Pete240885

Removing the switch and hard wiring would be the best option. It leaves far les chance of accidental shorts and also when you come to insert a new switch it will most likely be easier to remove this one.

you could always bur an inline switch instalit on the cable and use that until you can find a new one too...switch extention cord is another good solution

If your ever in the uk i'll hold you to that, in fact just make it LARGE bottle of jack

wolf1419

O.K., I hardwired the board at switch location(crapped out inside and unrepairable as you thought), hooked to a power strip with on/off, and everything came back to life, nothing feeding input, all controls counterclockwise(0), so step 1 is power is back.....

Input to IC8B = +1.2V(R25/C18) pin6 on IC8
Output from IC8B(pin7)+13.4V pin7(Main rails here, pin8=+15.4, pin4=-15.4)
Speaker out = +2.07V
R83=+15.3V/+1.5V; TR6 +2.8V; R82 +2.8V, +2.8V
R78=-15.3V/-1.6V, TR5 -1V, R79 -1V, +2.82V
Rails R77 & R75=0V; R84& R85=0V(no voltages....oh boy!!)
TR4=0V, TR7=0

Then something weird happened, everything in this section switched polarity and values switched.....

Input to IC8B = -1.1V(R25/C18) pin6 on IC8
Output from IC8B(pin7)-13.0V pin7(Main rails here, pin8=+15.4, pin4=-15.4)
Speaker out = -1.95V
R83=+15.3V/+1.5V; TR6 +1V; R82 -2.7V, +1V
R78=-15.3V/-1.6V, TR5 -2.7V, R79 -2.7V, +2.7V
Rails R77 & R75=0V; R84& R85=0V(no voltages....oh boy!!)
TR4=0V, TR7=0V

Turned off, ate dinner, turned on - changed polarity again to +(the meter is connected black to chassis throughout the tests and has never even been moved or unplugged, etc.

I'm totally baffled....

Pete240885

hmm IC 8b isn't working proberly either!, Its an inverting amp meaning put positive in get a negative out and visa versa.

not happy about this DC either hopefully this will dissapear if we can change IC8.

Acording to the pre amp schematic IC's 1,6,8 and 10 are of the same type, glancing at this also tells me that the other side of C18 is effectively grounded with the controlls all counter clockwise

Anyways borrow one of the above mentioned put it in place of IC8, if you whish as a kind of extra test you could check the voltages of the stage and then place IC8 into the borrowed IC's socket and see if any channges occur

wolf1419

Once again I may be off base here with signal flow, but isn't there a problem prior to IC8b since polarity keeps changing on the way out of the preamp at C18/R25(flow left to right), or does it react in reverse if IC8b is not working properly.
Is that only because signal flow is left to right, but current(electrons) are moving in many different directions.....?

Was that a favorable comment regarding C18 effectively grounded.........and any reason why I would have 0V on rails for TR8 & TR10......The main rails seem to be steady and proper polarity, so is it through the IC's that things are going wrong even though there is voltage present....?

I'm in deep, still not much of a repairman, but this may be an eye opening experience when all is said and done.

Pete240885

no i believe the fault to be located within the power section - this is DC we're dealing with here and it C18 should block it, and is doing, hence me being happy about the 0V at the far side of C8

wolf1419

Did you mean being happy about the far side of C18(being the preamp side) . ...? Is this capacitor keeping power side voltages from the preamp side of things, which would be the very low voltage put out by the guitar/preamp circuit..? (I'll work on that switching IC parts thing, and I'm also trying to find a supplier for my power switch(IT4100200-still made by APEM), in the meantime, but not having much luck for the small guy, only manufacturers in high quantities)

One more point on my DC measurements: I have measured all of these points with reference to chassis/gnd......should any of these be measured locally on each side of a specific part, or section of circuit, or am I o.k. referencing all to chassis/gnd...?

