hey its been a year and jcm800 2210 the same!!! not working

[Andre Oliveros]

hi guys, after almost a year and sending my amp to guys here in guatemala that were supoused to know about amps (only two guys in the country) they couldnt do anything to my amp to remember the problem is : when you turn on the stand by the tubes start to glow and run so hot, that if you let the amp like that blow fuses, also the tubes are new, and you can actually play like for 30 or 40 secs before the tubes start to glow that way too red. does any of you have any clue what could be or what can i do? sending the amp to someone its so expensive right now because if you add the shipping the cost of the fixing and the shipping again it probably will be better to buy a jcm800 at ebay.

so anyone can help me please, ill take pics or videos or wathever that help to someone that can help me doing it by myself , like assistance onlline and step by step as if you guys were the brain and ill be the hands.

thanks
Dre.

[Dynaflow]

 Sounds like the bias voltage is messed up somehow, the tubes are not being controlled and running to hot. Did it just start doing it, or were new tubes put in it or what are the particulars about how it started doing this? More knowledgeable guys will chime in, but something like a bad bias resistor or cap or something in that whole bias circuitry is where you'd want to begin your search I suspect.

Regards,

Dyna

[Andre Oliveros]

well here it goes the amp was fine, but i did one gig and the place was small and was so hot, so the amp blowed the fuse i asumed because it was so hot the place and it didnt have any ventilation so i bringed home then after that started to do that. i already changed the caps in bias area, and a diode, the amp has new filter caps, and screen grid resistors, thats all the changes.

[Dynaflow]

 Ok, was the value RIGHT with the resistors and did you re-bias the amp to specs? Diode in the right direction?

Regards,

Dyna

[Andre Oliveros]

p i changed with help of forums, now that rebias to specs i couldnt do because the amp gets hot so i have to turn it off, i changed al the things after it was bad so actually i dont know the next step

[prs_matt]

Start with this:

Pull your power tubes out.

Turn on the amp and set standby to run, measure bias voltage. Set your bias trim so that you get the most negative voltage from your bias circuit.

Report back on how much negative voltage you get.

[Andre Oliveros]

remind me something please im not pro and its been a long since i used to know the terms, how do i measure the bias voltage?? how i have to set the meter? and were i put the testers?

[Geezer]

>>how do i measure the bias voltage??

Look at "Biasing a fixed bias amplifier"
http://www.el34world.com/charts/fenderservice5.htm

>>how i have to set the meter?

DC volts

>>and were i put the testers?

Black to earth/ground, red to pin 5 of a power tube

Quote from the linked page (Doug Hoffman)......

"With your multi-meter set on dc volts, connect the black probe to a suitable ground. The chassis screw where the power transformer center tap is grounded works fine. Turn on the amplifier and let it warm up a few minutes. Turn on the standby so that the amplifier is up and running. Keep one hand behind your back because you will be dangerously close to some very high dc voltages on pins three and four. Touch the red probe of your meter to pin five of the first tube. Notice the voltage and then move the red probe to pin five of the next tube."

[Andre Oliveros]

ok doing this i have no reading on pin 5, the meter is just 0.00 or am i doing something wrong? because doing that procedure i have no reading at that pin

[Geezer]

You have a problem in the bias supply.

Since you said you did some component replacement in that area, it's possible you installed the new parts incorrectly.

Note the orientation of the D7 diode (the "line" goes towards R55/27k resistor) and also the C35 & C36 10uf capacitors.....the positive (+) goes to earth/ground, not the "normal" negative to earth.

http://www.schematicheaven.com/marshallamps/jcm800_splitch_100w_2210.pdf

[Geezer]

Post the voltages you get at each of the test points shown (A, B, C, D, E & F)

[Andre Oliveros]

well now theres no way i measured wrong so this is what i got
AC
A : 369
B: 367

DCV
C, D, E, F : 0.00

AC
also i read this ones wich i found to high
D9: 390 and + 1039 to 1040
D11: 390 and + 1030 to 1040

[Geezer]

Replace the D7 diode, making SURE it is oriented correctly, with the "line" on the diode body TOWARDS point "B" (R55).
Now what do you read DCV?

[LooseChange]

Replace the D7 diode, making SURE it is oriented correctly, with the "line" on the diode body TOWARDS point "B" (R55).
Now what do you read DCV?

That's gonna do it.

