Pete240885

Yup Indeed it should only conduct in one direction from + to - I'm pretty sure that replacing it would solve your problem, however there are a couple of things i'd like you to do :

First do another visual inspection of the board double check for any visible shorts, if you cant see anything obvious go ahead and unsolder DB1, then test it off the board if it still comes up as a short then you know that you need a new one.

Even if it does come up as a faulty, check the connections on the board and see if it they are still shorted let me know what results you get, i dont anticipate any other problems however it is good to be sure

wolf1419

Everything visually looked o.k. on the board, so I attempted to simply remove DB1, cleaning my iron tip/750 degrees/heat solder & pin, and tried to push it through, but I'm not doing well with that. DB1 is getting very hot to the touch, while solder is not really melting or letting go. What am I doing wrong...? HELP...

Pete240885

750 degrees...thats farenhieght right? I'm used to working in celcius so that sounds rather too warm lol

Do you have a solder sucker/ desoldering pump handy? they're very useful for removing anything with more than a few legs - especially if alot of solder has been used, have you tinned your iron (applied a small amount of fresh solder to the end after cleaning)...are you using a round tip or a flat one..... i find the small flat tips are alot better but thats personal preferance really

ragwood

Hi Guys, very interesting so far as I have the exact same amp in bits with the exact same problem!!! My knowledge with these amps and electronics in general is limited but, like wolf1419, I am keen to learn.
The board can be " flipped" upside down to test/remove components if the mains transformer to pcb wires (black,white,white) are unsoldered. This, however, puts stress on the other wires still attached, so, as you've already undone all the nuts on the pots and uncliped the pcb retainers (the hardest bit), make life easy and unsolder the remaining 8 wires... it's worth it!!!!
Also, testing DB1 in situ is not advisable. If another component is shorted or even on the same circut circut as DB1, it will affect any test results. This is a good rule of thumb for testing any component.

Instead, remove it from the board, (again, much easier with the pcb out) and test using a metre with a diode test facility (if your metre is digital, the ohm test won't work to test this, if analog (with a needle) ohms test may work). You should have continutity (a reading) when black probe (-) is conected to positive (+) on DB1 and red probe (+) is connected to any other pin. Also, if black probe is connected to either AC input pin (marked ~) on BD1 and red probe to negative (-) on DB1, again this should show a reading. Any other configuration of probe to pin connection should show an open circut (digital metre dosen't change from what it shows when not connected to anything). If any diode of the four in the rectifier are short circuted, it will cause a direct short between the outputs of the mains transformer, blowing fuses. With DB1 removed, it also isolates the auto transformer supply etc from the rest allowing more dectective work! My guess if DB1 checks out OK is the output transistors (TR8, TR10)

Sorry this is so long winded, but when I read how you tested BD1 earlier it didn't make sense to me so I opted for the long hand explanation. Have just removed and tested this on mine now, tested while on the pcb - made absolutely no sense and readings showed it was stuffed. Tested off the board and its fine... bugger!!! (although at it's one more likely canditdate off the list!!!)

Hope this helps

Pete240885

If no short had been found there would have been no need to take the component off the board at all, also i have asked wolf to test the component off board before ordering a new one.

I have a digital metre and it will test diodes on certain resistance settings, and on the higher settings its possible to tell by the readings.

wolf1419

O.K. guys, if my research is correct, and probes are properly polarized at your meter(red=+), my DB1 removed from the board reads as good per this test, Using An Ohmmeter under testing a bridge rectifier.
If this is correct my DB1 is good out of board.
With analog meter on ohm RX10 these are my results:
1) red @(+) to each adjacent reads 10
2) red @(+) to (-)black reads 40
3) black @((-) to each adjacent reads 10
4) black @(+) to (-)red reads "open"(infinity)

I went back and checked the same points on the board, with no DB1 aboard and was really confused.(ref(cw/ccw)=line +/-,-/+ as reference line)
1)red @(+) to clockwise point reads 60
2)red @(+) to counterclockwise point reads "open"(infinity)
3)+ to -, either way is shorted reading "0"
4)black @(-) to clockwise point reads "open"(infinity)=(infinity as above)
5)black @(-) to counterclockwise reads 55(nearly equal as above)

This is not what I expected! A short somewhere?, or just the way the circuit is interconnected connected(resisters/board, ground, etc). Please walk me to the next move.......

