Marshall s80 ValveState 8240: Boost Channel Not Working, Low Output on Normal Channel

Sir Technical Guru,

Marshall s80 ValveState (model 8240, serial #952268426, 2x 12" Celestion)

Amp still works on Normal Channel and Master Channel but only at approximately 20% original output. Boost Channel does not work. The amp still produces clear tones from the stratocaster and Telecaster, but at the diminished volume.

Leading up to today's symptom, a week ago the amp was turned on and as it warmed up, it started making noticeable sporadic popping and crackling sounds of varying volume levels. It was shut off and turned on several time in that state before it was discovered that the three prong female power cord end at the chassis was barely plugged in.

After securing the plug all the way in, the problem seemed to be resolved until today. The Amp has now developed the No Boost Channel symptom.

I pulled the chassis, inside is surprisingly clean, no noticeable dark overheated marks on or around capacitors, resistors, or soldered connections. The circuit is 20 year old solid state board. Amp has a single Marshall tube (presumed the original).

What do you suggest, failed tube?

With great respect,
Alex

More often than not it indicates a failed capacitor, from about 12 amplifiers i fixed, 9 of the amps failed because of their capacitors. Do any capacitors look thicker/bulged? Is the top of the cap a flat surface? Here is a pic with two of the most common faults in capacitors: http://jonrosensystems.com/computers/wp-content/uploads/2011/03/Failed-Capacitors.jpg
Hope to hear form you soon.

Alex..
Check and see if there are any dried up caps, change in volume ( you say the unit is 20 years old) and check also to see if there are any resisters that may have changed in resistance. Also; try to re-solder the connections.. When you have any amp that is very old ( solid state) these symptoms tend to occur....

Alex,
Fixing older amps is always somewhat hit & miss especially for those without training & experience with such repairs.
I'd suggest:
1) Replace all the caps because cap failure (to some degree) is probable, though cap degradation may not be responsible for the symptoms.
2) Dry solder joints also do develop over extended periods of time even on what appear to be well made joints. Re-solder those joints that you think are more likely to be subject to elevated temperatures in use.
Probably wise to also resolder other joints that are easily accessible, but attempting to do all joints is likely to be difficult & tedious.
3) I am unsure about valve degradation symptoms, in the old valve days you simply tried other valves in a process of elimination. If you can easily obtain a replacement equivalent valve at modest cost I would recommend doing so; it's one more possibility dealt with.
4) If all that fails take your amp to a specialist repairer; costs are generally reasonable for this type of electronics kit - assuming of course that you like the amp & want to keep it.

No real wisdom here but my two cents is that your poor connection killed a passive component and now you have an open in your circuit or it killed the tube. I say my two cents because I have had poor connections kill various amps and pieces of equipment!
I suppose the bad connection may cause surges when the circuit suddenly closes then starts struggling for power again as it teeders between open and closed.

How about lets have a circuit first and how about some voltage readings at various points , what about some scope readings and signal injections using a signal generator ??

Forgive my shear ignorance but Guessing the answer wont help will it !! Or perhaps Im wrong there too

Attached is the schematic for this amp -- happy hunting. You can pick up a replacement 12AX7 tube at a music store like Guitar Center for about $15. Wouldn't hurt to swap the tube. It's used exclusively in the Boost channel. But since the Normal channel is also degraded, it might be something else or something additional,

My father always said you start with the easiest fix because it's also generally the cheapest and work your way up in cost from there! lol. Last time my car broke down, turned out someone had stolen a 25 cent fuse out of it. Swap the tube and if it still fails, deal with checking continuity, resistance, current etc through the most likely circuit traces.

I'm a musician working on an electronic engineering degree and must profess, I reeeeeally hope to design and build gear rather than fix it.
Good luck!

I know Some one has suggested replace a valve but last week i had a classic example of someone who had done just that !! A month later it was back blowing all fuses PS He’d forgotten to set up the grid bias on the output stage or couldn’t be bothered to check it

Cost to customer for initial service this is to replace valves in output stage and what he classifies as a general service £185 that uh $400 us dollars roughly


The fault is possibly now the output transformer if not all 6 EL34's

So do your selves a favor and the customer
Get the circuit
Do some voltage checks
Scope the output
Inject a decent sign wave as the manual tells you to do
Make sure the grid bias is correct

And if you havent got a service manual and the customer is pestering you and swearing at you down the phone as they normally do with all the normall blistering barnacles " $$$ *** CC ,, FF " , Quite normal this !!

Make them bloody wait untill you do have one Other wise tell him to take it else where and charge him "£20 for wasting your time "

Equally most manufacture’s, suppliers to the trade will supply schematics


Hence why I always favored Panasonic UK when it came to service always helpful always sent you schematics and whats more they always offered free training even workshop training More than I can say for many orther firms in the UK


Most decent customers will appreciate this and will wait for the job to be done properly If they don’t they not worth having as clients you are better off without those type of people

This now means the customer can take you to court for loss of use , loss of business, , loss of earnings Equally other traders will now be able to take manufacturers to court for exactly the same


I have seen more of this in UK companies than you can ever imagine not mentioning how customers will deliberately and other traders attempt to set you up just like this

Finally put same warranty seals on screws label the valves and date them but above all do it properly and ensure you have signed resiepts from customers and addresses etc so that in the event of a dispute you have a signed record to indicate that they have are content with what you have done to date

One company after the other hath I come across who has either been set up like this or they never bothered to adjust something as simple as the grid bias

This is exactly how you wind up in court facing a massive legal suite

Up to you but me I know what I would do