The input is simply a dual differential pair, one NPN the other PNP. Each pair has it's own active current source. The outputs of the diff-pairs feed current mirror driver transistors where most of the Voltage amplification takes place. Bias for the output stage comes from a Vgs multiplier which matches the temp-co of the output MOSFETs much better than a Vbe multiplier. The source follower type output clips softly because the output Voltage can only swing to about 5V less than the rail Voltage. In addition, the zener diodes across the feedback resistor can cause soft clipping depending on the diode's characteristics.

Hard to tell what the open-loop gain is without modeling or building one, I don't speak German or trust any translation on the net.

Designs like this can be found on the diyaudio.com forum.

I would suspect Q1. Any big negative Voltage hitting the input will slowly degrade Q1 (making it hiss) as it has no protection.

Always plug the guitar cord into the guitar first, then plug the other end into the amp. That loud buzz you hear when you do the opposite is what damages the input transistor.

The LA4282 has a max load of 8 Ohms per channel so you'd have to use a 16 Ohm load in bridge mode. If you want something louder than 10W, use both channels each driving it's own speaker.

I like to use my meter's current mode to measure how much an effects pedal consumes. Simply use a 9V battery, unsnap one side of the battery connector and touch the probes between the battery terminal and the side of the connector you just unplugged.

Most meters have a separate input to connect one of the probes to when you measure current. There is almost always an internal fuse that will blow if you forget to move the probe back to the Voltage jack when measuring Voltage.

Hifi amps are OK for casual use, but will likely fail under heavy duty stage use. It would be best to operate the Hifi amps below clipping and below the maximum load.

You might try to locate some "14 pin DIP component carriers". These can be used to adapt other IC's to the socket on the Heathkit PCB or solder the old 7472 with the mangled legs to one to see if the chip still works. The 7472 is not a popular chip and may be expensive to find.

It looks to me like the chip is just used in the divide by two mode where J and K inputs are both a logic High and the Set and Reset inputs are active. An old practice was to just leave inputs unconnected to anything to act as logic High inputs. (A bad practice). Half of a 7474 chip can be adapted to perform this function.

443-4 looks like a Heathkit part number. Look for an assembly manual on the internet or possibly a Heathkit cross-reference. Be sure to observe the "1" printed on the PCB as the only indicator of which way to insert the IC. The socket doesn't appear to have any marking telling you which way to insert the chip.

Quote from: saturated on May 12, 2023, 09:45:08 AMback to the light switch question...they are all single pole switches right?

I don't want to give away the answer, and I applaud your determination to figure it out for yourself, so I'll give you this: At the Home Improvement store where they sell what you need to do the trick, they call the switches you need to do the common two switch circuit a "three way switch" and the one you need to add for the three switch circuit a "four way switch". Once installed and covered up with a face plate, the switches look and feel like the normal On-Off switches you see and use every day. (These names apply in the USA, not sure about other countries.)

I was thinking it's kind of hard to learn those circuits out of a textbook, I learned them from an instructor with a chalk and blackboard. I see an opportunity for a You-Tube series with an instructor, maybe even animated graphics.

One of the things you do in early electric/electronic class is learn how to analyze simple circuits usually with a battery, a switch or two and several resistors or light bulbs. You learn how to apply Ohm's Law to calculate Voltages, currents and equivalent resistances. If you skip this step or don't know how to do it, you can look pretty stupid on an internet forum.

Do you know how to hook up a light bulb so you can flip it on of off from two different switches no mater how the other switch is flipped ? How about three switches ?

If the amp has an Effects Loop, try plugging a short patch cord between Send and Return.

Is this one of those 'Guitar Amp Head' style PA's with a 260 series power amp across the back ?

What speakers do you usually use with the PA ? Does the back of the amp get warm ?

An intermittent type failure can make it difficult to track down the exact cause.

With the Sweep in NORMAL mode, there won't be a sweep unless you get a trigger somehow.

With EXT trigger source, you won't get a sweep unless you connect a signal to the External Trigger Input and adjust the trigger level control to get a trigger.

A drop or two of contact cleaner in each of those switches will free them up. Don't touch the end of the CRT tube shown in the picture.

The 7808 has an internal current limit. If it draws 3 Amps, that current limit isn't working and the chip is bad. To get to the current limit, you need a load resistor on the output, 8V/R = 3A. R needs to be 1.666 Ohms. Looking at the data sheet, With a 13V input the current limit at 25C is about 2.25A. 8V/2.25a = 3.555555 Ohms. Note that the current limit goes down as temperature increases. At 125C the current limit at 13V input is only 1.7 Amps.

Edit: I should mention that you need to use a heat sink when testing with any load current higher than about 100mA. The data sheet says the chip is protected, but they will fail if abused.

Edit2: You can use a small heatsink with a plastic "Vice Grip" ( made for holding wood as glue dries ) that has rubber jaws.