I've been a bit "out of it" for a couple days, but do have my parts and see the +V, -V rails connections, but I don't see how I can get the 10ohm resister "in series" like the diagram because they are "PCB traces"(I could do it easily if point to point wiring), but don't know how to physically hook these without still having connection through the PCB........I'm pretty sure it's expected that components must be removed and isolated to make this happen......but I'm not able to determine how to do that without major surgery......it seems physically impossible unless DB1, C61 & C62 are removed(not connected to rails/trace and other contacts wired back into the PCB with jumpers/wires,...... then direct lines(w/resistor) to collectors of TR8 & TR10....Yikes...........

I'm doing some preliminary tests for "opens or shorts" in the meanwhile, and now I'm getting resistance both ways across DB1 + to (adj)a/c, - to a/c, and + to -.......and a short from a/c to a/c(both directions, and also to gnd).....does that suggest another problem in this power stage before I even attempt the "live test"......(DB1 was fine off the board, and tested back on the board, powered up, when testing the preamp previously, and everything checked out), .....so I'm hoping this is just the reaction of all components installed(like with the transistors), but confused here also......I know it's 4 diodes(bridge) that should read in one direction only on each pair(+/-), open across diagonals, when off the board......please try to explain if I now have a problem with the circuit, or just confusion seeing the big picture when hooked in circuit.......or would this already "fail" if powered up with this short(or is it all proper gnd).........?........

I'm reading and reading, but still not able to predict measurements while in circuit on some of these components.

Thanks for the patience guys....my temptation is to just turn it on, but that would be too easy, and may be shocked one way or the other.......not knowing what to expect.

 

Hello Ron,

The powersupply itself works.(in your first post).
You stared in your first post that the +40 and -40 Volts are present:
3) check at R102 (+15.4v & +41.3v), at R101(-15.4v & - 41.3v).....good here..?

The strange measurements on DB1 with the ohm meter are caused by the transformer.
This has a very low resistance.

Are the powersupply and amplifier on one board?

There is a trace for the +VE connecting R84, R85 and the collector of TR8 that goes to the large capacitor C62.
You must cut the trace between TR8 and C62 to connect the 10 Ohm resistor.

There is a trace for thr -VE connecting R77, R75 and the collector of TR10 that goes to the other large capacitor C61.
You must cut the trace between TR10 and C61 to connect the 10 Ohm resistor.

Greetings,
Bertus

 

Hello Ron,

The powersupply itself works.(in your first post).
You stared in your first post that the +40 and -40 Volts are present:
3) check at R102 (+15.4v & +41.3v), at R101(-15.4v & - 41.3v).....good here..?

The strange measurements on DB1 with the ohm meter are caused by the transformer.
This has a very low resistance.

Are the powersupply and amplifier on one board?

There is a trace for the +VE connecting R84, R85 and the collector of TR8 that goes to the large capacitor C62.
You must cut the trace between TR8 and C62 to connect the 10 Ohm resistor.

There is a trace for thr -VE connecting R77, R75 and the collector of TR10 that goes to the other large capacitor C61.
You must cut the trace between TR10 and C61 to connect the 10 Ohm resistor.

Greetings,
Bertus

Hi Bertus,

You just made me very happy to hear about the strange readings as a result of being "in circuit", which I assumed, but didn't realize it was more a function of the transformer side(thought it was transistor side).....the a/c to a/c to gnd is correct...? I'm not able to justify that connection in my mind yet, though I do see the connection in the scematic through to R83/LED3 to gnd.......I'm missing the WHY and not getting the flow & concept as to what actually happens at DB1......even after reading about diode bridges......very unclear on the logic(electronics is tough stuff!! - it seems to be interacting in evey possible diection and difficult to conceptualize the whole pocess)

Yes...confident with power supply operation, which was first section verified at beginning of diagnosis(I had a friend from UK on the Marshall site last year-he is no longer posting for anyone), along with BR1, auto transformer, worked throughout until we found TR8/TR10 were shorted.........

No, power supply is mounted to case, not on amplifier PCB..

I've had all the connection points firmly in mind, as you stated, but didn't physically know the best way to make the connections......I've read about cutting the trace(minor surgery), but have never done it......that's physically cutting through the trace(just enough gap to disconnect trace) to "open" +VE & -VE, wire resistor in line(series) directly from capacitor(or DB1-optional) pin/solder point to transistor pin/solder point.......correct.....?

Is it easy to reconnect across that trace cut, without hurting the PCB when I get to the final conclusion......just connecting it by resolding the trace gap.......? Your student & friend, Ron

 

Hello Ron,

The gap made for the test can be "repaired" by cleaning the trace near the gap and solder a piece of blank wire (about 1mm) to the trace .
This provides a good link for the current uesed at that place (max 6 A).

