Got a 1986 Marshall JCM 800 to work on

[Platefire]

This amp was brought to me by someone else for the owner with a note that "Has Static & Hiss Sound when Powered on". When I powered it up I noticed it had a popping static that seems to deminish to some degree after the amp warms up. Also when you turn the master up to get some loud volume the opererating noise is pretty loud. It's pretty quite with pre and M volumes down.

I've been replacing the two pre-amps and PI tube with known good tubes but without much effect. Trying simple practical things first. Just thinking if filter caps haven't been changed since 1986, that may be part of the problem.

So maybe some of you have had experiance with this amp??? Thanks Platefire

[LooseChange]

Don't mess with the filter caps yet. figure out what gain stage it's coming from and check the plates and cathodes for noise.

[Platefire]

On the amp it reads it's a Model 4104. Haven't been able to turn up a schematic even on Doug's list yet.

Did a little jamming on the amp to get a feel for it. The thing sounds awful tinny/Trebly to me. To get a good sound I had to turn the presence, treble, middle almost completely off and turn the bass way up. This cut a lot of the hisss---when you turn the treb/mid/presence up--the noise really goes up and the amp sounds really thin. Don't know if that is normal for this amp but don't seem like it should be normal?

The knobs on this amp are really cheap. I think all of them are slipping on their shafts. That bugs the daylights out of me! No way to tighten them up, they would have to be replaced.

Once I dialed the tone in with those extrem tone settings, I really did like the way the amp sounded. This is my first time to jam on a tube Marshall and man I really do like the bottom thump--definitely not a fender--a different animal! I nice difference I think I could get use to.

I sure hope I can bring this thing around! Platefire 

[gldtp99]

Most Marshall JCM 800 (and other) schematics here: http://drtube.com/marshall.htm#JCM800

I have a similar JCM 800 1x12 combo, model 4010, that has push on knobs----- the pots and front panel jacks are board mounted in these JCM 800's with horizontal input jacks.
Check for poor factory soldering everywhere--- but especially on the tube sockets------noise could easily be coming from bad V1 plate resistors, bad/lifted PCB traces from previous repairs, and Bad Ground Connection. Poor soldering or dirty tube sockets could also be noisy. Dirty or bad pot could be the problem.
My 4010 came poorly modded (barely working) so i messed around and un-did their "mods" and put in the AFD #34 Mod with a PPIMV in the DI jack hole on the back panel.
Pics of my 4010: http://s46.photobucket.com/albums/f112/gldtp99/Marshall%204010%20AFD%20Mod/

PS--- be very careful soldering on these PCB's----- these boards are on the delicate side---i had to repair lifted traces from the previous mods---- they installed all Sozo caps (poorly and with mods that didn't make sense), some are still there---i'm considering going back into this one and re-doing the whole thing with a turret board and chassis mounted pots/jacks--- the #34 mod sounds very good but it adds some hiss from the added bypass caps--- i've found a 4 gain stage design (with plate driven tonestack) that i like much better than the AFD #34 Mod, anyway---better distorted tone, much less backround noise, less tendency to feedback--- no real clean tone available from either mod, however--- nothing wrong with a stock JCM 800 preamp built on a turret board, if that's what you're looking for---i'm not a big fan of the factory build quality in these amps---not like older Marshalls, even the older PCB models.....................gldtp99

Sound clip of my 4010 in it's current state----- it's one of the rhythm amps used in this clip---- all were different "AFD" modded amps---- this one can cover the tones heard here : http://soundclick.com/share.cfm?id=9924119

[TubeGeek]

Check the bias.

[Platefire]

I downloaded Doug's JCM 800 50 Watt MV Model 2204 schematic that looks like it should be close. Unlike Old Fender schematics, there is no voltages listed throughout the power supply to the tubes---how do you arrive at correct voltages?

Thanks Gldtp99 for the tips on the amp. I'm supposing this amp is un-moded but won't know for sure until I crack it open and do some checking. Don't really want to change anything from stock, just to quieten it down as the customer desires. Maybe you can tell me??--When the pre and master volume is cranked up to a loud 50% on each dial---what kind of operating noise should I be hearing when this amp is in top condition?