Pete240885

realised i didnt really answer this, have a look at VR13 on the shematic, and yes its useful to us in testing

Yup

kind of, however i don't think these voltages should be here in the first place

Thats good so far. If i want the voltage meausureing localy i will say somthing allong the lines of "whats the voltage accross component x" as oposed to "what is the voltage at point y"

Hmm it won't be cheapest place but it you can't find it visit a local amp tech he might be able order one for you or have one spare in any case

wolf1419

New test for my own satisfaction and a better feel for potential problems since things seem weird, before I pull or swap IC's, etc..... I've actually never been able to turn this amp on or hear anything from it, due to the fuses blowing(I purchased it DOA as a learning project).

With TR8 & TR10 still removed, hardwired @ switch, I wanted to see if I could hear some sound out of this thing, and found signal at FX send, hooked it to another amp, and found all channels operating well, including gains, volumes, tones, etc........even footswitch works.(clean/OD1 or OD2)....but I don't believe I have anything at "line out"(called direct out in schematic), and nothing to feed a guitar/bass amp input, and should be a decent level here......, but nothing, so that might help isolate, or verify that all problems are in fact in the power section.
This info should help narrow some things down, rule some things out, and confirm your diagnoses that it's not in the preamp stage(I just wanted to be sure I wasn't going to find a bunch of problems everywhere(input needed resoldering(fixed) & no reverb heard at this point, which comes after the FX section.....VR11 just before VR12....correct?).

Rails tested at 0V was actually correct due to TR8 & TR10 being removed. Is that right..? I'm trying to see how things come together with supply voltage vs. signal flow(weird stuff....but I keep reading and staring at those schematics).....and also looking at the best IC to use at IC8, without losing important parts of the amp, so it looks like IC10 just works in the reverb circuit, so that may be the one to play with...is that a good choice & is IC8 still the suspect even after my audio tests, or does it give you a different idea before I get in there...?

This live preamp test should also verify that the autotransformer previously questioned is good, powering my tube....Yes?

Pete240885

Doesn't really isolate any problems but yes direct out requires the main output transistors to function


Yup reverb is indeed after the FX loop


Oh dear it was early morning again - I must stop posting before i've finished the first brew of the day, especialy at 4am - 0V on rails is worrying... i would like GND to +VE and GND to -VE is that what you meausured? your notation was...interersting


That doesnt actually make sense i assume you meant input voltage? in order to this you will have to learn the basic behaviour of resistors, capacitors and transistors among other things, you want to study the circuits for a basic operational amplifieriers, you dont need to know how the work on the inside but you must be able to recognissed the basic circuit as you will come accross it ALOT in amplifiers, the 072 being a very popular choice is also quite cheap, i usually have at least 10 of these lying arround for prototyping aswell as repair purposes.

IC 10 is a good call, alternatively you could borrow one from another amp, Don't re-insert IC8 anywhere else unless the swapped IC does not make any difference.

wolf1419

Isolate is probably too general. I should say "rule out" certain parts of the circuit(by audio test) so that I can work my way toward isolating any issues that may exist, which may be more than 1, from what I've found so far.

Is there any difference between TL072S, which this amp calls
for, and TL072, which I also found in another amp/schematic of mine.....VS100, VS265R, if I remember correctly
The actual IC's on the 8100's board are JRC 072BD......what gives on all this....?
I can only find TL072ACP IC Dual J-FET Op-Amp 8 Pin DIP
from a supplier, without having to order by the hundreds.....
are they equivalent replacements for one an other..any issues using one or another, or is it just MFG, #'s...072 being the key?

Some of my other confusion is in the fact that voltage is the "potential push available" and current is the actual flow/quantity of electrons passing a given point at a defined time(Amps). Ohm's law defines the relationship of these 2 factors relative to resistance, but I can't picture it at work in these amplifiers and schematic circuitry yet, despite continued research, but I'll keep plugging at it.

You may be able to "jog my mind" into the proper way to comprehend this. I've read stats and descriptions for all the components you mentioned, but the "big picture", making the relationships of the components is not coming through. How do the terms "signal flow" itself, differ and relate to Voltage, Current, and Resistance, from the output of the guitar through the preamp & power amp to the output to the speakers. Please try to paint me an image that will make this all start to click, even if just in a very general way, and then I may do better with my actual curcuitry and components research, and make the connection to how these components relate to the changes required to alter and amplify sounds through an amplifier. I apologize for mosty writing and less hands on, but a lot of this is done from my bed, and needs to work in my head, which is the most difficult part - acquiring the practical knowledge!