[Andre Oliveros]

i changed the orientation of the diode and the reading was
DCV
0.10

i found that when i dont turn the stand by on, and measure the d7, the reading is - 0.18

and its having the same problem, also i think that the way it was the diode was correct, the body has a line like in the top, that was toward R55... at this point the same thing nothing its giving me correct readings... what else should i do? im thinking on posting pics so u guys can take a look to the guts.

Dre.

[Geezer]

Replace the D7 diode, making SURE it is oriented correctly, with the "line" on the diode body TOWARDS point "B" (R55).
Now what do you read DCV?

i changed the orientation of the diode

Did you REPLACE the diode (with a new diode)?

[hctrjs]

Hello, I dont know if anybody is still following this thread but I am trying to fix the amp mentioned above on behalf of the original poster. I measured all the points stated above, after removing the tubes. The results I got where:
A: 116.3 V AC
B: 116.8 V AC
C: 1.6 mv DC
D: 1.6 mv DC
E: Between 1.1 and 2.2 mV DC
F: Between 1.1 and 2.2 mv DC

When I measured the bias voltage I got between 1.1 and 2.2 mV DC, and even if I changed the value of the bias pot this values would not change. I checked the pot and the value of the resistor is changing between 0 and 22k as stated in the diagram.

The diode mentioned above was connected backwards, I changed it and I still have the same problem. I did not replace the diode with a new one, but I checked it with my multimeter and its working properly.

From the values measured, I believe there might be a problem with the bias circuit, but I don't know where I need to check or what do I need to change in order for the tubes to work properly. This is my first time working with a tube amp but I do have knowledge on electric/electronic components so any help would be great. Thank you.

[rzenc]

When I measured the bias voltage I got between 1.1 and 2.2 mV DC, and even if I changed the value of the bias pot this values would not change. I checked the pot and the value of the resistor is changing between 0 and 22k as stated in the diagram.

So... you don't say whether it is positive Dc or negative DC...and fixed bias amps, like the one you have run with NEGATIVE DC at control grid - pin #5.


The diode mentioned above was connected backwards, I changed it and I still have the same problem. I did not replace the diode with a new one, but I checked it with my multimeter and its working properly.

backwards with respect to which polarity? remember... you need negative DC in order to control electron flow. So, you can expect to see a reversed biased diode and also reversed caps, with the positive end going to ground instead of the usual way, that is, with positive lead connected to supply.

if you have the possibility to build a new bias board and derive it's supply from H.T. and see if the amps works properly - see attached schem copied from JCM800 2204 -

Hope this helps

Best Regards

Rzenc

[hctrjs]

They are all positive DC. Are you sure this is a fixed bias amp? The page where I bought the power tubes said that my amp needs to be biased when changing power tubes, and in the diagram shown (the links at the beginning of the thread) all the resistors are fixed except for RV1, so I suppose this is the resistor value I have to change to bias the amp.

I connected the diode as stated in the diagram, and yes the caps are have the positive lead connected to ground.

Sadly I can't build a new bias board since I am only doing this as a favor and don't have the time nor the components to do it, which is why I was hoping I could get some help from somebody on the board. If you have any other ideas, that would be great. Thanks for your time.

[sluckey]

Fixed bias doesn't mean the bias is not adjustable with a pot. It means that a negative voltage that is independant of tube current is applied to the control grid. In the 2210 this 'fixed' bias voltage can be adjusted to vary the idle current through the tube, but the bias voltage itself cannot be changed because of current through the tube. It's fixed. Confusing, ain't it.

Anyway, there are only a few components in the bias circuit. You could change them all in 30 minutes for less than $10. The components are R55, D7, C36, R54, R51, RV1, C35.

Pull V6, V7, V8, and V9 and don't put them back in until you can measure approx. -50vdc directly on pin 5 of each of those empty tube sockets.

Look at this schematic...
http://www.schematicheaven.com/marshallamps/jcm800_splitch_100w_2210.pdf

Check for 50VAC (or more) on the right side of R55. If you ain't got this AC voltage, the PT is bad. (It's possible but hopefully not likely.) Next, check for approx -50vdc across C36. Next check for approx -50vdc on C35. Then check for approx. -50vdc on pin 5 of each empty tube socket. If you don't get the voltage at any of these steps, stop and find out why.

Since you say you found the diode connected backwards, I'd put a new diode in right off the bat. C36 would have been stressed also and needs to be replaced. C35 may be alright, but if time is money, replace it too.

