Marshall MA 100C 100 Watt All Valve Amplifier

DrNomis · Post by **DrNomis** » 17 Nov 2013, 07:24

Just found this, the choke is rated 5 Henries at 120mA, it's basically a replacement for the chokes in Marshall 50W and 100W Plexi and JCM 800 Amps, wondering if this will work as a replacement for the 470 Ohm/7 Watt resistor....

http://grangeramp.com/shop/index.php?ma ... cts_id=117

bajaman · Post by **bajaman** » 17 Nov 2013, 12:26

the same choke fender use for all their 100w models OR a Marshall JCM800 100w amp choke OR a 9w neon florescent lighting ballast

bajaman · Post by **bajaman** » 17 Nov 2013, 12:28

yes that will work

DrNomis · Post by **DrNomis** » 17 Nov 2013, 12:33

Thank you for all your help bajaman, much appreciated.......

I'll order one of those chokes next fortnight......

DrNomis · Post by **DrNomis** » 04 Feb 2014, 07:52

Okay, latest update:

Last night I was playing my guitars through my Marshall amp and everything seemed to be working normally, today I fire up my Marshall amp only to find that it has somehow developed a new fault, here's what's going on with it now.....

The clean channel seems to be working normally as well as reverb, however, when I switch over to the overdrive channel I notice two things not right with it, at low gain settings the distortion sounds funny like one of the valve stages is a bit mis-biased, also the crunch ballance control doesn't seem to behave normally, that is it doesn't seem to be controlling the crunch level like it's supposed to, what should happen is that turning the crunch balance control up or down should control the volume relative to the boosted mode, I can turn the control up or down and there's no effect on volume, now, when I switch the overdrive channel over to boosted mode I get a very large drop in signal level, I have to turn the overdrive channel master volume up almost all the way just to hear a signal, when I switch the boost mode off the volume jumps up alot, this happens regardless of whether I have the footswitch plugged in or not, I have tried re-soldering all the solder joints for the pre-amp valve sockets and the pots on the PCB, as well as tested the two front-panel push button switches (they seemed to be working fine), with no change in the amp's behaviour, I'm thinking that there has to be something to do with V1A and V2B that's causing the problems I'm experiencing, maybe C8 (1uF/63V) may have gone bad and caused a short across R9 altering V1A's biasing, and there may be a partial short-circuit which comes into play when in boosted mode, as far as I can work out, could anyone offer some suggestions?.....

I don't think it has anything to do with the biasing of the power valves since the amp sounds fine when the clean channel is in use, also none of the power valves do anything abnormal, like red-plating.....

I've also tried swapping the preamp valves with known good ones, made no difference so I think it's obviously a fault somewhere on the circuit board, I really wish faulty electronic parts, or faults glowed bright neon yellow so we could find them easily....

The problems seem to be only occuring in the overdrive channel.....

bajaman · Post by **bajaman** » 04 Feb 2014, 10:36

I would be checking CON 11 on the switching mute board - bad connection on crimped cable ? - or perhaps a bad relay RL3 and RL1
RL1 and RL2 are series connected 9v types - how are they supposed to work from a +15 volt supply rail ???? - dodgy

bajaman · Post by **bajaman** » 04 Feb 2014, 10:42

RL3 is a 12 volt relay being run from the same +15 volt rail - wtf Marshalll ??

DrNomis · Post by **DrNomis** » 04 Feb 2014, 11:42

bajaman wrote:I would be checking CON 11 on the switching mute board - bad connection on crimped cable ? - or perhaps a bad relay RL3 and RL1
RL1 and RL2 are series connected 9v types - how are they supposed to work from a +15 volt supply rail ???? - dodgy

I'll have a look at that connector in the morning, and yeah I know what you mean about those relays, I was suspecting them too.....

The actuator coils in those relays should be working okay because the leds associated with them do light, maybe the switch-contacts might be a bit iffy?.....

