wolf1419

I thank you for the work on the VS100 and posted there also....

My power transistors just came. I couldn't get anyone(parts suppliers, Marshall, Korg, etc) to justify why the "letter designation" changed, but I ran into a Co. from your country/U.K. that had my original transistors in stock at great prices compared to my local(not in stock) and USA suppliers, even with shipping involved. They were so reasonable that I purchased 4 of each as spares, etc......(EBAY - Littlediode.com, (info@littlediode.com), and I recommend them for parts, as they have a no minimum policy, and communication was very good on this purchase-weird that I couldn't get this here done in the USA.
My question would be if I should place only one transistor first to see if that blows the fuse, if that is adviseable to isolate where there may be other problems(I part at a time). Is that possible....?

wolf1419

I stand corrected. The power transistors I received are the new part# 64c, 65c, so that's cool too...........

Pete240885

Ok this is what i think should be done as although the falirue was identified the reason why is not known it is entirely possible that they could be blown again although i'm hoping it was just a power surge that caused the problem:

First question is the cost of these transistors significant to you, if it is then get back to me i'm trying to analys current flow through the amplifiers output stage but its nature is slightly different to the output stages i have studied so i'm going to re-check my working tomorrow afternoon and make sure I agree with myself.

If not go ahead and replace both of them, let me know the result,

Pete240885

It is possible that the origonal parts have been outdated...the design is about 10 years old

wolf1419

I think I'd like to wait and check some points up to the transisters first, as that will also give me a chance at testing some components and follow signal flow, which might help my understanding.....I'll just wait to hear from you as you find time....no rush...

Pete240885

alright then,

couple of quick checks you can do

R75, R77, R84 ,R85..I don't expect faults but checking won't hurt, they may give you different values depending on the polatirty of your multimeter probes, see if you can work out why before you test them, you could test all the transistors in the circuit and all the diodes, tbh looking at it I don't think you'll find another fault.

Take out the ends of R98 and R99 connected to the output transistors, and then measure the voltages at the ends of the resistors with respect to ground.

I think your capable of judging weather diodes, transistors and resistors check out correctly, If you say thier ok i'll take your word for it, if you wish to post your results back on here thats cool but i'll expect your opinion on here with them

Teledeluxe

Hi Guys,
Here's the most common problem.....
Someone uses an instrument cable between the head and the speaker. An instrument cable is unlikely to be able to handle the power, and often overheats and shorts out, causing damage to the output transistors in the head, and blowing the internal fuse. A proper speaker cable should always be used, instead of an instrument cable. The output transistors on the VS100 are BDV64 / BDV65 type. Check for a short circuit between the speaker output socket tip and the black lead on the transformer. If it's short circuit then it's probably at least one of the output transistors to blame. The real culprit though is a faulty lead, so check this too!

wolf1419

This is a fair observation, and something that I'm very aware of, being a former live sound and studio engineer, and I know a lot of newbies that think anything with 1/4" plugs is o.k., because it works, not knowing that heavier gauge speaker cables are needed at this stage(also affected by length in sound systems or extension cabs). I got this amp in this condition and the prior owner "may" have used guitar cables, and high volumes, and fried the output transistors, and more....................

Pete240885

Yup he's a bit late though.... how far have you got with tasks set in my previous post?

wolf1419

I have only been able to do some of the reading/research concerning testing of resistors and diodes, and it's led me to believe that I must unsolder at least one end of each component in order to do the tests with the circuit out of the equation. I kind of hope this is untrue as I'm still not bouncing back back all that well and have put this off to do other simpler tasks that I have in progress. I do some work with guitars(frets, simple electronic & cosmetic repairs), and amps(cosmetic glueing/cleaning) and find that to be better therapy than the electronics in the amplifiers because of the concentration factors. ........

