Marshall JCM900 100W Dual-Reverb Full Restoration Questions

[GunalORSEL]

Dear Friends,

I am an owner of a Marshall JCM900 100W Dual Reverb Combo (Model #: 4102) which I bought new in 1992. Now, after 16 years of playing, the amp started creating problems, and I do not only want to fix occuring-problems and wait for new ones to arise, I want to treat it the way it deserves, and plan to fully restorate it, with best available replacement parts. Even as I do not play that hard as past, and playing more jazz and planning to have a Fender amp in near future, I want to definately keep this piece of equipment which witnessed my entire teenage years and all my girlfirends

My only problem is that I live in Turkey, and there's no repair shop nor a skilled professional to do the job: therefore I will be performing the task by myself, hopefully not alone, but with your assistance.

Just for your reference, my setup consists of:
Fender Stratocaster w/ Lindy Fralin Blues Specials
Gibson Les Paul Standard with factory pickups
Ibanez RG-570 with Seymour Duncan Live Wire Active Pickups connected to, in exact order:
Fulltone Clyde Deluxe Wah > Analogman TS-808 Silver Mod plugged in the amp, and some delays / choruses / flangers on the effects loop getting inbetween and out depending on the situation.

My amp's output is connected to a THD HotPlate, and then it's connected to the speakers. The amp had several tube replacements, all from Watford Valves in UK, with matched sets, but no BIAS adjustments have been done, as there's no one to do it.

My biggest issue with my amp is that it has started to switch to high gain channel by himself randomly while playing. The duration is also random, sometimes for a second, sometimes for a minute. Then it needs most of its pots to be replaced.

Once opened the box, and introduce the PCB once again with sunlight, I do plan to replace all resistors / caps / pots and tubes with new ones, as I do not have the skills to troubleshoot which component(s) do cause the problem, and finally planning to have my amp re-born.

I have already downloaded the schematics of the pre-amp and power amp sections, and prepared my shopping list regarding the specs on the schematics.

The first questions are:
- Which method to use to make BIAS adjustment for this specific amp? And, if possible, a link to a trustable article explaining the entire process?
- Is there any modifications for this specific amp to get better sound, once the soldering iron is hot and the PCB is out?
- Should I replace caps with Hovland Musicaps instead, for better sound?
- Any past experience of yours, if possible, to share with me so I do not discover the wheel once again?

Thank you very much, in advance, for your time and interest on the issue.

Best Regards

Gunal ORSEL

[GunalORSEL]

EL34s...

And a nice surprise, it has EL34s on board, not 5881s as expected.

[hamfist]

Firstly, unless you are very skilled at soldering onto PCB, I would avoid doing any component changes, expecially to the radical extent you are suggesting. You will probably end up just ruining your board, as they are pretty delicate things.
If the the amp is generally working fine, then just leave it alone is my advice. Resistors and non-electrolytic capacitors definately do NOT need to be replaced unless they are faulty. At 16 years old, some might argue that it might be reasonable to replace the electrolytic caps, as they do only have a limited lifespan.

What is seems you do need to learn how to do is to bias your power tubes.
I thoroughly recommend that you buy a bias probe of some sort, which will make the whole proces much more painless.
you will also need a digital multimeter. Now, you need to do a fair bit of searching and reading to find out how to do it safely, as there are lethal voltages around.

You'll need to be able to safely read the plate voltage (from pin 3 of any power tube, to the chassis).
You then need to know the resting plate current for each tube, and adjust the bias trimmer pot within the amp so that the tube with the highest current reading is within a reasonable range of power dissipation for an EL34 (reasonable is approx 60-70% of 25W for an EL34).

Here are lots of good links ;-

http://www.aikenamps.com/Biasing.html

http://music-electronics-forum.com/s...ead.php?t=1488

http://www.marshallampforum.com/foru...b16b621429e00a

[GunalORSEL]

Dear Hamfist,

Thank you very much for your time, I really do appreciate any help on the project.

