the silence is deafening

[brucer]

Hi TAN.

I’m a long-time lurker driven to post by desperate need. I’m totally new to amp building, but just finished a Two Stroke build based on a Weber Maggie kit. I’ve fired it up … and it’s SILENT. Nothing, no hum, zilch.

I followed the Guitar Amp Handbook (and TAN addendum) religiously (only change is a resistor on the first filter cap to discharge it when power turned off).

I checked everything for continuity.

I seem to be getting correct-ish voltage readings:

120V supply:

V1, pins 3 and 4: 114

V2, pin 3: 324
V2, pin 4: 274

V3, pin 3: 324
V3, pin 4: 274

V4, pins 2 and 8: 331
V4, pins 4 and 6: 322

125V supply:

V1, pins 3 and 4: 118

V2, pin 3: 339
V2, pin 4: 283

V3, pin 3: 339
V3, pin 4: 283

V4, pins 2 and 8: 346
V4, pins 4 and 6: 335

I’d appreciate any trouble-shooting advice that’s out there.

Thanks!

PS. I couldn’t figure out how to add photos to this post, but created an album here: https://www.tubeampnetwork.com/tan-community/533-brucer/photos/album?albumid=56

Comments would be appreciated. Thanks.

[brucer]

Okay, an update:

Just before pulling the board from the chassis to revisit the output transformer wiring, my son says: why don’t you try unplugging one speaker or switching where they’re plugged in?

Silly idea, won’t do anything, but it works …

One speaker plug seems to be the problem. Seems to be shorting out. If I open it up, then it works, no problems. If screwed together tightly and the wire or spring stress relief is wiggled, then sound for both speakers becomes intermittent.

To those more experienced than I am: does it make sense that shorted wiring in one speaker connection could stop sound from both?

Please, educate me.

[Robin]

Double check the negative feedback loop wiring (going from the speaker jack pos. lug to the 68k resistor) and be sure that you don’t have two of the taps from the OT secondaries linked together (check the output impedance switch wiring). Be kind to your OT, shorting the speaker output without a load and applying a significant signal will make it go up in smoke. Check the continuity of the speak wiring and be sure the plug in not shorted.

[brucer]

Thanks for the reply Robin. Much appreciated.

I’ve tried the checks you suggested, to the limits of Google and my abilities anyway. Unfortunately, what I’m seeing doesn’t make a lot of sense to me:

– The negative feedback loop wiring is showing zero resistance from the 68K resistor to the speaker input jacks … that’s good.

– Neither of my speakers show zero resistance from the positive to negative tabs whether plugged (5.6 – 6.6 ohms) or unplugged (0.7 – 2.7 ohms) … that’s good … I think.

– The negative feedback loop resistor is colour-coded 68k (and tested at 68K before I installed it … I checked them all), but it now reads 1.5K with a probe on either side … that seems not so good (I would have expected it to still read 68K … unless it’s receiving a signal through both the resistor and the circuit?).

– Resistance readings on the impedance switch tabs fluctuate, then return to what they were when I flip that switch … that seems not so good (I would have expected the “off side” impedance switch tabs to go to high resistance when the other side was selected, no?).

I’d appreciate any insights, even if that’s to cut my losses and protect the amp by finding a tech to check it out …

[Robin]

The components and voltages are probably fine, if you remove the 68k resistor from the circuit and it will read 68k. Because you can wiggle the power cord or torque the chassis and have the circuit become intermittent, it would indicate that you have a loose or shorting connection. Use the chop stick method (carefully) to tap and push around on the components and leads to try and isolate the problem. Usually, it will reveal itself, that’s what a tech would do. From your description, it sounds like a weak ground or there is something amiss in the output impedance wiring.

BTW, a current limiter should be used while you work with the amp turned on to ensure there is not a dead short. There are a number of web pages explaining how to construct one, I think there is a link somewhere in the TAN forums. They are simple, cheap and easy to build. I built mine from parts I had in the garage. It sounds like you are very close to having the amp sorted out.

[brucer]

Thanks for the suggestion Robin. I’m on it.

[beelzebum]

It sounds like your 1/4″ plug is shorting. Shrink wrap the wires separately before screwing the plug together.

[Robin]

I understood that both speakers are working correctly when they are plugged into the output jack, one at a time. Is that correct? And that you have an intermittent problem when the rear panel is screwed down or the AC cord is moved, please confirm.

[brucer]

Thanks Beelzebum and Robin for getting back to me:

Robin: the initial silent amp problem seemed fixed when I unplugged one of the two speakers (suddenly there was sound, the other speaker worked fine). Once I unscrewed the non-working 1/4″ plug, then the non-working speaker worked too so it definitely seems like a shorted plug was the problem.

Beelzebum: shrink wrapping the wires separately before screwing the 1/4″ plug together is a great tip! Thanks for that.

I’ve wiggled the power cord and other wires, torqued the chassis and poked components. All other connections seem good. It was only wiggling wires between the one non-working speaker and it’s 1/4″ plug that made things intermitent (or screwing that plug together).

One check I still haven’t done is for shorts in the impedence switch itself. I’m thinking that I need to remove the two wires from the OT before I can use my multimeter to check open/closed circuits when the switch is flipped? Right now resistance readings on the switch tabs just fluctuate, then return to what they were when I flip it. I would have expected the “off side” impedance switch tabs to go to high resistance when the other side was selected. Hrmmm.

[Robin]

Sounds like Beelzebum’s solution worked (as usual), If the shorted plug was the only issue, it sounds like you’re good to go.
Good luck with the amp

[brucer]

Thanks for the help!

[Author]

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