I believe those are input protection diodes. They'll limit any incoming voltage to about 1.2V peak to peak to prevent damage to the rest of the preamp in the event of someone hooking up something silly (within reason). They'll have no effect on sound unless your guitar is cranked up to super hot and you're picking like a fiend (though this depends a lot on the pickups), and then you'll hear the sound break up during the peaks. If you add those two diodes and find you can't crank things up like you want without clipping you can add another pair in series with them and that will double their effective range from 1.2VPP to 2.4VPP. If you're putting out that hot a signal though you'll find you have less room on the overdrive knob before it gets really dirty.

The short version: they are safe to omit if you're careful about what you plug in, though you might want to experiment with them just to see if you like what they do.

Hope that helps.

I came upon this a bit late, and I'm not entirely sure what you're after, but I think I see a possible problem. I'm somewhat of a noob, so you'll probably want a second opinion.

The circuit in your picture (second one) seems to be missing a bias for the splitter inputs. You can see R2 and R3 providing an evenly divided bias in the first circuit, where in the second it's missing.

DC coupling of gain stages can be trouble. I'm not much of an authority, but I'd recommend against it for audio. It's usually used where precise measurement or high frequency is important.

Hope that helps.

Quote from: EDWARDEFFECT1 on December 05, 2010, 10:05:21 AM
i have 166.3 vollts dc on one terminal of the light and -163 dc on the other. the light is 125volt ac.any ideas. i must have something leaking dc into it. there are no identifiers for dirrection with the caps.i have replaced all the large capacitors and the bridge rectifier with a 100volt 25 amp part. the amp works fine,but no pilot light.i have another post with pictures of the circuit board if it helps....thank any help apreciated......ed

That's about right, actually, with one exception. Assuming you didn't have a typo, you should definitely use a higher voltage rated bridge rectifier!
Pure sine AC will rectify to a higher DC voltage than it's original AC RMS value.
To figure the value, multiply the AC by the square root of two = 125VAC x 1.414 = 176.75VDC
There's usually a bit of voltage drop caused by losses in the rectifier and filter caps. Ten volts is about right, especially considering that you might be overloading your rectifier.
I'd strongly recommend at least a 400V bridge, as they're usually not much more expensive at 25A than a 100V version. Mains wiring is full of high voltage spikes of several hundred volts, and depending on your location they can occur from several times a day to a few times a month. It's not uncommon to see short transients of several thousand volts a few times a month where I live!

Regarding the pilot light, I'm not sure what you mean. If it's just a power indicator it might be burned out. It happens a lot with direct mains powered neon lamps, and they're fairly cheap to replace if you don't need to find the exact right shape and size to fit into something, like a particular switch. The only other thing I could think of was the heaters on a tube amp, but then it would hardly be working well!

Hope that helps.

It depends on the working voltage of the TDA7294. It can be powered from as little as 20V (+/- 10V) or as much as 80V (+/- 40V).
The preamp is capacitatively coupled (via C8) so it can be powered from any potential that's between 18V and 30V and the output can be ground referenced by the power stage, no worries.
This means you can run it off of any combination of power rails that won't blow it up, and add regulation (voltage regulator or zener regulator) to trim the supply to where you need it.

For example, let's say you're using +/- 35VDC to power the TDA7294.
Vcc sits at +35VDC
gnd sits at 0
Vee sits at -35VDC

Vcc and gnd is about 35VDC, a bit high for you. A linear regulator is going to drop that voltage down for you, and spend the rest as heat. Given that the preamp won't be using much power, this isn't a big concern, so let's say a target of 24VDC to power the preamp. This is a convenient value, as there are three pin regulators at 24V (google 7824) that you could wire up according to the datasheet and essentially be done. Alternately you can use a zener diode and a resistor, as explained here.

The output from C8 can be hooked directly to the input of a standard TDA7294 power stage, and essentially replaces C1 in the reference circuit on the first page. Capacitors will only pass changes in voltage, so the output of the preamp will be referenced to the ground of the power stage by R1 in the reference circuit.

I hope that helps, I'm a bit scrambled today.  :duh

Almost every cheap inverter out there will have problems like this. Mains voltage is usually a very well formed sine with almost no harmonics (just the primary frequency [50Hz or 60Hz], nothing else), but most inverters output something that looks a lot more like a square wave (even and odd harmonics as well). The higher harmonics don't really cause problems with much, but for audio amps with an unregulated DC supply (most guitar amps) the extra noise can play havoc.

The amp's power transformer will filter most of the higher harmonics because it's optimized for mains frequency, but some of the lower ones may get through. Then what remains goes straight through to the power rails and is picked up and amplified by the preamp stage.

