Gain control now clean like master volume in Prince Boogie copy amp.

[Paolo]

Hi Guys

I really appreciate all the time your spending on this and advice you're giving me!

I think i really have to step up to the plate now and get stuck in, now that that you guys have identified the potential faulty area and slung some theories out there. I don't understand most of the technical language but i'm sure it's informative for the other parties to speculate on.
Unfortunately, i don't have any one at hand with electronics knowledge that can assist me, but I definitely need to try and trace that part of the circuit myself and start learning about this stuff, as Phil suggested. I think i will probably have to draw it out in my own way though, as i can't relate schematics to actual pcb's.  In other words, it will be a drawing of the PCB
with linking lines denoting the connections.

I also think Teemuk and others are right about the quality of the photos, so i will do some new ones in the next day or two, focusing on the problem area. (Thanks for those photography tips Roly!)  As i said, initially i just wanted to get the whole pcb in the photos and i'm a beginer with this stuff, so i didn't know what people would want to see the most.

Until i get the new photo's done and try and check the circuit path, i have another (definetly simple!) question about the reverb, which i will make a new post for.

[Roly]

{Still do as Phil suggested as best you can, your own way.  One tip I can suggest is that you make a drawing/tracing of the copper side area of interest, then try to locate the components on the upper side onto that drawings.  Done as carefully and accurately as possible, that alone would be very helpful.  When trying to locate components a powerful desk lamp shining from the copper side can give you an "X-ray" view.

Resistor colour code charts;
http://www.google.com.au/search?q=resistor+colour+code

On-line calcs;
http://www.csgnetwork.com/resistcolcalc.html
http://www.hobby-hour.com/electronics/resistorcalculator.php
}

Okay, a few things you can check/try before posting photos.

See bottom view, attached.  Measure the resistance between ground "Gnd" and point "A" while varying the gain control.  The resistance to ground at this point should vary between nothing and whatever the value of the gain pot is (a few k ohms?).  If this appears to be open circuit try remaking the solder joint to the pot directly above "A".

Resistance swings up and down?  Good.

Next do the same thing from ground to point "B".  In this case the resistance should go from the upper value found above, plus about 1k, to around 820 ohms.

Don't get this result?  Then the 820 ohms (grey/red/brown) resistor is faulty, (but this is quite unlikely).

Do get this result?

Try replacing the electrolytic cap (shown in green circle in the top view) with the same value (this doesn't have to be exact, 10-20% more capacitance and/or voltage is fine).  Note value and post here as soon as you know.  This cap is polarised and must be replaced in the same polarity (use marking pen to mark on the board where black negative mark is on the cap before you remove it).

Cured fault?  Post and we can all enjoy a beer.

Same as before?

Now we get out of repair territory into mod territory.

When you go to buy the new cap, also buy the following resistors (if you don't have to hand or can't scrounge from some dead radio or such); 680 ohms, 560, 470, 390, 330.

Replace the 820 ohm resistor (blue circle in top view) firstly with the 470 ohm (don't cut the leads short, mount it up in the air a little bit, say half and inch).

The gain control should now produce more gain at the top end.

You may get the following results; just right, not enough, too much, wild oscillations near the top.

If not enough, try a lower value, if too much try a higher value, if wild oscillations try a much higher value.

Once (if) you find a value that gives you satisfactory results you can remount in down close to the board like the original (and make a note on the board of your mod, original value, the date, and your initials).

Post results.  {Drink well earned beer.  :dbtu: }

[Paolo]

Hi

I have been trying to draw/trace the circuit around the gain area, which has took around 6 hours!, but i've just looked at the photos i took, and i think the x-ray pictures are actaully better. I think this will leave no doubt where everything is, and someone will be able to see it as a schematic. 

Roly - I will try and work through your suggestions shortly, but i might not have the components at hand to try some of the suggestions and will have to order them.

Anyway, i hope the new photos are helpful! If anyone wants any components or connections  identified more specifically i will try to see what is written on them or where they join. The ony thing i can't tell from when i was drawing it out was, where the silver metal towers are connected underneath.

I think this site is terrible for being able to view pictures! Each picture should have a scroll bar to move the view along, and/or show them full size when you click on the thumbnail.  The best thing to do seems to be to click on the blue file name under each photo and open them in your prefered image viewer and make your own copies.

I have also updated and put photos on the reverb problem post.