On bias---on my schematic the EL34 cathodes goes directly to ground and I see it has a 22K Lin bias pot----how would I check this with my MM without the 1 Ohm resistors to ground? Thanks, Platefire

[Ed_Chambley]

Did a little jamming on the amp to get a feel for it. The thing sounds awful tinny/Trebly to me. To get a good sound I had to turn the presence, treble, middle almost completely off and turn the bass way up. This cut a lot of the hisss---when you turn the treb/mid/presence up--the noise really goes up and the amp sounds really thin. Don't know if that is normal for this amp but don't seem like it should be normal?

Once I dialed the tone in with those extrem tone settings, I really did like the way the amp sounded. This is my first time to jam on a tube Marshall and man I really do like the bottom thump--definitely not a fender--a different animal! I nice difference I think I could get use to.

I am a little confused, are you repairing it for a customer or are you repairing it for you.  If it is for you, pull the PCB and build a board.  A friend of mine bought a Fargen Ole 800.  This is the best modern sounding Marshall amp I have ever played, bar none.  I opened his up, made a materials list and built one for myself.  Fargen uses the most expensive parts you can get.  SoZo, Mercury Magnetic PT and OT.  I used a Hybour PT, but did spring for the SoZo vintage mustards.  The caps took me 6 weeks to get.  Dealing direct with SoZo is a nightmare. A/B the amps and they are exactly the same.  It is basically a 2204 with some tweaks.  With the Master and Gain at noon, it is punchy as hell with just a little hair, not a highly distorted amp at all.  I use it for country chickin pickin and it is great.  The preamp section is useable at any setting.  Never Boomy and never Thin, but it can get LOUD.

If you are repairing it (I have repaired many PCB Marshalls) so through circuit and find where the person who did "the mods" screwed up.  Chopstick it and look for caps which are not .022 or .0022 and .68.  Most of the time I find bad solder joints at the phase inverter where the new Master Mod has been installed.  This is usually the source of the noise and the loss of the preamp control is swapping caps to .1 or so in V2 or in the cathode bypass.  The stock PCB should sound good all by itself, but it does not have the distortion many Marchall guys are wanting, so instead of using a pedal they attempt to add gain in the amp and usually attempt it in one place only.

[Platefire]

Hi Ed

I'm looking at this for a friend of my Son who sent it by way of my Son who came by here Thanksgiving day. The guy bought it from Guitar Center and is still under 30 day warrenty. I talked to the guy on the phone while ago. He will be calling me back latter today after he decides to return it to Guitar Center or let me work on it. I told him I would have to break out the chassis to proceed and that might void any warrenty??? Don't know what their policy is. Lot of possibilities to be checked out that could be causing the noise. Just judging from what I heard jamming on it, sounds like a combination of bad grounding and amp hisss. So I let him know I haven't had any experiance with Marshalls so I told him if he decided to let me work on it I would charge him a maximum of 4 hours labor + parts. I would be willing to eat some labor time to get a little Marshall experiance. Platefire

[rafe]

Bias, caps......most likely the issues....the ones with vertical inputs are the ones you want though they are much better amps. Depending upon what he paid for it, if it had issues I'd return it to GC and look for one with verticle inputs......
4104 is the combo 2204 is the head ....same amp

[sluckey]

The guy bought it from Guitar Center and is still under 30 day warrenty.

Quit wasting time. Take it back!

[gldtp99]

After reading about the amp situation, i'd also vote for "take it back to GC"---- the fix might be simple (dirty pot/tube socket, bad tube, bias off, etc) but it might be poor solder joint(s) anywhere.
When working 100% these amps don't have excess backround noise--- but getting it back to 100% might or might not be an involved process.
Stock JCM 800's, some feel, are too bright unless cranked up to very loud vol levels---- single coil pickups can add to the brightness.
Since taking it back to GC is an option, it might and up being more trouble than it's worth to open this one up--- it is fixable but it could be an expensive, involved job. (or not---hard to say for sure)
After reading Ed's post, i'm more motivated to re-do my 4010 with a turret board.............gldtp99

[Platefire]

Well they elected to leave it with me. So it begins. The amp works pretty good as is but you put the pre and the MV both at 12:00, you got a lot of hisss. The amp is way bright but when I adjust the treb=9:00, Bass=4:00, Mid=9:00, Presence=8:00 it is close to my sound I get on my other amps but that seems kind of extreme settings IMHO. I talked to amp tech friend right down the road from me that has owned a JCM 800 before and he said the way the tone stack is acting don't sound normal. Even with the extreme tone settings the hisss remains at the high volume settings. Increasing the Treb/mid/Presence increases the hisss dramaticly. Platefire

BTW-On the loose knobs I pulled them off and put a small piece of foil on the end of each shaft and put the knobs on over that and it tightened them up nicely.