Pete240885

Sound like a short i'll have another look at the schematic, whilst i'm thinking could you test:
Black @(+) to each adjacent
Red @(-) to each adjacent

On the diode bridge?

PS how did you solve your soldering problem?

Pete240885

Ok whilst the brige is still out can you check the resistance from + to ground and from - to ground?

Pete240885

thinking about it only need one of those checks scene as they're shorted anyway

wolf1419

DB1 as you requested out of board reads all "open" (infinity) , in this reversed mode, which I believe is good.
1) At he board(DB1 removed) reads 50 = to gnd/- to gnd
2) a/c(prior open) still open to gnd
3) a/c(prior 55-60) reads 9 to ground

Your other question about desoldering was a difference in techniques. One of my sites said not to tin again after cleaning, but I went with your suggestion, which was my previous technique to get better heat transfer(A few years back I designed and soldered a full 24 track analog studio(multitrack/FX/TT patchbays, etc............hundreds of connections). It was still very difficult removing DB1, even using my desoldering bulb(just enough solder remained to make it stay put I guess) I had to heat and lightly pry from the opposite side a little at each leg until it was all freed.

Pete240885

Hmmmm ragwood could be on to something, two tests for you measure the resistance of TR8 and TR10 in from collector to emmiter in both directions, though i find it unlikely both would have failed at once they do handle alot of power so lets eliminate them early

wolf1419

I can see on TR9 the bce(base, collector, emitter-left to right), so assuming they are the same for TR8 & TR10, they might both be shorted.....?
Using ohms RX10 again as reference:
TR8 still on board:
1)b(red) to c(blk)=120, b(blk) to c(red)=17
2)b(red) to e(blk)=120, b(blk) to c(red)=17
3)c to e(shorted="0")red, or blk(both directions)

TR10 still on board:
1)b(red) to c(blk)=17, b(blk) to c(red)=120
2)b(red) to e(blk)=17, b(blk) to c(red)=120
3)c to e(shorted="0")red, or blk(both directions)

TR9 still on board:
1)b(red) to c(blk)=120, b(blk) to c(red)=15
2)b(red) to e(blk)=60, b(blk) to c(red)=15
3)c(red) to e(blk)=26, c(blk) to e(red)=80

I'm not sure if they need to be removed, but this is what I've read in place.

Pete240885

that doesn't sound good...c-e short in both directions ok same procedure as before... after a conversation with adwex in another thread i realise he is more experienced than me and so I'd like to try his suggestion of the suto transformer wihlst its convenient... ok i think we'll do things in this order: -

1: If you have put DB1 back on the board take it off again

2: Remove D3 - Be carefull your in the high voltage area here! If you haven't discharged those caps already get a 100 ohm resistor and connect it across C59 and then C53 - I have specified this order as C59 holds more charge, I would recommend holding the resistor with a pair of insulated pliers whilst you do this, half a seccond will be more than sufficient to discharge them to a safe level.

2: Put in a new fuse, switch on, if it doesnt blow the autotransformer is good. If it does carry on as normal

3: Re-insert D3, test it if you whish

4: Remove TR8 and TR10 perform off board tests

wolf1419

I know this is child's play to a real technician and I can now appreciate the labor costs of repairing these things. I am so grateful for your help and time also!!

I have discharged the caps, tested for safe voltage(DCV I hope), removed D3.
Wow, how do you guys get into those tight spots........??

I probably need to retest resistances at position DB1(without it installed, and no power transformer yet)to see if there are any changes there.....? Here it is and one point has changed without D3!!!!!!!!!!

I went back and checked the same points on the board, with no DB1 aboard and D3 removed (cw/ccw)=line +/-,-/+ as reference line)
1)red @(+) to clockwise point reads 60
2)red @(+) to counterclockwise point *now reads 120*
3)+ to -, either way is shorted reading "0"
4)black @(-) to clockwise point *now reads 120*
5)black @(-) to counterclockwise reads 55(nearly equal as above)

*No changes in values on TR8 & TR10(still shorted both ways c-e)while still on the board for reference with D3 removed*

I'm also checking to be sure that my originally tested power transformer must go back in to get power to the autotransformer before testing, correct?
When that is answered and done, will it be ready without DB1 & D3 in the circuit to try powering it up.....? Anything missing before I proceed..because I have a lot of stuff disconnected at this stage?