I decided to check CON 11 and it looks okay to me, none of the wires pulled out of the connector when I gently tugged on them, so it's looking like it could very well be the relays, what about those BC182 transistors (TR40 and TR42) that control those relays?, could TR 40 be faulty?.....

DrNomis · Post by **DrNomis** » 05 Feb 2014, 02:16

Not being able to think of anything else to try, I'm going to try replacing transistors TR40 and TR42, from what I can work out from studying the schematics, TR40 switches Relay RL3 on or off, Relay RL 3 is responsible for switching the boosted overdrive mode on or off (it's actually controlling how much negative feedback is applied to V1 A, and the crunch balance pot (VR5) gives us a way to vary the ammount of applied negative feedback), TR42 is responsible for switching Relays RL1 A and RL2 A on and off, these two relays switch between the clean and overdrive channels.....

I don't happen to have any BC182 transistors, but, I do have some BC547C transistors and I've compared the specs and they should work fine as replacements, they have the same pinouts as the BC182 (CBE), both are NPN Silicons.....

One other thing, the main HT supply in this amp is 450V DC, I noticed that the valve stage coupling caps in the preamp are all rated at 400V DC, I was wondering if changing them to 630V DC Greencaps and 3KV DC Blue Ceramics Caps would be of any benefit?......

Stay tuned for more updates....

DrNomis · Post by **DrNomis** » 05 Feb 2014, 11:01

Okay, I've replaced TR40 and TR 42, made no difference, incidentally, I checked the two BC182 transistors I removed with my semiconductor component analyser, they tested fine, I also tried replacing C10 (the original was marked 471 which I interpreted as 470pF, didn't have any 470pF/400V caps in stock but I had some 1nF/3KV caps so I used one as a replacement.....

I did some more thinking and have come to the conclusion that I think the relays are working as they're supposed to otherwise firstly I wouldn't be able to switch from clean to overdrive, secondly if Relay 3 was faulty then the signal levels wouldn't change so dramatically ( there appears to be a huge loss of signal when I switch to boosted overdrive so something else is probably shunting signal to ground somewhere), I desoldered VR5 and measured it with my multimeter, it appeared to be working properly.....

So, at the moment I can't seem to figure out why the crunch channel sounds bad at low gain settings, the crunch balance seems to have minimal effect when it should be affecting the volume-balance between overdrive and boosted overdrive, and there seems to be a big loss of signal level when I switch to boosted overdrive (what should happen is that the avalible gain should increase, what actually happens is that the signal level goes down and the sound gets cleaner rather than more-gainier).......

Note: Swapping both preamp valves made no difference, so that rules out bad preamp valves....

DrNomis · Post by **DrNomis** » 05 Feb 2014, 13:57

Tomorrow I'm going to try tracing through the preamp signal path with a signal-generator and an oscilloscope in order to find the cause of the faults in my Marshall amp, I happen to have an 8 Ohm 50 Watt Dummy load which I can plug into the amp chassis so I don't inadvertently destroy the output transformer, I know my dummy load is only rated at 50 Watts but as long as I dont turn up the channel master volumes there shouldn't be any problems as I'm only interested in the preamp section rather than the power amp.....

Stay tuned for updates.....

DrNomis · Post by **DrNomis** » 06 Feb 2014, 02:39

Update:

Just done some voltage checks on the valve sockets and found that there's no anode voltage on pin 1 of V1A when the amp is switched off standby, all the other anode pins in the preamp section have what look to be correct voltages on them, so my conclusion is that there's either a short to ground from pin 1 of V1A, or R10 (220k/.6W) ,which is V1A's anode resistor, is most likely open-circuit, I've just measured R10 with my digital multimeter set to the 20M range, I get a very high resistance reading, so I'm satisfied that R10 is indeed open-circuit and if I replace it with a 220k/1 Watt carbon film resistor that should get my Marshall working normally again, stay tuned for more updates......

