Marshall 1959SLP Problems - Please Help

[Peter Nerves]

Hi All,

I am new to this forum so hello to you all.

I have a Marshall 1959SLP 35th anniversary re-issue. I have had a minor problem recently with no sound coming out of the amp when I flicked on the standby switch, which was solved by flicking the power switch off and on quickly and it worked fine for a few hours until it would cut out again.

I had a look around a few sites and decided that it was best to start with replacing the valves and see what would happen. I put the new valves in and I could only get the amp to work intermittently, cutting in and out all of the time. When I get a sound out of it I have to be really careful not to whack the strings, as this causes the amp to cut out again. I put the old valves back in and the problem stayed.

I tested the valve seat voltages and some of them seem far from what was indicated on the Marshall 100w voltage chart (although this chart may be the wrong one for the SLP) I was getting 380Vdc at the grid of valve 2 when the chart said 170Vdc.

I have been through all of the resistors in turn and tested their resistance to the values marked on them and came up with some supprising results. The 4 resistors that couple the 4 guitar inputs to the 2 grids of valve 1 were half of their marked values 34k (marked value 68k).

Could using effects infront of the guitar input cause this? I use an ebow and the output of this screams into the amp!

I have been testing the amp with a clean guitar signal straight in, all of the leads and connections are fine. It is definatley an amp problem!

I pray on your expert knowlege to give me some more advice on whether these results would cause the problems above, or if you have any further suggestions I would love to hear them.

Thanks for reading!

Pete.

[Ray Ivers]

Pete,

This could possibly be a case of the valves making intermittent contact with their sockets - which can be one of the easiest problems to fix (and it really makes sense to try the easy things first). There's a product called Caig ProGold:

http://www.tubesandmore.com/new/scri...?item=S-CG5S-6

which has saved my bacon many times. You'd pull all the valves, carefully spray the pins, then insert & reinsert them a few times to spread the ProGold around. Even if this doesn't solve this problem this time, IME ProGold is an excellent item to have around to clean plugs, jacks, AC connectors, etc. It's not cheap, but the best things never are. Caig also makes DeOxit and PreservIt, which are similar (but not identical) products.

If this doesn't fix you up, I'm afraid it may be a bad component and/or connection inside the amp, which may or may not be 'user serviceable'.

Ray

[Peter Nerves]

Thanks Ray

Hi Ray, thanks for your reply.

The Pro Gold product looks really good and I would like to get hold of some. I had problems with the above link and I could not add the product to my basket. I am in the UK and will try and search for an outlet that I can order it from.

I understand what you say about user serviceable parts, but I have prior experience in electronics and it has always been a hobby of mine, and I will not give up and put the amp into a service shop as I really want to track down the problem and repair it myself. I hope I can rely on the help of the members of this board to help guide me through this.

Did the bad resistor values listed above concern you?

Thanks,

Pete.

[Ray Ivers]

Pete,

Hopefully you can find some (or all) of the Caig products on your side of the pond - they really are exceptional.

Did the bad resistor values listed above concern you?

If you disconnected one end of each resistor and then measured it, then yes, each resistor is either incorrectly marked, or identically bad (FWIW, I've never seen the second condition in 28 years of working on amps). If, on the other hand, you measured them in-circuit, then your readings are completely normal, as each 68K pair are normally paralleled (for a reading of 34K). Do you have a schematic for the amp? If not, the one below may (or may not) be close enough:

http://www.schematicheaven.com/marsh..._100w_1959.pdf

The 380V reading concerns me more (you're absolutely positive this was at a grid socket pin, either 2 or 7?), but as this could be caused by a bad socket connection if the reading was actually at a pin 1 or 6, I figured that socket cleaning/treatment was the best course of action, at least to start.

Ray

[Peter Nerves]

Thanks Ray

I tested the resistors in circuit so that explains the half readings. I thought there was no good explanation for all 4 resistors to go faulty!!

