Got a problem with an amp

[teemuk]

...Or the troublesome part of the circuit runs cooler when boards are not mounted to the chassis. (This usually results into better circulation of cool air). I would assume that a short circuit would exhibit some problem symptoms constantly - not after a delay. Delays are often sign of thermal problems.

If the chassis is a crucial part of the heatsink assembly DO NOT run the amplifier without it: Heat is the worst enemy of semi-conductors! Reducing the junction temperature by 10 degrees of Celcius will double the life of a transistor and vice versa: Running it 10 degrees of Celcius hotter will halve it's lifetime. The transistor operation is also very much affected by the temperature and reliability of most designs is based on assumption that the transistors are attached to at least a moderate size heatsink.

I would be interested in seeing couple of photos. Maybe they could tell something that the schematic fails to show.

[SolidxSnake]

The only part of the electronics that are cooled by the chassis are the four transistors (Q12, Q13, Q15, Q16).

I'm positive it's not a short in the chassis, there's not a single thing that could cause a short besides the two jumpers that contact the screws for the PCB.

I'll take some pics and host them.

[SolidxSnake]

Pics are here:

A pic of everything (chassis, PCB, speaker and everything else):
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0898.JPG

A shot of the PCB:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0899.JPG

Shot of the fuse, power switch, transformer, and related areas (connections between transformer and PCB):
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0900.JPG

Shot of the DSP and footswitch jacks:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0901.JPG

Shot of the resistors I suspect to have problems.  The three towards the left from front to back are R89, R88, R90.  The two to the left from front to back are R64, R61.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0903.JPG

Shot of capacitors C39 and C40, and another angle of suspicious resistors:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0905.JPG

Shot of R72 on the left and R74 on the right, AP1 to the far left:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0906.JPG

Shot of the four transistors Q12, Q13, Q15, Q16.  The metal plate they are connected to had a thermal paste between it and the chassis when attached.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0907.JPG

Shot of the Insert and External Speaker jack.  Note the Insert jack is missing (moved over to the external speaker jack which broke), but the only contact that is supposed to be shorted but wasn't is jumpered with a wire.  I haven't bought a switching jack to replace the lack of jacks.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0908.JPG

Shot of the speaker:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0909.JPG

Shot of the chassis and transformer.  Note the thermal paste in the BG, that is where the four transistors and their "heatsink" lay.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0910.JPG


In the image of the suspicious resistors, the three on the left (R89, R88, R90) were getting extremely hot.  The two to the left (R64, R61) were still very hot, but not as hot as the three to the left.

If you need any more pics or sound clips (I couldn't recreate the problem while the amp wasn't in the chassis), shoot me a message.

[teemuk]

My first impressions:

I assume the photos did not show the real heatsink? Don't tell me your only heatsink is that attachment plate, which is mounted to chassis...

The resistors R88, R89, R90 in the power supply section look more burned than normal. Either Crate has done a poor design job or the preamplifier circuit is drawing excessive current. Do the resistors excessively heat that big capacitor beside them (C39?) ? If so, this is also a very bad thing since capacitors hate heat and their lifetime seriously decreases in elevated temperatures.

Q8 might not be tracking the heatsink's temperature accurately. It doesn't seem to be in much contact with it.

[SolidxSnake]

The only heatsink that amp has for the four transistors is the metal plate and the chassis.  Nothing else.

The resistors do not heat up C39.  C39 and C40 are both just lukewarm, nothing else.

Now that I look at those resistors, they do look a bit charred.

It does seem a large possibility that the preamp circuit is drawing too much current.  Any ideas how that could have happened?

[teemuk]

The only heatsink that amp has for the four transistors is the metal plate and the chassis.  Nothing else.

Wow! What the hell have the designers of this amp been thinking? Just take a look at these three amps and their heatsinks. They're all about the same power as your Crate. You likely will realize why your amp seems to run better in open air...

http://home.swipnet.se/malman/lowtim_circuitboard.jpg
http://www.littlefishbicycles.com/poweramp/monoblock_oblique.jpg
http://sound.westhost.com/p27-amp3.jpg

The resistors do not heat up C39.  C39 and C40 are both just lukewarm, nothing else.

That's good.

It does seem a large possibility that the preamp circuit is drawing too much current.  Any ideas how that could have happened?

Well, either Crate did bad poor on calculating the dissipation requirements of those resistors - which I seriously doubt - or there truly is a "soft" short in the preamp.

The simple Zener regulator is the greatest current hog of the preamplifier circuit since there is a constant shunt to ground over over it. During normal operation the circuit will draw about 90mA ((40V-15V)/270R) and the rail voltages should be around 15 - 16 V. Did you measure them? In idle state (and when the load impedance presented by the preamp is high) most of the current flows through the Zener (which dissipates about 1.4W). The series resistor constantly dissipates about 2.25 watts (25V*90mA) but since its rated for 5W it should be Ok. This is a pretty poor regulator circuit but that's another story.

If there is an excessive current draw (that is required to really cook up those resistors) the load circuit in parallel with the Zener becomes more dominant and begins to hog most of the current instead of the Zener. Even when the load is about 270 ohms (the size of series resistor) the regulator still works like described (drawing about 90mA). However, most of the current now flows to the load instead of the Zener.

