JCM 800 2203 dead...

[stokes]

Yeah,like Tom said 2k aint right.Are you sure you are taking the measurement at the right pins.I would think a fried primary would read as a dead short or get an open reading.I dont see what would cause more resistance insise the winding.I would open the amp and disconnect the CT and with no tubes in measure from the CT to each end of the primary.If you are trying to find pin 3 from the outside of the chassis make sure you are counting the 3rd pin in the right direction from the locator slot.It will be opposite than if you are looking from the inside of the chassis.

[MitchK]

Yeah I'm pretty much sure I'm reading the right pin. I'm looking at the bottom of the socket (from inside the chassis) and counting clockwise the third pin from the notch on the tube base.

is there any sure fire way of checking both the PT and OT, I'm kind of bummed about it and just want to know if I'm going to have to take it apart and rewind it or buy a new one or whatever I can do.

Stokes, so you are saying measure the voltage between the center tap and the primaries of the PT? or should I just have it powered off and check the resistance?

[stokes]

Not voltage,I suggested taking the CT off the cap,and removing the tubes to make sure there is nothing else in the mix,and wasnt sure if you were taking the resistance from the inside of chassis.It just seems to me that a bad secondary would be either open ar shorted.Reading the resistance from CT to each end should read like 1.5 ohms or less,I am no expert on trannies,but I just dont see what could cause more resistance in the secondary going bad,but if it is reading 2k,something aint right.

[MitchK]

Its a miracle guys, its up and running. by miracle though I just mean I'm a fool and should pay more attention to what you guys say.

First Steve, you totally called exactly what happened. Upon some further investigation I've realized what happened in the first place what made this all go wrong was when I was getting the "blues" off the power tubes, it was because the speaker cable connecting the head to cabinet had crapped out and I just didn't realize it. last thing I would have thought to be honest.

As for the dead transformer and the 2k ohm reading. the reason it was 2k ohm dead on? pin 3 on this amp is connected to pin 5 and pin 5 goes to the OT, BUUUUT a 1K ohm resistor connects pin 3 and pin 5. one on each side = 2k ohm. I took reading directly from the transformers and all the windings read normal. the new tubes got here plugged them in, no blown fuses and it sounds fine.

I learned a LOT from this thread, thanks guys!

[stokes]

That 1k you describe doesnt sound right.Pin 5 is your input grid.What you describe has your input connected to the OT with a 1k going to your plate.That cant be.Glad to hear the amp is working and it was just a bad speaker connection,but whats up with that 1k between the grid and plate?I can see the 1k being from pin 6 to pin 4,6 has no internal connection and 4 is the screen grid that should have a 1k on it.Or from pin 3 to pin 4 which would mean you are using a triode type connection,but it cant be as you describe.

[MitchK]

That 1k you describe doesnt sound right.Pin 5 is your input grid.What you describe has your input connected to the OT with a 1k going to your plate.That cant be.Glad to hear the amp is working and it was just a bad speaker connection,but whats up with that 1k between the grid and plate?I can see the 1k being from pin 6 to pin 4,6 has no internal connection and 4 is the screen grid that should have a 1k on it.Or from pin 3 to pin 4 which would mean you are using a triode type connection,but it cant be as you describe.

hey stokes, yeah I quickly realized this when I went to bias the amp, I was confusing pin 3 for pin 4.

I do however have a grave problem. I think I just blew my new tubes which i have to say I'm ecstatic about. While biasing the amp it took a little while to figure out that I was reading the wrong pin and then to reconnect my resistor to pin 8. I finally took some readings and got 437 for the plate and.... 80mv across the resistor?? yeah way too high, so I then look at the bias trimmer. it has printed on it 22K. I read the actual resistance of the trim pot and with it turned all the way I get a max resistance of about 16K. Meanwhile, the HT fuses have blown again. I'm really really hoping this did not kill my tubes. I juuuust bought them.

Sooo what I'm wondering at this point is could this just be the trim pot its self? it just seems to simple and I don't see why/how it would start acting this way. could anything else cause all of this? it worked fine until the problem with the speaker cable and when playing it with just 2 tubes at max volume for about 30 minutes it totally conked out again, opened it up and saw that it had really high value fuses inside. any clues?

and is there a way to test my tubes to see if they are blown and to see which ones are blown?

[stokes]

You say you were confusing pin 3 for pin 4,but what about pin 5?Where does pin 5 fit in here.'Cause if pin 5 is connected to pin 4 via a 1k that aint good either.If that is the case you are putting a couple hundred volts dc on your input grid.Getting to those readings, if you have one 1ohm resistor connected to one tube on one side of the OT,that would be the reading for two tubes so 40ma each tube,which is about right for an EL34 at 437 plate volts.But first lets get back to that 1k resistor,is it connecting pin 4 to pin 5?If it is,somebody definately F&*#ed up that amp.Where did you have the 1ohm resistor connected before you realized it was on the "wrong pin"?I hate to say this,Mitch, but I am starting to think you may be in over your head here,but lets back up here and see what we can make of this.Clear me up on this 1k resistor going to pin 5 first.And let me know where you had that 1ohm when it was on the wrong pin.

