I tried to get on MEF today and Microsoft Security Essentials popped up a window telling me that it found something it didn't understand and ask me to send it to them. Perhaps some kind of malicious code. There was also something called Exploit:JS/Blacole.KV that was quarantined.

Is it that hard to prevent this crap on the website end, or just expensive? I don't really want to start using a different browser. Last time I tried going on the Sunn forum (a child of MEF on the same server) the key click on my computer started going off like I had pressed on a dozen links.

Unless you have a cable you can cut up, you can wrap a length of wire around the long barrel part of the plug and connect that to ground. Then simply touch the end contact of the plug to the pot terminal. You'll only get one channel but we just want to see if the power amp will pass a signal to the speaker.

Two things look wrong. The voltages on C17 and C18 are too low. This could mean that the Rectifier is bad (looks like the schematic calls it DB:) or the transformer is bad. There are other possible causes but the power amp should still make some sound.

The voltages on C2 and C6 should not be zero. this will prevent any guitar signal from getting through. Measure the resistance across C2 and C6. You might get an unstable reading, but not measure a short or low reading. Now measure the resistance across R6 and R11. Again you might get a strange reading but it should not be higher than 1000 ohms.

I would add to what JM said that before you apply power to the amp, you measure the ohms from pin 4 to ground. You may get an unstable reading but not a short or reading less than 1 ohm.

Quote from: GIMAGUITARS on January 12, 2013, 11:53:27 AMCan you suggest an alternate testing method?

You can use the headphone output of your computer if you have the cable to make the connection.

Your voltages suggest a problem in the preamp. Try to measure those voltages again with the preamp IC removed. Are you sure that you didn't create any shorts on the solder side of the PCB when you replaced components? Is the IC plugged in correctly? Plugging it in backwards will damage the IC, you'll need another one.

Follow the links in this thread: http://www.ssguitar.com/index.php?topic=2555.0

That should get you a little familiar with a DVM. Measurements should be taken on the 20V or 200V DC Volt range. The black lead should be connected to ground (the chassis) and the red lead touched to the indicated point. This will tell us the general health of the preamp and power amp circuits.

Do you have a DVM? Need DC voltages at the speaker output (Speaker disconnected), and pins 1, 4, 7 and 8 of the IC.

It's been ok since Monday but today there was several trojan attacks and a java script attack. I think I'll stay off there for a while.

The problem with most transistor power amps is that the Safe Operating Area protection circuits get activated when the amp clips with a speaker load. Driving a resistor dummy load is not a problem. The Polytone has no protection circuit other than a simple fuse. Drive it hard and it will eventually toast the output transistors. Most protection circuits are too simple to allow a big enough operating area unless the instantaneous voltage across the output transistor is limited to about 50V. There are some exceptions, and MOSFETs don't have this problem. Chips like the LM3886 have a much more complex protection circuit and they perform better. There is a graph on the LM3886 data sheet that shows the SOA. It's pretty impressive. Current limiting sounds bad. Voltage limiting is good. You need to stimulate a speaker with voltage and let the current go where ever it wants.

One other thing to look out for is overdriving the input of any chip. On the chip data sheet there will be an Input Common Mode range. Don't drive the inputs beyond it or the output will not obey the input. The output usually slams the opposite rail. It sounds really bad but you won't be able to diagnose it without an oscilloscope.

I have a schematic for that amp from 1985. Two stage opamp preamp, diode clipping and a single transistor stage. Descrete 7 transistor power amp with +/- 21V rails and headphone jack.

Try using a pair of headphones or just insert a phone plug into the headpone jack a number of times. If that doesn't work, you probably have blown transistors in the power amp section.

That transformer is too much voltage for a TDA2040 and is too light in the current department for an LM3886. A 48V transformer will make about +/- 33VDC. At that voltage an LM3886 will make about 60W at 8 ohms. The transformer is rated at 57.6 VA. If the transformer VA is equal to the power output of the chip, it will run very hot and will probably eventually fail if you play loud. If the VA is twice the power output, it will run warm but probably survive. If it's four times to power out, it will run cool and never give you a problem. If you increase the transformer voltage above the stock value, some changes to the rest of the amp will be required to the circuits that make +/- 15V to keep them from burning up. As Roly suggested, build an external power amp.

Isn't it supposed to be the Marlin? See if you can find a schematic and post it or a link.

It runs off of a 15V 2A regulated wall wart. Weight of the amp is 0.85kg. 20W seems like a little bit of a stretch...

What country are you in and what is the nominal power line voltage? I think some of the replys assumed you were in the USA. The alligator clip might be the best and only option but you still may get a shock if you touch another guitar player or microphone if they are not also grounded.

From a collector/dealer point of view what are the models to look for and what are the ones to avoid?

I'm not seeing any problems with a single rail design. The opamp is happy. Time constants could use a little tweeking, but a negative rail won't fix that. JFETs really start to open up when you get above 15V. All I can say is that a single rail design is more tube like, warts and all The subsonic artifacts of a tube amp are often overlooked.

Now I need a chip power amp with 100K input impedance (at least) and a Presence control.