[tubenit]

If you were willing to run the amp at 50watts instead of 100watts, you can remove two tubes and cathode bias the amp.
It won't be that much quieter. Unless you're playing outdoor venues, 50 watts is alot of volume.

So if you don't need all the power,  you might find the simplest solution for you is to turn the amp into cathode biased and forget about fixed bias. If you have a pair of 6L6's try a 330 ohm/10w resistor with 47uf/100v cap to ground on the cathodes.  Then take the 220k resistors and put them to ground. I took a friend's Marshall and cathode biased it and he preferred it that way.

If you want an example of what I am talking about look at the schematic attached.

With respect, Tubenit

[hctrjs]

I checked the voltages. I was getting around 100VAC to the right of R55 but on the places I'm supposed to get -50VDC I'm getting 2 mVDC. I measured the values of the caps and resistors, and they were all showing the correct values. Either the caps are not working well and are not charging or the diode is not working and I'm not getting the negative voltage needed to charge the capacitors. I will buy new components tomorrow, change them, and let you know if the problem got fixed.

If that does not work I will talk to my friend to see if he minds changing from 100 to 50 watts.

Thanks for your help.

[sluckey]

The diode is the most likely suspect.

[hctrjs]

I changed the diode, capacitors 35 and 36, and RV1 and I'm still getting 2mVDC where I should be getting -50VDC. On the right (and left) of R55 I am still getting over 50VAC. Any other ideas??

[hctrjs]

I noticed something else which makes me think that the bias circuit is not the one that is not working. Before I turn on the standby switch I can measure 30VAC to the right of R55 and -8VDC on C36 (I can change this value by changing the pot). When I switch on the standby switch I get the 100VAC on the right of R55 and 1mVDC on C36. Could this be a problem with one of the transformers? Or is something else messing up my ground and thats why I cant get the diode and capacitors to work properly??

[sluckey]

You do have the 4 output tubes pulled right? Pull the HT fuse also.

Disconnect one end of D7. Measure the resistance across C36. It should be approx 60 to 80K depending on the setting of RV1. What is it?

Post a hi rez pic of all these components.

[hctrjs]

Yes, I did remove the tubes and the fuse. The resistance across C36 was between 62K and 82k depending on the value of RV1.

You can find the picture here:
https://tech502.com/Archivos/Components.jpg

Whenever I flip the standby switch on, the bias circuit stops working. Before I flip it on, it does work but I don't get over 60VAC on the right of R55, I only get 30VAC. When I flip on the switch, I do get around 100VAC on the right of R55 but the bias circuit stops working and I only get 2mvDC on C36. I am starting to believe that C49 might be causing some sort of problems, VDC on C49 is around 400VDC so the Cap is working, but it might be causing problems on the bias circuit. Could it be that this might be the cause of the problem?

[hctrjs]

Is anybody still there? I still cant figure out what the problem is, does anybody have any ideas?

[sluckey]

Looks like the PT leads have been disconnected at some time and just tack soldered back to the board. Are they all connected correctly?

[Direwolf]

That's what I was thinking, Sluckey. I don't like the looks of the soldering in that area. That needs to be corrected before anything else. If those are cold solder joints, you may be frying any components before they get a chance to work!

Also, D11 looks like it may be fried.

[rafe]

looks like they just soldered from the front  of the board ! Who did that? Can you post the rest of the board?

[hctrjs]

I dont know who did the soldering there, it was like that when I got it. Its not my amp and I am just fixing it for a friend, so I didn't want to change that.

Yes, the connections are correct. I have checked them several times. None of the diodes are fried. I have also checked them with my multimeter and they all have a forward voltage of about 0.5V, which is the average for these types of diodes.

Is there anyway of knowing if the PT is working? What should the voltage be on the leads? Is it ok that the lead that goes to R55 is around 30VAC and when I flip the standby switch on it becomes 115VAC?? The two leads that go into D9 and D11 have a voltage of 350VAC, yet C49 has a DC voltage of 500V. I'm pretty sure this is not correct, but I could be wrong. As I said earlier, C36 is losing voltage when C49 gains voltage. I am convinced that there is a problem with the ground, the reference level is changing and that is why I have changing values on the leads of the PT. The lead that leads to R55 should always be the same, since the standby switch does not affect this connection. If the reference value changes, D7 will not let any negative voltages through and the capacitor will not charge and I will never get -50VDC on C36. 

