Mesa Nomad 45 heater issue?

[g1]

Some of your other measurements suggest you might have been in the wrong meter mode.

Either this or the AC range on your meter is broken.
You said you measured 0VAC at the heater winding (green to green). Yet V1 is lit up and therefore there MUST be AC at the heater winding.

[Enzo]

Are the tubes still in their sockets? Pull them out. If you try to take voltage readings with the tubes in place, they will mask a missing side. Pull the tubes and see if your 3v on either side still exists.

[boogiebomb]

First I want to say thank you to everyone that's helped so far, I really do appreciate it!
I removed all the tubes, there is voltage between the two green heater wires, but only 2.9VAC. I checked between V2-V5 preamp pins 4/5 to 9 and only got 2.9VAC, while V1 has 12.8 VAC.

[g1]

Do you mean V1 has 12.8V DC ?
I don't know how you could make that from 3VAC.
Can you measure the AC going into the bridge that makes the DC for V1 ?

[boogiebomb]

Do you mean V1 has 12.8V DC ?
I don't know how you could make that from 3VAC.
Can you measure the AC going into the bridge that makes the DC for V1 ?

Will check the bridge rectifier when I get home for lunch (there are a few groups of 4007 diodes I need to search around).

I apologize, I left out that with no tubes in (as with my last post) V1 pins 4, 5, and 9 to ground measure 6.2VAC. If I check DC, then it is -3.1VDC on V1 pins 4 and 5 to ground, and +3.1VDC from pin 9.

Nothing on v2-5 pins 4 or 5 to ground. Pin 9 gets +3.1VDC.

Thanks again guys. please let me know what/how I should check for anything else.

[km6xz]

Something is not being measured right. So there is no AC 6.2V between 4 and 9? So you have a open connection between the green wires and the corresponding pins on V2 through V5. With the amp turned off, what is the resistance between pin 4 and 9? It should be a very low value. If it is not, work forward from the green wires which you DID measure 3 ohms in an earlier post. Trace the wires and pc board traces to see where the connection is broken. It will be a bad solder joint or break in a trace. Don't bother with any more tests of V1, it is just confusing you and you proved it was fine.

[boogiebomb]

Amp is working great now!

I checked the PT heater wires where they are soldered to the pads where the power tube heater wires emerge, and part of one of them had a break. I guess the original overheat/burn up of the molex connector was deeper than I thought. I redid the job and I get the 6.2 VAC between heater pins, except V1, which is still 12.8VAC between 4/5 and 9, but I guess it's meant to be that way?

Thank you very much, I appreciate the help and will definitely enjoy this amp!

Is there any reason as to why the heater leads would burn up in the first place? poor contact in the molex connector?

[km6xz]

Congratulations!

Molex connectors, once they oxidize increase their contact resistance. That drops voltage across the connection which generates heat, heat further oxidizes the connection and higher heat is generated. The only way to repair them is to cut them off and install both the male and female connectors....or wire directly around them.

The V1 reading is wrong because you are measuring DC voltage on an AC calibrated range. It should be 6.3 volts DC. You can measure DC on a DMM that has a AC meter bridge rectifier but it will read twice the voltage. Getting 12VAC means that really there is only 6.3 volts DC on it, but since DC is going through the bridge rectifier, only at +6volts, the AC meter is expecting both a positive and negative voltage on each AC cycle, so is calibrated by assuming the negative value there(as it would be if the voltage was actually AC but it is not)
Anyway, the V1 DC heaters are working, the amp is working, and it plays....time for a beer...

[g1]

Thanks for that explanation km6xz. Another example of weird illogical readings caused by a cheap meter.
It caught me out again (like they always seem to). One day I'm going to remember about meters like this.

