Slow Build...

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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lavrgs
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Re: Slow Build...

Post by lavrgs »

As I wait for parts I spent some time re-routing the transformer wires and next I will run shielded cable to the tremolo channel inputs.
I am somewhat gunshy about testing voltages at this point because I don't know if it is possible to blow tube by shorting the wrong pin or did I have some other problem.
My problem tubes are a EZ81 rectifier which appears to get the wrong voltage on pin 3. The measurements are low and unstable. The other tube problem is one of the EL84s which presented itself as I re-did the dim bulb test. When I took it from the V5 position to V4 the problem followed the tube. I also had reduced output. New tubes are on the way but I want to be very careful that it's not another problem that caused the previous tube failure . My only check on the was to make sure the heaters were continuous and no pins were shorted. Not sure I can do much more.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

I was waiting for some parts so I spent the last couple days checking components, solder joints etc. I replaced the socket on V5 because the DBT told me to. Of coarse when I fired up the amp...nothing, well I knew where to look. I found the heater wiring had a bad solder joint. The good news is that I'm back to where I was a week ago. The bad news is that I'm back to where I was a week ago.
My problem is the "motorboating" so I'm off the figure out why - you may recall it starts when I connect to pin 6 on v3...
I have added my latest voltage check
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colossal
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Re: Slow Build...

Post by colossal »

As I said on page 3, there is an obvious problem with your phase inverter . Pin 6 shows 323V. For the amp to be working correctly, Pin 1 and 6 should be closely matched. So that tells me that you have something wrong with how your phase inverter is wired, a bad tube, a bad 100k resistor, a bad solder joint on that resistor, bad wiring, or a combination of these. Carefully trace your amp in that region, check your solder joints, and check your lead connections. Go through the amp with a copy of your layout and a highlighter and mark off each and every lead and solder joint, one at a time, on the layout, as you go through the amp to verify that you have everything correct. You will need to consider under-board connections as well.

The bottom line is your voltages have to look like the references voltages. Anywhere there isn't a match within 10% or so, you have an issue.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

OK let the fun begin... I have removed the board so I can try and remember what I did three years ago. I am definitely finding issues. I originally started with an early Mojotone layout and recent started using a newer version...I convinced myself I had shifted components over by a turret but the old version had one more turret...
1. I had double wired connection that were supposed to be under the board - I left the ones on the bottom.
2. found a huge solder ball on the connection where I was having motorboat problems when I connected the wire to pin 6 V3 - was probably touching the chassis...I now have made the connection without huge pulsing.
3.MJ provided a 130ohm 3 watt bypass resistor I changed to 150 ohm 5 watt
4. the bypass cap measured at 210uf vs 250 - not sure that matters - ?
I have checked all the other component values and all are within tolerance.
I am investigating the high voltage on the PI load resistors - the voltage across the 100 ohm resistor feeding pin 1 dropped from 328 to 210 and the drop across the resistor feeding pin 6 went from 328 to 323??? the resistor measured ok and I did not find any mis-wiring other than the double wire feeding the 328...

I'm going to put the board back in the chassis after I clean up the socket wiring which seems like it would be easier with the board out.

So progress is being made.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

colossal wrote:
Fri 01/31/20 10:25 pm
there is an obvious problem with your phase inverter . Pin 6 shows 323V. For the amp to be working correctly, Pin 1 and 6 should be closely matched. So that tells me that you have something wrong with how your phase inverter is wired,
a bad tube, I swapped tubes no change
a bad 100k resistor, it measures ok as does the bypass cap
a bad solder joint on that resistor, if the joint is bad then I should just give up - frustration show through 8-)
bad wiring, this is where I need help - I feel the socket wiring is good - what else should I check?

Carefully trace your amp in that region, check your solder joints, and check your lead connections. Go through the amp with a copy of your layout and a highlighter and mark off each and every lead and solder joint, one at a time, on the layout, as you go through the amp to verify that you have everything correct.
You will need to consider under-board connections as well. I lifted the board yesterday and found some issues but I am now confident it's correct

The bottom line is your voltages have to look like the references voltages. Anywhere there isn't a match within 10% or so, you have an issue. I understand the concept
I read that leaking caps could let voltage pass so I lifted the cap and it measures 0.01uf the resistor is 100k. I was going to swap resistors but I convinced my self the problem is somewhere else.
Correct me if I am wrong but I believe the voltage supplied to V2 pin 6 comes from the resistor marked 'Hi" (323 measured)in the attached. I am having trouble understanding why the voltage drop across the right resistor is ok (210V measured) and only drops a few volts across the left hand one. Is it getting voltage from somewhere else? I removed the components I expect I should have no continuity and I don't. Please understand that I am new to this electronic stuff. I will highlight the schematic to complete my review. I am just too focused on the voltage problem...
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colossal
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Re: Slow Build...