Pete240885

Yeah thats right, reconnect your power transformer, new fuse, and power up ,


then Power down and re-insert DB1 + D3 .

wolf1419

O.K. I replaced the power transformer, soldered primary wires, powered up, rechecked secondary voltages, ok'd, soldered secondaries, powered up and it still held. No lights came on at all though. I expected a power LED if I am even close to reading the schematic, and I'm sure there is voltage at the secondaries.
DB1 and D3 are "both off the board", and signal flow should be still going through the autotransformer and ending there, and power just before entering DB1.........correct? How can I verify power(AC, or DC) to these points to be sure were're good to this point...?
Now, If these are two totally separate paths for current, should only DB1, "or" D3 be put back, because I would assume that just putting both back in would not isolate the problem, just blow fuses, and be back at my starting point. Am I thinking correctly, and should I still check TR8 & TR10 "off the board" before I do anything else.....?

Pete240885

Hmm good point actually it would be worth testing with Just D3 re-insserted, then removing the transistors if the fuse holds

As for the power LED (LED3) take a closer look at the schematic, one side connects to ground and the other side to +VE, at + VE there will be no voltage without BR1. Have you researched bridge rectifiers or power supplys yet? if not don't worry about it i'm only asking out of curiosity

wolf1419

I see the connection from +15V through DB1 to LED to gnd so it is not getting current yet.
I resoldered D3, checked continuity for haha's, diode continuity/resistance +(red) to -(blk), then open from +(blk) to -(red), as it should be if I understand something of diodes now, or at least a how to do the amatuer test.
Powered it up and fuse held, so I went one step further to be sure I got a charge at C59 & C53 to be sure I have checked current at least to that point, and discharged them.
The next thing is to check TR8 & TR10, out of the board. I'm seeing very small bits of the signal flow, but a hard time getting from D3 through to TR8 & TR10 to see what other possibilities might lie in that circuit. Oh well, a small bit at a time, and follow your teacher......

Pete240885

Yeah once you have TR8 and TR10 of board they should test like two diodes - C-B B-E

Each transistor is a different type so the diodes should be orientated oposite ways in each transistor,

C - E reading should be 0

once you have removed TR8 and TR10 i'd like you to do the onboard short tests aswell, I've a suspicion that these are not actually the cause as when you tested b-e and b-c you didnt get a short.

So if these test fine it will be power up without them plugged in anyway and probably observe a blown fuse, however if the fuse doesn't blow with out TR8 and TR10 don't throw them away it my be other components in the chain causing this short. Do you know the basics of transistors?

wolf1419

Assuming order bce(left to right), Ref RX10 on my meter, I get
TR8
1) b(red) to either c, or e = "open"(infinity)
2) b(blk) to either c, or e = 18
3) c to e = 0("shorted")either way

This should be a NPN transister(2 diodes)

This is measuring the same points on the board(transistors removed)
1) b(red) to c=1000, e = 2000
2) b(blk) to c=50, e = 1500
3) c(red) to e = 1000, c(blk) to e = 500


TR10
1)b(red) to either c, or e = 18
2)b(blk) to either c, or e = "open"(infinity)
3) c to e = 0("shorted")either way

This should be a PNP transister(2 diodes)

This is measuring the same points on the board(transistors removed)
1) b(red) to c=35, e = 2000
2) b(blk) to c=2000, e = 2000
3) c(red) to e = 1000, c(blk) to e = 300

This appears to be what we should be expected except those readings of "0", Shorts don't seem good, correct....?
Resistance on the board, but no shorts should be good, correct?

wolf1419

I went back and checked the same points on the board, with no DB1 aboard and D3 on board, TR8 & TR10 "off the board" (cw/ccw)=line +/-,-/+ as reference line)
1)red @(+) to clockwise point reads 60
2)red @(+) to counterclockwise point *now reads 60*
3)+(red) to -(blk) *now reads 85, flipped +(blk) to -(red)=1500
4)black @(-) to clockwise point *now reads 60*
5)black @(-) to counterclockwise reads 60

This seems to point to TR8 & TR10, since removing, and testing board at DB1 is now "not shorting out + to -", and legs are now balanced for resistance.......make sense? Or am I still missing something else......

So DB1, TR8 & TR10 are all "off the board", powered up, fuse holding, so I'm wondering if I should put DB1 back on to test that circuit now, without TR8 & TR10.....................Is that where I should go..?

Pete240885

Yup that does point to TR8 and TR10, Have you worked yout how to test them from my previous post? Bi-polara Junction transistors (BJT's) are basically 2 diodes - one from C-B and one from E-B.