Thanks for the schematics, those are the ones I have been working from all day.

The 380Vdc is present at pin 7 on the 2nd 12ax7, I have tested all of the valve bases for correct voltage, and they seem to differ either a lot, or slightly from the 100w marshall voltage chart that I have. All of the 12ax7 pin 2's have no voltage which is correct, and the only 12ax7 which has voltage at pin 7 is the 2nd valve. I am testing the voltages from the valve socket to chassis, and I have not confused the pin numbers. Pin 1 on the 12ax7's is at 240Vdc on all sockets. Pin 3 on the EL34's is measuring 505Vdc on all sockets.

Thanks for your help, does this sound like anything that you have come across?

The fact that I am in the UK (230v outlet) should not cause any difference, as the voltage selector is set to 230v, so I expect that the amp's internal voltages would be the same as in the US?

Thanks for your help!

Pete.

[Ray Ivers]

Pete,

Try checking R9 for an open-circuit condition, and/or trying another new tube in the V2 socket. It seems like V2a is not conducting any current, causing its plate voltage (and V2b's grid voltage, as they are wired together) to skyrocket.

Ray

[Peter Nerves]

Thanks Ray,

I must add that the voltage readings that I took were without any of the valves in place. Sorry if this has caused a red herring. Should I have tested with the valves in situ?

Thanks,

Pete.

[Ray Ivers]

Pete,

I must add that the voltage readings that I took were without any of the valves in place. Sorry if this has caused a red herring. Should I have tested with the valves in situ?.

Yes - valve-out readings on preamp tubes, while not exactly meaningless, aren't really all that helpful when troubleshooting (at least IME).

Ray

[Peter Nerves]

Thanks again Ray, I will test the voltages again with the valves in place. I will search for some progold at the same time. I will post again soon with an update. Its a good job I have a 1972 Hiwatt Custom 100 to fill in for the 1959! I will try all I can, but I may have to put it in to a repair shop, only on the condition that I can watch & learn though!

Thanks again for your help,

Pete.

[tim]

peter, do a small experiment and take a short shielded guitar cord and jump your fx loop with it (plug one end into the send and the other into the return) and see if problem goes away...if it does, then you have either dirty contacts (usually on the return jack) or cracked solder joints under the jacks if they are soldered to a circuit board. I can't remember if they are on the reissue.

[Enzo]

Operating tubes have among their attributes a virtual resistance. The current flows through them, and voltage is dropped across them. They are part if the curcuit. With no tubes in place, most voltages in the amp will either be very high, or absent altogether. I have never seen a schematic or test point chart that referred to a no-tubes condition.

More than a red herring, that is a whole fishing boat full of mis-direction. A RED fishing boat.

Being a Marshall, I would ask if the first few preamp tube heaters were glowing when the amp is silent? If not, there's the problem. If this is the case, ther is a square bridge rectifier for the preamp heater current. It is in the middle of the voard somewhere. Try removing it, scraping the legs clean and resoldering it to the board. Or just replace it.

Otherwise, next time it happens, don't flick the switch to restore it, the symptom is troubleshooting gold. IN THE FAILED CONDITION, check for missing or very wrong voltages at the tubes.

[Peter Nerves]

Hi Tim, thanks for your advice, but my model has no FX loop on it, they came a couple of years later. Mine is the 1959WSP (white super plexi??) made in 1997.

Thanks,

Pete.

[tim]

OK then...carry on. I was looking at the schematic I had and it showed one but it must've been a later model add on as you said.

[Peter Nerves]

Hi Enzo, thanks for your reply. I now know not to test without the valves present! The flicking of the power switch was only something I needed to do a few times in the last couple of weeks, before then the amp was fine. I changed the valves and now it just cuts in and out (mostly out) all of the time.

Any further sugesstions?

Thanks,

Pete.

[Peter Nerves]

Hi Tim, Thanks for looking for options for me, I really appreciate it!!

Thanks,

Pete.