Post by colossal »

Correct me if I am wrong but I believe the voltage supplied to V2 pin 6 comes from the resistor marked 'Hi" (323 measured)in the attached. I am having trouble understanding why the voltage drop across the right resistor is ok (210V measured) and only drops a few volts across the left hand one. Is it getting voltage from somewhere else? I removed the components I expect I should have no continuity and I don't. Please understand that I am new to this electronic stuff. I will highlight the schematic to complete my review. I am just too focused on the voltage problem...
There is no way there is a 3V drop across that 100k resistor marked "HI" unless something is miswired or that resistor is bad. Figure that out. My guess is you have bypassed that resistor accidentally or the resistor is blown. I would unsolder it and measure it, then check that 0.01uF cap next to it. Ohm's Law is not being violated, something is wrong.

To your credit, you are doing the right things. But do not hop around, trying this or that. Do not convince yourself of anything. As I said earlier, mark that layout up with a highlighter. Take DC voltage measurements at EVERY SINGLE POINT and write them neatly on that layout and repost your results. The error will reveal itself. Stay focused and eliminate assumptions, one variable at a time. It can be intensely frustrating though when something supposedly simple is not working out. That layout is baffling though and violates some basic tenets of good design. Hang in there.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

I've done the voltage check that I don't yet have an "ah-ha" moment. The voltage at pin 6 is still high and if I have a wiring problem I don't see it. The component values for all resistors are within spec and the poly (?) capacitors check out but I have trouble measuring the electrolytics...
I had the board removed a few day ago and took a picture - I verified the wires on the back match the layout and will include the picture and a marked up layout showing what I followed.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

One more thing... I found a thread that showed a mojotone error on the schematic but I am unable to decipher if it's causing the issues I see - I don't understand the error.. I see two foot switches - what is the error in the red line?
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colossal
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Re: Slow Build...

Post by colossal »

lavrgs wrote:
Tue 02/04/20 8:21 pm
I've done the voltage check that I don't yet have an "ah-ha" moment. The voltage at pin 6 is still high and if I have a wiring problem I don't see it. The component values for all resistors are within spec and the poly (?) capacitors check out but I have trouble measuring the electrolytics...
I had the board removed a few day ago and took a picture - I verified the wires on the back match the layout and will include the picture and a marked up layout showing what I followed.
First of all, your voltage labeled schematic is EXACTLY what everyone should be doing when asking for help 8) . I thank you for it because it makes life just so much easier and paints an imminently clear picture of what is going on. I stand by my guess that either the inverting side 100k resistor is bad or incorrect value or that V2 Phase Inverter tube is wildly imbalanced and must be replaced. With that small of a voltage drop across that resistor and high plate voltage on V2a being nearly that of B+ it makes sense that that resistor could be just 1k or 10k (check the color code on the resistor against the known working side!). It is an easy mistake to make; subbing a 10k for 100k. It may not be the case here, but worth double checking.

Another quick test for V2 would be to swap a known/working 12AX7 from V1 into that position and see if the error follows. If not, bad tube. If yes, bad parts/wiring/something else.

p.s. I'm purposely avoiding the tremolo question until your amp is working correctly.
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Re: Slow Build...

Post by JMPGuitars »

lavrgs wrote:
Tue 02/04/20 8:30 pm
One more thing... I found a thread that showed a mojotone error on the schematic but I am unable to decipher if it's causing the issues I see - I don't understand the error.. I see two foot switches - what is the error in the red line?
To answer that question, you need to compare it to an accurate schematic...assuming they have one.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

Done... my problem was the 4-5 pins on V2 were badly soldered- not contacting each other. OK I'm not really done because I still want to fix up a better grounding scheme. I'm getting short wave signals from China... :mrgreen:
I now have a better understanding of what the high voltage on pin 6 of meant...no current flow and since the heater in that tube must not have been working I can understand...V=iR also the tube was rather cool during a swap.
I'll be working on cleaning up the hums, radio interference etc...

Ohhh I almost forgot, the other thing that works is the tremolo - a freebie in this troubleshootin' fiasco...now I just have to make a shorting plug to keep that OFF.