Paolo

[Paolo]

Hi Roly

1. Measure the resistance between ground "Gnd" and point "A" while varying the gain control.  The resistance to ground at this point should vary between nothing and whatever the value of the gain pot is (a few k ohms?).

A: This reads from zero to 2.7 ohms while turning the dial.  The gain pot itself says B3K ohms.

2. Next do the same thing from ground to point "B".  In this case the resistance should go from the upper value found above, plus about 1k, to around 820 ohms.

A: At full clockwise it reads 820 ohms, then gradually gets into the 1000's as you turn it back to Zero. (I have to point out here that i always have a real problem reading the meter, as i'm never sure which setting to put it on in the ohms catagory. i don't know the difference between ohms and k ohms and stuff like that! The meter i have is also very basic).

3. Try replacing the electrolytic cap (shown in green circle in the top view) with the same value (this doesn't have to be exact, 10-20% more capacitance and/or voltage is fine).  Note value and post here as soon as you know.  This cap is polarised and must be replaced in the same polarity (use marking pen to mark on the board where black negative mark is on the cap before you remove it).

A: The only writing on the elec cap is Tracon and 10/16.

    These are the spare elec caps i have:   4.7 uf/25v
                                                        10 uf/35v
                                                        22 uf/10v
                                                        22 uf/50v
    Should i try any of those?

As for the mods, my playing style doesn't need loads of overdrive, as i like a gritty, open medium break-up overdrive. Sort of keith richards to george thorogood tone. So i probably don't need even more overdrive than the amp does already! The overdive i was getting with the EHX signal booster pedal sounded pretty good, a bit like my Fender tweed Bronco, but with it's own 'unique' 70's Japan Solid State touch!

[Roly]

Photos much better, thanks.  You really only need to work out the first bit, first two controls or three transistors from the input ('tho having an entire tracing to work from and offer others is helpful with any problems down the track, and to others struggling).

"10/16" will be 10uF/16V, but the value nor the voltage rating are critical here, anything within 2:1 will be fine.

Reading 1. is inconsistent with reading 2. but your comment explains that; "k" = "kilo" = x1000.

Reading 2. is as it should be.

Any of those caps will do, it's mainly a matter of what will physically fit.  I'd select the 22uF/10V on the grounds of voltage first, then the 10uF/35V on grounds of value, then 4.7/25 on voltage.  Just watch you get the polarity right - the black stripe means the negative lead and will almost certainly go to the 820 ohm resistor+pot.

What this bit of the circuit does is change the voltage gain of the first preamp stage, and the 820 ohm resistor sets a minimum resistance which sets a maximum gain.  By reducing this resistor you allow the stage to go to a higher gain, more crunch.

If you carry this to extremes (such as putting in a wire link instead of any resistor) the gain will go to a very high value.  This may work but the amp may also become unstable with so much gain, and oscillate at high gain settings.

Something for you to experiment with.

Try and find some sort of old electronic junk, radios, Hi-Fi amps, whatever, you can recover components from.  Anyone who is into electronics has a well stocked "junk" box (although the class of the junk varies quite a bit).  It is just shocking how many perfectly good components go to landfill every day. I haunt my local rubbish transfer station and tip shop, garage sales, op-shops (thrift stores), roadside hard rubbish collections, &c&c.

Keep us posted on how you get on.   :tu:

[Paolo]

Hi Roly

I've tried a few things from your suggestions with some positive results!

1.  I changed the electrolytic cap with a couple of the ones i had spare (22uf/10v & 10uf/35v, but this didn't seem to make any difference at all.  Eventually, after the whole testing session, i put the original back in as i don't think it was the problem?  Is this what you thought was the main problem?

2.  I took out the 820 ohm resistor and I tried a 467 ohm resistor. This didn't seem to make much difference.

3. I didn't have any resistors of a lower value, so i just went straight for a jumper wire. This gave the massive overdrive that i got when i used the EHX clean booster pedal before it. This was in the 'Boost' input, but i also plugged into the 'Normal' input and got similar results at a lower volume and reduced gain.  So now we know the amp works on the same basic principle as most amps with a high and low input.

4. From 1 - 8 on the gain knob, nothing really happens different from before (just gets louder), then the overdrive kicks in dramatically between 8 - 10. , with little scope for much fine adjustment.  I assume this is because there is no resistor, just the jump wire? 
The overdrive itself is pretty raw in a crunchy marshall distortion way, with an undercurrent of fuzz. There is artifacts in the sound at higher volumes, but this is probably due to vibration from the amp being open and not all screwed down. There i also a high pitched whine when using the headphone socket with the gain on full using the Boost input.