If the load drops below 270 ohms the load and series resistor effectively form a resistor divider and the Zener begins to drop out of the equation. The current draw will be higher than 90mA and the amp will start to experience some supply voltage sag in the preamp rails. When the voltage has sagged below 15V or so the Zener can't conduct and its current draw drops to zero. The preamp load will now solely dictate how much current is drawn.

So, if the supply rails have lower voltage than about 15 -16 V it's a pretty good indication that there is for some reason excessive current draw. You can also measure rail-to-ground resistance (which is the "load" I'm talking about) but since this value is practically unknown it can provide only slight clues; it should likely be in the order of kilo-ohms but at least higher than few hundred ohms. A load of approximately 23 ohms would cause a current draw (and sag) that would result into full 5W over the series resistor.

[SolidxSnake]

Starting the amp cold, I measured across the Zener diodes, one has 16.15vDC, the other is 16.65vDC.

The amp is giving me a constant hum, starting cold or leaving on for a while.  The hum doesn't go away.  It did once, after I shook the chassis (and in turn shook the PCB).  The hum I can record for you if you might want to hear it.

I rechecked the voltage across R72, and guess what, it dropped down to 5mV...  I readjusted it.  After letting the amp warm up a bit, still idle, I checked the Zener diodes again.  The first was 16.7vDC across and the other dropped to 15.3vDC across.  This is without any input, only the speaker connected.  Also, this is with the amp back in the chassis.

[SolidxSnake]

Sorry for the double post, but..

Turns out all my measurements in my past post were wrong.  Turns out the solder joints for C39 (very large capacitor) were cracked.  The broken solder joints were also what was causing the hum, reheated the joints and let the solder reflow and now the amp works like it used to when cold.  I measured the Zeners after playing for a bit.  One was a little over 16v (I believe 16.34vDC), the other was nearly 17v (16.97vDC).  My electronics teacher had told me that the Zeners should be extremely accurate to 16vDC, +- a few hundreths of a volt.

I haven't measured the rail voltages.  Where would I go about measuring them?  As you may have guessed, I am not the most electronically able person in the field of theory (such as, I don't know the basics about different kinds of components and how they should act, etc), but with some guidance I most of the time am able to troubleshoot problems if I know where to specifically look.

Edit:  Looking at the schematic, I see four things which I assume to be rails.  There is a +40 and a -40v rail, as well as a +V and -V rail.  I assume you mean to measure the V+ and V- rails, and they should be ~16v?

If the +V and -V are the correct places to be measuring, then I have measured them.  With the chassis as a reference point, +V (negative side of the Zener D25) is roughly 17V after a bit of playing, whereas the positive of D26 is around 16.3v.

Are D8 and D9 Zeners?  Their schematic symbol don't look like Zener diodes, but they are reverse-biased, no?

[teemuk]

I am not the most electronically able person in the field of theory (such as, I don't know the basics about different kinds of components and how they should act, etc

I got to say what's on my mind: Are you sure you can fix this amp mostly by yourself with the current level of technical knowledge you got? I mean, likely the people at this forum can only help you a little and the main amount of work is done by you. Is this a "learning project" or do you need it badly? In the latter case I would take it to a tech.

Helping to troubleshoot circuits in an internet forum is hard enough already - and becomes even harder if the guy (or girl) who's trying to fix the thing doesn't know a lot about the circuit in general. If there is anything but an obvious problem you might get plenty of different answers and most of them are speculations of what could be wrong - and people have to speculate because they can't have your amp and measure it. If the answer is obvious (like it was in the case of that bad solder joint you had: hum stops when you shake the amp etc.) people are usually too lazy to even bother answering.

Don't get me wrong, I don't mind answering basic questions but some of those answers... well, could be learned from theory books as well.

There is also a limit in how much I can help. I don't have the amp and I can't measure it. ...And at the current point I'm a bit confused what's wrong with it anyway: You fixed them hum - which was actually a very important fix because the concerned capacitor was pretty much the only effective filter for positive rail - but you also mentioned overheating and sound cutting in and out. Have you located if the latter problem is in the preamp or in the power amp yet?

What problems did fixing the solder joint fix? For the record, Crate is infamous for making amps with poor solder joints so you should check all of them, starting from the components under mechanical stress: Jacks, potentiometers, switches, heavy components such as big capacitors etc. Then proceed to components carrying high currents. Don't assume they are OK just because they look so: You really need a magnifier glass to inspect them properly so you might as well redo all soldering just to be sure.

Anyway, a fair warning is also in place: If you don't know the correct procedures of working with an amplifier you can easily ruin it to a point where even a certified tech wouldn't touch it. For example, it doesn't take much effort to ruin those "new type of" PC boards with extremely thin copper and no solder reinforcement. Even worse, you can shock yourself badly: That amplifier stores very high currents and the voltage across the rails is pretty high (although not considered lethal). ...And then there is mains circuitry.

Edit:  Looking at the schematic, I see four things which I assume to be rails.  There is a +40 and a -40v rail, as well as a +V and -V rail.  I assume you mean to measure the V+ and V- rails, and they should be ~16v?

