Rowe AMI MM4 - No sound - Amp Rebuild

[nobbsy]

Hi All,

I have just purchased a MM4 and absolutely love it. Everything works as you would expect, except for the fact there is no sound. I can hear the song coming off the needle but nothing coming out of the amp/speakers.

I'm going to have a try and rebuilding the amp myself with the help of my father and wife who are both electrical engineers.

I have a feeling the amp (R2255a) that is in the jukebox is from an MM3, and is solid state not valve like I was expecting.

Are there rebuild kits for the 2255? I'll be doing some point-to-point testing tonight to rule out cracks in the board.

Any hints or insights into best way forward here would be very much appreciated, I'm keen to do this myself although haven't completely ruled out sending it away for a rebuild.

Thanks all
Ben
Brisbane, Australia

[Ron Rich]

Hi Ben,
I am not familiar with that amp, but if it uses the "slide on" type contacts-- see the sticky above on common problems. Ron Rich

[nobbsy]

Hi Ron, thanks a lot for your response. I'll give the suggested 'fix' a go and see if that gets me going although I do notice a bit of damage so not confident there.

I'm also open to the idea of just purchasing (upgrading) this 50W mono amp to a stereo solid state stereo amp.

I've read on here in my searches that the R2179A is an alternate I could use. Are there any others (just to broaden my search and improve my chances of finding something).

Thanks again
Ben

[Ron Rich]

Hi Ben,
Sorry-- I do not know enough about this to advise you-- hopefully Alan, or Norman, will chime in here--- Ron Rich

[jukenorman]

Hi Ben,
I also am not familiar with your particular jukebox. However I can recommend the archive at jukebox-world.de - it's a wonderful resource when you are wanting to know what original equipment was used in any particular machine. A quick look there indicates that the Australian MM4 had the option of a 240V, 25W mono amp. Check that archive out, it will let you know you which amplifiers were the original AMi ones.
Norman.

[nobbsy]

Thanks a lot Rich and Norm, I really appreciate you trying to help. Norm I'll have a look at Jukebox World, it sounds promising (or at least interesting haha). That makes a lot of sense as the 50W I keep seeing mentioned is actually stereo. So half being 25W for mono?

[wdln]

Pop for the service manual... it will have full schematics for the amp. Definitely plan on replacing all the electrolytic capacitors. If they're not leaky already, they will be soon enough on a 50-year-old machine if they're original equipment. If you have a way to test the tubes (valves), that's a good start as well. The book will also give you some info on voltages at different test points that you can look at to help determine what's going on. Since you have no sound at all, it could be a power issue. I'd start with the transformer and rectifier circuit.

It's doubtful, btw, that there's a circuit board in there. Most tube equipment was wired point-to-point rather than printed circuit boards.

You could also start with the volume control. Could be a bad pot or the factory pot on the rear panel may have been disconnected. That would be an easy fix.

I would vote to keep it original, rather than swapping the amp. I'm guilty of doing that myself, but I tend to do it only on newer models without as much character.

[nobbsy]

Thanks for your reply wdln. This one is a solid-state, I have the service manual for the MM4 but the diagrams don't have this amp. Quite strange. Although the 50W stereo is almost identical to it except approx half the caps etc on the board. I'm going to do all the tests you recommend, and will replace all electro caps. I agree with you in keeping it original as possible. Thanks mate appreciate it

[Ron Rich]

Hi All,
For reasons unknown to me, Rowe dropped off all amplifier information in some manuals. They forced the purchase of a (very expensive) "shop manual", in order to get schematics. Ron Rich

[nobbsy]

Looks like i found my culprit(s).

On the driver board there is a leaky swolen cap (I'll swap this out). But even more likely is a small transistor on the driver board that had broken off at the base on two of the three wires. Its one of the ones with an alloy heatsink on it. Struggling to ID it with minimal info on the amp but I can tell its a Teledyne symbol. It has the codes S-310-C and 7141

Any idea on a replacement part? Thanks a lot gents

[nobbsy]

Oh if it helps, it was in the Q72 location on the Driver Board. And its a Rowe L 5771 B Driver.

[Ken Layton]

Transistor marked S-310-C is Rowe part number 7-00310-03 and is npn. I cannot find a cross reference in NTE guides.

[nobbsy]

Oct 28, 2020 3:52:09 GMT Ken Layton said:

Transistor marked S-310-C is Rowe part number 7-00310-03 and is npn. I cannot find a cross reference in NTE guides.

Hi all

Thanks so much for your help. I replaced the transistor with the one Ken recommended, and swapped out a few of the dodgier looking caps. Put the amp back in and it sounds great!!!!

I want to go through it again and replace the remaining caps and test all the resistors etc.

Thanks again!
Ben