Last night, I have removed the amplifier off the combo cabinet. The number of resistors and caps, considering that not all of them are electrolytic, convinced me not replace them all. As you have also advised, I will be replacing only the electrolytic caps, all potentiometers and all the tubes, then I will be biasing the amp.

On soldering and working with electronics side, I'm totally confident, as I do regularly do it. The thing is I have never worked on these types of analog circuits, and amplifiers where we have some serious voltage levels.

Discovering the tube amp world once again, I will be using this thread as a blog and project archive, where will I be explaining each step I take, its results, photographs and all documentation, so other rookies like me today can benefit.

I hereby invite you all to get involved, and drive me whenever you need its necessary.

Best Regards

[GunalORSEL]

Capacitor Types and their Values

Dear Friends,

Could you please be so kind as to inform me about the differences between:
Stacked Metal Film, Silver Mica, Metal Polyester Film / Foil and Polypropylene capacitors?

Electrically, they all are caps, but sonically, what should be my order of preference to be used on the project? Could I use polypropylene caps instead of electrolytics (where available) to postpone re-cap job in the future?

If the exact capacitance/voltage value is not available for a specific cap, can I substitute it with a higher voltage cap? What would be its effects technically and sonically?

Marshall schematics underline some caps (specifically the C14 and C15) to be "MKT Radial" but I was not able to find any on suppliers in Europe?

Thank you very much, Best Regards

[hamfist]

Dear Friends,

Could you please be so kind as to inform me about the differences between:
Stacked Metal Film, Silver Mica, Metal Polyester Film / Foil and Polypropylene capacitors?

Some cap construction materials are more suited to certain value caps of an appropriate size. Hence you will tend to see Silver mica caps in values of about 680pF and lower.
Different material caps in some circuit positions can give tonal differences. Personally, I believe this to be fairly minimal between some types. The value of the cap is way more importnt than the construction materials.
For Fender-type amps, people often prefer polypropylene caps, like the Sprague Orange Drops. For Marshall-type amps, most prefer polyester film caps , like Mallory 150's

Electrically, they all are caps, but sonically, what should be my order of preference to be used on the project? Could I use polypropylene caps instead of electrolytics (where available) to postpone re-cap job in the future?

Personally, I'd go for Mallory 150's for a Master volume Marshall circuit. If you want to spend more, go for Sozo's.
I think you'll find that electrolytics are used in various positions because non-electrolytic caps of the same value would be prohibitively large. Even a 1uF Polyester film cap is getting pretty big to squeeze into a guitar amp.

If the exact capacitance/voltage value is not available for a specific cap, can I substitute it with a higher voltage cap? What would be its effects technically and sonically?

yes, just use a higher voltage cap. It will have negligible effect sonically, but will just be larger.

[GunalORSEL]

MARSHALL JCM900 Potentiometres

• VR1 POTS220KB-B220K 220 K Linear Preamp (Ch.A) Vol. 16 mm Split Shaft PCB Mount
• VR2 POTS1MEGA A1M 1 M Log/Audio Gain (Ch.B) Enhance 16 mm Diameter Split Shaft PCB Mount
• VR3 POTS220KB B220K 220 K Linear Treble 16 mm Diameter Split Shaft PCB Mount
• VR4 POTS22KB B22K 22 K Linear Mid 16 mm Diameter Split Shaft PCB Mount
• VR5 POTS1MEGA A1M 1 M Log/Audio Bass 16 mm Diameter Split Shaft PCB Mount
• VR6 POTS10KB B10K 10 K Linear Presence 16 mm Diameter Split Shaft PCB Mount
• VR7 POTS220KB B220K 220 K Linear Reverb (Channel A) 16 mm Diameter Split Shaft PCB Mount
• VR8 POTS1MEGA A1M 1 M Log/Audio Master Volume (Channel A) 16 mm Diameter Split Shaft PCB Mount
• VR9 POTS220KB B220K 220 K Linear Reverb (Channel B) 16 mm Diameter Split Shaft PCB Mount
• VR10 POTS1MEGA A1M 1 M Log/Audio Master Volume (Channel B) 16 mm Diameter Split Shaft PCB Mount

[Dave Curtis, dB AudioTech]

Why do you want to change all the pots? Just change the ones that don't respond to a good, proper cleaning (Deoxit, rotate several times, then Fader Lube).