[rafe]

  Do yourself and your buddy a favor ....rip out the board. install 6 new pots and 2 new jacks and this board.....New caps and tubes.He will then have something to make you all proud ....the rest of the stuff
should all be pretty decent trannies ,speakers,cabinet . I think other wise you may be opening a very nasty   can of worms ...maybe not ....just MHO

http://www.dreamtone.org/pdf/NEW2204LayoutSchemBomDreamTone.pdf   (link would help)

[gldtp99]

To keep the push-on knobs from falling off, just insert a small screwdriver between the two halves of the knurled pot shaft and gently open the gap enough to securely hold the knobs---get rid of the foil.
One used Marshall (a '76 model 1959--100 watt Superlead) bought from GC by a local guy lasted one show before one of the old 6550's blew. This one needed new output tubes (i converted the amp to EL34's because the guy had spent almost everything he had on the amp purchase from GC---- EL34's are much cheaper than 6550's)----- I also cleaned the tube sockets, jacks, and pots------ this was all that this one needed: new matched quad of EL34's, replace a few bias resistors, set bias, clean sockets, jacks, pots--- and a new HT fuse.
The amp sounded terriffic and he brought it back after the next show complaining that it was now "too loud"- --- it was just working as it was supposed to---i explained to him that a 100 watt non-Master Vol Marshall is a very loud amp and by the time the amp is distorting (he liked "natural amp distortion" not any pedals) it will be extremely loud---- then he tried it with my Keeley-modded TS9 at lower volumes---he ended up getting a Bad Monkey OD pedal and using the amp for several yrs of shows/rehearsal without another amp problem.
Try all the usual "easy" general amp maintanance before ripping too deep into that amp----- make sure all tubes are strong and not noisemakers--- output tubes are biased properly----- pots, jacks, tube sockets are clean---- nothing is burnt up inside.
This schematic has voltages--- it's a JCM 800 2204 but it is similar to your amp: http://mhuss.com/MyJCM/JCM800_2204.gif

[pzung]

hello,

- i have a mid 80's 800 head that get's all kinds of noises and problems simply because the nuts for the pots get loose. make sure all the nuts are really, really tight. basically several signal wires become unshielded and noisy and the tone stack won't work because it's ground is lifted.

- if the noise is controllable with the gain knob and you can't get it to go away by any other means, try attaching the input wire's shield to the first tube's plate with a 10pf cap, after disconnecting it from wherever it was grounded so the shield is only attached to the plate through the cap. i find this works well for any amp where you're going for high gain and are putting extra gain pedals in front of it.

[Ritchie200]

The amp sounded terriffic and he brought it back after the next show complaining that it was now "too loud"- --- it was just working as it was supposed to---i explained to him that a 100 watt non-Master Vol Marshall is a very loud amp and by the time the amp is distorting (he liked "natural amp distortion" not any pedals) it will be extremely loud---- then he tried it with my Keeley-modded TS9 at lower volumes---he ended up getting a Bad Monkey OD pedal and using the amp for several yrs of shows/rehearsal without another amp problem.

Yeah, everybody wants to say they got a 100watt Super Lead Marshall, but they have no clue as to how to deal with the sonic assault!  Tell him to go back to his Line6 pod and use the Marshall as a backdrop - just like "Eugene" Malmsteen!!!

Plate, what gldtp99 said, simple stuff first - tension/clean sockets, tubes, etc...

Jim

[Platefire]

Built a 2"x4" "U" rack to mount the pulled chassis on top of cab. Pulled chassis and started checking. Had to rig up a speaker wire extention to make it reach with the rack. Looks all stock to me. I see what you mean about the board---Man!  Could you even buy those little caps and resistors if you could get them out of there???? It looks like you would have to pull the board to change any components. Don't see nothing burnt or out of order after one close going over. Seperated some wires, cleaned pots, have already cleaned tube sockets. I used my chop stick to probe arount to try to turn up something but found nothing. The only thing that I noticed was a reduction in the noise when I put my MM hot probe on one of the EL34 plate socket leads to get a voltage reading, the noise dropped everytime I would touch the socket plate lead with the MM probe. It didn't have that same effect on the other EL34 plate lead??? Makes me wounder about that tube! I don't have any reference on what the Marshall voltage levels should be, so here they are:


V1b-392V before 100k plate resistor & 286V after(high input)
V1b-392V before 100k plate resistor & 349V after(low input) This seems high??? maybe bad resistor?
V2a-405V before 100k Plate resistor & 220V after and 220V to V2b grid input
V2b-405V No Plate resistor, 405V to plate
V3a-405V before 82K resistor & 280V after
V3b-450V before 100K Plate resistor & 265V after
V4-Plate=490V & Screen=487V
V5-Plate=490V & Screen=488V

Bias Voltage -50 before & -48 after 220K resistors

Bias in mA-Using a Rudy BiasMater got a reading of 16.5mA and 12.1 mA on power tubes(cold! should be 25 to 42 according to BiasMaster Spec sheet) I adjusted it up to 35mA/22.4mA kinda un-balanced!
No change in the noise.

Thanks for your help!! please don't quit on me now Platefire

[firemedic]

V1b-392V before 100k plate resistor & 349V after(low input) This seems high??? maybe bad resistor?
V2a-405V before 100k Plate resistor & 220V after and 220V to V2b grid input

Bias Voltage -50 before & -48 after 220K resistors

Bias in mA-Using a Rudy BiasMater got a reading of 16.5mA and 12.1 mA on power tubes(cold! should be 25 to 42 according to BiasMaster Spec sheet) I adjusted it up to 35mA/22.4mA kinda un-balanced!
No change in the noise.

I bet the V1b cathode resistor has opened up to a higher value, or plate resistor has a lower value. What is the cathode voltage on that triode? Should be just one or two volts.

On V2b, 220v is pretty high grid voltage, even for a 12AX7's max @180v, though it probably was designed that way. IIRC. That tube may be worn out. Have you tried any new preamp tubes yet?

 

[sluckey]

Quit wasting time. Take it back!

Sorry for that reply. I read the message too hastily and only just now did I realize this was an old amp, not a current production amp.

[Platefire]

Well sluckey, even though it's a 1986 amp, I thinking of calling the customer and telling him I can't do anymore with it. Just looking at that thin PBC board with those small special components, if there is a problem on the board---I don't have any confidence I could do anything with it. Most of the capacitors on the board I don't even see any leads--just a tiny square looking midi cap sitting right on the board---guess the leads are underneath the cap and would have to be desoldered underneath board. If you were to get it out---where would you find those special little components at?

The noise sounds like a terrible grounding hum that gets louder as you turn the amp up. All grounding connections I checked have continuity with chassis and appear to be good. I have tried a varity of preamp tubes with no change. I don't have any of my own EL34's to try.

The LCR filter caps look original and has a number at the bottem, 86 05 under the UK---is that a date?. I could change the filter caps and tubes but after that expense, if the hum still persisted, I would be handicapped on doing anything on the board--looks very fragile, delicate and with mini components. I would hate to charge the customer for new caps, tubes and if that didn't help, hand the amp back to him with the same problem. So not being able to manage that board in my mind is a real show-stopper! You guys tried to tell me that but I guess I had to figure that out myself 

Thanks, Platefire
 

[woolly]

If you're going  to do amp repair I suggest you build a "Doug's listening amp"

[gldtp99]

Preamp voltages are very high, dangerously high to the 12ax7's----- were these voltages measured with output tubes installed ? If that's what the voltages are, with EL34's installed, then there is a problem in the power supply--- check dropping resistors.
If you're not comfortable working on this particular PCB amp then don't do it---- yes, the whole board has to be pulled to do anything at all----- remove knobs from pots and nuts from pots and input jacks and gently push PCB (with pots/jacks connected--be careful) back enough to flip it up---- the PCB traces and the "through hole" component mountings should be easy to see at this point.
The EL34's aren't matched very well and that can contribute to hum but the too high preamp voltages would seem to be the real concern here (if the measurements you gave were with all tubes installed).
The date codes on the filter cans mean 5th month of 1986----- orig cans---- wouldn't hurt to replace them but they could have nothing to do with the noise problem. If they're not bulging they are probably still working well enough---i replaced the Filter Caps on my 4010 with new F&T's---- when i measured the old ones after removal they tested very close to spec (with my Fluke DMM--- not the most exact way to measure these parts)--- but there was little change in amp behavior.
If this were on my bench i'd try a matched pair of EL34's, find out (and fix) why preamp voltages are way too high---- i'd use a magnifying glass and good light to check for poor solder joints everywhere on the PCB, and re-flow/touch up any suspect ones-----i'd take a good look at all the dropping resistors in the power rail, they may not measure correctly in-circuit---also that 100k plate resistor on V1b could be bad and causing a lot of noise.
If you don't want to work on this PCB amp i understand---- it's different than vintage turret/eyelet construction---- if you don't feel comfortable with it, then let it go back to the owner----- remember: #1- Do No Harm

[Platefire]

gldtp99

Thanks for going over my readings. To answer some of your questions:

1-Voltage Readings were taken with all tubes in place.