[MitchK]

You say you were confusing pin 3 for pin 4,but what about pin 5?Where does pin 5 fit in here.'Cause if pin 5 is connected to pin 4 via a 1k that aint good either.If that is the case you are putting a couple hundred volts dc on your input grid.Getting to those readings, if you have one 1ohm resistor connected to one tube on one side of the OT,that would be the reading for two tubes so 40ma each tube,which is about right for an EL34 at 437 plate volts.But first lets get back to that 1k resistor,is it connecting pin 4 to pin 5?If it is,somebody definately F&*#ed up that amp.Where did you have the 1ohm resistor connected before you realized it was on the "wrong pin"?I hate to say this,Mitch, but I am starting to think you may be in over your head here,but lets back up here and see what we can make of this.Clear me up on this 1k resistor going to pin 5 first.And let me know where you had that 1ohm when it was on the wrong pin.

sorry what I should have said is that I was reading all the pins wrong. thinking pin 4 was pin three and that pin 6 was pin 5. I was just counting from the wrong pin, couldn't quite see the notch in the tube base. so by that I had the 1ohm resistor hooked up to pin 1 at first.

and I see what you mean about taking half of the reading for some reason I thought that was only the case for the OT shunt method, and when I read the resistance on the trim pot it seemed to make sense. so, if that is normal, what else could it be?

[stokes]

Okay,so the immediate problem is that the HT fuse is blowing,right?First lets eliminate the caps as the cause.To do this remove all the tubes and power up the amp,if the fuse doesnt blow I want you to check the volts at each filter cap starting at the rectifier and all the way down the rail.If you have a current limiter we can use that for the next step,but for now check those voltages and post them.When the fuse blew this last time,was the amp idling or were you playing it?If it was idlong how long was it on before it blew?

[Amp Kat]

I think he just got his pins crossed there Stokes. I'm sure he meant 4 & 6 however 2k would be ok but he may also be looking at the grid resistors going to pin 5 where the bias connects. You are lucky Mitch because this open cable is exactly what I was talking about in this thread that is so dangerous to your output transformer and if the volume would have been cranked on 10 and the load open your OT would have been toasted in a short amount of time. The thing is that it happens and you don't realize it. If it had blown and techie guy says oh the switch was on 16 and your running 4 ohms well that's what blew it when in reality it was the open that blew it. Just a point I was trying to make. As far as the 2 KT-88's go you should now know that the Marshall trannies aren't to stout so run 2 only and adjust your bias. But to be honest if you run 2 EL-34's and then add the other two you won't notice but about a -3db SPL difference in dynamics. Glad you found the problem as that's the main thing

[MitchK]

Alright so I just took a reading, I get 50v on the hot terminals on first cap after the rectifier bridge, and 24 on its ground pin. its tied to another can and on that one I get the 24v where its tied but the ground reads 0. all the other caps all read zeros everywhere.

what had happened was the amp died at a show. I opened it up and saw that the ht fuses and center tap fuses had blown and that they had super high value fuses in them. So I replaced those, turn the amps power on, heard some buzzing, turn the standby off and pop, the HT fuses blew right away. Then I replaced all the tubes, put new fuses in, turn it on, no buzzing, took it off standby and it came on it was pretty buzzy, but I did have the chassis open. I then started to bias it as described in the last posts and after about 10 minutes of having the amp on the HT fuses blew again.

[stokes]

I want to see the voltages with the amp on.By on I mean the power and standby on,in play mode.I want to see that the voltage is the same at all the filters.This is to be done with no tubes in the amp.Make sure you have the right value fuses in the amp.

[MitchK]

I want to see the voltages with the amp on.By on I mean the power and standby on,in play mode.I want to see that the voltage is the same at all the filters.This is to be done with no tubes in the amp.Make sure you have the right value fuses in the amp.

ahh right I hadn't yet put new HT fuses in, I'll get some and tell you get when I get off work in a couple hours.

thanks sooo much,
mitch

[MitchK]

Alright so I took a picture to make this easier:



here is what I got for readings, they all look normal to me but of course that doesn't say much.

I- 470v same as the plates
Ig(ground pin) 230v

II- 470v
IIg - 230v

III - 230v
IIIg - 0v

IV- 230v
IVg- 0v

V- 330V
Vg- 0v

I read both terminals on each cap and they were all the same as their counterparts.

I also took a reading from the big cap in the pre section and that one reads 280v and 0v on the ground.

one other thing that I noticed. the bias fuse, gives me a resistance of aobut 35 ohms where as none of the otehrs did. I checked the fuse for continuity and no dice, its a small value fuse only 100ma but I'm assuming I should still be able to get something from it, I'm not really sure just thoguht I'd get that out there too.

[stokes]

Mitch,you are missing the point here.What we need to do is find out what is blowing the fuse,when a fuse blows it means there is a short somewhere.This test is to determine that there is no short in the filter caps.I need you to take all the tubes out and turn the amp on and see what the volts at each cap is starting at the rectifier and working your way down to the last filter in the preamp.There is no need to measure the 0volts at ground,also it appears that the first two caps are stacked one cap grounded and another in series with it,I dont need the volts in between where you see the 470 is the B+ feed that 230 is where the two caps are connected together,we dont need the 230.With no tubes there is nothing to draw current so the voltages should be the same at each cap,if we see a voltage drop at one of the caps it will indicate that cap is drawing current,and is either leaky or shorted.Look at the schem.you will see the rectifier feeding one cap then the choke then another cap then a resistor then another cap another resistor and finally the last cap in the B+ rail.These are the voltages I need to see.