Pete240885

That should be fine... TL072 is the core of the part number the other letters can indicate things like packace (type and material), manufacturer temperature ranges... as long as they are in an 8 pin dip type package it will be fine.

Consider a tank of water, with a pipe coming out of the bottom. In this case gravity appears as voltage - the force causing the water to move

The amount of water flowing is the amount of electrical current.

That just leaves resistance.

Now the resitance can be approximated as the diameter of the pipe. The larger the diameter of the pipe, the LESS it RESISTS the water flowing through it and so the MORE CURRENT flows.

A tap is like a variable resistor, when you close it its internal diameter is effectively reduced, increacing its resistance and reducing the water current flowing through it.

Hope this analogy helps somewhat

Another thing o note is that Ohmss laws stated form (V = I * R can be re-arranged into 2 way with a little algebra;

I = V / R
R = V / I


Learning how to trace signal flow is not going to happen over night. you will need to be able to:

1. Identify the individual stages of the amplifier
2. Have a basic understanding of these stages
2a. Understand the principle of negative feedback
3 Recognise components that can be ignored.
4. Have basic knowledge of the individual components behaviour

If you really want to learn send me a Private message with your MSN address and i'll get back to you - note the time i will be able to give you will vary at time maybe a couple of hours a day, at other times a few minutes a week

I spent four years doing my degree, don't expect to pick this up overnight! perseverance wil be needed and probably some patience aswell

wolf1419

Peter,
Thank you for clearing up the 072's, and the analogy using water and "gravity" as a key principal, which should, in our case, be the potential force to go from - to + as the nature of DC current.........yes...?
I'll continue with your final comments and try to learn more so we'll be able to have more educated discussions before we go to MSN, which I'm assuming is the "live chat" MSN Messenger. In the meantime, I'll see what happens with the IC's, do more study, and give you the results that you've asked for throughout this thread......
Thank you, Sincerley, Ron

Pete240885

Yup

And your welcome

Pete240885

ok had my first re-read of this now your back, so we left off checking out the preamp whilst you were looking for new output transistors?

Also i believe you wanted to learn how to follow a signal through an amp?

I'll have another look on sunday i'm on nights untill then! so it might be monday before I actually post (i'm trying to avoid the effects of tired posting and from tommorow this week is going to be hellish!!

wolf1419

Hi Pete,
Yes, a year is a long time, but seems like a much shorter time for me, but hanging in there.....I've only accomplished as much as I used to do in a week's time......that's just the way it is....I'm positive though..
I'm ready to get back into this...have the parts, even got the switch I needed from a very nice woman that got me a sample part(they only deal with thousands of parts at a time)....she was a saint, and the only person that was willing to help, even though the part is still being made...neither the maker, nor distributor could help....this one woman(after contacting at least 15 distributors) had a simple answer...she ordered a sample and sent it to me....very Cool!!!
No rush on any of this as it would just wipe me out anyway...so get your rest, take care of yourself and others....stay safe!!....

wolf1419

OK, started with replacing that switch...success...

Hooked FX out to an external amp again and had some fun with the preamp stage..still sounds good.....

Now, back to reality and where we left off....
1) TR8 & TR10 still out of the board(BDV64C/65C's available)
2) Board is still out but connected for powering up
3) We left off by checking voltage at various points to check for other potential problems and were getting wacky values.....
4) Can we pick up here again and proceed with tests again, as needed...?