[km6xz]

Actually is it not a function of being a cheap meter, although it happens mostly with cheap meters because cheap meters out number precision meters 20:1. What is amazing about cheap meters is how good they are, really. A meter with as accurate calibration, stability of calibration, resolution and precision costing $30 would have been science fiction 40 years ago, as implausible as pocket as communicators ala Star Trek.
Reading a DC voltage with a meter using a conventional meter bridge rectifier will give a reading but is will be wrong because of it is expecting both positive and negative sides of the waveform(low distortion sinewave only) to add so is calibrated to reflect that . An exception is the "True RMS" detecting meter. Even some cheap meters have RMS detectors which make them really useful for audio, particularly when dealing with the output of a guitar amp which is seldom a clean sinewave. A RMS detector or converter can accurately measure distorted waveforms(within limits) by measuring the equivalent heating power of the an AC signal compared to a reference DC voltage. That can be done several ways, but the oldest, used years ago, is the easiest to visualize. A DC voltage across a set pure resistance, say 10 volts, will generate heat, measure the heat and note it. Then apply a AC voltage of any waveform across the same purely resistive load, and increase the voltage until is generates the same amount of heat. Both voltages are producing the same work. The RMS voltage of the AC will be 10Volts. Regardless of the amplitude of the AC waveform or its crest factor or duty cycle, the RMS value will be 10Volts if it does the same work as a 10VDC current across the a set load. So, you can measure the power output of a guitar amp when running into a non-reactive load even well into clipping, which is not accurate when using a averaging calibrated meter like most AC meters.
As with any measuring device we have to view the reading in context of how the device sees it, not how we assume it is. That is in all sorts of measurements in all fields since the way a reading is derived is just as important as what is derived when considering what it really means. I try to explain this to photographers who think their light meter should tell them how to expose their scene, which it does within limits and according to a reference that does not seem intuitive, it is referenced to 18% grey. Which means for proper exposure of subject that are not 18%(that is the value of black that is needed to add to pure white to visually appear grey or half way between the blackest black and the whitest white), they need to visualize what the meter is actually seeing from its point of view, how it sees the world, not how the photographer sees the world. So a white wedding dress will to a exposure system in a camera will appear too bright and it will adjust the camera to under expose the whole scene so the bright white wedding dress is a middle grey. To properly expose the same scene the photographer must know this and intentionally over-expose the scene to get a white dress looking white. The scene did not change, our eyes did not change when a white dress walked into the scene but how the meter works, there was a big change when the bride walked in front of the camera. A dark scene or black suit or tea-shirt according to the meter will have to be exposed much higher, to get it to appear grey instead of black so the photographer intentionally underexposes the scene to make the black objects look black. Same with pilots reading their air speed indicators, and expect it to mean their actual speed referenced to some point in space on their axis of movement. It does a pretty good job under defined normal conditions and in controlled lab conditions it is very accurate. But not when they need it most.
So we must be familiar with the way an instrument sees the parameter being measured and how that relates to the information we are needing. We have so many parameters to measure but they all come down to joules per coulomb and time. All other electrical properties can be derived from having two standards of known calibration so a shop can use the best instrument in the shop to calibrate the others even if what they measure is different. If you know time and and you know voltage potential you can calculate capacitance for example.
The above was added to show how almost all our measurements are derived, indirect and not of the phenomena itself but a method of inferring it. Knowing that inference was arrived on in the measuring instrument gives us a good idea of how it measures influences what relevance the reading is to what information we seek. For example what is read out is influenced by the method of attaching to the circuit. In the old VOM days, meter sensitivity played an important part because lower cost meters had less sensitive meter movements sp higher currents needed to be sampled within the meter so the impedance of the instrument itself really impacted the circuit, by placing a higher load on the circuit under test. A 1,000 ohms per volt meter was going to give a different reading on low current, higher impedance circuits than a 10meg/volt impedance VTVM for example while both would read closely in a low Z circuit under test. That all comes from Thevenin's Equivalent circuit loading.
Most cheap DMMs have a fixed 10meg input Z so readings at low voltage ranges will have less circuit loading than a typical analog meter but more at higher voltage ranges. A high quality z bench analog multimeter like the HP 410C could have 100meg input z while a lab specialty electrometer can be higher than 100Tohms(10^14 ohms). Why would anyone need such a meter in audio? Well, the circuit loads by any normal meter in condenser mics or when measuring pc board leakage make those measurements extremely inaccurate are two examples. A condenser mic has series resistors connected to the diaphragms in the 100-200 megohm range so even a HP410c would give enough loading to make the unit under test stop working and give a very low reading.