Colossal you said all the right things but it just takes me some time to wrap my head around it. There are still bits about how "this is sharing that" in the Mojotone circuit which doesn't compute yet, but I'll figure it out - Thanks for your help!!!
Last edited by lavrgs on Thu 02/06/20 10:20 pm, edited 3 times in total.
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lavrgs
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Re: Slow Build...Nice Resource

Post by lavrgs »

Here is a nice resource. Stewart-McDonald sells the Mojotone Tremolo (Mojotone) kit but they have developed a set of instructions...https://www.stewmac.com/freeinfo/i-1073 ... 74X-F4.pdf
They wired the speakers differently than the layout I used. My questions is; it seems like they have it wired for an auxillary speaker. If you plug only into what they call the extension jack would it be a problem?
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colossal
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Re: Slow Build...

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lavrgs wrote:
Thu 02/06/20 8:20 pm
Done... my problem was the 4-5 pins on V2 were badly soldered- not contacting each other. OK I'm not really done because I still want to fix up a better grounding scheme. I'm getting short wave signals from China... :mrgreen:
I now have a better understanding of what the high voltage on pin 6 of meant...no current flow and since the heater in that tube must not have been working I can understand...V=iR also the tube was rather cool during a swap.
I'll be working on cleaning up the hums, radio interference etc...
Fantastic! That makes sense. It's odd, because you had conduction on the other half of the tube, so Pin 9 (heater) was still limping the tube along. Well, that's great you figured it out. As for picking up China, now it is time to solder your grid resistors on the tube sockets. And now that the amp is working properly, cleaning up your grounds is a good idea as is chopsticking your leads and cleaning up the dress. You're in the home stretch!
Ohhh I almost forgot, the other thing that works is the tremolo - a freebie in this troubleshootin' fiasco...now I just have to make a shorting plug to keep that OFF.
Excellent!
Colossal you said all the right things but it just takes me some time to wrap my head around it. There are still bits about how "this is sharing that" in the Mojotone circuit which doesn't compute yet, but I'll figure it out - Thanks for your help!!!
You're very welcome. It's frustrating when things that are seemingly simple don't go well, but in the end, it's usually something simple, as you found out. Definitely go through the amp and reflow some of your solder joints. I recommend a good brand of solder like Kester with a flux core.

Well done! Time to rock.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

OK The saga continues...this morning when I woke up sleeping beaUty the vibrato channel was suffering from... something...I'll call Trumpitis. After a few pokes and prods I found the V2 socket was the cilprit... do you solder with tubes installed...trying to validate the internet debate of do or don't,,, So I'm gonna replace that socket because it's like Trump - it needs to be replaced hurray

UPDATE: pin7 on the V2 socket was frozen - welded into place what ever you want to call it. So now I am past the part where the vibrato channel doesn't work - WITH CAVEATS...WTF...? When I plug in the foot switch and turn off the vibrato it's sorta like the normal channel as far as volume goes. When I turn on the vibrato via the switch the volume decreases dramatically - so I will have to check grounds. But yesterday the amp was noisier that the macy's day parade - with signals getting all the way in from outerspace telling me I should buy this or smoke those cigarettes... you know... Now I'm ad free- Yeah!!! I better not fix everything tomorrow or I'll have to start another amp project...
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lavrgs
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Re: Slow Build...SNAP CRACKLE POP

Post by lavrgs »

OK almost there. I've always had a bit of a pop when I turned the amp on so I decided to go CHOPSTICKING...As it turns out whenever I touched the board I got a loud pop like touching something metal to your guitar cable...I mean ANYWHERE I touched the board. It took a couple of probing sessions but I found the culprit and now the amp is dead quiet!!!
C H O P S T I C K I N G I S Y O U R F R I E N D !!!!!!!
The lil b*ast*rd was a resistor lead touching a filter cap
chopstick lightning.JPG
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lavrgs
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Re: Slow Build...Voltage Check

Post by lavrgs »

Latest Voltage check
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Re: Slow Build...

Post by JMPGuitars »

Your B+ is okay at 350, but your cathode voltage should be 12V. What's your cathode resistor value?
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Re: Slow Build...

Post by lavrgs »

I'm using a 150 ohm resistor
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Re: Slow Build...

Post by JMPGuitars »

lavrgs wrote:
Sat 02/08/20 3:56 pm
I'm using a 150 ohm resistor
Try a 180 ohm and see where that puts you.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

Parts received and installed. I have a question about the voltages on V3 (trem) they change with volume and intensity pot. I took the V3 voltages with full on and off of intensity - is on or off the default place to test or does it matter?
The voltages of V4 and V5 are significantly different when calculating the dissipation do you use the V5 voltages or both...?
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