2-Filter caps cans look good in appearance like new

3-In regards high voltage I have measured the two 10K power rail reistors and 100K plate resistors in place and find they are all close to their rated tolorance. In fact I have measured all the resistor on the whole board and found them right on the money with the exception of the following:

R23 on schematic appears to show 4K??? The resistor in pace measures 9.1K and color code indicates 10K. Please look at the attched schematic at R23 and see if you agree it indicates 4K?

R22 which appears to be a negative feedback resistor shows 100k on schematic and has a brown/black/yellow color code indicating 100K but reads in place with a MM reads 9.1K. The NFB loop is directly tied into R23 to ground that also reads 9.1K. Not sure if that means anything but is a depature from other accurate resistor readings?

R30 220K on schematic, the bias voltage dropping resistor is actually on board a 220K resistor in parallel with a 1 meg and measured in place is 180K. This is another departure from the attached schematic.

I'm just checking everything very close to try to find something out of whack that might be causing the noise problem. Again I have attached the 2204 schematic and I hope you can take the time to look at it and comment on a possible reason for those resistor issues mentioned above. Thanks, Platefire

[gldtp99]

R23 is supposed to be the 4.7k (or 4k7) tail resistor for the PI----- R22 is the 100k Negative Feedback (NFB) resistor, this one won't measure correctly in circuit but i doubt it's the problem------R30 is the bias resistor--- Marshall used different output tubes in JCM 800's over the yrs, EL34's or 6550's-----each type of tube needs a different range of bias voltage (neg voltage) so you'll find either a 220k here for EL34's or a 120k for 6550's-----the resistor in series with the bias pot is 56k for EL34's and 47k for 6550's---the bias splitter resistors are 220k for EL34 and 150k for 6550 (these are the most commonly found values but this doesn't mean that's all Marshall ever used here)-- often output tube conversions are done on these amps and changes are made in the bias circuits----sometimes people will just slap another resistor in parallel instead of removing the board and doing it right so you'll see almost anything in the bias circuits of older Marshall amps (you'll see much half-a$$ed work in Marshall bias circuits)---- but it's possible that Marshall did this to get the bias in range for whatever output tubes were orig supplied with the amp.
None of this explains why the voltages are so high on the preamp tubes---this extremely high preamp voltage is a red flag--- somethings wrong in the Power Supply circuit---- were all preamp tubes installed (along with the EL34's when the measurements were made ? (just checking) If so, there seems to be a bad dropping resistor even if it reads in spec with no load--- there can't be 405V on the V2b plate ---- this is way too high.
The voltages should be similar to the ones shown in this mhuss Marshall 2204 schematic: http://mhuss.com/MyJCM/JCM800_2204.gif
They may not match exactly but there's no way they should be that high--- check the 10k/2watt dropper between the PI and V2 supplies.

PS--- some newer 50 watters had changes to the Power Supply vs older 2204's--- i changed my 4010 to lower preamp voltages with 22k/2 watters where needed----closely compare the schematic you linked to with the one i linked to------ there are differences in the Screens and V3/V2 power supplies-----your preamp voltages need to get into safe ranges for the 12ax7's.
This may be farther than you want to go with this amp, and i wouldn't blame you----i said earlier that it might be an involved process and also working with a delicate PCB---i pulled the PCB on my 4010 several times just dealing with Power Supply issues (getting preamp voltages in, what i feel, are a safe range for 12ax7's: less than 300V--or close)..........................gldtp99

[Platefire]

gldtp99

I found the same schematic on the net last night you attached, thanks. My voltages are running 30 to 50 volts higher accross the board than that referenced schematic. My heaters measure 6.74VAC. My wall voltage measure about 123 to 124VAC. One thing though I would say about the new referenced schematic with voltages, there is a additional 10K power resistor in the power rail between the PI and preamp tubes for a total of three 10K's where mine only has two!!!

