MOD 102 Kit not powering on at all. Any suggestions?

[martin manning]

The scratchy pot in your latest vid... sounds like DC on the pots. All DC should be blocked by the caps in the tone stack before the volume pot.

At this low plate voltage it's possible that this is caused by grid current.

[Stephen1966]

I had a few minutes during lunch today and popped it open to try the continuity test again. I got "beeps" for continuity between all of the pots and chassis ground. I also got a picture of the inside front panel.

When I have more time this afternoon I'll go through and check voltages and also check for AC in the pots

Continuity is good. That's shielding your signal running through the pots. If you get any voltage above the ground reference voltage though you have a leak somewhere so...

Check these leads:

Untitled-2.jpg

They look as if they are making contact with the pot cases, the 6k8 will make a ground loop if it is touching the case... they shouldn't. If you desolder these and cut a piece of insulation off a strip of wire cut to length, you can use the insulation to cover the leads and isolate them from the pot backs.

The scratchy pot in your latest vid... sounds like DC on the pots. All DC should be blocked by the caps in the tone stack before the volume pot.

It's hard to get a good picture, but none of the component leads are touching the backs of the pots. I even tried lifting them away a bit, but no change. The black cap is farther away from the pot that it appears in the new photo.

While I was testing the amp to see if that worked, I chopstick-ed around a bit to see if I noticed any changes. And while I didn't notice any changes with the movement around the pots, I noticed anytime my hand/finger got near either the input jack or volume pot, the buzzing significantly increased. I tried to take a new video of this as well. Skip forward towards the end if you just want to hear "the finger buzz"

https://youtu.be/AFDs__7LxN8

Please check the voltages on the filament string. Do you have a sheet of metal you can place over the open chassis, anything will do, an old metal dinner tray. Again, without the benefit of 3D it's hard to tell how far your hand from the open chassis was when it induced the extra hum. A metal shield should block that though. It won't solve your problem. But it tells us something about the nature of the hum. The blue wire (primary from your output transformer?) passes over the B+2 connecting wire (white). There appears to be some distance between them but it wouldn't hurt to shorten white B+2 wire so that it is well away from the blue OT wire. Not sure that will solve the problem either - the voltages are high there (so be careful if you just decide to move them around first).

Untitled-3.jpg

[vipor3D]

Continuity is good. That's shielding your signal running through the pots. If you get any voltage above the ground reference voltage though you have a leak somewhere so...

Check these leads:

Untitled-2.jpg

They look as if they are making contact with the pot cases, the 6k8 will make a ground loop if it is touching the case... they shouldn't. If you desolder these and cut a piece of insulation off a strip of wire cut to length, you can use the insulation to cover the leads and isolate them from the pot backs.

The scratchy pot in your latest vid... sounds like DC on the pots. All DC should be blocked by the caps in the tone stack before the volume pot.

It's hard to get a good picture, but none of the component leads are touching the backs of the pots. I even tried lifting them away a bit, but no change. The black cap is farther away from the pot that it appears in the new photo.

While I was testing the amp to see if that worked, I chopstick-ed around a bit to see if I noticed any changes. And while I didn't notice any changes with the movement around the pots, I noticed anytime my hand/finger got near either the input jack or volume pot, the buzzing significantly increased. I tried to take a new video of this as well. Skip forward towards the end if you just want to hear "the finger buzz"

https://youtu.be/AFDs__7LxN8

Please check the voltages on the filament string. Do you have a sheet of metal you can place over the open chassis, anything will do, an old metal dinner tray. Again, without the benefit of 3D it's hard to tell how far your hand from the open chassis was when it induced the extra hum. A metal shield should block that though. It won't solve your problem. But it tells us something about the nature of the hum. The blue wire (primary from your output transformer?) passes over the B+2 connecting wire (white). There appears to be some distance between them but it wouldn't hurt to shorten white B+2 wire so that it is well away from the blue OT wire. Not sure that will solve the problem either - the voltages are high there (so be careful if you just decide to move them around first).


Untitled-3.jpg

So I just went through and checked the voltages (or at least tried). The instructions give voltages for when the tubes are both in and all controls are at zero, and that's used as a basis to compare my readings to. I put together the results in an excel file for ease of viewing (image attached).

Either something is wrong with the high voltage, my meter doesn't work right, or I don't know how to use it properly. I have a 6000 count meter which I thought would cover something like this, but every high voltage connection just gave me the "0L" on the meter. In addition to checking the tube pins, I also checked at the filter caps and got the same 0L. The highest DC value on the schematic for any of the filter caps is 264V so I would think that would show on my meter. If it's not my meter, does that suggest something is wrong with the power supply?