Some people would love this tone as is, but i need less (or the more scope to go from less to more) and a more gradual sweep on the Gain dial.  It's amazing how one tiny part can make so much difference!

I've sent off for some of the other resistor values you suggested, so i'll try those when i get them.

Only recently, i chucked out alot of old hi-fi equipment/ ghetto blasters that were partly broken.  Some good made in japan stuff from the 70's. I took out some electrolytic caps and normal caps and some other parts, but didn't take out any resistors because i didn't think i would be able to identify them, or even need them for that matter.

I really appreciate all the help and advice so far. Getting some results now!

Paolo

[Roly]

1. okay, this suggests that there isn't actually anything wrong and it is working as designed, just not to your satisfaction.

2. no it wouldn't be dramatic, roughly a doubling of gain. (820 / 470 = 1.7 times)

3.

so i just went straight for a jumper wire.

Yikes!  Oh well, okay, so now you know what it will do with the preamp first stage pedal-to-the-metal.  8|

Just guessing on the numbers you gave I'd say that distortion due to overload will start with the "gain" control flat out with a gain-limiting resistor of about 330 ohms (orange/orange/brown), so if you want a bit of crunch right up the top end of the gain control I'd go for something like a 220 ohm (red/red/brown).

I assume the "467" was from your ohmmeter (maybe with your fingers in parallel?) and it will be a nominal 470 ohms (yellow/violet/brown).  Holding the meter probes onto the resistor lead with your fingers works up to a few k ohms, but above that you start to get errors due to you body resistance being about 20k, and not constant at that either, so you can only hold one end.

The high pitched whine may be the oscillation I was worried may happen with very high gain, but it could be that it is external electrical crud (light dimmer, electric drill or blender, &c&c) and you can now hear it because the amp has a whole heap more front end gain to amplify it (and it may be reduced or eliminated when the amp is reassembled).

Yes, well, the pot sets the first stage gain and the resistor limits how high that gain can go, so replacing the limiting resistance with a bit of wire seriously supercharges the first stage.  Now it's a matter of tweeking the value of that resistor until you get the result you want, or at least can live with.

I have a large box full of printed circuit boards ripped out of sundry stuff.  The cases are generally useless and just take up space, but you can get 10 or 20 PCB in a small cardboard box, and whenever you are short of a cap, LED, resistor, or even transistor you can hunt one down and recover it.

Learning how to read the resistor colour code on sight is very helpful, but you are normally only dealing with about a dozen values and their multipliers.  Get a colour copy of a resistor cheat-sheet and pin it up above your soldering iron, and pretty soon you'll be picking up resistors and seeing their values without looking them up.

It's one of Murphy's very many Laws, you'll need tomorrow what you threw out yesterday.

Have we now moved closer to what you originally wanted?

[Paolo]

Hi Roly

Yeah i think we/you are really starting to solve this problem!  I just want to get the amp to sound how it should have done originally or better.  I disagree though that the gain knob was designed to not produce any gain! I think the fact that we can get a big overdrive sound now proves that it's always been there?   

I'm also learning alot from doing this. While i'm on the subject, could you explain what the electrolytic cap does on my amp gain and why you thought of changing it? I'd like to learm as much as possible. I still don't really know what causes the overdrive other than the resistor controls the amount.  I only know about diode and LED clipping from pedal mods.

Yes i probably had my fingers contributing to the ohm reading! I've included a picture of the meter i use. As you can see on the left, there are several settings for testing ohms and as i said sometimes i'm not sure what to set it on, but i think i'm getting the hang of it more now. Prior to problems with my amp i'd only used it to test the resistance of pickups and to test battery power.

The 467ohm resistor was blue with dark blue stripes.  I got sent some with some LEDs i ordered for a pedal mod.

Perhaps the real problem with the gain on this amp (as i've mentioned over on the reverb problem post), lies with the ghost overdrive, which is still there under the regular crunchy overdrive. It's like two guitarists playing the same thing through the same amp, one using a distortion pedal and the other on overdrive pedal where the batteries in it are dying and it's like a splatty fuzz tone that has a longer sustain than the first guy's guitar. 

