I would only replace the resistor if the amp is not sounding right, if you have enough highs coming out then leave it.(Maybe clean up the mess a little bit)
If you do decide to change it then you might need a higher wattage for the resistors you put in, and then you also have to make sure that the L-Pad can manage the higher load as well.
I am presuming the pot you mentioned is a L-Pad or a wirewound high power potentiometer that controls the tweeters.

The 50R resistor might have been a late change in the factory or it could have been performed as an upgrade or recall from the factory but performed at the local service center.(back of the local music shop).

I have never seen one of these amps, but I have looked at the schematic a few times and wondered if I should try making one for the fun off it.

The original schematic is fairly hard to read, but I gave it a go and re-drew it.
I have most likely made some mistakes,and I left some values off because I can not read them if wanted to.

When you replace the capacitors you only need to replace the electrolytic ones, they look like little cans.

They should be easy to check by just lifting the lid of the amp and having a look, please do not go by what I think the schematic says or even the original schematic as Marshall sometimes changed components to make the amps more reliable.

These changes did not always make it onto the schematic for years till they made a new revision of the schematic.

If you find any components I have marked wrong then please let me know and I will upgrade the schematic,
Cheers
Mick

The 0.22v is the 1kHz test signal you are putting in, if you have a signal generator then you should be able to turn the voltage (volume) up and down on the signal.
 
In the instructions it says to put the tone pots on 5, volume and gain pots on 10, balanced line out pot on 10, and then I can't make out the test signal clearly.(with 600ohm load. 1kHz input?)
On the chart RHS it says to leave the limiter off and inject a ac signal at 1kHz with a voltage of 0.22v peak to peak.

Peak to peak (vp-p)means that the signal should be that voltage from the top of the sine wave to the bottom of the sine wave. That is best seen on the scope as the multimeter will be measuring volts rms which is a lot lower then p-p.
When you see peak to peak (p-p) it will always be an AC voltage that is being described, all audio signals are some form of AC but the wave shape can differ.

Good luck

It looks like you have a great power supply on that bench.

Quote from: WimWalther on August 19, 2023, 08:48:19 PMThe amp uses a pair of Motorola KSN-1005A piezo tweeters

The amp should have an Ampeg custom 15" speaker or an EVM-15L speaker,if there are any tweeters in it they would have been added by someone at a later date.

If it sounds good, leave the 50 ohm resistor in there, two tweeters would likely be too loud with a lower resistance to them.

Thinking of the wrong amp, must stop posting at 5am and get more sleep.

I would inject a signal and then test all the testpoints with the scope, if you leave one channel on the input signal and then probe around with the scope using the second channel you should soon see where the signal distorts.

I would start with the limiter off and inject 0.22 volts on the input like the chart says to do.
Please note that there is an AC signal shown and a DC voltage as well, when it shows a DC voltage it means the AC signal is centered at that voltage so do not earth the probe you are using to test the checkpoints.

Earth the probe on the input signal to the chassis or the ground tab of the input jack, but again, DO NOT EARTH THE SECOND PROBE.

There should be a way to bring the trace from probe 2 up to the center of the screen even when the center of the vawe-form is at -16v or -11v, otherwise it will be at the bottom of the screen.

Good luck and keep us updated, we love to hear of a win.
(Have you tried to get a wave-form from your tongue yet?)

If you have a signal generator, use the CRO to check the signals to the power amp.
But first, check the speaker output for the DC volts with no input, if you have a DC voltage of even a few volts then do not leave the speaker connected.You cannot view this attachment.

Schematics

They are usually very clean sounding amplifiers as they were aimed at Jass Guitarists more then anything else. They are known to clearly reproduce the sound of the guitar and not "colour" the tone, something that most other styles are looking for.
The are also good with other instruments where you want the clean true sound to come through the speaker.
Acordians, mouth organs and piano etc used them when amplification was needed as well.
Its just a case of the right tool for the job.

Those wirewound resistors are 5 watt and 10 watt for a reason, so they will get hot.

Mybe spray some DeoxIT in the slide switch and work it for a bit to clean the contacts.

Do you have any wall warts with AC output ?
A nice small 9v AC would be a good test to do.
The older heavy DC ones usually have a transformer in them, you could test the DC voltage on AC and look for a ripple.

That one looks like a nice simple circuit to make, should not cost too much to make.
Please show us all some pictures of your build when you finish.
Good luck

Cheers
Michael

According to the SUNN BETA service manual:
The Sunn Beta has 1.75v P-P out at the master line output, but each individual channel has only 1.1v P-P at the preamp line output.
Sunn Beta basically has 3 effect loops.
If you made a single channel the output to the power amp is only 1.1v P-P which should be ok, and you don't have to crank the volume pot all the way around.

If anything you might not get enough signal to drive the TPA chip to full power since the Beta figures are peak to peak.

I forgot, BAJAMAN  has made a number of circuits for preamps over on Free Stompboxes,
https://www.freestompboxes.org/index.php?sid=244283fce956bdeb138bb1af61af85ef

Check his 59 Bassman layout, it might be worth a look, all his designs are made for stompboxes but you could adapt them for mounting in a box.
https://www.freestompboxes.org/viewtopic.php?t=28915

I think BajaMan must have designed a dozen or two of these circuits emulating popular amplifiers and they get some good praise.
You could carry a dozen different amps in a small case in stompbox form.
Cheers
Mick