Any thoughts?

[sluckey]

Its not my amp and I am just fixing it for a friend, so I didn't want to change that.

Whether fixing it for a friend or a customer, I'd definitely fix that sloppy wiring. You may even discover the problem while doing do.

Is there anyway of knowing if the PT is working?
What should the voltage be on the leads?

Just measure the voltages for each of the secondaries.

1. For the HT winding you should have about 350VAC BETWEEN the two blue wires that connect to the diodes. Note, the schematic shows different colored wires and that makes me wonder if this is the original and correct PT. There is also a yellow center tap associated with the HT winding and it should not be connected to anything.

2. The heater winding should be 3.15VAC from each black lead to ground. There is a green center tap associated with this winding and it should be connected to ground.

3. The bias winding is a totally separate winding. One lead is white and should connect to R55. The other lead is green and should connect to ground. The voltage between that white wire and ground should be approx. 50-100VAC (I don't know the exact amount) and it should not change.

Is it ok that the lead that goes to R55 is around 30VAC and when I flip the standby switch on it becomes 115VAC??

THAT'S A PROBLEM! That voltage should be constant and totally independant of the STBY switch.

The two leads that go into D9 and D11 have a voltage of 350VAC, yet C49 has a DC voltage of 500V. I'm pretty sure this is not correct, but I could be wrong.

That's perfectly normal. I assume you're measuring the 350VAC BETWEEN the blue leads that are tack soldered to the diodes?

As I said earlier, C36 is losing voltage when C49 gains voltage. I am convinced that there is a problem with the ground, the reference level is changing and that is why I have changing values on the leads of the PT. The lead that leads to R55 should always be the same, since the standby switch does not affect this connection. If the reference value changes, D7 will not let any negative voltages through and the capacitor will not charge and I will never get -50VDC on C36.  

Something is wired incorrectly or the PT is bad. If you can't find a wiring error, then replace the PT. I'd like to see a few more hi rez pics that show where every PT secondary wire is connected, even that HT center tap that should not be connected.

[hctrjs]

I checked and I got the voltages you said I should get. Yes, I was getting 350VAC between the blue wires (the reason they have a different color from the schematic is because the PT leads go to the standby switch, and from the stby switch to the diodes they have a different color).
The heater windings have 2.75VAC each.
The only problem was the bias winding, which is giving 30VAC and it changes to 110VAC when the standby switch is turned on.

I checked all the connections you told me to check, and they are all wired correctly. I haven't had time to post the pictures, but the PT leads are all connected properly. The center tap is not connected.

I'm guessing the PT is bad. There might be a problem with the green bias lead, which is connected to ground, but if there is something wrong with that lead (a bad internal connection or something like that) it makes sense that the voltage on R55 changes. Besides, the amp was working correctly until one day it stopped working, so I'm guessing it makes more sense to think that the PT is bad rather than a bad connection.

I will try to post the pics this weekend.

Thanks a lot for your help sluckey, I really appreciate it.

[sluckey]

The correct thing to do would be to replace the PT since you are sure there are no wiring errors. However, there is a cheap alternative, assuming the bias winding is the only thing wrong with the PT. There's a simply mod you can do to convert your bias circuit to the JCM900 bias circuit like in this schematic...
http://www.schematicheaven.com/marshallamps/jcm900_21xx_25xx_100w.pdf

You simply remove R55 and put a .047µF/600V cap where R55 was located. Then connect a 56K/1W resistor between D7 cathode and ground. Then totally disconnect that bias winding. Now connect the .047 cap to one leg of the STBY switch (on the PT side), thus feeding the bias circuit from the HT winding. This will save having to replace the PT just because the bias winding is funky. Hopefully the rest of the PT is OK. Of course the owner would have to approve.

Take a look at this drawing and see if you are interested.

EDIT... I just looked at your pic and your board is set up to easily do this mod already.

[hctrjs]

Hey, that definitely looks like an interesting and cheaper solution, I will be sure to check with my friend if he is ok with that.

I only have a few questions, the 0.047uF cap that u placed, it shouldn't have to be a polarized cap right? It could just be a regular ceramic one with no polarity?

The second question I have is regarding the bias winding. You mentioned earlier that the winding has a green lead that is connected to ground, so I am assuming that this is the reference of the bias winding, is this correct? I disconnected the bias winding, and I am only getting 8VAC from that winding when it is not connected to anything else. When I turn on the standby switch, it goes to 25VAC, still not connected to anything. I guess that this only confirms that the bias winding is funky so it definitely needs changing.

