't be any. I am worried about a few volts here, not a vew millivolts.

Now that I am looking at it, I am not going to worry about the 8-leggers at this time. The four by the input jack are split between the channels. I doubt the same thing is wrong with both channels. Frankly I suspect U9. If we want to change U9, until new parts come, we can use U10 or U11 in its place, just for testing. if we do that, flip the graphic off, since that is what those to ICs do.

The two input ICs are common 5532 types. The TL604 ICs are not available, but we can deal with it if we have to. The two warmer ICs were the crossover circuit for the biamp outputs, whuch we are not using, are we?

TL074 is common, Mouser sells them for about 80 cents. They come in different shapes, so make sure to get one that ends in CP or CN.

[Swampy]

Okay, I've measured for voltage at the jacks as you suggested, and theres nothing substantial going on there.

So I checked the tempurature of the IC's, and U3, U7 and U9 are all hot, with U9 being hotter then the other two. Then I measured the corners of the 14 leg IC's - there was no DC voltage at any of the corner pins on U9, but there was around 7 volts at each corner on U10, and the same for one corner of U11 (pin 14).

Out of interest I measured the 8 legged ICs also and found around 15 volts at some of the corner pins on all of these...is this normal?

Matt

[Enzo]

Yes. The center pin on each side of the 14s are power. Most 8 legs you see will have +15 on pin 8 and -15 on pin 4. All dual op amps are that way. Your TL604s and 478 chips are different. I'd be trying new TL074s in there.

[Swampy]

Sweet, I replaced the TL074's but after measuring I get the same results. The only difference I've noticed now is that the pop/hiss rhythm I described before is slightly different.

Any more suggestions?
Matt