By the way, refer to this schematic:

DrNomis · Post by **DrNomis** » 06 Feb 2014, 08:23

Update:

I've just finished testing the amp out after replacing the resistor and.......the amp is fixed!!!......

Hard to believe that all that trouble with the amp was caused by one single resistor (R10) going open-circuit, but there you have it folks.......

DrNomis · Post by **DrNomis** » 05 Mar 2014, 21:57

Update:

My Marshall MA100C seems to have developed a serious fault in the power amp stage, I took my amp in to Uni on Tuesday 4Th March this week, plugged it in and it was working fine, then suddenly it blows the main HT fuse again (1A 250V), I replace the blown fuse with a new one, the amp powers up normally but as soon as I switch the amp off standby the fuse immediately blows, so, something is causing the amp to draw more than 1Amp of current from the HT supply, I tried using my multimeter to check for short circuits to ground where the standby switch connects to the PCB but I can't find any, none of the caps look like they're bulging, so maybe my output valves have gone bad or they're not getting any -V biasing voltage......

Just wondering, could R68, R69, R72, R73, R76, and R77 have gone open-circuit?, could the bias supply have failed?........

I'm starting to get a bit frustrated with this amp, how could Marshall UK have allowed their Vietnamese manufacturer to design/manufacture such an unreliable amp?, that's what I'd like to know, I know this is supposed to be their entry level amp but still for something like Au $1300.00 which I paid for the amp, I expected much better reliability than what I've had to put up with, Marshall should seriously think about stopping the manufacture of these amps in Vietnam if they want to avoid tarnishing their reputation......

blackbunny · Post by **blackbunny** » 06 Mar 2014, 00:58

Bias supplies do fail, but it should be easy to troubleshoot for gross (obvious) faults). Your Marshall bias circuit is based on the ones used in most 100 watt valve models since the late 90's, with separate bias adjustment trimpots and bias monitoring points for measuring current flow through 1 ohm resistors in series with the cathodes of each pair of EL34 output valves.

Probably due to the terrible reputation early DSL and TSL JCM2000 amps have for bias problems [ believe me I know; I've owned both and had such bad experiences that I will never buy another Marshall made after 1986

], it looks like Marshall have tried to improve the bias circuit in your model so that if a trimpot fails, the bias voltage goes "high", which is safer for the valves and output transformer than having very low bias voltage or none at all, although it will result in excessive crossover distortion, making it impossible to get a clean sound at any volume level.

Power up with the amp on standby, and after a minute or 2 warm up time measure the voltages on either end of the 4 x 5k6 bias feed resistors, R72, R73, R76 & R77. If you measure between -38 and -42 vDC on all 4 resistors, and there's not more than a volt or so difference between any 2 readings, then the bias supply hasn't failed and you should look elsewhere. If the bias supply voltages are very low, test the electro caps , resistors & diode in the bias circuit.

If the bias supply looks like it's OK, it's time to test the valves and power supply. Take all the valves out or their sockets, fit a new 1 amp HT fuse, power up and hit the standby switch. If the fuse blows, test all the rectifier diodes & look for shorted / low resistance across the terminals of electro filter caps.

If the HT fuse doesn't blow, power down and re-fit only V1 and V2, the 12AX7's nearest the input, which have a DC filament supply. Power up, warm up and hit the Standby switch. If the fuse holds, power down and fit the other 12AX7 (PI / driver) and one pair of EL34's, fitted to either the inner or outer sockets, power up, warm up and so on, until you have cleared all the valves - or found a faulty one.

Sorry if I'm telling you things you already know, but this is standard troubleshooting procedure for valve amps and could be useful info for any FS members who own a valve / tube amp and know how to use a DMM.

DrNomis · Post by **DrNomis** » 06 Mar 2014, 09:42

blackbunny wrote:Bias supplies do fail, but it should be easy to troubleshoot for gross (obvious) faults). Your Marshall bias circuit is based on the ones used in most 100 watt valve models since the late 90's, with separate bias adjustment trimpots and bias monitoring points for measuring current flow through 1 ohm resistors in series with the cathodes of each pair of EL34 output valves.

