jukebox amp, noise like a harley ?

[casso]

Hello everyone, I googled "my amplifier's broke, and this superb site
came up, I hope someone can help me. I bought a jukebox that was "working" but needed TLC. Ok, I have fixed all the electro/mechanical controls, keypads and selectors etc, but I know little about amps.
I found the volume was being controlled by a pot wired directly to the speaker cables? and there was no output on the right channel.
The original volume pot was missing from the rear of the cabinet, and the original five wire harness for this pot had been all butchered up.
I took the amp to a repair shop, and asked for the volume control to be fixed as it was, with a new pot, and to fix the right channel.
A few days later they rang me to say the volume control was done but
two ICs needed replacing, and they were obsolete. I got two within a week. He gave me back the amp, with the new volume control pot wired onto a plug for the amp, and the two old ICs (4739cp).
when I tried it, it sounds like there is a motorbike running through
the speakers, although, unlike before, it is at least coming from all the speakers. With the volume in the lowest position the noise is horrible,
with the volume plug removed, it is slightly better. and it makes no difference if the phono plug is in or not. The amp is made up of two small 4" square driver pcbs, and a larger preamp board, a heavy alloy finned cooler and some small capacitors fastened to it.
I have been over everything with a magnifying glass, and when I tested those capacitors they all read as they should.
Has anybody got any ideas I can try next or any advice .
I thought about changing the amp, but this one has a plug marked mute that is connected to a micro switch on the turntable to mute the noise from the record changing mechs, and I don't think this is on normal amps? anyone know of any?
I hope this is not too long for my first post, I would really appreciate any help at all. I love to fix stuff and I hope that with your help, I can stop sending my amps out for repair. and hopefully
help you guys in return.
Thank you. Casso.



I

[lineup]

.

it sounds like
there is a motorbike running through

This sound effect is called
motor boating
( quite nice! if you fancy motor boats or Harley Davison, HD MotorBikes )


I am not sure I remember,
but maybe can be cured

by a small ( 1000 ohm ) resistor
connected so that
the INPUT transistor BASE pin,
never brings into 100% zero resistance contact
with
Volume Potentiometer
or GROUND

[bikehorn]

i believe sometimes this can be caused by insufficient power supply capacitance, or dying capacitors in the power supply. after all those years(i assume it's pretty old) any electrolytic caps in the amp have probably reached "retirement age". i would replace every electro i found, but at the very least, go for the main PSU filter caps and maybe even replace them with slightly larger value ones, maybe put some small-value film caps in parallel with them. YMMV.

[casso]

I just want to say a heartfelt thanks to the lads who have responded with advice to the problem I posted about my amp.
One person has even offered to come round to my place and take a look at it for me, I never expected that, I am bowled over. Andy, I will E.mail you if that is ok?
Reading through some of the many thousands of fascinating threads,
has made me realise how little I know, about how the music gets from the record to the speakers, so I don't think I will be giving much advice
here!! Quite a bit of the forum is more my field/strength so hopefully I can do my bit there! I am also good at obtaining/ sourcing obsolete
components, so maybe I can help there as well.
Thanks again, Casso