Can you check if C7 (2.2 mF) in poweramp is OK?

May be no heatsink compound on the power transistors. Only mica washers?

If you reverse the phase and neutral, does the hum goes away? If it works, you still have to rewire the on/off switch correctly that it disconnects the live end for continued safety.
Regards.

Rod designs are good and proven. But one big bottle neck is some board designs do not have mounting holes. The board is supported by either the power transistors clamping bolts or by the potentiometers. To me they are not that much flexible to use when using TO3 transistors or separate potentiometers. Just my view. There are many who have successfully used his boards. Regards.   

Great information in one place.
Philips produced TDA1022 and TDA1097 BBD chips
And there was SAD512 chip from some other manufacturer
These were originally developed for telephony applications.
Thank you very much.
Regards.

Just a small explanation of the name bucket brigade delay.
Imagine a line of 10 fire fighters. One side is fire and the other side is water. Buckets of water has to be moved from the water source by buckets to the other to kill the fire.
Since the fire fighters are in a line, when they receive a bucket of water, they can't deliver to the next person. Similarly when they deliver water to the next person, they can't receive water.
A BBD chip in similar, has tiny line of capacitors (say 511) inside representing the buckets. Electronic switches (say 512) are in between each capacitor and the  switches represent the fire fighters.
The odd numbered switches and even numbered switches are turned On and OFF alternatively from the square wave signals.
When the odd numbered switches are turned on, they transfer the charge from the left side capacitors (input side) to the right side capacitor. When the even numbered switches are turned on, they transfer the charged capacitor value to the next and so on.
At the output, switches 511 and 512 outputs are connected together so there is a continuous signal flow. Now there is signal at the output but chopped at the same frequency as the square wave.
Since this chopped frequency is 30 Khz or more, a low pass filter of 20 Khz is inserted at the output to filter out all higher frequency above audible range. If the portion of signal is fed back to the input to create a flanging or echo, another low pass filter is required at the input before the BBD.
Since the stages work similar to the bucket brigade fire fighters, hence the name bucket brigade delay line.
To prevent loss of signal (leakage of capacitors) during modulating frequency transition, the driver chip is interlocked internally cmos or ttl gates so the rise and fall of the wave forms never cross each other.
Regards.

All those early chorus pedals used a BBD chip (bucket brigade delay line chip) and are obsolete now.
If you want to service, :
The BBD chip has variants. Some with 512 stages, some with 1024 or 4096 stages. 4096 stages usually for producing longer delays (echo).

All the circuits using the BBD chip have these common components.
Signal path : An input buffer > low pass filter (20KHz) > BBD chip > low pass filter (20KHZ)> output buffer, mixer
Modulation path : A Low frequency oscillator (0.5 Hz to 10 Hz) > A square wave oscillator with 2 outputs with 180 deg opposing phase

The square wave oscillator IC drives the BBD chip. (one for odd numbered stages and other even numbered stages). This frequency determines the delay time. This square wave oscillator frequency is varied (modulated) with a very low frequency oscillator triangle wave (LFO), thus the delay time is also varied.
The instrument signal is buffered and passed to a low pass filter (around 20 KHz), then to the BBD chip. The output from BBD is passed through another low pass filter (around 20KHz). This signal is mixed with the original and buffered to output.   

Depending on the delay time, feedback and mixing methods, same circuit does the chorus, flanger, delay, echo etc.

Hope this is of little help.
Regards.

Year 2001 application note from ON semi regarding mounting considerations for power semiconductors. Regards.

Guitar speaker expert from Jensen interview on YouTube.
Have a watch. About 50 minutes

Regards,
Mandu.

https://www.youtube.com/watch?v=ZAUpWQWlTe0

Sorry wrong post

This is not a standard in most guitar amps including Marshall. The low pass filter is set to pass signals below 20 kHz. The drive circuit produces lots of harmonics and may be well beyond audible range. The filter may be part of circuit to produce that Marshall sound for this model or the designer added this to his taste. For guitar applications, Low pass filters are generally provided before and after digital delays or BBD circuits. Regards.

When you replaced Q1 and Q2, probably the transistor pins are not same as the replaced ones, even if the same number. The pin connections some times vary between manufacturers. Regards.

You can get Zener diodes for almost any voltage and wattage. They work as shunt regulators. Don't get easily damaged by over voltages like the 78xx. But voltage will dip on over current by load. Very widely used in many instrument amplifiers to drop from higher voltages above 35V DC. 78xx/79xx are Series regulators limited to selected voltages and prone to damage by higher supply voltages and some tend to oscillate under less load. Application to use 7xxx or Zener depends on the designer. All diode/transistor junctions are noisy when reverse biased. 78xx/ 79xx use zeners internally.

Did you check the 470 mfd emitter bypass capacitor? If low value, then less gain at the very low frequency setting and the oscillator will start and stop at power up. You can temporally place a similar capacitor to test. Regards

The tremolo oscillator is on the right side of your snap photo, called as the phase shift oscillator . You can test if the output voltage on the collector is changing using an analog multi meter. The 3 nos. 1 mfd capacitors, 3 resistors (two are variable potentiometers 3.3K+50K pot) and the transistor form the oscillator. The transistor gain must be greater than 29. The phase shift happens from the collector to the base by 3 x 120 deg. with the 3 capacitors and 3 resistors. Varying one or two resistors changes phase shift and thus the frequency. If the phase shift fail, ex. dried capacitors or high resistance, the oscillation will stop. Regards.