Depending on the type NPN (TR8) - the most comonly used - or PNP (TR10) - works the oposite way round, will depend on how the diodes are orriented inside.

If you want to know how to tell the differnce on a chematic its by the direction of the arrows

If the arrows point from the Base (the thick black line) to the outside of the transistor it is NPN. and i'm sure you can figure out the rest for yourself

Oh the leg with the arrows on is the emmiter btw

I've not seen this exact symbol before but i would assume the double arrows on the transistors leg indicate a darlington pair (the extra diodes D4 and D5 support this thoery two ) - this should test the same as an ordinary transistor off board so don't worry about that - but i would like some off board readings for these transistors and your findings please

The time has come to re-insert DB1 and test with the power on. before you do i would like to see if your meter is capable of reading R118 - you most likley will ned a low setting in order to do this...my meters lowest is 2 ohms and i would use that if yours has one. If it is measure the resistance Between the two holes for the emmiters of TR8 and TR10 on you lowest setting and higher settings you might find it interesting.

wolf1419

I had a couple of posts in a row. Please check post#50 which I believe to be the true results of TR8 & TR10, "off the board", my diagnosis and type as I figured them out. Please tell me if I have that work correct and I will work through your other requests also................and thanks for hanging in with me. Resistance measured with analog meter on RX1, at R118 reads approx 0.5 ohms, showing that when resistance measurements are very small, I check my RX1 setting and may read small resistances & also see open(infinity) relationships better, and get true resistance measurements, and I appreciate every post and lesson!

I just replaced DB1, powered up and it held fine, got that power light and channel select lights/working with push button. So this part of the circuit is looking good....!!

Now, somewhere in that other circuit path through TR8 & TR10 is at fault. Is it those 2 components, or is there more to check and diagnose to rule out any other issues....?

Pete240885

Ok re-read that definitely need new output transistors , hopefully just a power surge that caused it if they go again let me know i'll keep m,y subscription to this thread open

wolf1419

So I should just order and replace them.....no other possible problems to check first?
While I'm waiting for parts if you could take a look at 2 other posts I have, they would help my learning also. VS265R - 1 side problem & VS100 head - clean volume loud, won't reduce......any ideas, checks......?
Thanks, Ron

Pete240885

Yup just order them, won't look aat those posts today got ssome over people to helop out and its been a long one, post me direct links to the threads in here please.

Quick question what do you think the different RX settings are for?

wolf1419

Pete,
I have a slight problem choosing the best power transisters. The schematic shows BDV64/65(60V), but PartsisParts.com, Marshall's official U.S. parts supplier states BDV64C/65C(120V) as the replacements for this 8100, my VS265, and my VS100.
Here's a link to the difference in specs between the two: http://www.bourns.com/pdfs/bdv65.pdf

I can't thank you enough, and I know how drawn out this was for you. I also see that you have a lot going on in this forum, and WE ALL THANK YOU!!!.
The RX1=actual ohms as read, RX10=ohms read x 10=value, RX100= times 100, etc(RX1 for very small resistance values, RX1k for very large values, and you need to pick your proper range to get real ohm values.....correct?

When you get a chance the links are:

VS265 stereo chorus amp - 1 side problem!!

VS100 head - Clean volume loud-won't reduce level!!!

Pete240885

Hmm your multimeter could be different to mine however mine works as follows, the Ohms value represents the highest possible impedance to read on that setting, lower settings are more sensitive and will therefore read a lower value more acurately, so yes picking the proper range does help you get good values

As for the transistors I personaly would go with the suplliers suggestion, they will have the latest schematics and possibly more accurate versions than we have access to via the net. Let me know how these work out.

Your welcome to my time i've actually enjoyed helping you out

PS if your ever in the uk you can buy me a pint

wolf1419

Pete,
O.k. with the meter settings, and you helped me to figure out a lot of basics, which is the building block for better things. I'm going to take your advice with my supplier, even if the cost is a little higher, but they are available to speak in person, offer great service, good stock on hand, and support. They had marshall mode Four MF350 knobs that no one seemed to have. (PartsisParts.com)
I hope I can get to the U.K. someday, and if I do I'll look you up and we'll have a few pints on me.........
If you find time I left links as you requested in my prior post to those issues that I need to tackle.................

Pete240885

Just going to have a look at those links