Also, I do have a chassis cover for the amp that I can screw on and have been when I test the amp. I just didn't have the cover on while the amp was on my desk. Makes sense that I didn't notice that "finger buzz" issue until the cover was off

[LOUDthud]

I suggest the following change to grounding. Remove the wire on the terminal strip that ties the preamp filter cap to the other filter caps and ground it in the preamp.

Mod_102_Gnd_2.GIF

[Stevem]

The red and yellow wire from the secondary side of the PT should first go right to the ground side of the 47 uf filter cap on that terminal strip.
From there then a ground wire should run to that center ground terminal on that strip where it bolts down.

The ac power cable ground should have a lug on its end and that should be bolted down there also.

[Stephen1966]

So I just went through and checked the voltages (or at least tried). The instructions give voltages for when the tubes are both in and all controls are at zero, and that's used as a basis to compare my readings to. I put together the results in an excel file for ease of viewing (image attached).

Either something is wrong with the high voltage, my meter doesn't work right, or I don't know how to use it properly. I have a 6000 count meter which I thought would cover something like this, but every high voltage connection just gave me the "0L" on the meter. In addition to checking the tube pins, I also checked at the filter caps and got the same 0L. The highest DC value on the schematic for any of the filter caps is 264V so I would think that would show on my meter. If it's not my meter, does that suggest something is wrong with the power supply?

So the question is, why aren't you getting any meter readings above 6V? Check that you have not got your meter set to read milli-volts. Better yet, read the manual that came with it and thoroughly understand how you measure volts with it, including how you plug in the probe leads for the different kinds of readings. It is always possible that when you probed the plate of V1 (pin1) your probe was actually touching a layer of flux from the solder which can insulate the joint, in which case, scratching it with the tip of the probe will remove the flux and allow the probe to make contact with the conductive joint. You might expect this from one joint - but all of them!? You would not be able to hear your guitar if none of these were carrying any volts. They are. So don't go touching them with your bare fingers when the amp is on. Also, on a safety note, it would be remiss of me not to mention your caps have to be fully discharged before you do any work on them. It is better to ask if you are not sure how to do this.

Without good voltage readings it is going to be very difficult to troubleshoot this little gem. I am looking at the kit schematic and the voltages are slightly different to those you mentioned as from the kit guide. That is not that important, wall voltages will vary and you will be lucky to get a voltage dead on with the published figures but even so, your readings should really be no more than +/- 5%.

Once you have figured out what the problem with the voltmeter is, you can chase down - and eliminate - possible problem areas by comparing your voltage readings with the comprehensive set of readings provided with the kit. Before you do anything else now, I suggest you solve the problem of your voltmeter first.

mod102_schematic.pdf

[Stephen1966]

The red and yellow wire from the secondary side of the PT should first go right to the ground side of the 47 uf filter cap on that terminal strip.
From there then a ground wire should run to that center ground terminal on that strip where it bolts down.

The ac power cable ground should have a lug on its end and that should be bolted down there also.

Respectfully, that's what the layout shows with the AC ground, but the AC ground should have its own screw, and no other grounds should share the fixture.

[martin manning]

The red and yellow wire from the secondary side of the PT should first go right to the ground side of the 47 uf filter cap on that terminal strip.
From there then a ground wire should run to that center ground terminal on that strip where it bolts down.

The ac power cable ground should have a lug on its end and that should be bolted down there also.

Respectfully, that's what the layout shows with the AC ground, but the AC ground should have its own screw, and no other grounds should share the fixture.

I made the same two comments a couple of pages back. The red/yellow should go direct to the reservoir cap negative lead, where the layout has its return through a short distance of chassis. That may not cure the buzz, but it is the best practice.

[vipor3D]

So I just went through and checked the voltages (or at least tried). The instructions give voltages for when the tubes are both in and all controls are at zero, and that's used as a basis to compare my readings to. I put together the results in an excel file for ease of viewing (image attached).

Either something is wrong with the high voltage, my meter doesn't work right, or I don't know how to use it properly. I have a 6000 count meter which I thought would cover something like this, but every high voltage connection just gave me the "0L" on the meter. In addition to checking the tube pins, I also checked at the filter caps and got the same 0L. The highest DC value on the schematic for any of the filter caps is 264V so I would think that would show on my meter. If it's not my meter, does that suggest something is wrong with the power supply?