Here goes...
1) voltage at R98(-40.7v), R99(-40.6v).....so that's not good
2) (power rails +VE, -VE) at R84/R85(+40.4vv), at R77/R75(-41.4)....good here..?
3) check at R102 (+15.4v & +41.3v), at R101(-15.4v & - 41.3v).....good here..?
4) check at R97 (+2.12v), R117 (+2.12v)....not good here..?
5) check at IC8 TL072 pin4 (-15.5v), pin 8 (+15.5v)....good here..?
6) check at IC8B TL072 pin5 (+2.11v), pin6 (+1.19v), pin7 (+13.57)...not good here..?
7) check at R25/C18 (+1.19v)......opp side of C18(0.0v)no input...good here..?
8) check at speaker out (+2.09v)...good here..?
9) check at IC8A TL072 pins 1,2,3(0.0v)....no input...good here..?
10) check board at C(rails) at TR8 (not installed)(+40v) & TR10(-40v)....good here..??
11) check BCE at TR9(still on board)(all read-40v)....not good here..??

Turned off, waited, then on..."polarity changes"(voltage values approx. same)
1) voltage at R98 & R99 (both now +v).....so that's not good
4) check at R97, R117 (both now -v)....not good here..?
6) check at IC8B TL072 (pin 5,6,7 now -v)...not good here..?
7) check at R25/C18 (now -v)......opp side of C18(0.0v)no input...good here..?
8) check at speaker out (now -v)...
11) check BCE at TR9(still on board)(now all read+40v)....not good here..??

Tried 3 more times and got same as last test throughout (R25/C18= -v)......?????????
Tried again, polarity back to original throughout (R25/C18= +v).......??????
Tried again and again and polarity changes at will throughout(R25/C18= +-v)......?????????
What could be causing the polarity changes at input R25/C18, etc...or am I thinking backwards.?
Does TR8 & TR10, not installed, have anything to do with this polarity weirdness...or is it all in IC8 that is causing all the havock....?

wolf1419

OK, I couldn't get any takers on that last post, so I went forward and installed the power transistors, and 8ohm 10w load resister at speaker output, after finding on another forum that the crazy polarity/voltage readings I was getting were a result of not having TR8 & TR10 aboard, due to the feedback nature and incomplete circuit......

I got it together, turned it on, and VOILA...no blown fuse..... but R101 & R102 were the first parts to get heated, after approx. 10 minutes at idle.....quite hot to the touch(not enough to burn, but hot enough to be uncomfortable....so it's somewhat hot for sure). I turned it off, let it cool, then turned on again, tested for sound at the line out, which is the same as speaker out, ran to an external amp, and everything sounds good, even reverb is fine..................so that's cool......

The big question for me now is if R101 & R102 should be this hot, and if there are any known tests for voltages that I can do to be more confident that all is running properly and not a problem somewhere else, causing overheating, that would cause this to fail when I put it back together and run with speakers. I don't want to replace components that are not failing, but would like to eliminate any obvious issues while it's apart, if something is causing excessive heat(or is this normal). Any good tests with predictable results that I can test this further........................????

wolf1419

Looks like I'll be answering this myself for whomever it may help....this has been a long haul....:..thanks Pete, Adwex, Ragwood..

I have a VS65R with similar resistors in the same stage of the circuit, got the same amount of heat, and another test for voltage at the speaker output of the 8100(should be close to 0V), measured 13milivolts(very low from opamps, etc...should be OK), then ran it for 1 1/2 hrs and all is smooth so far.
Last step will be to reassemble, hook to speakers, and make some music(easy at first, and adding more gain & output volume til I get it up there in the "live realm").....then run it through the paces for a few days.........

wolf1419

All assembled, ran for hours, on/off...working up to higher volumes....THIS AMP ROCKS!!!!!!!!!! ...................
"All tube guys"(I have some of those also.... JTM30, JCM800, 900, 2000) will still want that character, but Nice job on this Marshall.....design is simple and awesome..............single PCB, not all those various motherboards like my VS Series.....which are also decent...(but don't like the MG series..Cold/brittle/poor tone/lifeless sound)

The 8100 offers Clean/Crunch channel & OD1/OD2 channel...from super clean....really nice..almost Fender like character to light Marshall blues/classic crunch....then Marshall classic rock OD to over the edge OD2 metal/thrash distortion...tight, or loose using contour control, altering midrange totally getting different character as you desire, and all with good tone(no kids buzzsaw distortion here) and a lot of sustain..... ......this can really work for some. Try it, you might like it!!