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Discussion in 'Marshall Amps' started by islandjimmy, Jan 11, 2019.
Bingo. This is why i asked if there was 15 pos/neg at the channel switching opamps.
No, half power lifts the cathode connection from two of the tubes ground hookup, triode changes the point where the screen grid draws B+ from.
this reminds me off a mesa express i had..iswapped out the speaker , reg ol swap and i went to play..hum..faint noise ..but nothing, no sound ..but hummm..ordered tubes and went on all kinda diagnosing..then one day,, opened it up..just because and when i re hooked up speaker , i used the other 8 ohm jack..and ta da !! back to normal..the one jack was dead
Another reason i asked about the 15 volts is the second loops return jack has an opamp buffer behind it and if there is no juice you won't get sound by plugging the guitar into it. IC308B
....plus doesn't the FX loop have a series/parallel option with their associated line levels? If something is faulty here, could that prevent a signal from passing through the amp? But you could easily test this by plugging a guitar into the FX-loop return, so I don't know.
Also, it looks like you guys have different circuits/boards (did marshall use variations on the 6100?). If you look at R18 on each picture, one has a hole in the board for a wire while the other (his) does not. So now, I don't know what to think.
Yes indeed, I see at least five fuses inside the chassis, in addition to any external fuses. Each needs to get pulled and metered! One other thing that has not been mentioned (unless I missed it) is the possibility of dirty/corroded/oxidized jack contacts as well as switch contacts.
Yes they used different boards between the LE's, Anniversary, and LM's. This is also why i asked about the solder pad at EC1-A position as there appears to be a broken off wire there. Some of the earlier heads used this point to make the ground connection. They also tacked it onto one of the caps ground leads and then they put the through hole connection in on the LM's. Even though these boards have solder pads and trace on both sides, most of these connection have to be soldered on the bottom as some of these top traces don't do anything.
This is from the op's pics:
I tested all 7 fuses in this amp and they all tested good. I used a continuity setting with an alarm sound if the connection is good, they all passed. There was one fuse in slot F403 that was slow to beep but it did eventually, not sure if that matters.
I took a close look at that hole and it is super clean. It does not look like there was ever solder there. The hole is perfectly round and unobstructed. That picture is a bit deceptive because it looks like there is something in the hole but there is not.
Just curious, did you remove the fuses when testing them? Sometimes, depending on the circuit, you can get false readings on a fuse that's still in the circuit (readings through resistors...). It's always best to remove fuses when testing them. It's also good practice to take voltage readings on each side of a fuse before removing it, to assure there are no stored voltages present (charged caps...). Just connect the negative lead of your meter to the chassis and probe the fuse holders with the positive lead (meter set to DC volts (500). Always best to be safe, than sorry.
Here's the board ID number on mine.
Yes, I removed each one for testing. I also needed the number off of each one so I can order more.
Ok, can you check those voltages that i asked about?
I don't get any value when I test the voltage on these pins. I know where pins 4 and 8 are on each prom chip so lets make sure I'm doing this correctly. After setting my meter to voltage, I chose 20, do I put the red probe on 4 and the black on 8? Or vise versa? That's what I did both ways and got 0. Dumb question #2 is the amp supposed to be on standby when I do this?
The amp can be in Standby or out of Standby. You want to measure the pins with reference to ground (negative meter probe on chassis or PCB ground).
If you have 4 or 5 different PCB boards, each will have its own 'name'. There's a lot going on under there. Did you try thoroughly cleaning everything yet? After sitting for 25 years, this would be job #1.
Can you tell me what types of cleaning I should do? I have a can of Deoxit D5. Should I be cleaning pots and pin connectors?
I'd recommend this to clean, and this to lube afterwards. Many people like DeoxIT including myself.
IC201 pin 4= .43 pin 8= .48
IC202 pin 4= .43 pin 8= .48
Meter set to voltage 20, negative (black) on chassis, red on pin.