Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: The Newriginals on June 10, 2017, 10:41:59 am
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I picked up a JTM45 clone, tested everything, all the voltages were perfect, biasing was perfect etc. with the exception of a VERY high reading at the standby switch ~1000 volts. I stupidly decided to play for about 20 minutes until the PT started smoking. Powered off, drained caps, disconnected the PT. I figured it was a bad transformer. So I installed a new one and did some quick tests and the standby switch still reads over 600 volts and sometimes jumps even higher than that. I figure something is really fundamentally wrong here that isn't the power transformer. But all the other voltages test perfectly. Does anyone have any ideas?
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I picked up a JTM45 clone, tested everything, all the voltages were perfect, biasing was perfect etc. with the exception of a VERY high reading at the standby switch ~1000 volts.
Kinda hard to believe everything was perfect except for the high reading at the standby switch. Maybe your meter is lying? Which make/model do you have? Can you try another meter?
Maybe the voltage select switch is in the wrong position? Can you show us a schematic and layout? Post some hi-rez pics.
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Yeah, I found it hard to believe myself, until the original PT started smoking despite maintaining a good sound... It's an old Metro build. I'll post pics shortly.
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Is it possible you're putting 220V into a 120V primary?
Otherwise use a trouble shooting flowchart or checklist.
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Sorry for the delay, I tried rewiring the standby terminals and filter cap and rectifier socket. Now I'm reading 0 at the top standby switch terminal and 630v at the bottom (in the photo). I'll post the transformer specs in a second.
http://imgur.com/oNlr55x (http://imgur.com/oNlr55x)
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transformer schematics:
PT
https://store.metropoulos.net/collections/transformers/products/metro-spec-jtm-45-pt-drake-1202-55 (https://store.metropoulos.net/collections/transformers/products/metro-spec-jtm-45-pt-drake-1202-55)
OT
http://www.classictone.net/40-18039.pdf (http://www.classictone.net/40-18039.pdf)
Choke
http://www.classictone.net/40-18058.pdf (http://www.classictone.net/40-18058.pdf)
Is it possible you're putting 220V into a 120V primary?
Not at all, I've checked it a dozen times.
Otherwise use a trouble shooting flowchart or checklist.
Been at that for the past couple hours, no luck so far.
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I've narrowed it down to the tube sockets in the PT region or the solder connections in that region. I resoldered everything around the PT and removed and reinstated the tubes to sort of feel out the sockets and the voltage came down to normal! But now when I hit the standby switch the voltage is too low and there is no sound... :help:
p.s. I had several zero readings on the turret board, is that bad?
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I am no expert but looking at the picture your red/yellow CT goes into a fuse then out ground.
Make sure that fuse is not open and make sure it has a high enough voltage rating. Looks like a black spot on it.
Good luck hope this helps.
John
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I am no expert but looking at the picture your red/yellow CT goes into a fuse then out ground.
Make sure that fuse is not open and make sure it has a high enough voltage rating. Looks like a black spot on it.
Good luck hope this helps.
John
Nice catch and thanks! I was indeed getting a zero reading from the fuse until I cleaned up the soldering. Now I'm getting a 219v reading on the PT end and zero on the ground end. Still having same issues, but I think that's in the right direction, I'm not getting any readings from the wires connected to the main fuse. I also notice a tiny metallic *plink* sound from the top of the chassis when I shut off the power.
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Set your meter to AC mode when measuring AC voltage such as found on the mains fuse, or PT secondary voltages. Tell us again what your present symptoms are. Measure voltages on all tubes, every pin (even if zero) and post those here.
Did you build this amp? If not, what do you know about it? Did it ever perform normally?
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Nice catch and thanks! I was indeed getting a zero reading from the fuse until I cleaned up the soldering. Now I'm getting a 219v reading on the PT end and zero on the ground end.
Which means the fuse is blown, no? It should be the same on both ends when the fuse is good.
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Set your meter to AC mode when measuring AC voltage such as found on the mains fuse, or PT secondary voltages. Tell us again what your present symptoms are. Measure voltages on all tubes, every pin (even if zero) and post those here.
Did you build this amp? If not, what do you know about it? Did it ever perform normally?
The amp is my first build unfortunately. It only "worked" when I first tested it, but I replaced PT after the smoke. I have a generic meter that has always been set to 500v~
Present symptoms are strange voltages and no sound.
Here are the voltage readings @ 500v~:
V1===>1:61, 2:0, 3:0, 4:3, 5:3, 6:57, 7:1, 8:0, 9:0
V2===>1:59, 2:0, 3:0, 4:3, 5:3, 6:97, 7:59, 8:3, 9:0
V3===>1:86, 2:1, 3:6, 4:3, 5:3, 6:82, 7:1, 8:6, 9:3
V4===>1:0, 2:3, 3:132, 4:131, 5:0, 6:137, 7:3, 8:0
V5===>1:0, 2:3, 3:131, 4:130, 5:0, 6:131, 7:3, 8:0
Rectifier=>1:0, 2:134, 3:0, 4:61, 5:0, 6:28, 7:0, 8:133
Powerswitch terminals: 117v
Standbyswitch terminals (off): 438v, 0v
CT Fuse: 0v both terminals
Main Fuse: 0v every terminal
Strangely the fuses look okay? Thank you so much for the advice!