This is also present in the reverb and increases when the reverb knob (and/or the gain) is turned. It's always been like that.  It's not present if the gain knob is completel off.  I'd rather have the gain working than reverb, so it's not a big deal but i'm hoping we can fix this issue also, once i get the right ressistor to set the gain at the right balance.  Do you have any ideas what causes the ghost fuzz? 'Phatt' seems to think it can't be fixed (at least in the reverb).

I'll definitly have to start saving old cicuit boards. It's a good idea.

I really appreciate all the time you've spent on this and the step by step advice.  You have a lot patience!

Paolo

[Roly]

Paolo, I've been working on guitar amps for fifty years odd and I can sadly assure you that amps that are poorly designed are far too commonplace.  Even very well known names like Marshall have amp models that are dogs to live down.

The fact that we have found no actual fault tells us that this amp is working as it was out of the factory.

"Gain" is just another word for "volume" and as I understand your posts this control has been changing the volume, just not as much as you would like.  "Overdrive" means enough gain to drive the amplifier into clipping or limiting with the attendant crunchy or grinding tone of distortion due to the amp being driven past its clean range.

This amp has two inputs, "normal" and "boost".  Well actually what it has is one input that cuts the input signal back a lot, "normal" and another that cuts it back a bit less, "boost".  These two signals are then taken to the same place, the input of the variable gain stage where the gain is set by the gain control.

Why did I suspect the electrolytic?

A couple of reasons, firstly that these types of caps "dry out" over time and lose capacitance which would, secondly, be consistent with a remark you made that suggested that the gain control was acting like a middle/treble control.

This could be simply subjective, or it could be caused by a cap in that position that wasn't fully open circuit but had only lost a lot of it capacitance, so I got you to test it by substitution.

Since this produced no change we could then assume it was okay, and move on to modding the circuit to get more gain from the first stage, i.e. by reducing the resistor that sets the upper gain limit on the gain pot.  When you put in the wire link you experienced how high the gain could go.

As I said above, once the gain got high enough some stage down the chain ran out of headroom and started clipping, giving you "overdrive" gunge.  As it happens (and this is very common with solid-state amps) this clipping didn't sound too nice, and may give us a clue as to why the manufacturer selected a resistor value in series with the gain control that prevented you getting into this gain territory.


The resistance ranges, like the other ranges on your meter, define the highest reading on that range, so "200" is for measuring resistance up to 200 ohms, "2000" to 2k, and 20k, 200k and 2meg, ditto.  It is particularly important with these last three higher ranges where you have to be careful about your fingers - just try measuring your own resistance between your hands and you should see why - you are in parallel with what you are measuring, and you ain't no insulator. 

Just another point about resistance measurement is that they must be made on circuits that are dead - voltage free - or you will a) get incorrect readings, or b) blow up your meter.  All of these style of meters are also quite prone to dirty connection on the range switch, so if you get readings that don't make sense, just try wiggling the range switch a little bit.

A resistor with all blue stripes would be 660megohms.  Google "resistor colour code" and learn how to work out the value of resistors from their colour bands.



...and...

http://www.michaels-electronics-lessons.com/resistor-color-code.html

Phil has been paying much more attention to your reverb problem than I have, so I would trust his advice/opinion.

HTH

[Paolo]

You know alot more about this stuff than me, so i'll just have to accept it was manufactured like that.  As far as i'm concerned though, it's been fixed, BY YOU, ha ha!

Well the main part anyway.  I still don't know what is causing the ghost overdrive/fuzz, and you didn't mention it in the last post. Unless you are calling this "Overdrive Grunge", in which case if it happens "further down the line", i'll just leave it, as any more messing around by me might damage the amp.

When you suggested changing the elecrolytic cap, i thought this would make the overdive happen, so that is why i said it didn't do anything.  I wasn't listening for changes in the mid/treble. Should i try replacing it again, once i've got the right value resistor in place.

In a way, the gain knob working in this mid/treble manner was a good thing as it contributed to the woodyness of the middle and the chime of the top end at low volume.  Some of that will now be lost in favour of a fixed overdive and an improved reverb (with new tank).  The gain knob will have to be on zero, to avoid the ghost overdrive coming in for clean tone with reverb.

Ideally, i'm hoping for the great clean tone to remain, with good reverb available to that clean tone. I don't use reverb with overdive so that shouldn't be an issue really. Then also i'm hoping for a nice range and sweep of overdrive from the gain knob (after i've found the right resistor value). Obviously, given the manufacturing limitations, these two things will have to remain strictly seperate to both work effectively.  If i get all that i'll be really pleased, and we can say we have made a great improvement to this amp!

