"RESET" is not a procedure.  it is a common function in digital circuits.  All it is is a wake up reset.  When power is first applied, the circuit waits a second before letting the DSP program start to run.  That insures the power supply has had a chance to stabilize.


Apply a nice strong signal to the amp input.  Is signal then present on pin 7 of U4?  Or at pin 7 of J11?  (Electrically those are the same)  If there is no signal there, then the FX has nothing to work with.

Likewise the signal at pin 8 of J11 - the output from the FX card - is ther signal there?  Does it make it to the top of the FX control P17?   How about the wiper of P17?

Inject a signal at the wiper of P17, even if it is just hum.  Does it come out the speaker?

Those tests check the signal path to and from the FX card.

If the card itself is faulty, then verify the 5v regulator on the card is indeed producing 5v.

There are two reset generator ICs, U4 and U8.  Look at pin 2 of each, and verify that teh voltage there is something like +5 and not ground.

Quotewhat do the high priced IEC cables have that a standard shielded cable doesn't?

Marketing.

Why are the diodes stupid?  And if you don't like D1,D2, I see no reason you wouldn;t hate D3,D4 and their LED buddies as well.

Those are clipping diodes, when the signal reaches a certain amplitude, they start clipping it - it adds distortion.  They also serve to limit signal excursion into the later stages.


I would agree that replacing the op amps won't accomplish much.  Nothing at all wrong with TL072s.  Keep in mind that this amp is not a hifi amp, no guitar amp is a hifi amp.  The amp is not designed to faithfully reproduce the input signal, in fact it is not designed to RE-produce anything.  The amplifier is a primary producer of sound, it is part of your instrument.   Guitar amps are of limited bandwidth.  A guitar speaker will be rolling off the highs over 3kHz to 5kHz, and the amp circuits generally are not much interested in anything over 5kHz.   SO "improving" the circuit to have video bandwidths and lighting fast slew rates won't do anything for you.

ANything is possible, but really.  WHy not just set the amp on clean and use the pedal?

Look further into skin effect.  COnsider at what currents it comes to light, and in what sorts of conductors, and at what frequencies.  Further, read up on who has to consider skin effect as an important consideration - in other words where does it come up in daily use.  A 9v battery circuit need not concern itself with it, but a high tension cross country power line might.  I think putting it in its proper context might shed some light onto whether it applies to your amp's power cord in the slightest, or not.

I won't quibble with the technical analysis.  But the original question was why do transistors and tubes sound different.  Understanding how the circuits work will of course add to the understanding of this issue.  But I would point out that the sound of an amp is a whole lot more than the tube or transistor curves.  Those are DC characteristics.  SO for example trying to fathom performance in an audio circuit, such things as inter-element capacitances and their effect will not show up in that graph.  And other arcane parameters like response to transients won;t show up in the curves.

Looking at the horsepower and torque curves won;t tell you what it feels like to drive a particular car, and reading the recipe won;t tell you what the meatloaf is going to taste like.  I encourage you to continue your research, but caution that it won't be so simple as finding some numbers or a formula, or a set of curves to explain the dynamics of the compared systems.

You can compare curves between some semiconductor and some tube, and you might see that they approximate each other.  That is fine, you could then build a circuit around each, and probably get the sounds to approximate each other.  But the devil is in the details.   Approximating is not the same as each other.

The 705-18720 was used in the 400BH power amp as used in the COmbo300 Bass AMp.

If the 400BH board is intact, it is a sturdy and reliable power amp.   I'd use it, and build a custom preamp if I were into a project.

The schematic is too large a file for the forum limits.  We have it posted over at Music-electronics-forum.com.  DO a search there.  or contact customer service at Peavey.

The voltage is approximate 50v is just as good as 45v.

That 32vdc with 16vac on it is the dead giveaway that you have a missing filter - caused by that cracked solder.

You can look for a 50k reverse audio pot at Mouser, not sure you will find a D shaft.

Try www.tubesandmore.com for a chance.  They have a number of D-shaft Fender pots.  You might adapt and use a linear instead.   And try www.fenderparts.com see if they have it.

Always a good idea to CALL these places rather than email.

I see 019712 as a part number.

028576 is a 250k log pot.

OK, no speaker for now.