So the question is, why aren't you getting any meter readings above 6V? Check that you have not got your meter set to read milli-volts. Better yet, read the manual that came with it and thoroughly understand how you measure volts with it, including how you plug in the probe leads for the different kinds of readings. It is always possible that when you probed the plate of V1 (pin1) your probe was actually touching a layer of flux from the solder which can insulate the joint, in which case, scratching it with the tip of the probe will remove the flux and allow the probe to make contact with the conductive joint. You might expect this from one joint - but all of them!? You would not be able to hear your guitar if none of these were carrying any volts. They are. So don't go touching them with your bare fingers when the amp is on. Also, on a safety note, it would be remiss of me not to mention your caps have to be fully discharged before you do any work on them. It is better to ask if you are not sure how to do this.

Without good voltage readings it is going to be very difficult to troubleshoot this little gem. I am looking at the kit schematic and the voltages are slightly different to those you mentioned as from the kit guide. That is not that important, wall voltages will vary and you will be lucky to get a voltage dead on with the published figures but even so, your readings should really be no more than +/- 5%.

Once you have figured out what the problem with the voltmeter is, you can chase down - and eliminate - possible problem areas by comparing your voltage readings with the comprehensive set of readings provided with the kit. Before you do anything else now, I suggest you solve the problem of your voltmeter first.


mod102_schematic.pdf

So I should note, I am aware that the filter caps need to be drained before working on the amp. The build has resistors tied to the filter caps, so they drain whenever the amp is turned off. But I always check with the meter, drain with a resistor, then check with the meter again before touching anything in the amp.

I determined the issue with my meter not getting the high voltage readings was due to my error in setting it to manual mode. I didn't realize I had to tap the range button again to move the decimal for higher voltage readings. After taking a minute feeling like a dummy, I rechecked the pin measurements. I updated my tables from my previous post with the readings I got this morning and have attached that here. I also attached the table from the kit guide that I've been following. It's on the last page of the kit instructions. This is what I compared my tube pin measurements to. I did reference the schematic for the filter cap voltages, though, and they do seem a little low. The schematic says there should be 264V at the 47u cap, but I'm only measuring 248.4V.

[martin manning]

What is your line voltage? If it's not 120VAC as shown on the schematic, you should scale all your measurements by the ratio Vline/120.

[vipor3D]

What is your line voltage? If it's not 120VAC as shown on the schematic, you should scale all your measurements by the ratio Vline/120.

Just tested it and got 119.4, so pretty much 120

[Stephen1966]

So I just went through and checked the voltages (or at least tried). The instructions give voltages for when the tubes are both in and all controls are at zero, and that's used as a basis to compare my readings to. I put together the results in an excel file for ease of viewing (image attached).

Either something is wrong with the high voltage, my meter doesn't work right, or I don't know how to use it properly. I have a 6000 count meter which I thought would cover something like this, but every high voltage connection just gave me the "0L" on the meter. In addition to checking the tube pins, I also checked at the filter caps and got the same 0L. The highest DC value on the schematic for any of the filter caps is 264V so I would think that would show on my meter. If it's not my meter, does that suggest something is wrong with the power supply?

So the question is, why aren't you getting any meter readings above 6V? Check that you have not got your meter set to read milli-volts. Better yet, read the manual that came with it and thoroughly understand how you measure volts with it, including how you plug in the probe leads for the different kinds of readings. It is always possible that when you probed the plate of V1 (pin1) your probe was actually touching a layer of flux from the solder which can insulate the joint, in which case, scratching it with the tip of the probe will remove the flux and allow the probe to make contact with the conductive joint. You might expect this from one joint - but all of them!? You would not be able to hear your guitar if none of these were carrying any volts. They are. So don't go touching them with your bare fingers when the amp is on. Also, on a safety note, it would be remiss of me not to mention your caps have to be fully discharged before you do any work on them. It is better to ask if you are not sure how to do this.

Without good voltage readings it is going to be very difficult to troubleshoot this little gem. I am looking at the kit schematic and the voltages are slightly different to those you mentioned as from the kit guide. That is not that important, wall voltages will vary and you will be lucky to get a voltage dead on with the published figures but even so, your readings should really be no more than +/- 5%.

Once you have figured out what the problem with the voltmeter is, you can chase down - and eliminate - possible problem areas by comparing your voltage readings with the comprehensive set of readings provided with the kit. Before you do anything else now, I suggest you solve the problem of your voltmeter first.


mod102_schematic.pdf

So I should note, I am aware that the filter caps need to be drained before working on the amp. The build has resistors tied to the filter caps, so they drain whenever the amp is turned off. But I always check with the meter, drain with a resistor, then check with the meter again before touching anything in the amp.