[GunalORSEL]

Resistors

Would I go with Carbon Film Resistors or more expensive Carbon Component Resistors? Would the price difference benefit my sound?

[GunalORSEL]

Dave, the pots, they all have been already dead, a local shop have already opened them individually and tried to recover them by cleaning, changin some internal components already, I think it was 3 or 4 years ago. That time, I was unable to find the correct pots over the web. By changing them all, I just want to make sure that I will not be re-opening the box in the near future once again.

[Enzo]

Caps - and other things - that have their leads come out the ends are called axial leaded components. The wire leads come out the component's axes. Your common resistors are axial leaded parts. The filter caps in old Fenders are axial leads as well. SOme parts have their leads all coming out the same end or the same side. For example the filter caps on a circuit board that are mounted standing up. Both leads come out one end. Those parts are said to have radial leads.

The ONLY difference between radial and axial packaged parts is the convenience of mounting. Otherwise electrically it doesn't matter which end the wires come out.

So when Marshall built the amp, for whatever reason they chose various caps. SOme have axial leads, in fact some smaller ones look like little pale green resistors. But you also often see some caps in Marshalls that are little rectangluar things standing on one edge. The leads come out near each end, but both on one side. Thus you can mount the cap down tight against the pc board with its wires through the holes. This is then considered a radial part.

I don't think Marshall specified the radial part on the schematic for sonic reasons, I think they were specifying it for identification. I think it suited their manufacturing needs to spec the particular caps at their particular values and ratings in that package form.

Here is a picture of one:


It is a Vishay part number MKT1822368254 and I happened to select .068uf as an example. It is a metqalized polyester film type cap. Note the "MKT" in the part number. Check your suppliers for Vishay film caps in radial packages.

[GunalORSEL]

PCB Photograph

Dear Enzo,

Thank you very much for the explanation.

Here, I am attaching a photograph of the inside PCB of the Marshall JCM900 100W Dual Reverb Combo amp (with EL34s) for everybody's reference. It seems there are non available on the net, so here it is...

I am now measuring all the resistors' resistance values in order to make sure they all are OK. I have already found a dozen which are far out of the tolerance, so they will be replaced.

On the weekend, I will also be measuring the capacitance of all the caps (electrolytics will be replaced no matter their measurement values), and will also replace out-of-range pieces.

Another question is, for the resistors, let's say their tolerence is -+ 5%, and after the measurement some are on the 5% limit, but some are tight and they have exact values indicated. Considering that these resistors are cheap, I have already ordered them with 10 extras in each value, so I can measure their resistance and have the option the mount the closest ones to the required values. Replacing the resistors which are near their tolerence values with the ones at exact values would make difference in sound and worth the job?

[hamfist]

I am now measuring all the resistors' resistance values in order to make sure they all are OK. I have already found a dozen which are far out of the tolerance, so they will be replaced.

Silly question, but are you measuring these still soldered into the circuit ?

You need to actually desolder one end, and isolate each resistor to get any sort of accurate reading, unless you know the circuit well enough to know that the circuit is isolating it for you.

[GunalORSEL]

... would have figured it out it wouldn't be that easy: I was measuring them all on-board

[Enzo]

And really, this is not something we would do in the service business. You might for a 50 year old Fender where we EXPECT the parts to have drifted, but not a recent era MArshall. The very act of removing them from the circuit board and reinstalling them, just to measure them, is a stress to the parts, and is far more likely to CAUSE a problem than to prevent one. Plus the possibility you will damage or crack a copper trace on the circuit board.

If you are THAT concerned a resistor might fail, then buy a selection of them to cover what is used in the amp and keep them in your shop or toolbox.