Probably due to the terrible reputation early DSL and TSL JCM2000 amps have for bias problems [ believe me I know; I've owned both and had such bad experiences that I will never buy another Marshall made after 1986 ], it looks like Marshall have tried to improve the bias circuit in your model so that if a trimpot fails, the bias voltage goes "high", which is safer for the valves and output transformer than having very low bias voltage or none at all, although it will result in excessive crossover distortion, making it impossible to get a clean sound at any volume level.

Power up with the amp on standby, and after a minute or 2 warm up time measure the voltages on either end of the 4 x 5k6 bias feed resistors, R72, R73, R76 & R77. If you measure between -38 and -42 vDC on all 4 resistors, and there's not more than a volt or so difference between any 2 readings, then the bias supply hasn't failed and you should look elsewhere. If the bias supply voltages are very low, test the electro caps , resistors & diode in the bias circuit.

If the bias supply looks like it's OK, it's time to test the valves and power supply. Take all the valves out or their sockets, fit a new 1 amp HT fuse, power up and hit the standby switch. If the fuse blows, test all the rectifier diodes & look for shorted / low resistance across the terminals of electro filter caps.

If the HT fuse doesn't blow, power down and re-fit only V1 and V2, the 12AX7's nearest the input, which have a DC filament supply. Power up, warm up and hit the Standby switch. If the fuse holds, power down and fit the other 12AX7 (PI / driver) and one pair of EL34's, fitted to either the inner or outer sockets, power up, warm up and so on, until you have cleared all the valves - or found a faulty one.

Sorry if I'm telling you things you already know, but this is standard troubleshooting procedure for valve amps and could be useful info for any FS members who own a valve / tube amp and know how to use a DMM.

Cheers for that mate, much appreciated, I could do with a bit of refreshing of my knowledge of valve guitar amps anyway so it's cool, actually I feel like I'm still learning and have loads more to learn......

I'll give your suggestions a try and see if I can service my amp and get it working again, I might have to bring my dummy load into the music department.....

bajaman · Post by **bajaman** » 06 Mar 2014, 09:46

most likely to be a bad EL34 - try powering the amp up as Blackbunny suggests - fit ONE EL34 and turn the standby switch on - if the fuse doesn't blow, switch to standby and insert another EL34. Keep doing this until the fuse blows and you will have found the bad tube

I have a tube tester which takes a lot of the guesswork away. What make are these EL34 tubes Simon ?

DrNomis · Post by **DrNomis** » 06 Mar 2014, 09:55

bajaman wrote:most likely to be a bad EL34 - try powering the amp up as Blackbunny suggests - fit ONE EL34 and turn the standby switch on - if the fuse doesn't blow, switch to standby and insert another EL34. Keep doing this until the fuse blows and you will have found the bad tube
I have a tube tester which takes a lot of the guesswork away. What make are these EL34 tubes Simon ?

As far as I can tell all the EL34 tubes in my Marshall are marked with the Marshall logo if that's any help, they could very well be a make that's known to have issues with quality Bajaman, I'm going to try blackbunny's suggestions and see how I go......

I'll tell you what, I'll see if I can take a pic of the EL34s in my Marshall amp and post it in this thread tomorrow.....

If it does indeed turn out that I've got some bad output tubes, what brand would you recommend?, I was thinking of replacing all the valves with a set of ElectroHarmonix valves including the preamp/phase-splitter valves......

bajaman · Post by **bajaman** » 06 Mar 2014, 10:20

NO - try the Chinese 12AX7B for the preamp tubes and either the JJ EL34 or the Winged C type for output tubes

DrNomis · Post by **DrNomis** » 06 Mar 2014, 10:35

bajaman wrote:NO - try the Chinese 12AX7B for the preamp tubes and either the JJ EL34 or the Winged C type for output tubes

No worries, thanks for that.....