I determined the issue with my meter not getting the high voltage readings was due to my error in setting it to manual mode. I didn't realize I had to tap the range button again to move the decimal for higher voltage readings. After taking a minute feeling like a dummy, I rechecked the pin measurements. I updated my tables from my previous post with the readings I got this morning and have attached that here. I also attached the table from the kit guide that I've been following. It's on the last page of the kit instructions. This is what I compared my tube pin measurements to. I did reference the schematic for the filter cap voltages, though, and they do seem a little low. The schematic says there should be 264V at the 47u cap, but I'm only measuring 248.4V.

Good! I am happy to stand corrected... Safety first! I meant no disrespect by it.

It does appear your wall voltages are a little on the low side but at least now proportionally speaking, the voltages look to be around the mark.

Thinking a little more about the AC ground connection. It really is best practice to isolate it from the other grounds in the chassis. Under normal circumstances it shouldn't make a huge difference if it connects with another ground somewhere else but when I think how some of these old houses are wired, it makes me wonder if this might be part of the problem. Building codes vary as I understand them, from state to state in the US, in the UK where I'm from there are a different set of codes and here in the Czech Republic, they are different again. That said, it is common for the earth and neutral lines to be bonded somewhere along the line. I think in modern property builds that's usually at the main circuit breaker but when I came to rewire my old house, I found all kinds of shenanigans and the earths were bonded to the neutrals in the socket outlets!!! That makes for a very noisy supply.

Modern kits like these are usually well thought out but they often try to get away with the fewest components, every single component adds cost to the unit and to stay competitive manufacturers have to look at every possible saving. For the cost of an extra screw, nut and washer though, and the time it takes to drill an extra hole near to where the cable enters the chassis... it makes you wonder what they were thinking. Anyway, it is something you might try. Even the little distance you move the AC ground away from the other grounds in the chassis, might make a difference. If you do that, just make sure the green cord is longer than the live and neutral cords. The idea being that if for some reason the AC cable is pulled from the chassis, the ground will be the last to break.

[Stephen1966]

The schematic says there should be 264V at the 47u cap, but I'm only measuring 248.4V.

Different tubes can give you different voltages at their plates so I wouldn't worry too much about them. They look close enough. Maybe check the resistor values in the dropping string after the big cap though and remember, different voltage and current draw downstream can affect the voltages you read upstream.

[vipor3D]

So the question is, why aren't you getting any meter readings above 6V? Check that you have not got your meter set to read milli-volts. Better yet, read the manual that came with it and thoroughly understand how you measure volts with it, including how you plug in the probe leads for the different kinds of readings. It is always possible that when you probed the plate of V1 (pin1) your probe was actually touching a layer of flux from the solder which can insulate the joint, in which case, scratching it with the tip of the probe will remove the flux and allow the probe to make contact with the conductive joint. You might expect this from one joint - but all of them!? You would not be able to hear your guitar if none of these were carrying any volts. They are. So don't go touching them with your bare fingers when the amp is on. Also, on a safety note, it would be remiss of me not to mention your caps have to be fully discharged before you do any work on them. It is better to ask if you are not sure how to do this.

Without good voltage readings it is going to be very difficult to troubleshoot this little gem. I am looking at the kit schematic and the voltages are slightly different to those you mentioned as from the kit guide. That is not that important, wall voltages will vary and you will be lucky to get a voltage dead on with the published figures but even so, your readings should really be no more than +/- 5%.

Once you have figured out what the problem with the voltmeter is, you can chase down - and eliminate - possible problem areas by comparing your voltage readings with the comprehensive set of readings provided with the kit. Before you do anything else now, I suggest you solve the problem of your voltmeter first.


mod102_schematic.pdf

So I should note, I am aware that the filter caps need to be drained before working on the amp. The build has resistors tied to the filter caps, so they drain whenever the amp is turned off. But I always check with the meter, drain with a resistor, then check with the meter again before touching anything in the amp.

I determined the issue with my meter not getting the high voltage readings was due to my error in setting it to manual mode. I didn't realize I had to tap the range button again to move the decimal for higher voltage readings. After taking a minute feeling like a dummy, I rechecked the pin measurements. I updated my tables from my previous post with the readings I got this morning and have attached that here. I also attached the table from the kit guide that I've been following. It's on the last page of the kit instructions. This is what I compared my tube pin measurements to. I did reference the schematic for the filter cap voltages, though, and they do seem a little low. The schematic says there should be 264V at the 47u cap, but I'm only measuring 248.4V.

Good! I am happy to stand corrected... Safety first! I meant no disrespect by it.

It does appear your wall voltages are a little on the low side but at least now proportionally speaking, the voltages look to be around the mark.