Thanks a lot.

[sluckey]

I only have a few questions, the 0.047uF cap that u placed, it shouldn't have to be a polarized cap right? It could just be a regular ceramic one with no polarity?

The cap is not polarized. Just use your favorite cheap coupling cap rated for 600vac.

You mentioned earlier that the winding has a green lead that is connected to ground, so I am assuming that this is the reference of the bias winding, is this correct?

Yes. Since that PT uses a separate bias winding, there must be a wire connected to each end of the winding. The schematic shows a white wire and a green wire.

I disconnected the bias winding, and I am only getting 5VAC from that winding when it is not connected to anything else. I guess that this only confirms that the bias winding is funky so it definitely needs changing.

I agree.

It still bothers me that flipping the STBY switch would change your bias voltage. That would indicate that there is a short (or at least some continuity between the HT winding and the bias winding. While you have that bias wire disconnected, check the ac voltage on the bias winding while flipping the STBY switch. Does the ac voltage change? Also, with power off, check for resistance between that bias wire and the HT wires. Do you get any resistance reading?

If there is resistance between the bias winding and the HT winding, there could be other problems with the PT,,, problems that may not show up until you get the amp working enough to test it. Keep this in mind as you proceed. If the owner has the $$$, it may be better to just replace the PT.

[hctrjs]

I disconnected the bias winding, and I am only getting 8VAC from that winding when it is not connected to anything else. When I turn on the standby switch, it goes to 25VAC, still not connected to anything.

There is resistance between the bias winding and the HT winding, when I measure resistance from the bias winding to any of the HT leads (including the center tap) I get around 10M ohms of resistance between them.

What exactly does this mean? Will your solution still work? Obviously the best solution would be to change the PT, but I am trying to come up with all the possible solutions before I ask my friend what he wants to do.

In case I do need to change the PT, where could I buy the PT for this amp?

Thanks.

[sluckey]

I disconnected the bias winding, and I am only getting 8VAC from that winding when it is not connected to anything else. When I turn on the standby switch, it goes to 25VAC, still not connected to anything.

Can you identify that green wire associated with the bias winding and disconnect it from ground? Then, with both bias leads flying in the air, measure ac voltage between the flying leads.

I would not worry about 10MΩ resistance. Go ahead and try the bias mod. I don't know where to source that PT. Maybe some of the more experienced guys here can give you a lead.

[hctrjs]

The measurements I took and posted are with both leads flying in the air.

When I disconnected ONLY the green lead from ground, I would get 30VAC on the bias winding (the same value I get when the green lead is connected to ground). The 30VAC I measured are from the white bias winding to ground (which means that the green lead connected to ground is not affecting this voltage). When I flipped on the standby switch (green lead still disconnected from ground) I would get 100VAC from the white lead to ground (the same value I get when the green lead is connected to ground). When I disconnected BOTH leads (both where in the air) I got 8VAC between them, and when I flipped the standby switch on I got 35VAC between them.

[Shrapnel]

That power transformer sounds really suspect, IMHO. Perhaps if we can get a photo of the transformer, with all the leads visible (colors especially) and the numbers found on the transformer, we can find out if it is even the right one or see if something looks amiss.

[hctrjs]

I tried the bias modification, it did not work, I am getting the same results. I'm guessing the PTs bias winding was not the problem. I always get the correct voltage before the diode, but after that I don't get the expected result.

Before turning on the amp, I can measure continuity with my multimeter between the two black leads from the heater winding and between the black leads and ground. When I turn it on I can measure 2.75VAC between the black leads and ground, is this correct?

Any other ideas?

I will get the pictures of the transformer, but I'm pretty sure its the original transformer that came with the amp, or so I've been told.

[sluckey]

Any other ideas?

Replace the PT.

[hctrjs]

Yeah, that's what I was thinking, but since the modification you posted did not work it made me think that maybe the PT is not the problem, unless all the windings are not working, I just want to make sure that the PT is the one not working before I change it, since it is expensive.

[Andre Oliveros]

Hey im the owner of the amp, the guy told me that he changed de PT, and still not workin, i already wrote you renz and ill wait for your answer jaja, but if anyone has any ideas please tell me, thanks.

Dre.