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[Paolo]

So is this thermal paste issue contributing to any of the problems with the amp (gain and/or) reverb)?

Anyway, i'll have to get some of that paste to refresh the connections.

Thanks for the advice on that.

Paolo

[joecool85]

So is this thermal paste issue contributing to any of the problems with the amp (gain and/or) reverb)?

Anyway, i'll have to get some of that paste to refresh the connections.

Thanks for the advice on that.

Paolo

I doubt it.  If it was a thermal paste issue it would only start acting up after being on for a while and fully warmed up.

[Roly]

Nah, something isn't right there; the TD526 comes up as a half watt transistor in a small D-package, and that certainly isn't what you've got there.

Not sure what you mean by...

I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right.

These two transistors act as a "push-pull" pair, one driving the speaker during the positive half of each signal wave, and the other during the negative.

The output stage of your amp will look something like this (with thanks to Rod Elliot)...



At rest the join of R10, 11, and C5 is at about half the overall supply voltage (35/70V), therefore this is called the "half rail", and it's also the output node.

The output pair are Q4 and Q6 (the ones bolted to the heatsink).  When the input swings positive transistors Q1, 2, 3 and 4 all turn on, causing the output via C5 to swing positive and drive the speaker cone in one direction.

Similarly when the input swings negative the transistors above turn off and Q5 and 6 turn on pulling the output negative and driving the speaker cone in the other direction from its rest position.

There are several different ways output stages can be arranged, but this would be one of the most common.


The white gunk is thermally conductive gunk intended to fill the small voids between the nominally flat surfaces and improve heat transfer.  Once  it's in place drying out doesn't matter much since the solids are what do the thermal conducting, it's only a paste form so it's nicely conformal when it goes on.

When you remove a device however if the paste has dried out you need to clean of the old residue and apply a dob of fresh.  Any place that sells power transistors should also sell mounting kits, variously shaped insulators, and stepped insulating washers for the mounting bolts.

This stuff was particularly necessary with mica washer insulators, but there are now grey floppy rubbery insulators that are conformal and thermally conductive and don't actually require the white goo.

(PS) Joe mentions silicon grease thermal coupling compounds, and there are several different types.  The older silicon grease was a milky paste and not sticky at all, but the while gunk that I get, "Unick", is like artist's white paint and sticks to everything, and it sounds like Joe uses something else again that is rather more friendly.

Remounting the output transistors with fresh goo certainly won't hurt, but be careful of the insulating wafers and washers, the mica wafers are fragile and it all must go back so the tab of the transistor is isolated from the heatsink (test with ohmmeter after remounting).

Thanks for the explanations and further advice. You have a real knack for explaining things in a way that i can understand!

My pleasure.  I have been doing this stuff since the days of valves (tubes) and for much of that time I have had trainees.  When it comes to electronics it's us imperfect humans against Murphy and the very picky universe, so I do like a win, and the moreso when I've helped an "improver" to a win and a better understanding.  And you can always award chip points, top left. 

Far too many musicians are far too often victims of their gear.  Rock 'n Roll has always been somewhat technical with amps 'n all, so really the modern guitarist needs to also know enough about electronics to survive.

[Paolo]

Here are some photos of the area where the heatsink wires connect to the board.

Assuming that the input wires to the heatsink are front left, those wires are on the right on the PCB in the pictures. 

INPUT to heatsink: Right hand yellow and the top blue and the (partially hidden) orange wire underneath that.

OUTPUT from heatsink: The next blue wire under those and the lefthand yellow and left hand orange beneath the two big red resistors.

For orientation, the controls pots from left to right in the picture are Tuner/Reverb/Bass/Treble.  From the underside of the pcb they are featured at the top of the picture so you can see where the front corresponds to the back.  There is also an x-ray view of the area.

I hope these photos are ok.  If not i'll take some more.

[Paolo]

Not sure what you mean by...

I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right.

[/color]

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Ahh, i see what you mean.  I thought you mean't transistors on the pcb in your previous explanation/post.  I suppose i've overexpained then in my last post!

[Paolo]

Here is a picture of the holy grail for those non-believers!

(I put the 'L' pen mark on by the way, because i thought i might have to unscrew those to get the PCB out. Glad i didn't now i know about the thermal paste! (which i think i'll leave until everything else is sorted)).

[tonyharker]

Thats a 2SD526 transistor many mfg leave out the 2S part. Data sheet here http://www.datasheetcatalog.org/datasheet/toshiba/3449.pdf
The T is the manufacturer's logo - Toshiba.

[Paolo]

That's great Tony.

Thanks for the ID on the transistor.  Roly should be happy with that info.

Paolo

[Roly]

Thanks Tony, that's much more like it and helps a lot.

Paolo, take a look at your (excellent, crisp, and well lit) pic 874 - a bit below the middle there are a pair of red-ish resistors connected to the yellow wires to the output pair, and marked "0.5 ohm".

These are a dead giveaway as R10 and R11 (Rod's cct above) and when you are dealing with an unknown amp you look for something like these to help orient yourself.

Another thing you look for is if the type numbers on the output transistors are the same (quasi-comp, generally both NPN as here), or different but perhaps by only one number (fully comp NPN/PNP pair, e.g. BD139/BD140, 2N2955/2N3055).

That would make the two blue wires just to the right the output transistor base connections (between the two 150 ohms brown/green/brown/gold, like R8 and R9), and the two transistors to the right will be the drivers (like Q3 and Q5 above).

A couple of variations; your amp appears to use a split rail supply and direct speaker connection rather than a single rail and output coupling cap (C5), so the supply will have a grounded mid-point between the two caps at left, just above the four diodes for the power supply bridge rectifier.