Quote from: trialabc on November 24, 2010, 12:28:50 AMI have tested the voltage for the inverter output. It does not reach 230, but 190 (AMP is turned on). Not sure if this will affect the performance of the AMP.

If you are not using a true RMS multimeter (most sub $100 meters) then it will read incorrectly for non-sine signals. The inverter is probably not producing a square wave, but something partway between square and sine, so it should read lower than nominal on such a meter. Odds are fairly good that the inverter's operating fine, it's just that it's difficult to get clean power with one.

Noise is pretty much inevitable. You might be able to minimize it with some careful filtering, especially if it's just getting into the input stage. It might be possible to use an inline filter as well, though I don't know if there are any designed to filter frequencies in the audio band. Usually they're used for filtering out 100kHz + EMI.
There are inverters out there with sine output, but they tend to be expensive.
Another possible option is buying or designing a DC to DC converter and powering the amp directly from the battery that way. Designing such a power supply for any significant load is very difficult and more than a little dangerous, but you might find something cheap on the surplus market. These tend to use a switching topology as well, just like inverters, but they're designed to output as clean a DC signal as possible. If you found one that was a good voltage match for your power rails it would essentially plug right into them and off you go. Any noise problems would be easy to combat by filtering between the converter and the rails.

Hope that helps some.

Quote from: joecool85 on November 23, 2010, 04:03:53 PM
Looks like this one would work: http://www.radioshack.com/product/index.jsp?productId=2062483

Am I right in thinking so?

Should work just fine (though I hate RS's prices). It'll use a bit more current (~60mA), but that's no big deal.

Also, corrected my math above. I read mW and wrote mA, sheesh.

Quote from: joecool85 on November 22, 2010, 04:33:20 PMThat does help.  Still at a bit of a loss as for what relay to use.  Where can I even get one?

For something like this I'd usually use All Electronics. Here's an example from them that should work fine, plenty in stock, and an easily found data sheet. They've got a wide selection, here's a 12V DPDT if you'd rather use 12V (and it's datasheet).

Quote from: joecool85 on November 22, 2010, 04:33:20 PMAlso, from looking at the sheet it looks like I will need a DPDT not DPST relay.

Quote from: J M Fahey on November 23, 2010, 10:12:58 AMHere you go.
You'll need a 12V DPDT relay, because your DM uses a pushbutton DPDT to switch distortion in, and you need to replace it.
Pull it from the PCB and route its contacts to the corresponding contacts in the relay. . .

That's doing it the right way.
It's true that you'll need a DPDT relay, that was my mistake. I should not post from memory, something made me think DPST would work.  :duh
Using a 12V relay is probably preferable over a 5V because it will not stress the zener regulator as much (less voltage drop). This may not be the case if you use very large 12V relays that require a lot of hold current though (automotive relays perhaps), as the higher current will also cause stress. The small signal type relays above are well suited to the job. The 12V version above uses 140mA 140mW nominal, 79mA 79mW minimum, so you'll probably want to design your regulator for 200-250mA 200-250mW (assuming 12V ~16mA, 20mA would be fine) just to be safe. The 5V relay uses roughly the same current, but you have to shed more voltage to use it. If you were designing for production you could balance the loss between the zener (voltage drop) and the resistor (current demand), but it really doesn't matter too much here so long as you use parts that won't burn up.

Note: Many of these relays are sized right to fit in DIP sockets, which can be useful, but you always have to check the sheets. The 12V version above, for example, would fit in a standard DIP socket (like this, but preferably with 10 pins), but the 5V version would require a skinny DIP socket (0.2"[5.08mm] wide as opposed to 0.3"[7.62mm]).

Sorry, I had a lot of coffee this morning so I'm kind of babbling.  :loco

Quote from: joecool85 on November 23, 2010, 02:19:00 PMJuan, I guess I'm a bit confused.  What is the "left switch" and "right switch" about?

Those represent the relay output, and you'd wire them in to the six pins where the old switch came from. They get wired in to the amp's mainboard where SW1A and SW1B are on the schematic.

Quote from: joecool85 on November 23, 2010, 02:19:00 PMAlso, wouldn't something like this make more sense for the jack/switch situation?
Wouldn't this mean that if the foot switch was plugged in it would control the relay and if not, the switch on the front of the amp would control it?

I think you've got the right version. Switching always makes me a bit dizzy, but it looks like yours would behave as you describe.

Bleh, I hope that made some sense. Sorry about the mistakes in my first post.  

Edit: Corrected zener calc math above (blue text).

I did something a little like this for a friend once. We used a small reed relay removed from an old modem and wired it in parallel. This sets it up like an OR gate, if either the footpedal or the front switch is on then OD is on, and they both have to be off (or the footswitch disconnected) for clean.