Pete240885

its not completely true, it depends on what the resistors are connected to as i mentioned some of the resistors values will change pepending on polarity of the test i think you'll figure out why if you think about it and study the schematic, heh i know what lack of concentration can be like and it is annoying as hell

wolf1419

Maybe I can just get in there and see what happens without disconnecting anything and see if I reach any conclusions......

Pete240885

Go for it, oh it shouldnt be necessary to disconnect one side of R98/99 at all before checking the voltage on the power transistor end if you haven't yet inserted the new power transistors

wolf1419

O.K. I'm trying to get a handle, but feeling pretty dumb. All I can determine is that I do have differing values. but I don't have an open, or a short on R75, R77, R84 ,R85(in either direction)...so I assume that's a good thing.

As far as voltages are concerned at R98/R99, I first did continuity to ground and had no short/no open, then realized you said "voltages". I expect that I have to power up the amp to do that....?(scary thought...) I need help here before I fry myself or my amp....

Pete240885

Yup thats what i expected, look at the schematic when your brain is functioning you should be able to get it

Yeah wait till your alive for that one, basically you need base - emiiter voltages, DC with no input

wolf1419

I'm having trouble figuring the symbols for resistance values (ohm symbol is fine, k=1000-fine), but what is R, or 8, 2, etc...after the value..?


That will be with A/C power on..with nothing feeding the input, or am I having brain fog?

Pete240885

In this notation

R means Ohms (don't ask me why Omega is not used here)

K is Kilo Ohms (as you said)

M is is Mega Ohms

Now your going to kick yourself when you realise how simple this notation is (i did when i found out)

The letter is placed where you want the decimal point so 4.7KOhm resistor becomes 4K7 a 2.2 Ohm would be 2R2 etc

Thats correct

wolf1419

Funny stuff actually. Sometimes they use Omega, sometimes R, right next to one another in the same schematic, and they are equal.......

The #'s are in weird order, but I guess it eliminates the decimal, so I can see where they're coming from there, and yes, that's fairly simple....

I'll do some more testing of components just to get a good feel and see if I can make the relationships with the schematic. I did notice that some resistors read the same R both ways, some differ, and that the highest resistance reading is closest to the actual value of the resistor. As long as I don't find "opens", or "shorts" through components in either direction and get values that are reasonable vs. schematic values while "on the board", the components are not detrimental/fried.

My next area of misunderstanding relates to all the seemingly "multiple paths" of circuits. I'm trying to follow signal paths, but they just look like they go in many directions at once(like many vehicles making a journey at once vs. one vehicle following a certain route). I'm not able to follow the actual "flow" of these circuits. I'm not following the "logic" of signal/current, making it difficult to actually "see" where a problem may be in any part of the circuit. there seems to be so many paths all at the same time. Give me a "clue"..

Pete240885

Have you worked out why the Resistors i pointed out measure differently?



Don't expect to trace the signal flow through this, if you whish to learn how that works i'll dig through my notes from uni and find somthing a less complex.

If you really want to work out how this works consider this:

IC1B is an inverting amplifier. R58 and C29 are there to control its gain and frequency response and should be considered part of this amplifier stage.

R78,R83,D1 and D2 form bias volatges for TR5 and TR6 this voltage is constant.

Since the Bases of these two transistors are held at a constant voltage it follows that their emmiters are.

Now if the voltage at the input to the circuit changes what happens? (Note R100 is also i feedback resistor and should be ignored here)

wolf1419

I've tested many resistors and values in the power section and they are within reasonable values with no shorts/opens so far, so I'm ready to move forward with either replacing the BDV64/65's(still not on board), or trying the live voltage tests you were mentioning. What safeguards do I need to be concerned with.......
With my limited understanding of circuits, but research abilities I am going to say that when the input voltage is changed that these are switching transistors on/off. How that actually all works is beyond me at this point, but I'm assuming that at some point it passes voltage to the power transistors that I will be replacing at some stage, that are used as amplifying transistors(power amp stage).
Am I even close.....?.....

