Nice little forum you guys got going here. This is my first post, but after perusing your site for a few days I think I am probably in good hands with my issue.
I played this amp for 5-6 years with no problems, then just recently the boost channel stopped working. It also had a real scratchy master volume pot, so I purchased some contact cleaner and a new 12AX7 and went to work, hoping this would take care of it.
I cleaned all the pots and all the jacks and installed the tube, but still no boost channel.
Here is what I know;
Old tube was a Telefunken ECC83 30170, new tube is a JJ T-12AX7-S, EEC83 S
Volume pot issue is fixed
Upon a good visual inspection of the viewable component side of the PCB's, everything appears good. No blown caps or burnt resistors. Nothing obviously fried.
When pressing the boost channel switch, nothing happens and LED for boost channel does not come on, and none of the knobs work
LED for footswitch (a Crate 2 pedal switch that worked before) does not come on for either distortion or chorus
Chorus appears to work
4x12 cab is good
The f/x pedal I use in the loop doesn't seem to make any difference in the sound
It also appears that someone may have hot-rodded this thing, there are two resistors wired onto the PCB near the input jack that don't look stock to me.
I have some electrical knowledge and skill. I am real good with cars, and can wire my house. I have also done some PCB soldering, and diagnostics down to the component level but only for a year or so.
I have never worked on an amp.
I did pick up some Marshall schematics for an 8280 from another site that I think I found linked here, approximately 5 of them, but really dont know much about this type of circuitry.
I do have a DVOM and soldering iron and all the necessary tools to repair it, except the know-how.
I like this amp, so any help would be much appreciated.
Last edited by Trfsrfr; 03-19-2016 at 10:20 PM.
Panel SW2 is the boost channel switch. It is powered by +12 volts, which comes in via CON 1. So make sure that connector CON1 is solidly pressed into the socket, etc. In fact just make certain that all of the connectors are solidly seated all through the amp.
If you have a voltmeter and feel safe in working on the amp while it's plugged in, read the voltage at R14 to see if you have +12 volts dc there. R14 connects to the panel led so look for it where the led is mounted. The channel switch will ground the voltage there when the channel is set for the clean channel, so test the voltage with the switch in both positions.
Stay away from all of the high voltage areas of the amp and be careful with test leads on the live pc board. Accidentally shorting things with the meter probe can make for a really bad day.
Okay, thanks again for that info.
This is what I have;
6.37v at one side of R14, and
1.715v at the other side, and
.083 and .077 volts when switch is pressed.
I also have 11.87v at one side of R49.
Edit: I noticed while checking voltages that the LED was coming on, so I plugged the guitar in and guess what?
It appears to be working now.
I will put it back together and play with it later and let you know what I find.
Last edited by Trfsrfr; 03-20-2016 at 05:55 PM.
Excellent, thank you JPB!
I'm not sure why my attachments didn't work.
Update- Amp is working now. Not sure why. Maybe a cold solder joint?
You pushed unconnected parts together when pushing with your meter tip, but you can't trust that, it will come out with vibration, specially during a hot solo or something, so just redo all nearby soldering adding a tiny bit of solder, you don't want to create a blob which shorts something unwanted.
Stay ay least 5 seconds on each solder or make sure it actually melts.
Many, for fear of burning the PC stay too little so fresh solder added sticks on top of old (cold) cracked one, without actually repairing it.
Juan Manuel Fahey
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