In my case we made a small pcb daughterboard and crammed it in as close to the switch as possible to keep the leads carrying signal short. I soldered small leads from the relay pins directly to the switch pins on the solder side of the amplifiers board. It's a bit tricky and kind of kludgy but it keeps the modification to the amp minimal.

Any DPST relay should work, and you can use a zener regulator to drop your rail voltage down to 5V or 12V for use with a small relay. The smaller relays tend to operate on lower voltages and currents, and this keeps you from having to run full rail (or both!) through a TRS jack in the back for switching (disaster in the making). The relay's datasheet should list the hold current, and you can plug that into a zener regulator calculator to see what power rating you'll need on your zener.

I recommend including a protection diode across the relay coil, any 1N400X series rectifier will work fine. This will keep your amp safe from voltage spikes when the coil switches off, and reduce the chance of the switch causing audible noise in the amp's output.

Hope that helps

The big output transistors look like MJ2501 and MJ3001. It fits the profile, as these are TO-3 darlingtons with the appropriate power rating. These are probably not manufactured new anymore, but you might still be able to find them (though they're likely to be overpriced). They were second sourced by a whole bunch of manufacturers, here's the datasheet for the ST Micro version.

The datasheet should give you a great start on finding replacements if you're used to matching spec sheets. If you need more help I can find something, though the pros here could probably tell you off the top of their head what to use. Just let me know if you need more help.

Hope that helps.

It seems pretty unlikely that your hiss is related to your speaker wiring. By the time a signal reaches your 8Ω output it's been amplified dozens to thousands of times and somewhere in there something is going wrong. Often the hiss is generated in an input stage and is grown by amplification across the whole signal chain, but this would respond to playing with the volume and tone controls. If it's not responding to any controls then you've already narrowed it down (probably) to the final power amp stage. This is a good start!

I think I've found the schematic to your amp, but I'm not completely sure. You may wish to cross check the board drawing in the pdf with your amp.

I'd start with a visual inspection of the board, both sides, and see if you see any leaking caps, bad solder joints, or anything else fairly obvious. If it were me I'd then be poking around with a scope and a meter looking for trouble, but the pros here might have better advice on where to start. Pictures always help!

Hope that gets you started.

I poked around for a while and found a bit of info on the Pearl brand. It seems likely that your amp is rebranded from a different manufacturer, which would make it very hard indeed to find info under the original name. It's likely that there was no effort by the company to duplicate the support materials from the original manufacturer, and that any problems were handled by techs without schematics to refer to unless they recognized the equipment (or it's guts) by sight.

If you have a problem you might have better luck posting about the problem itself. There's lots of highly experienced folks here who could help you sort things out even without a schematic. It always helps to post lots of pictures!

Quote from: Enzo on October 21, 2010, 07:49:28 PMI have used these fiberglass brush erasers for about 35 years now . . .

I didn't know about these, neat stuff! That looks like just the thing for stripping enamel off of fine magnet wire too. I'll definitely have to pick one up.

There's an IPC video somewhere on this, but they're methods are a little extreme and meant for 'as new' board manufacture. They use flat copper trace and epoxy it down, but there's a lot of intermediate steps to make sure you don't harm lower layers on a multilayer board. Often the simplest way to go is to trim the broken area out, clean up the ends so they'll take solder well, and just jumper in some wire that's not too thin for the current, i.e. the same cross section or more.
You can use pretty much anything, but stripped solid core wire has worked best for me in the past. You can bend it to whatever shape you need and flatten it a bit with a piece of pipe so it'll lie flush on the board, solder it in, then glue with epoxy or fingernail varnish (whatever's handy). Just about anything will do so long as it's not layout sensitive, so try to keep in in the same general area the old trace was. If you reroute the trace, say by using free hanging wire, you might pick up more interference or hum than the trace would have. Board repair gets sketchy at higher frequencies too (tens of MHz) but that's rarely an issue with amplifiers.
Most traces break from having to support heavy components. Banging a board on a table edgewise when it has some heavy inductors soldered in (especially smt components) is a common example. This is why you see a lot of heavier components hot glued or gooped to the board to help provide support. Boards also expand and contract with heat cycling, though they're designed to do it as uniformly as possible. Using too much heat when soldering can wreak all kinds of havoc too. There's probably a million ways to damage traces.

Hope that helps.

Quote from: morgoth2006 on October 19, 2010, 07:48:57 PMEvery time someone calls me to check a computer problem I have a hard time troublechecking the hardware because everything works! I call it the "fear of the doctor" syndrome.

I've got a bit of this too. It's kind of like a reverse Pauli Effect. I always thought that if they named this after an experimental physicist then there's hope for anyone to pursue their dreams.

Peavey usually responds rather quickly to schematic requests, I'd drop them an email. They've got a nice fellow named Gene that will sort you out.

Hope that helps.