mrmuzic

I just bought a SL100 made in 1973 and I am having a problem.

1) loud hum and nothing out of Channel 2
2) low volume out of Channel 1
2) I replaced all tubes and the same thing

I suspect the output transformer ? Can anyone help me here. If it is where can I get a dropin in replacement or a more robust one if needed.

Kevin

plexipaul

1) is the amp still stock?
2) did you check all solderpoints, tubesockets in particular?
3) a clear gutshot may be helpful.

PaoloJM

Hi,
I'm new around here but maybe I can help.
Have you checked your filter capacitors for bubbling? When caps age and start to go they stop filtering the AC ripple in the B+ line and also reducing the B+ value. This can cause excessive hum and also reduce your output level. Take a look at the positive (+) end of the caps. If you see a bubble or even leak then they need replacing.
Please, please, please ensure that you discharge you caps before fiddling around. If you don't know how to do this then don't open up your chassis.

A quick test for your output transformer is to measure the DC resistance on the primary and secondary. You should read somewhere between 60-100 ohms on the primary between the center tap and either side. If you read a short ie. 0-1 ohm resistance or much more than 1k ohm then your OT would appear to be screwed. The secondary should read about 1-1.5 ohm depending on the winding 4/8/16. Remember the actual value of the DC resistance is fairly meaningless but it gives a reasonable indication and is easy to do.

Also can you post a gut shot, it may help and a bit of amp porn is always welcome!!

Hope this helps. Let us know how you get on.

Cool amp BTW.

Regards,
Paul.

marks

I am a new member so hi to all,
I have a JCM900 4100 Dual reverb model, that i bought in about 1987, and a couple of years ago the overdrive went really weak and ponsy. I changed valves Pre amp and power to no avail. The amp has been to Marshall UK for a seperate power issue, where i highlighted the weak distortion only for it to come back working again but still weak.They too changed all the valves as i was incorrectly sent the wrong spec valves the first time! Can anyone help me get my great big ballsy overdrive sound back?. I have been using a Korg pedalboard instead and really miss that sound. Any ideas appreciated.

seba-AMPS

A 4100 dual reverb in 1987 ????

Are you sure ?? JCM900 came out at 90's,first with the master volume 2100 model I think