Greetings,
Bertus

 

Hello Ron,

The gap made for the test can be "repaired" by cleaning the trace near the gap and solder a piece of blank wire (about 1mm) to the trace .
This provides a good link for the current uesed at that place (max 6 A).

Greetings,
Bertus

That sounds simple enough................
I also found a better site for explanation of DB1......
http://www.tpub.com/neets/book7/27c.htm
This should explain the connection a/c-a/c-gnd, in circuit/on PCB.....and how the bridge rectifier works.........no explanation required....

 

Hello Ron,

Yes, the publications of NEETS are very good.
Here is a link where you can download them as PDF's.
http://jacquesricher.com/NEETS/

Greetings,
Bertus

 

OK....that's cool...I'm well armed with info for now and will do this when I'm ready......still laid up....crazy CFS........up, up, positivity.......

 

Hello Ron,

This is a bit off topic, but I found some video lectures on basic electronics:
http://www.learnerstv.com/lectures.php?course=ltv052&cat=Engineering&page=1

It is made by the Indian Institute of Technology Madras.
So it is a bit of indian english.

Greetings,
Bertus

 

Bertus, we both have some of the same radar/searches...and not off topic at all, since my goal is learning............I started watching some of these lectures last week, while searching for more info.....they are very good. They are quite long(indepth explanations) and helpful.....I found them on Youtube, saved individual links in my Favorites, but your link to all the lessons in one place is much easier to work from...
Nice investigative work..........I should be back on my amp within the next day or two..........CHEERS.....

Hello Ron,

This is a bit off topic, but I found some video lectures on basic electronics:
http://www.learnerstv.com/lectures.php?course=ltv052&cat=Engineering&page=1

It is made by the Indian Institute of Technology Madras.
So it is a bit of indian english.

Greetings,
Bertus

 

Hello Ron,

More links to videos can be found in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=29157

I hope you will get well to work on the amp soon.

Greetings,
Bertus

 

OK, I got feeling a little better, made the 8ohm speaker load jack(gd), put a jumper from R25 to gnd as planned.....then things got hairy....I know where to tap into circuit(bypass with 10ohm resistor, from each collector(TR8/TR10) to capacitor(C62/C61), but with the board still tied to power supply, etc..., I couldn't get a good feeling about "cutting" the PCB traces, due to space concerns, clarity, and possibility of ruining the board.

Now I know this was the cowards way out, but with all the testing done on all components, after all was reassembled, I decided to just take the chance(figuring that it's the 2 power transistors were the only components actually "fried"-and I had spares just in case I was wrong).

You guessed it....I turned it on briefly to see if the fuse would hold.....
Well it held, so I turned off, tried again..........then let it warm up for about 10 minutes(as I was going to see if I had any output at the line out, as a "live test" for proper sound here, which should be the same sound as the speaker out, without frying a speaker.....but I kept an eye on components for heat or smoke...everything seemed fine until I checked R101 & R102 and they were quite hot to the touch(not enough to burn your finger, but hot enough for minor pain if you where to leave it long enough), so I powered down to avoid any damage. ......

I assume there's a lot of current and resistance there and these resistors in particular should put out more heat than the others......is this a proper assumption...?

Curiosity still got the best of me, so after cooling, I hooked up line out to an external amp and powered up again..............before any heat was produced I played some guitar through it..........everything sounds good to this point, even reverb............is this fixed(sounds good), or can you give me some voltages that can be verified(please specify point to point, or point to gnd and some explanations of values to expect...I'm up for a little math and further knowledge) to feel confident that all is well before I mount this back in the case and hookup speakers..............Sorry I got spooked about delving deeper into the PCB, but had to weigh risk vs. reward and this was my decision.....I think the guy I got it from(DOA) may have pushed massive power thru this with no speaker load and "fried" the power transistors...................

 

Hello Ron,

R101 and R102 are the resistors to drop the voltage from 40 Volts to 15 volts for the preamp.
The stabalization happens using the two zener diodes ZD1 and ZD2.
These resistors can be warm.
There could be something wrong with the capacitors C40 and C41.
There also may be somthing in the preamp part that is drawing to much current.

Greetings,
Bertus

 

Hello Ron,

R101 and R102 are the resistors to drop the voltage from 40 Volts to 15 volts for the preamp.
The stabalization happens using the two zener diodes ZD1 and ZD2.
These resistors can be warm.
There could be something wrong with the capacitors C40 and C41.
There also may be somthing in the preamp part that is drawing to much current.