Check the power supplies.  The amp runs on +/-45vDC.  A convenient place to check is the cases of the four power transistors.  With respect to ground, the cases of two on one end are -45v and the cases of the two on the other end are at +45v.

Now flip your meter to AC volts and measure at the same points.  You should see less than a volt of AC on those transistors.

If one of them shows substantial AC voltage, and most likely one of th DC voltages is considerably lower, then a main filter cap is suspect.  The filters are the two cans next to the fuses.   Any cracks in the solder under those cans?

You can pull a speaker, but since you will have to pull the chassis to do anything, I wouldn;t.  Top screws come out, and a few screws along the bottom rear of the chassis, and it slides out the rear.  There will be a blue and yellow pair of wires feeding into the speaker enclosure.  They connect to the circuit board with a white Molex plug.  There may be a cable tie around the wires - clip it if so.  With the speaker disconnected remove the chassis entirely.

Now you can fire it up on the bench and check for DC at the Molex pins.

Unlike your tube amps, solid state amps do not require a load.

Don;t sweat the circuit board.  From that era, most Peavey amp socketed their ICs.  If you have to change out a transistor or resistor, then yes, you will have to solder on a board.

Isolate the problem.


There is a power amp in jack.  Plug a cord into that and apply a test signal.  CLip a speaker to the molex posts if you need to hear it.

Does the symptom remain?   Likewise, you can run a cord from PREAMP OUT to some other amp for a listen.  Is that signal affected?

Assuming the power amp is the problem, the preamp and power amp are on separate boards.  The power amp will run standing alone, you can disconnect the unbilical to the preamp board and clip a test signal to the power amp at the exposed pins.

There are three ICs on that board, two 4558 dual op amps, and a "87478" used as a compressor.

You can remove the 87478 from its socket during test.  Note its orientation.  ANy change?

U1 is the signal; IC, is there undue DC offset at either output pin?

Muting JFET Q1 could be faulty, remove it and see if the problem goes away.

Note that U1 has its own zener derived +/-15v rails.   There are also 7815/7915 dervied rails for the preamp.  make sure U1 has +/-15v.

Due to file size limits, I can't post the schematic here, but customer service at Peavey will send it to you.













xxx

Isolate the problem.

Turn on the amp and set ALL the controls to zero.  Let it sit there running.  Does it still pop?  If so, the power amp is doing it.  Could be a filter, but I don;t usually look to filters for popping.

If no pop, then turn up the master volume only.  Does it start popping now?

What we want to do is find ANY controls that have ANY effect on the pop sound.

For example, maybe we cannot stop the pop, but we do find that the tone controls affect the sound of the pop.   That would mean that the pop was entering the system before those controls.

Cranking the screws down tight guarantees the vibration from the cabinet will be fed into the reverb pan.  if your pan is not in a reverb pan bag, consider getting one.  Cut a piece of cardboard to cover the open side of the pan and tape it in place.   And it can matter where in the cab the thing sits.  Try moving it around - as far as space allows.

ANother trick is to get some of that rubber weather strip like you put along a garage door.  Peel off adhesive back, rubber about 1/2 to 3/4" wide and maybe 3/8" thick.  Clean off the wide flat top surface of the pan.  GLue a stripe of the weatherstripping down the center of the flat surface.   That will keep the flat side from resonating.  Auto makers do this inside large car body panels to cur down on sheet metal noises.

The reverb, does the siren come on immediately when you turn the reverb on, or does it take a couplpe seconds to build up to a roar?  Reverbs can feed back acoustically, and if it takes a couple seconds to get there, that is what is happening.

If it is immediaste, the connections to the pan could be reversed, or other odd things wrong.

That's why I get the big bucks here.  DOuble my pay, please.

Solder is a weak link alright.  Check solder on each and every control and jack, plus all the rectangular cement power resistors - those 5 and 10 watt things.  They vibrate and break their solder.

Crate uses those Cliff style jacks, and the cutout contacts on those is a weak link.  I automatically measure the resistance of each cutout contact.  If it is more than half an ohm it needs to be cleaned or burnished.  If it measures like 1 or 2 ohms, it will probably work fine, but the very fact the resistance is up that high means the contacts are not pristine, and 2 ohms at the moment can turn into 2000 tomorrow.