Marshall Popular 1930

[HotBluePlates]

Looking at your pics and I can not quite see it - is that a solid wire connecting the plates on V1?  Where is the 100K resistor?

It's on the board, connected to the 220kΩ resistor for the other 12AX7 plate.

What is the resistance end-to-end of the Intensity pot?

I was thinking this would be 5kΩ based on the schematic, but that doesn't seem possible with your numbers...

I measured on the middle lug of the intensity pot to ground: 0K on 0 to 1.38K on 10... Far from 5K!

Okay, now what is the resistance end-to-end of the on-board trimpot? Is that also ~1kΩ?

[SleepLess]

Looking at your pics and I can not quite see it - is that a solid wire connecting the plates on V1?  Where is the 100K resistor?

It's on the board, connected to the 220kΩ resistor for the other 12AX7 plate.

What is the resistance end-to-end of the Intensity pot?

I was thinking this would be 5kΩ based on the schematic, but that doesn't seem possible with your numbers...

I measured on the middle lug of the intensity pot to ground: 0K on 0 to 1.38K on 10... Far from 5K!

Okay, now what is the resistance end-to-end of the on-board trimpot? Is that also ~1kΩ?

It's 0KΩ to 4.85KΩ...

[HotBluePlates]

A-ha!! I made a wrong assumption that the 5kΩ pot would be the panel control. So your measurement of the panel control at a bit over 1kΩ is correct.

Am I right that you still have nothing in place of the 0.068uF cap you found defective?

Here's what I'm thinking: the "popping" sounds like motorboating, but rather than being associated with a filter cap, it is simply the trem oscillator overpowering the 12AX7 gain stage. The 0.068uF cap would have reduced the trem strength (too much, from what you said), but "no cap" is leaving the signal strong enough to cause motorboating.

Approach 1:
Get a handful of different cap values: perhaps from 500pF to 0.047uF. Start with the smallest-value cap, and place it in the circuit where the 0.068uF cap went. Increase the cap value until the motorboating stops, but you still have trem (a cap substitution box would help speed this, but it not mandatory).

Approach 2:
The 5kΩ trimpot sets the bias of both the trem oscillator and the 12AX7 gain stage. If its resistance is increased, the oscillator gain falls somewhat, and the bias voltage of the 12AX7 gets bigger, which allows the application of a bigger (trem) signal without overloading. Turn up that onboard trimpot to eliminate the motorboating.

Most likely, you'll need a combination of the two, just as the original circuit used a combination of the two. It's up to you to strike the correct balance.

[TIMBO]

Hey Sleepless,
How did you go.......
I have since built a clone of my own and have encounter the same problems
 Marshall 1930 Popular (el34world.com)
Getting myself up to speed with how I left it some time ago.
I do remember having some problem with the term
So I returned the circuit to as stock as I could as per schematics and photo's found on the net
And there was quite a few of different circuits.
So this is what I came up with


When I returned it to what I believed to be stock, the trem was making the speaker flap.
The 5k trim was set about mid rotation, turning the trim in either direction reduced the flapping but also reduced any tremolo effect.
By chance when I was trying the quiet down the fizz/hum by putting a voltage divider before the phase inverter, this deleted the flap and a good tremolo effect was achieved.
The tremolo effect was fast and this seem to reduce the effect, so a .022u was slotted in and this also made it stronger and more articulate.
Again not all 12AX7 would just drop in, most were noisy and microphonic.
With the trem in a good place and some of the noise reduced by the voltage divider there was still some hum/fizz that could be reduced.
A .01u cap at the junction of the 220k and 100k plate resistors seem to do the job.
Doing these mods has had on ill effect on the overall sound of the amp.
Hope to have my stunt guitarist give it a test soon.
Hope this helps