Comparing voltage drops of the 10K's from my measurement and the ones on the new schematic, the 10K's are doing their job and are dropping similar amounts. Also my overall voltage is higher at 492 before it ever hits the choke compared to the new schmatic at 468---24V higher.

So what I'm seeing is my PI and power section is about 15 to 20 volts too high which is not too bad but the preamp section is 40 to 50 volts to high. What occured to me is if I could find a simple way to insert another 10K(like the new schematic) between the PI/Power amp section and preamp, that would bring it more in line. Right now I can't seen any simple way to do it? I first thought of disconnecting a wire to a filter cap and inserting one on a termial strip, but I don't seen anyway to do that looking at the board arrangment.

To answer you question again, yes all preamp and power tubes were in place when measurements were taken. I might would be willing to tackle the board if I could figure out without a doubt in advance what I needed to do--then I could prepare for it and take it very carefully. I'm trying to find out as much as I can before I call the cutomer so I can provide him current status of the amp so he can weigh his options on how to proceed. Platefire

BTW-With V2b without any plate resistor how could it help but get the full 405 volts from main power supply?

 

[gldtp99]

BTW-With V2b without any plate resistor how could it help but get the full 405 volts from main power supply?

 

 

It can't help it--- that's why the supply voltage in the Power Rail needs to come down @50 volts---- the high wall voltage in your area isn't helping thigs but not everybody can haul a variac around with them so the amp can be fed 120 VAC at all times.
Yes, the power supply in your amp (and my 4010) are not the same as the earlier 2204's--- I'll open up my 4010 and see exactly what i did ---i can't remember and my pics don't show enough details- this is one of the reasons i'll be converting my amp to a turret board--- i'll be better able to dial in the power rail voltages and circuit mods.

[Platefire]

Well I called the customer last night and had a good talk with him explaining the high voltage problem best I could and also explained my reluctance to work on that board not wanting to make things worse. I told him I could replace the filter caps, tubes and it my not even effect the noise problem and I don't think it would help the voltage problem either. He understood and is coming to pick it up tonight. It is still under warrenty, so he has time to take it back.

I can't help but wonder if this amp has had this voltage problem since it was new? Seems the power supply is putting out more power than the amp needs---so I'm thinking it would take some tinkering with differnent power rail resistors to adjust the power down to safe.

First time I ever gave up on an amp repair, so it don't make me feel good! ---but I would feel a lot worse if I pulled that board trying a lot of things to correct the problems and messed it up. I wouldn't mind on the job training on my own amp but on someone elses, might not be a good idea. I think the customer appreciated getting the info that the amp has a problem and my honesty regarding my lack of experiance with these type boards. Platefire

[rafe]

Last thing I'd want to do is void a warrenty on an uncertain repair....you probably made the wise choice you may suggest that he call you for advice before he purchases an amp.
I sold an HRD because it's long term quality was iffy, it sounded great but repairs were difficult.
Again , if there are any problems with his returning the amp, you could offer to install a new board
When I purchased my used JCM800 back in 87 it was dumb luck that I got a verticle input version
It has a seperate board and that board has good components and is easily serviced as opposed to the one your friend has HTH

[Platefire]

rafe

I did the same thing with my original 1996 usa made HRD. That amp sounded and worked fine but I sold it to avoid dealing with the inners when it eventually would go down.

Well The JCM800 is gone now! I have had a bit of Marshall experiance now    I might run next time I see one coming! You guys tried to warn me but I just wouldn't listen  

It will take me a while getting over not being able repair it. My trouble shooting confidence has been temporary shattered. I'm limping along on one MM probe and a chopstick! Platefire

[gldtp99]

Don't sweat it----that era JCM 800 (horizontal input jacks) isn't the easiest Marshall to work on---- the earlier vertical input JCM 800, JMP, JTM amps are much easier to deal with--- my '81 JMP 2204 was an easy rescue even though it has a PCB, different from the amp you had on the bench and my 4010--- the '73 and earlier amps are turret board...........................gldtp99

[Platefire]

Hay Thanks! I appreciate your and everybody else help/input on this. It's hard to calculate just how much all you guys help is worth when it comes to something like this but bottom line, it's worth a bunch. And speaking of trouble shooting confidence, you guys are a big portion of my trouble shooting confidence because I know when I get in unknown areas, you guys will always pitch in and share your experiance. So don't think it is un-appreciated maybe because I don't follow your advice and go my own direction--it is very much appreciated. Thank You Very Much! Platefire