Which means the fuse is blown, no? It should be the same on both ends when the fuse is good.
I'll swap out the fuses to double check, but they look fine.
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Installed New fuses, the standby terminals shot way back up to over 600, but still no readings on any of the fuse terminals.
Update: sound works and sounds awesome! Still worried about the PT burning tho with the standby voltages high.
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Something to consider making. Current limiter.
It may help and will cost $10 maybe.
Do you have a schematic and some more close up pictures of the wiring?
My first time messing with tubes I blew up my Acoustic G20 amp. :BangHead:
Made my wife and kids jump and the dog run in his cage.
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I have a generic meter that has always been set to 500v~
~ means AC. You must set the meter to measure DC for everything in the amp ***EXCEPT*** mains voltage, any secondary voltages out of the Power transformer, or filament voltages. Everything else in the amp is DC and your meter cannot read those voltages if it's set to AC mode.
Since the amp sounds awesome the voltages must be pretty good. You just got to figure out your meter. Tell us which make and model meter you have and we can probably tell you how to set the switches.
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From the picture looks like you have a wiring issue.
Your hot leg comes in and goes to the power switch then jumps back to your fuse. It should go through the fuse then to the switch.
It also looks like the neutral goes to a tube?
Is is this the layout you used? If so from what I can see that first tube and power wiring is a big part of the issue. But I can'see all of the wiring in the picture.
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~ means AC. You must set the meter to measure DC for everything in the amp ***EXCEPT*** mains voltage, any secondary voltages out of the Power transformer, or filament voltages. Everything else in the amp is DC and your meter cannot read those voltages if it's set to AC mode.
Since the amp sounds awesome the voltages must be pretty good. You just got to figure out your meter. Tell us which make and model meter you have and we can probably tell you how to set the switches.
Here's my multimeter: http://tekpower.us/multimeter/dt830byellow.html (http://tekpower.us/multimeter/dt830byellow.html) only difference is mine has 500v~ instead of 750v~. I tried getting readings from the left settings but didn't get anywhere, it could be a dud.
From the picture looks like you have a wiring issue.
Your hot leg comes in and goes to the power switch then jumps back to your fuse. It should go through the fuse then to the switch.
It also looks like the neutral goes to a tube?
Is is this the layout you used? If so from what I can see that first tube and power wiring is a big part of the issue. But I can'see all of the wiring in the picture.
I'm triple checking the schematics right now.
Edit: It looks correct, the fuse is post-power switch and that neutral wire is going to the rectifier tube. Here is a correctly built Metro JTM45 gutshot: http://imgur.com/4nXpCTx (http://imgur.com/4nXpCTx)
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Set your meter as shown to measure DC voltages...
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I'm triple checking the schematics right now.
Edit: It looks correct, the fuse is post-power switch and that neutral wire is going to the rectifier tube. Here is a correctly built Metro JTM45 gutshot: http://imgur.com/4nXpCTx (http://imgur.com/4nXpCTx)
If this: https://store.metropoulos.net/collections/transformers/products/metro-spec-jtm-45-pt-drake-1202-55 is the power transformer that is now in the amp, and is the one shown in your photo, then shouldn't the neutral wire from the incoming AC cord be connected to the orange (common) wire on the PT, rather than to the rectifier tube? (It's shown as blue rather than orange in the layout diagram linked in post #15, but the PT in that diagram has a different color scheme than the one that is in the amp now.)
It would really help if you would take several well-lit, clearly focused photos of your amp from various angles, making sure to cover the whole thing, and post them directly in this thread (rather than to a file host that just adds an additional layer of bother for the people trying to help you).
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If this: https://store.metropoulos.net/collections/transformers/products/metro-spec-jtm-45-pt-drake-1202-55 is the power transformer that is now in the amp, and is the one shown in your photo, then shouldn't the neutral wire from the incoming AC cord be connected to the orange (common) wire on the PT, rather than to the rectifier tube? (It's shown as blue rather than orange in the layout diagram linked in post #15, but the PT in that diagram has a different color scheme than the one that is in the amp now.)
It would really help if you would take several well-lit, clearly focused photos of your amp from various angles, making sure to cover the whole thing, and post them directly in this thread (rather than to a file host that just adds an additional layer of bother for the people trying to help you).
Oh jeeze, I'm sorry about claiming it went to the rectifier, the neutral wire actually goes to the alternate main fuse terminal next to the main fuse terminal with the orange common. Also I tried posting pictures directly, but the filesize was too large even after compressing them. Ironically, I guess I need an older cell phone?
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Set your meter as shown to measure DC voltages...
Absolutely, I've been doing that and getting no readings or very tiny readings (zero for power switch, around 300 for standby switch). I just changed the battery too; I might try picking up an new one, it could be faulty. It doesn't seem like it's working at that setting.