Thinking a little more about the AC ground connection. It really is best practice to isolate it from the other grounds in the chassis. Under normal circumstances it shouldn't make a huge difference if it connects with another ground somewhere else but when I think how some of these old houses are wired, it makes me wonder if this might be part of the problem. Building codes vary as I understand them, from state to state in the US, in the UK where I'm from there are a different set of codes and here in the Czech Republic, they are different again. That said, it is common for the earth and neutral lines to be bonded somewhere along the line. I think in modern property builds that's usually at the main circuit breaker but when I came to rewire my old house, I found all kinds of shenanigans and the earths were bonded to the neutrals in the socket outlets!!! That makes for a very noisy supply.

Modern kits like these are usually well thought out but they often try to get away with the fewest components, every single component adds cost to the unit and to stay competitive manufacturers have to look at every possible saving. For the cost of an extra screw, nut and washer though, and the time it takes to drill an extra hole near to where the cable enters the chassis... it makes you wonder what they were thinking. Anyway, it is something you might try. Even the little distance you move the AC ground away from the other grounds in the chassis, might make a difference. If you do that, just make sure the green cord is longer than the live and neutral cords. The idea being that if for some reason the AC cable is pulled from the chassis, the ground will be the last to break.

No worries, no offense taken.

I actually live in a modern build, but it's a multi-unit building. They throw these things up so fast that it wouldn't surprise me if something with the electrical wiring was off.

I'm going to try moving the red/yellow PT wire to the grounding point of the 47u cap as suggested. However, the red/yellow wire itself isn't long enough to reach that point. Just want to be sure, is it okay to solder and heat shrink a piece of 20awg wire to extend the red/yellow?

[Stephen1966]

So I should note, I am aware that the filter caps need to be drained before working on the amp. The build has resistors tied to the filter caps, so they drain whenever the amp is turned off. But I always check with the meter, drain with a resistor, then check with the meter again before touching anything in the amp.

I determined the issue with my meter not getting the high voltage readings was due to my error in setting it to manual mode. I didn't realize I had to tap the range button again to move the decimal for higher voltage readings. After taking a minute feeling like a dummy, I rechecked the pin measurements. I updated my tables from my previous post with the readings I got this morning and have attached that here. I also attached the table from the kit guide that I've been following. It's on the last page of the kit instructions. This is what I compared my tube pin measurements to. I did reference the schematic for the filter cap voltages, though, and they do seem a little low. The schematic says there should be 264V at the 47u cap, but I'm only measuring 248.4V.

Good! I am happy to stand corrected... Safety first! I meant no disrespect by it.

It does appear your wall voltages are a little on the low side but at least now proportionally speaking, the voltages look to be around the mark.

Thinking a little more about the AC ground connection. It really is best practice to isolate it from the other grounds in the chassis. Under normal circumstances it shouldn't make a huge difference if it connects with another ground somewhere else but when I think how some of these old houses are wired, it makes me wonder if this might be part of the problem. Building codes vary as I understand them, from state to state in the US, in the UK where I'm from there are a different set of codes and here in the Czech Republic, they are different again. That said, it is common for the earth and neutral lines to be bonded somewhere along the line. I think in modern property builds that's usually at the main circuit breaker but when I came to rewire my old house, I found all kinds of shenanigans and the earths were bonded to the neutrals in the socket outlets!!! That makes for a very noisy supply.

Modern kits like these are usually well thought out but they often try to get away with the fewest components, every single component adds cost to the unit and to stay competitive manufacturers have to look at every possible saving. For the cost of an extra screw, nut and washer though, and the time it takes to drill an extra hole near to where the cable enters the chassis... it makes you wonder what they were thinking. Anyway, it is something you might try. Even the little distance you move the AC ground away from the other grounds in the chassis, might make a difference. If you do that, just make sure the green cord is longer than the live and neutral cords. The idea being that if for some reason the AC cable is pulled from the chassis, the ground will be the last to break.

No worries, no offense taken.

I actually live in a modern build, but it's a multi-unit building. They throw these things up so fast that it wouldn't surprise me if something with the electrical wiring was off.

I'm going to try moving the red/yellow PT wire to the grounding point of the 47u cap as suggested. However, the red/yellow wire itself isn't long enough to reach that point. Just want to be sure, is it okay to solder and heat shrink a piece of 20awg wire to extend the red/yellow?

Multi-occupancy buildings can have all kinds of stuff going on. But modern building/modern building codes - there's only a slight chance that is causing the problem.

Did you keep your offcuts from the transformer wires? Use the same AWG and if you solder them maybe put a couple of lengths of heat shrink over them. If you have a hardware store that deals with electrical, they might have some butt-splices (not as painful as they sound) which you crimp and then heatshrink - they're very sturdy.