Yes. First few things about the higher rails: The supply is loaded by the varying resistance of output transistor and the speaker load. Together with the internal supply impedance they form a voltage divider. The high voltage rails (measurement points TP14 and TP15) are at the joining connection of the internal power supply impedance and transistors. This is why you measure about +-35V instead of +-40V when there is an 1kHz, 1Vpp sine wave at the insert tip. In short, the rail voltages drop/vary according to severity of loading. This is known as "sag".

The +-15V/16V supply rails are regulated, meaning they have (or should have) a constant voltage regardless of loading. The voltage is constant because Zeners shunt anything above 15V-16V to ground.

If the +V and -V are the correct places to be measuring, then I have measured them.  With the chassis as a reference point, +V (negative side of the Zener D25) is roughly 17V after a bit of playing, whereas the positive of D26 is around 16.3v.

The Zeners are rated 16V so anything above this is not right. The voltage at these rails should stay constant under any normal operating condition. Check the Zeners and their solders.

Are D8 and D9 Zeners?  Their schematic symbol don't look like Zener diodes, but they are reverse-biased, no?

I must be blind but I couldn't locate these. Where are they?

[SolidxSnake]

The amp at the moment is essentially a learning project, as this amp has been dead for a while and I got new one in the meantime.

Fixing the solder joint got rid of the hum.  When it was humming, a guitar plugged into the input jack would not be heard, only the hum; changing anything (gain/clean channels) did nothing.  Fixing the solder joints did allow the amp to "work" as i was previously.

I work with electronics quite a bit.  I've got plenty of practical experience, but theory-wise I don't know so much.  I've got knowledge about how to handle electronics and such, so I should be fine actually doing the physical work.  However, I'm not very able alone to figure out the problem based on the measurements and other inspections I find.  The amp however is basically a learning project, so fixing isn't on the top of my priority list.

I can't test TP14 or TP15 at the moment.  I did have a function generator, but no dummy load to replace the speaker (I need a 4-ohm load, it is far too loud with the speaker connected).  I will probably end up building a switchable 16-8-4ohm load soon enough however.

I'll check the Zeners and their solder joints.

For the past few days, I've been playing guitar for an hour or so and the amp gave me not a single problem.  However, the voltage from +V is 17v still, and the 5w resistors are still extremely hot.

[teemuk]

The resistors are supposed to be (at least) fairly hot. Currently there is (in the worst case scenario) a 23 V drop over the resistors: 40V - 17V = 23V. According to I=U/R, I = 85mA so P=UI means P = 1,96 W. Well within the rated 5W. Likely the heat is "normal condition". Each resistor still dissipates about 2 watts. That's the problem of shunt regulators in general; they are pretty inefficient.

If the solder joint in the mains filter capacitor was failing it could have caused some occasional volume drop but likely you would have heard the signal being modulated by hum as well - and some loud "pops" and "crackling" noises. (Up to the point the solder initially failed and you started hearing only hum). It's difficult to say anything certain about this. I still think it's a thermal problem, maybe a solder joint of a resistor failing somewhere.

Good luck on the project. I'll try to help if I can ...and for starters (on improving that theory part) you might want to take a look at this:

http://www.pacificrecone.com/JackDarrBook.html

It's not the best book to learn about solid-state power amplifiers and guitar circuits but has a strong focus on troubleshooting and fixing stuff. You might find it helpful.

[SolidxSnake]

Previously, my amp was giving me the symptoms you were describing (with the solder joints breaking).  Sound cut out, hum appeared, pops, crackles etc.  Haven't gotten any of those since I started working on the amp again (from the start of this thread).

You say the resistors are supposed to be fairly hot.  I would assume they are too hot at the moment however.  I can hardly touch them for more than a second or two without my fingertip nearing the scorching point.

I'll post back if the problem arises again, but at the moment the amp seems to be working fine.

Thank you very much for your time.  It is highly appreciated.

And thanks for the link.  I'll definitely check it out.  I know the basics of the amp (preamp->poweramp, what those stages do, etc.).

[teemuk]

Ok then. I guess it would be fairly safe to assume that the failing solder joint was causing those volume drop issues as well. Anyway, you'll know for sure if the problem appears (or doesn't appear) again.

The resistor issue is a bit more troublesome: According to your measurements (about 17V at the regulator side) they should not dissipate power thar exceeds their ratings. However, I would suggest that you measure the higher rail voltages just to check they are within about 40V - 35V. You don't need a dummy load for this, just check the voltages without signal applied - at this state the voltages are highest anyway. Anything exceptionally higher than +-40V would explain the very hot resistors - and indicate about more serious problems.

If those voltages are correct then it's likely just too much power for those resistors to handle. This isn't practically very ideal situation since too much heat can melt the solder joints (and that capacitor in close vicinity to resistors won't like it either). If this is indeed the normal operating condition - and if you want to solve this issue - you can substitute the resistors with something that can handle more power (i.e. two 5W 560 ohm resistors in parallel per each 5W 270 ohm resistor). Personally, I would rebuild the regulator circuit to be active type but that's another story.