MOD 102+ build trouble

[martinlb]

Hi,

I've just finished, or thought I'd finished, a MOD 102+ amp kit.

When I turn it on, the power light lights up, and a fairly loud buzzing comes from the power tranny.  I'm pretty sure that's the source because the buzzing changes when I touch it (the transformer).

I'll try to post a picture or two in case anything jumps out.

[tubeswell]

How many amps have you built?

Can you take voltages without killing yourself?

Did the MOD people send you a link to their schematic and suggested layout?

[Esquirefreak]

Since the buzzing changes when you touch the PT, I'd suggest you make sure the PT bolts/nuts are tight.

/Max

[martinlb]

How many amps have you built?

Can you take voltages without killing yourself?

Did the MOD people send you a link to their schematic and suggested layout?

I've built a few, but I've got to admit I need training.  I've developed a loose sense of how things work but not nearly what I want/need.

I could take voltages and I believe, or I've heard, that MOD provides voltage checkpoints.  I don't think they'd mean much to me though.

[martinlb]

Since the bussing changes when you touch the PT, I'd suggest you make sure the PT bolts/nuts are tight.

/Max

Yes, thanks, they're tight.

[scstill]

See schematic below
Odd that MOD would put the power switch on the neutral.
Also why no grid stopper resistor first stage?

Strange That a new transformer would buzz.
I have fixed buzzy vintage transformers by using varnish or shellac to bond the plates that had come loose.
Try tightening the two long horizontal xfmr bolts that hold the laminated plates.

Schematic below has MOD design voltages in red for you to compare yours to
https://www.modelectronics.com/sites/modelectronics.com/files/media_files/mod102_schematic.pdf
https://www.modelectronics.com/product/mod-102

[tubeswell]

Thanks for that scstill.

So its a simple single ended amp with 1 x 12AX7 and 1 x EL84.

@martinlb, 1st step is to use your voltmeter to check that the DC voltages shown in red on the schematic are what you are getting on your build. Be careful to not electrocute yourself. Best way to take these voltages is to use insulated test clips attached to your meter and to attach and remove these with the amp switched off and unplugged from the mains. Only switch the amp on to get the reading, then switch it all off again and unplug it before reattaching it to get the next reading. Do not put your hands inside the chassis when the amp is switched on, and do not do anything that would make your body part of a circuit between the high voltages in your amp and the ground. Be careful when attaching the test clips to the filter caps, even when the amp is switched off. Make sure your meter is set to measure DC volts* (and if it is not an auto-ranging meter, make sure it is set to the appropriate measuring scale i.e., the hundreds-of-volts setting to measure the high voltages, and the tens-of-volts or ones-of-volts settings to measure anything under 10 volts). Report back on the voltages you are getting

*This won't be appropriate for checking a parts of the circuit that operate in alternating current (i.e. the PT secondaries and the heaters) which you have to use the VAC setting on your meter for

[martinlb]

You guys are great.  I really appreciate your detailed help, and please believe me that I'll get to testing.  For now, I guess I'm discouraged enough to set this amp aside while I try the other kit I've got.

Thanks again, and I'll be back at this soon.

[Carlsoti]

See schematic below
Odd that MOD would put the power switch on the neutral.
Also why no grid stopper resistor first stage?

Strange That a new transformer would buzz.
I have fixed buzzy vintage transformers by using varnish or shellac to bond the plates that had come loose.
Try tightening the two long horizontal xfmr bolts that hold the laminated plates.

Schematic below has MOD design voltages in red for you to compare yours to
https://www.modelectronics.com/sites/modelectronics.com/files/media_files/mod102_schematic.pdf
https://www.modelectronics.com/product/mod-102

The schem for the 102+ is a little different than the docs linked, but I'm not sure how to link the proper one. The 102+ has got a couple push/pull pots, etc.

[martinlb]

After looking more carefully I discovered I'd miswired one of the tube sockets.

I'm realizing how easy it apparently is for me to get confused there, and I'll be paying much closer attention to that in the future.

The amp works now, except for the volume pot being sloppy with a lot of movement in the push/pull before it switches.  The signal cuts in and out with that movement.

The other two pots aren't like that.  I'm going to ask MOD for a replacement.