The transistor just above the centre of the pic will be doing the same anti-crossover bias and thermal compensation job as diodes D1, 2 and 3, and the transistor just to the right of that will be the VAS - Voltage Amplification Stage, Q2 above.

I'm guessing, but I'd say that the two transistors on the right-hand edge are a differential "long-tailed pair" doing a somewhat better job of the task of Q1.



Here Q1 & 2 are the long-tailed differential pair, Q4 is doing the bias/thermal comp, and in this circuit Q6 & 7 are providing overload current limiting protection (generally not used in combos where the speaker is permanently connected and not externally accessible).

It's a bit of guesswork, but it looks like your amp uses a transistor bias rather than the string of diodes, but yours doesn't appear to be adjustable, which means that it will be fairly conservatively set, and could well be the source of your "ghost fuzz" - crossover distortion.  Tell me, is it worse at quieter levels than louder?

(http://www.ozvalveamps.org/repairs/solidstateamprepair.htm)

{Here endeth the guided tour of your output stage - postcards and tourist geegaws are on sale on the other side of this post.}

[Paolo]

Awww Roly, you've gone all JM Fahey on me  with those technical terms and complex schematics!

I didn't understand hardly any of that and i can't read a schematic.  I know a few of the symbols but that's it.  Mainly beacuse it's non-linear, non-heirachical, or any other historically acceptable form of information transferrence! I don't think i will ever understand them.  For a start there are no direction arrows from the input through to the output, or between components.

I have included a diagram for everyone's amusement, but if it was like that i could understand it! 

It's going to take weeks to figure anything out, if at all. At least they've sent me the correct resistors this time, so i will be able to get that original gain problem sorted.

[Roly]

{You're kidding, right?}


Signal flow goes;

INPUT -----> OUTPUT

By convention forward signal flow is from left to right.

In the "elecfree 100W" (cough cough) amp above the signal flow goes - Input, C1, Q1, Q5; it then splits, the +ve half wave going via driver Q9 and output transistor Q11 and R17 where it recombines with the -ve half of the waveform which has gone ia driver Q8 and output transistor Q10 and R18.  Then the recombined signal goes via L1 to the speaker.  See attach; forward signal flow in red, negative feedback in blue.

Q4 provides the bias offset between Q9 and Q8 to reduce crossover distortion.

Q6 and Q7 are overcurrent protection transistors (which you can ignore for the moment), and Q3 is a load for Q5 which you can also ignore.


One thing you can do is to fire up your amp, and after it has been running for about ten minutes with the volume and gain at zero, measure the voltage across each of the 0.5r resistors.  It will be quite small, in fact it's possible it won't be anything much at all.  What we are hoping for is about 25mV (0.025 on the 2VDC range) across each.  I suspect it will be lower.

[teemuk]

The input stage.

[Paolo]

Here are the answers to a couple of tests Roly suggested:

1.  One thing you can do is to fire up your amp, and after it has been running for about ten minutes with the volume and gain at zero, measure the voltage across each of the 0.5r resistors.

A.  (Looking at the picture)
     The left red resistor reads 0.015
     The right red resistor reads 0.013


2.  "ghost fuzz" - crossover distortion.  Tell me, is it worse at quieter levels than louder?

A.  Increases with the gain knob. It's about the same on the volume knob.  The more the reverb knob is turned up the worse it gets, i suppose because the sound is being echoed.

I'm pretty sure that it's not so prominent (in general) when the reverb unit is disconnected. (I'm actually suprised the amp still works with the reverb wires disconected!)

Bearing in mind, i still only have a jump lead in place of the first resistor, there is a high pitched whine when a cable is plugged into the boost channel (but not the guitar) and the Gain is between say 8-10 (even with the Volume knob low). It's not feedback from the guitar obviously.

Also we have to remember that the ghost fuzz was there at the begining with the original 1st resistor in place, so it's not because of the jump wire (although it ain't gonna help).

I really should get that resistor set!

Paolo

[Roly]

Just a quicky 'coz I'm away from home at the moment.

Applying Ohms Law to your 0.5 ohm resistor readings;

I = E / R, = 0.015/0.5, = 0.03 amp, or 30mA, which is a very respectable idle current for this output stage and tends to make my guess about crossover distortion due to insufficient bias look wrong.  This should be about right.

Your observation that "ghosting" is less with the reverb spingline disconnected suggest that at least some of this effect is due to vibrations from the speaker being directly picked up by the reverb springline, which I'm sure you have discovered is mechanically sensitive.

If that is the/a source then you need to improve the physical isolation of the springline by resilient mounting and by padded enclosure - there really is no other answer since this is much the same as the feedback you get by bringing a mike too close to a PA speaker.

The reason the amp continues to work with the reverb disconnected is because the reverb is in what we call a "side chain".  The amp "dry" or un-effected signal goes straight through, but some of this signal is sampled off and fed to whatever effects ("Fx") are fitted, or via external Fx loop connectors.  The "wet" or effected signal is then brought back and mixed with the dry signal down the signal path a bit, giving a mixture of dry and wet which goes on to the main amp and speaker.

[Paolo]

Like i said, the ghost fuzz is more like a cheap distortion pedal with the batteries dying mixed in with the amps overdrive. You can hear it as a seperate thing as it has a longer sustain. It's metallic like some one playing down a ventilation shaft. It's not like feedback from standing too close with a guitar or microphone.

I can't say for sure about more or less with the reverb tank attached. The full on gain with the jump wire was clouding the issue.  I'll have to try it again.