MOD 102+ Troubleshooting

[JJ Beicher]

I have finished a MOD 102+ kit but have problems. No sound: 1) DC voltages are off spec 2) tubes do not light up (AC voltage on heater wires is 6 volts), 3) transformer makes a mechanical hum (not overly loud - but noticeable). Actual voltage readings are attached.
Also have attached pictures. Have checked wiring multiple times, but have not found the problem. Suggestions would be greatly appreciated. I have assembled 2 more complicated kits (Princeton, Allen Encore), they both fired up right away. This one is frustrating

DC Test Points				Actual		Target
Power supply		273	251
power supply		274	249
power supply		267	198
plate		265	126
grid		0	0
cathode		0	1.1
plate		265	132
grid		?	0
cathode		?	1
plate		275	242
screen grid		?	197
control Grid		?	0
cathode		?	5.3
? = close to zero but fluctuates

[Helmholtz]

Please post schematic.

[loudthud]

Some old links to the schematic are broken, but I found this: https://www.tubesandmore.com/sites/d..._schematic.pdf

[g1]

You say you have 6VAC for heaters, but heaters are not lit. So start with that.
Measure from green to green at each tube socket. Should be about 6VAC. Double check that the correct pins are connected to the greens. (pins 4 & 5 for EL84, pins 9 & 4/5 for 12AX7/ECC803)

[casey73]

Check to make sure you didn't mix up the 1 watt 470 ohm grid resistor with the 470K ohm bleed resistor in the power supply. I worked at CE Distribution for 12 years and helped customers troubleshoot their kit builds. I remember a similar problem a customer had with his MOD102+, and in the photos he sent I could see that dark brown 470 ohm resistor was bleeding much of the power to ground. I think the only reason it didn't fail was because it was 1 watt. We sent the customer two replacement resistors and never heard back so I assume he got it up and running.

mod_102_schematic.pdf (modelectronics.com)

[JJ Beicher]

Thanks to everyone for their feedback. Made some headway.

Despite checking multiple times, I wired the heater wires to pins 5 and 6, rather than 4 and 5. Corrected that and it works. VDC Voltages are now at spec. There are 3 knobs on the unit, all with pull switches. The volume control and pull function ("boost") works. The treble control does not work, but the bright pull switch does. Neither the Bass control nor the mid boost pull switch appear to work. Any thoughts on what I should check out?

[g1]

Sounds like the tone stack is disconnected. Check the 6K8 and 10K resistors that go from the bass pot to ground.
From the lug of the bass pot that connects to the .047, you should measure around 16K8 to ground, and 6K8 to ground when mid boost switch closed.

[JJ Beicher]

Got it working right, finally! I did not make the connection between the switch lug and the ground lug on the 10k resistor. Hard to see on the diagram. Took care of that and everything worked as advertised. Nice practice amp.

Thanks once again to everyone for your help.