Pete240885

In a vague kind of way that makes sense but it could describe almost any amplifier circuit, you have to think in terms of current flow in response to a voltage and also remember that transistors maybe switches in digital circuits but in analogue thier natural function is a current amplifier....urg i really should stop trying to explain things at midnight ^_^

Live voltage tests, very simple...don't touch stuff with your bare hands...or elbows...etc, in fact the safest way is to use rubber gloves preferably somewhat thicker than the ones your mum has in the kitchen, alternatively just be very careful, attatch a crocodile clip to one of your meter's probes and where possible make sure that you only have one hand near the live stuff at a time, this way any shocks should be restricted to one arm, provided that no part of your body is grounded of course


P.S. really was hoping you could tell me about why those resistors changed in value, from looking at the schematic... if you cant figure it out read back through the thread there is a massive clue in here, not telling you where though

wolf1419

Bookwork first: I believe that I got changes in R value due to the fact that these are not simply resistors hooked in series, but most is parallel circuitry, interacting to give different values than might be expected if I tested each one off the board out of the circuit. Resistors by themselves have no polarity, but based on how they are hooked in the circuit I may get different values in either direction at times as other parts have different values on each side of the resistor, affecting the individual parts hooked to them....each part is like a worker of different ability, but all connect to form a united job....

Pete240885

True but: don't forget series, and which component is causing the change with polarity?

wolf1419

The diodes, D1, D2....

How do I differentiate when to read A/C or DC current by schematic so that I will know about testing R98/99....and I've followed the end to test on the side of BD64/65....and how do I know how much current I should measure at that point by scematic.....? These voltages will be measured from that point to chassis, which = gnd.......

Pete240885

nope, look closer to the resistors, remember previous tests, and stop guessing!! if you slow down a bit you'll figure it out.

DC for the moment, sorry should have clarified



You really don't want to try measure current from there to chassis...you will cause a short circuit, on the current setting your meters impedance becomes VERY low as curent readings are taken by inserting the meter into the path of the current being measured. That is to say if you wish to meausre the current through a resistor you wire the meter in series with it.

Chassis is correct

wolf1419

Ah Ha, I'm confusing the "terms" current and voltage, so I went back to the basics, voltage being the push, amps being current flow, and of course resistance which we've been dealing with. I need to measure voltage, not current, from each of the 2 points to chassis/ground, while powered up.

Just did it! Checked proper polarity of my probes at the meter(red=+), then put black to chassis/gnd, red to R98/R99 and my needle was pegging backwards.....so I switched red to chassis, black to R98/R99 and read 40V at each. Is that because of flow from +(R98/R99) to -(chassis/gnd) is the direction of flow in my circuit.....?

Your other question about changing values may be due to polarity differences(reverse flows -arrows) in the transistors TR4, TR5, TR6, TR7....and yes, that's still a guess, as I've looked, slowed down, studied, and that's all I can see that should affect these resistors while on the board. I still can't picture it though, and not finding it through quite a bit of research......my wife is getting lonely while I try to get a handle on all of this....luckily she's supportive and a great lady! This must be making you pretty crazy too.....I'll blame it on CFS or maybe it's those drugs from my youth....LOL

Pete240885

Thats not good R99 should be positive and R98 should be negative,

What voltages do you have on the power rails?

What voltage is coming out of IC8B?

your meter has a needle? spend the 10$ or so it costs to get yourself a digital one, they're more accurate, and have less effect on the circuit being tested...


ok your on the right track, ....tried measuring R84/85 as a pair yet?

wolf1419

O.k., I got a digital meter and hooked black to chassis/gnd, and red to R98, then R99 - both are -40.5 volt DC, which is not good......

I tested at ZD1 & ZD2 as beginning of power rails, o.k.'d @ -15.4V & +15.4V on one end only, and at far end of IC Supply Arrangement, which I assume you mean by "the rails", and reading -15.4V & +15.4V at R64 & R65 both ends, respectively.