Yeah, Phil has been the main guy focusing on the reverb issue, i might attempt his stand alone reverb tank one day.

Thanks for the explanation of the gain, the components and the resistor chart.  Very useful info. I've learned alot from this forum.  I even have some idea now of what is probably wrong with the excessive bass on my Fender Princeton Chorus. So keep an eye out for that post! I even have a schematic!

What does HTH mean at the bottom of the post Roly?

[Roly]

Well, apart from playing around with the resistor value in the preamp you can also play around with the value of the capacitor.  For large values the gain control will effect all frequencies equally, but as you reduce its value the gain will tend to be tilted towards higher frequencies.

As the original cap was 10uF you could try some larger values and this might make the bass end more responsive to the gain control (and maybe not, maybe it is already large enough) but you can certainly try some lower values such as 1uF, 0.1uF and 0.01uF.

The smaller values will be non-polarised types, such as "greencaps", that can be put in either way.  These will confine the effect of the gain control to higher and higher frequencies.


My failure to mention your "ghost overdrive" was deliberate.  Where ever possible in electronics you try to deal with only one thing at a time, or things can very rapidly get confusing.  Now that you seem to have the preamp under control to your satisfaction we can move on to the next problem.

I suspect that what you are calling your "ghost overdrive" is actually some distortion arising later in the amplifier, and my guess would be in the output stage.

You will notice that there are two output transistors.  What happens is that the signal is split into the top half and the bottom half, and each transistor amplifies its half, either the positive half or the negative half.  Since the signal consists of waves that swing back and forth between positive and negative so each of the transistors take turns to amplify the output.

Now ideally when one stops, the other should take over seamlessly, however that isn't always the case and you can have a tiny pause in between.  Since the signal is crossing over from one to the other this sort of distortion is called "crossover distortion" and happens to be a rather nasty sounding form of distortion even at very low levels.

To overcome this, or at least minimise it, the two output transistors are biased so that they are both slightly on or conducting when at rest, so that when driven one starts to conduct a bit before the other stops (somewhat like the next runner in a relay race starts running a bit before the hand-over).

In inexpensive amps (and even some expensive ones) it is easier to leave a bit of crossover distortion rather than put in the components to set the bias correctly, and to hold it correct for changing temperature.

Lacking a circuit it is hard to say, but I suspect that this is the cause of your "ghost overdrive".

If you follow the wires from the transistors on the heatsink back to the board, it could be helpful to have a photograph of this area of the board where they go.

Also, the transistors on the heatsink will have type numbers such as 2Nnnnn or 2SAnnnn, 2SBnnnn, 2SCnnnn, or perhaps just A, B, or C followed by some numbers - these would be handy to know since they will give a hint to the circuit being used.

HTH = Hope This Helps.

[Paolo]

Well, i might have the preamp under control to my satisfaction, if the electrical store could get my order right. They've sent me the wrong thing twice now, so i still haven't been able to settle the resistor value equation. But that should be solved in a day or two.

What i can do though at the moment is tell you the codes on the heatsink transistors, which are TD526, then underneath that code is the code 01 E.  I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right. I will take some photos of the area later today.

The white gunk underneath those transistors that you mentioned previously is bone dry.  I don't know if that has any bearing on anything?

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

Get back to you soon

Paolo

[joecool85]

The white gunk underneath those transistors that you mentioned previously is bone dry.  I don't know if that has any bearing on anything?
Paolo

If by "bone dry" you mean crumbly, then that's not good.  I would unmount the transistors, wipe off the crud and reapply some new thermal paste.

[Paolo]

Hi, thanks for your interest.

Well from Rolly's initial description i was assuming it would be a thick sticky substance, but it's a thin coat/film. It's not exactly crumbling, as it's not thick enough to really crumble! It IS bone dry though. Probably to be expected after 30 years or more?

Paolo

[joecool85]

Hi, thanks for your interest.

Well from Rolly's initial description i was assuming it would be a thick sticky substance, but it's a thin coat/film. It's not exactly crumbling, as it's not thick enough to really crumble! It IS bone dry though. Probably to be expected after 30 years or more?

Paolo

It wouldn't be sticky, at least not in my experience.  Silicone based thermal paste is slippery and should stay gel-like (similar to toothpaste, but slippery like I said) to work properly.