Marshall 6100 help

[Braudy]

I've had a bad run on luck with amps, first my VM and now my 6100. Can anyone help me figure out why my 6100 has died? She will power up, with all tubes glowing, but no sound comes. It played fine yesterday but today I switched impedeance (16 to 4,) and she died. Can anyone give me some ideas , its my favorite amp!

[ClubAndCountry]

Check that you haven't accidentally pushed the Power Amp Mute button! (Between the Send and Return jacks.) I know it sounds stupid but I've come across this more than once :-).

What do you mean by switching from 16 ohms to 4? The amp only has 16-ohm and 8-ohm settings. (8-ohm and 4-ohm on the combo version.) I hope you haven't mistaken the "16(4)" panel labeling to mean that you can run *either* 16 or 4 ohms from the 16-ohm setting... this is one of the worst panel labelings on any amps (and the similar JCM900s) I know of, because it seems to imply exactly that. It's so misleading that when I service these amps I deliberately remove the incorrect number with some solvent, to avoid confusion - I've seen too many examples where owners have run them wrongly - and sometimes blown things.

Running the 16-ohm amp setting into 4 ohms could have done some damage. If the valve failure lights aren't on, try undoing each of the fuse holders and switching the amp on - if the lights still don't come on, there's a failure in the main HT supply somewhere. It's very difficult to be more accurate at this stage, these are complex amps. Also make sure that the impedance, the Triode/Pentode and the High/Low power switches are fully locked into whichever setting you're using. If any of these isn't making proper contact the amp won't work properly (or at all, depending on which one).

[Braudy]

I did run the amp into a 4ohm head using what I thought was the 4ohm setting! I removed the fuses and the lights came on and all the switches seem to be fully engaged. If you have any more ideas please help! And thanks for setting me straight on the 16,4,8 selector.

[ClubAndCountry]

I hope you mean 4-ohm *cab* :-).

If the valve fail lights come on you have a functioning HT supply, and output transformer on the primary side at least... that's good.

Is there *any* hiss through the cab when the amp is on - you may need to listen very closely, but you should hear some, and at least a pop or change when you flip from Standby to on (these amps normally hum *more* on standby than they do when running). If no, the problem must be in the speaker circuit somewhere - the output transformer secondary (not completely impossible, but extremely unlikely), the impedance switch, the speaker jacks, the cable or the cab... or a bad connection inside somewhere.

If yes, the problem is not in the power stage and running into the 4-ohm load was almost certainly just a coincidence. Try connecting the guitar directly to the FX return, and turn up the FX Mix and Master Volume controls - this bypasses the preamp and will allow you to check the phase inverter and power section. And do check that mute switch, if you haven't already!

[Braudy]

Thanks for all the help. I did'nt notice any hiss or pop when going from standby. My cab works fine with my VM so I don't think the problem is there. The mute button is'nt on either. I may have to take her to a tech.

[ClubAndCountry]

No problem for the help. If the valve fail LEDs are coming on but no sound at all comes out of the amp (and you know your cable and cab are OK) it does sound like a final power output failure of some sort - and running at the wrong impedance could definitely be a contributing factor for this sort of fault, since it's this part of the circuit which will be stressed by doing that.

Be warned that a lot of techs don't like working on these amps - they're complex, can be difficult to diagnose and it's a pain to get the boards out. (Though luckily yours is the head, the combos are worse.) If you're in the UK and you have any doubts as to the competence of whoever you can find to look at it, consider sending it back to the Marshall factory for a full service - they're fast, do a great job and it's not expensive. I've done this with a couple I've worked on where the problem was difficult to trace or in the MIDI-controlled switching system, which I'm not knowledgeable enough about.

[Hump]

Trouble with blowing fuses,and replaced the WOO8 rectifiers, solved that issue. After trying to power up the amp, the control lights sequenced on. So after 2 tries, they don't come on anymore and there's no B+
So, I intend to try the filter caps next, but where do I look for the power to the control lights??!! I will appreciate any input or help on this matter. Thanks in advance!

[Anitoli]

First off these amps will run a 4 ohm load, you just need to know how to switch the jumpers on the power amp board to change 16 to 4. You switch the black ( 4 ohm) with the yellow (8 ohm) to give you a 16/4 output selector, or you switch the green and black to get 8/4 which coincides with the labeling on the back panel. Now for the sake of argument unless you were really cranking this thing on the impedance mismatch, i highly doubt playing it at bedroom levels or slightly louder would cause any damage.

Now what are you referring to when you say you removed the fuses and the lights came on? What fuses did you remove? Do you have a multi meter? What lights came on? Need more info.

Check my sig.

[Anitoli]

Second order of business is don't automatically assume that these issues are caused by the capacitors and just start changing away. When it comes to power filter caps running high voltages they degrade slowly as in the ESR gradually increases with time which will reduce it's ability to filter out AC, it's capacity to charge and store current will be reduced resulting in an amp that gradually becomes more and more dull and lifeless, if there is excessive leakage they make these farting sounds that you hear in the out put, and in the event of a total short the thing will just explode through the top vent. The first three don't kill your amp unless they are ignored for an eternity and the last one will be a no brainer.

[Anitoli]

Christ i wish i would have noticed the dates on the beginning threads, necro post revival.

[Hump]

I'm sorry but I posted in two different spots on here, I read if I remove the two fuses on back panel and their respective LEDs don't illuminate there is issue with the B+. And they do not light up when I do this. Also, what area would I look at because the 3 control lights on front panel no longer sequence upon startup. Yes I do have a multimeter.

[Anitoli]

Gotta back up a bit. This started as the amp was blowing fuses? What fuses were being taken out?

[Hump]

Originalky the mains fuse was blowing,this was cured by replacing WO08 rectifier (one was shorted so I changed both DB1 and DB2,problem solved. Info online said remove output fail fuses on rear panel and LEDs should light,they do not.

[Anitoli]

Ok, with the power tubes out, you place the black meter probe on the chassis ground and the red goes to pin three which is indicated in this pic. You do this with the power on and standby off ( play mode). Be careful, there is high voltage in there and if you brush up against the top of those big filter caps you're gonna be renamed sparky.

[Hump]

Per your instructions it starts out at about 23vdc and goes down to 9vdc after a minute or two. I am bewildered...