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Marshall 1987S Hiss/Hum Problems

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Tmay	Marshall 1987S Hiss/Hum Problems	Lead	[-]
Posts: 1 (09/03/09 03:02) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Tags : None Hello all, I've finally decided to have a go at fixing my old Marshall which has been plagued with problems from the day I bought it, but sadly my knowledge and experience with amps is woefully inadequate so any help would be greatly appreciated. At the moment the problem is hiss/hum and intermittent volume swells (which may be a loss of high frequencies which give the illusion of a loss of volume) The hiss/hum is only present on the high channel and is not affected by turning up the volume, it is however affected by adjusting the treble, middle and presence controls. If I play through the low channel the sound will intermittently drop in volume/lose high frequencies. What I know is defiantly wrong with the amp is the pot values, the 1M controls are right but all others a 470k which isn't. I've checked the preamp tubes by tapping them with a pencil and one of them is defiantly passed its best, but when I tried to test them by pulling them out one by one to see if the hiss would stop the sound cut out completely. Is this what is normally supposed to happen? From I've read I should be able to pull them out and the amp should still work, but this is defiantly not the case, there is no sound at all. Heres a link to the schematic http://www.thinlizzy.de/Bilder/1987Schematic.gif Any ideas on what might be wrong would be a big help, Cheers.
StoSoStu		#1	[-]
Posts: 608 (09/03/09 05:05) South of the Border..way South! Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Fixing valve amp problems is often hard when it's on the bench in front of you, it is even harder by email, BUT, that said I'll make a start. Firstly, all the preamp valves (12AX7's) are in the signal path so if you pull any one of them out you won't get any sound from the inputs, you may get some internally generated noise, like your hum and hiss. I'm not sure where you think there is a 470k pot (a most unusual pot value) in your amp, it has 2 x 1M volume pots, a 250k, 1M and a 25k in the tone section and a 27k bias adjust pot. Anyway, finding the source of the hum and hiss. FIRST OF ALL, YOUR AMP HAS LETHAL VOLTAGES INSIDE, EVEN WHEN IT IS UNPLUGGED. USE EXTREME CAUTION AND WORK WITH ONE HAND IN YOUR POCKET. Is the hum and hiss there when you plug in your guitar but not when it is unplugged? If so, you may have a problem in your guitar or a bad guitar lead. If the noise is there even without anything plugged into the inputs, then: Set up the amp with no inputs and adjust so that you can hear the hum and hiss. Then turn off the amp and unplug it from the mains, then remove V1. Turn on the amp. Is the hum and hiss still there? If not then the source is before V1. If the hum and hiss is still there, turn off the amp, unplug it and replace V1 and remove V2. Is the hum and hiss still there? If not then the source is before V2 but after V1. If it is still there, turn off the amp, unplug it and replace V2 and remove V3. Is the hum and hiss still there? If not then the source is before V3 but after V2. If the hum and hiss is still there, turn off the amp, unplug it and replace V3. If the noise is still there with V3 unplugged then it is coming from the power amp section, the two EL34s, or from the power supply (unlikely with a solid state power supply unless you have bad capacitors, in which case I would suggest that is a job for a trained valve amp technician). We should now have isolated the source of the hum and hiss. Try replacing the particular valve that caused the hum and hiss to stop with a known good one. If that doesn't fix the problem, then with the amp off and unplugged and after discharging the power supply properly, check the various components associated with that section of the amp. Once you have gathered all this information, get back to me with the results, assuming that you haven't fixed the problem. If you have, then all is sunshine and light, pick on. Bob The road goes on forever and the party never ends
Tmay		#2	[-]
Posts: 2 (09/03/09 05:44) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Thank you for the speedy response, its muchly appreciated Ok, well i can safely say the guitar is fine, i dont have any problems with it with any of my other amps. The amp does have 470K pots in it, i know it shouldnt; i bought it second hand so im guessing someone put them in at some point which is beyond me, but they are there. The results of your tube test showed somewhere before V2 and after V1 so i tried replacing the preamp tubes and something slighty odd happened. I have a fully working boogie preamp with 5 ECC83 in it, so i pulled three and put them in marshall to see if that sorted the problem, what it actually did was nothing, the amp was silent. Took them out again and put them back in the boogie and its still working fine. Did i do something wrong here? They lit up but no sound. I then put the faulty ECC83's back in the marshall and it came back to hissy life, but the hiss has now changed, it is now increasing with the volume. I dont really know what im talking about but am i right in assuming that the faulty tube must now be in V1? So any ideas why the other tubes wouldent work? they're definatly the same model tube and working fine in the boogie. Last Edited By: Tmay 09/03/09 06:02. Edited 1 times.
StoSoStu		#3	[-]
Posts: 610 (09/03/09 19:14) South of the Border..way South! Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	That is unusual. Firstly, where are the 470k pots? It is possible that you have a valve in the MB that has one triode not working, they are not all used in the MB, but all are necessary in the Marshall. It would appear that at least one of the 12AX7/ECC83s is faulty, if it were me I'd invest in a new set and see what happens in the Marshall then. Bob The road goes on forever and the party never ends
Tmay		#4	[-]
Posts: 3 (09/03/09 23:03) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	The Treble, Middle and Presence knobs all have 470K, i cant see this contributing to the noise but i think it does explain why the bass control does very little. You are spot on, i checked the boogie schematic and it does only use one triode, so i tried putting them back in the marshall and found a combination that worked (two seemed to have a broken triode). The hiss is still exsactly the same, so although i think the marshall tubes need replacing i dont think they are the primary source of the noise. Im going to replace the plate resistors and see if that sorts it out, if not, im going to take it to a technician. Thank you for your help.
StoSoStu		#5	[-]
Posts: 611 (09/04/09 05:01) South of the Border..way South! Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	No problem, my pleasure. You should definitely change the tone pots back to the original values, that could be the source of the problem, the values are on the schematic. The plate resistors more often cause crackle rather than hum or hiss. I would also check the 68uF capacitor on the cathode of V2 and the two cathode resistors, 820R and 100k. Also give the valve base a good clean with contact cleaner (make sure the power is off and the caps are discharged). Bob The road goes on forever and the party never ends

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