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So I think I found part of the problem. Going by what you said earlier about the new PT 1202-55.
Went here http://valvestorm.com/JTM45%20Kit (http://valvestorm.com/JTM45%20Kit)
Has all kinds of info INCLUDING a 35 page manual with schematic and voltage readings UNDER INSTRUCTIONS.
Then from there went to "prerequisites"
Which braught me here http://valvestorm.com/Transformers#JTM45 (http://valvestorm.com/Transformers#JTM45)
This is where it gets wierd
When you go to metroamp 1202-55 it gives you what you posted. Supposedly adound 400vdc at B+
But when you go to Merren 1202-55 you get this
http://www.merrenaudio.com/marshall_power_transformers (http://www.merrenaudio.com/marshall_power_transformers)
Note the 500v rating
Good luck
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So I think I found part of the problem. Going by what you said earlier about the new PT 1202-55.
Went here http://valvestorm.com/JTM45%20Kit (http://valvestorm.com/JTM45%20Kit)
Has all kinds of info INCLUDING a 35 page manual with schematic and voltage readings UNDER INSTRUCTIONS.
Then from there went to "prerequisites"
Which braught me here http://valvestorm.com/Transformers#JTM45 (http://valvestorm.com/Transformers#JTM45)
This is where it gets wierd
When you go to metroamp 1202-55 it gives you what you posted. Supposedly adound 400vdc at B+
But when you go to Merren 1202-55 you get this
http://www.merrenaudio.com/marshall_power_transformers (http://www.merrenaudio.com/marshall_power_transformers)
Note the 500v rating
Good luck
That's interesting... I just ordered the PT from Metro, technically it's a "Heyboer" I think. Valvestorm is where I got the kit originally, they took over most of Metro's component stock now that George Metropoulos manufactures the Metroplex exclusively.
Anyway, thanks for all the help guys! I'm going to do another round of playing watching the PT more closely this time, and if there's still an issue I'll take it to a local tech. I think I've done everything in my power to fix it except start over from scratch. Much obliged for the electrical wisdom.
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The only way I can see that you're getting such high voltage on the standby switch is if somehow the bias supply is miswired or something is touching what it shouldn't. Forming either some sort of "voltage doubler" or throwing that negative voltage on one leg of the stby switch. That would explain (to me) why the amp "sounds great" but your transformer is overheating... I assume that's what the smoke is from, the varnish cooking?
Does this theory make sense to anyone else?
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I think at this point the only problem he has is a faulty $5.00 meter. And lack of experience using a meter.
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I think at this point the only problem he has is a faulty $5.00 meter. And lack of experience using a meter.
Maybe so. I still haven't went in for basic 'scope training!
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The only way I can see that you're getting such high voltage on the standby switch is if somehow the bias supply is miswired or something is touching what it shouldn't. Forming either some sort of "voltage doubler" or throwing that negative voltage on one leg of the stby switch. That would explain (to me) why the amp "sounds great" but your transformer is overheating... I assume that's what the smoke is from, the varnish cooking?
Does this theory make sense to anyone else?
This was actually my initial guess.
I think at this point the only problem he has is...lack of experience
I'm pretty sure it's this though.
To end on a note of levity, here is a gross picture of the original PT:
http://imgur.com/JjqP4ry (http://imgur.com/JjqP4ry)
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I didn't mean that in a derogatory way. Hope you didn't take it as such. We all had a lack of experience at one time.
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I didn't mean that in a derogatory way. Hope you didn't take it as such. We all had a lack of experience at one time.
Not in the slightest, I'm the one that's making light of the situation, now that it's more or less out of my hands. It was my first ever build after all, I'm surprised things didn't go even worse.
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The only way I can see that you're getting such high voltage on the standby switch is if somehow the bias supply is miswired or something is touching what it shouldn't. Forming either some sort of "voltage doubler" or throwing that negative voltage on one leg of the stby switch. That would explain (to me) why the amp "sounds great" but your transformer is overheating... I assume that's what the smoke is from, the varnish cooking?
Does this theory make sense to anyone else?
This was actually my initial guess.
I think at this point the only problem he has is...lack of experience
I'm pretty sure it's this though.
To end on a note of levity, here is a gross picture of the original PT:
http://imgur.com/JjqP4ry (http://imgur.com/JjqP4ry)
Smokin!!!
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I just noticed this.
To end on a note of levity, here is a gross picture of the original PT:
http://imgur.com/JjqP4ry (http://imgur.com/JjqP4ry)
Do you have the diagram for this PT?
Just asking because of the fairly clean burn lines.
Also looking at the original gut shot pic you posted the blue wire goes down towards the board but then almost looks like it goes off and touches the chassis. Make sure whatever wires you are not using are capped off and not touching anything else.
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Oh shit! I just realized the original PT I had the unused lead ends wrapped together when I insulated them.
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Oh shit! I just realized the original PT I had the unused lead ends wrapped together when I insulated them.
That'll let the smoke out.