I found IC8 on the board, did some more research and figured that pins are #'d counterclockwise, verified & measured pin4=-15V,
pin8=+15V, output at IC8B, pin7 = -12.8V............then turned off.
Now it won't turn on and the fuse is not blowing......WTF......What did I blow now!!!!!!!

I did some testing and I think it's the main switch this time by checking continuity. I've got continuity measuring on the underside of the board at switch lugs across N(blue), none across L(brown) to the fused portion. The switch has 6 contacts(3 pairs), the pair closest to the face of the amp/switch have continuity(on or off), the next set have continuity(on)(off=open), the final set do not have continuity(open on, or off), so the last pair of contacts at the switch appear to have an issue, not allowing the amp to power up. I don't know why that would happen all of a sudden. Is it safe to use some type of jumper across the last pair to get by this for now, to be sure...........and it looks like the switch "may" be able to be taken apart and checked/serviced.

Pete240885

....Sorry i ment the other power rails...labeled +V and -V, that the output transistors collectors connect to

Switch not turning off is an issue, yes i'm sure its the switch. Now its probably two sets of three rather than three pairs - Look at it on the schematic - Disconnect it from the transformer you should have no continuity from one set of three to the other, or from any outer pin to another outer pin, a middle pin will connect to either of the outer pins depending on switch position. I wouldn't bother taking it appart If its broken it probably can't be fixed easily.

Thats a rather large DC offset :| Is the input to IC8B measureable? (between the R25 and C18)

Also can you measure the voltage at the output to the speakers, again with respect to ground?

wolf1419

Clarification: The problem with the switch is not that it won't shut off. It's that it won't turn on. This is a push type switch, seemingly unavailable so far, as I've searched tirelessly on the net, so I may be forced to jump it or fix it.

I did a lot of testing to narrow it down to why it wouldn't turn on. I originally thought I had some other problem in the circuits, and was determined to find the problem myself with what I'ved learned to date. It was clear that power was not available throughout the circuits after the transformer, or even at the fuse, so I traced everything around the switch and found that that last pair of pins is what needs to connect to complete A/C cuircuit feeding the transformer.

The first pair of pins appear to be merely anchors for the switch as it's part of the metal bands holding the top/bottom switch halfs together and anchoring the front end of the switch as both active pairs of pins are too far from the front end to be reliably connected mechanically. Continuity from this front pair of pins is "open" to any other part of the circuit or parts of the switch, and there are no "traces" on the board to suggest connections here. I am able to follow the flow and traces on the board at the 2nd and 3rd pair of contacts. One side(right side looking at bottom at solder side) of each of these 2 pairs is connectd(trace) to the wires entering the board. The opposite side(left) is connected(trace) to wires leading to transormer and fuse link path.

This last pair, with switch on, is not connecting to the fused portion now, which is why I was considering soldering a small jumper here. When I push switch on, the other pair would connect and the amp should go on. When switch is off, one pair will be disconnected, shutting the amp off. Should this work safely to get me to do my other tests....?

The other option is to remove/bypass the switch, hardwire each of the 2 pairs, and plug in/out to turn the amp on/off til I can get the amp working and lastly try to find the proper switch when I get everything else working properly. (If the switch is bad anyway, it wouldn't probably hurt to take a look in there and see if there's anything that could possibly be fixed, or hardwire that last pair inside the switch as a last resort, if my other scenario is sound, using one pair for on/off.

I'm not able to test any of those other voltages without power to the board, so which option would you suggest............?

Sorry to be so wordy with all this, but I'm trying to paint a full picture for you, so I guess that's worth a thousand words.......

I know it's not much, but at least I was able to isolate this problem with some diligence and able to describe how/why it works. I guess I'm better at the more mechanical/simple flow end of this, and a little hazy on how the individual resistors, diodes, caps, transistors, etc. interact to make what makes a complete well-performing amplifier.

I may have to buy you a whole pub full of pints for hanging in there with me and helping me through all these learning stages.........