Greetings,
Bertus

As my 1st test of voltage at the preamp, I've verified -15V/+15V at R64/R65......then poweramp section voltage at the rails, collectors of TR8/R10 measures at +41V/-41V, respectively......

 

Hello Ron,

R64 and R65 are together with C24 and C26 an extra filter for the input opamp (IC1).
The voltage from R101 and R102 go to this point and all the other ICs.
I do not know how much current is drawn by the 10 opamps,
but I can imagine the R101 nad R102 will get very warm (they are 5 watt types).
When you calculate the voltage drop and how many current flows through R101 and R102,
you can calculate the power dissipated.
Even without the opamps the current will be (41 - 15)/330 = 0.079 A.
The power dissipated will be 0.079 X 0.079 X 330 = 2.06 Watts (this will generate some heat).
I think the heating of the resistors R101 and R102 is OK.
The stabalization current adds also to that (the current used for the zeners ZD1 and ZD2).

Greetings,
Bertus

 

Hello Ron,

R64 and R65 are together with C24 and C26 an extra filter for the input opamp (IC1).
The voltage from R101 and R102 go to this point and all the other ICs.
I do not know how much current is drawn by the 10 opamps,
but I can imagine the R101 nad R102 will get very warm (they are 5 watt types).
When you calculate the voltage drop and how many current flows through R101 and R102,
you can calculate the power dissipated.
Even without the opamps the current will be (41 - 15)/330 = 0.079 A.
The power dissipated will be 0.079 X 0.079 X 330 = 2.06 Watts (this will generate some heat).
I think the heating of the resistors R101 and R102 is OK.
The stabalization current adds also to that (the current used for the zeners ZD1 and ZD2).

Greetings,
Bertus

I can see the logic at R64/R65 & C24/C26 & IC1...also the feeding of all the opamps IC2-IC10 arrangement through the rails +V, -V from the dual power supply....I'm following the voltage drop calculation V(drop)/R=I.

I'm not yet familiar with the power dissipation equation, or how to estimate that in terms of heat to expect.

I thought I could check between this Marshall 8100(100w) & a Marshall VS65R(65w), which I also repaired recently, but it is a totally different circuit arrangement(attached schematic-don't have a clear .pdf).......but I'm following logic........ and find R17/R18 as common stage resistors that should be equal to the R101/R102 to compare physical heat just for comparison....and see if I get more confidence before I go any further....................

 

Hello Ron,

The circuits round R17/R18 of the 65 Watt version
is comparable to the R101/R102 of the 100 Watt version.

For calculating the power take a look ath this page of the AAC eBook:
http://www.allaboutcircuits.com/vol_1/chpt_2/4.html

Greetings,
Bertus

 

Hello Ron,

The circuits round R17/R18 of the 65 Watt version
is comparable to the R101/R102 of the 100 Watt version.

For calculating the power take a look ath this page of the AAC eBook:
http://www.allaboutcircuits.com/vol_1/chpt_2/4.html

Greetings,
Bertus

Thanks, that confirms my logic....and the equation P=I(I)R(Joule's law)......I had read this page in my studying..never applied it to something I am working on...need to get these equations into real practice.

My test should tell me something about the heat to expect as "normal"..

I just tested my VS65R and have the same heat...pretty hot to the touch....so that is the normal heat generation for these power resistors..........I think we're good for now....I'll get it back together, burn it in for a couple of hours, keep an eye, on/off a few times, and if all stays running I'll hookup to speakers, play guitar and start applying power slowly and work my way up if all goes well and put it through the paces to see if it can hang in there under playing conditions......when I can make some noise!

 

Hello Ron,

Looks good for now.
Did you measure the voltage at the output with the link accross R 26?
Is it about 0 Volts?

You will have to remove the link accross the R 26 before you can hear something.
Start with low volume to test the amp.

Greetings,
Bertus

 

Hello Ron,

Looks good for now.
Did you measure the voltage at the output with the link accross R 26?
Is it about 0 Volts?

You will have to remove the link accross the R 26 before you can hear something.
Start with low volume to test the amp.

Greetings,
Bertus

If I'm understanding you correctly, I would put a jumper back, on across R26, effectively grounding the output of the preamp(0V here), and then check voltage output at speaker output(+ to gnd), or across speaker 8ohm resister(being the same test basically)...is that what you mean...?

 

Hello Ron,

Yes, that is the check if there is no un-balance in the amplifier itself.
If the input is grounded, the output should be 0 Volts.( some mVolts are OK).
If that is not the case there is something wrong.
A DC offset could be bad for the speaker as it is forced one way with no input
AND there is a current flowing through one of the transistors without input.

Greetings,
Bertus