Fender FM212R Problems

The way my amp started playing right the other night after I changed U8 and then went back to being fuzzy after a few minutes, and the fact that U8 doesn't come into play unless I engage the Mid Contour, makes me think that all I did was disturb whatever part is causing the problem. Be it a bad solder joint or cracked track or damaged component. Hmmm...
 
Exactly. When you change some part and it works a minute than goes back to not working, it often is not the part you changed, but instead the act of working on it waqs teh issue. Whenever you do anything, you wind up flexing teh board, moving wires, etc.

With regard to the transistors, all I was saying was that Q21 being hot is only a clue. Q21 could be defective, or Q21 could simple be running hot because something else is not right. You can test transistors forever, and maybe will find a bad one and maybe not. Testing does not put real world stresses on transistors. This ciscuit puts what 30-40 volts across these parts at least, dooes your test meter do that? And we could have a resistor that is funny or a solder joint that looks wonderful but is not making good contact with a wire. SO that is all I was saying, Q21 AND the circuit it is in the middle of need closer attention.


The reason we plug a cord from preamp out to power amp in, is because they may not be connected together already. They may be SUPPOSED to be connected together, through the cutout contacts on teh jack. But what if those contacts are dirty of oxidized? Dirty patch jack contacts are one of THE most common problems in guitar amps. So we take 30 seconds and do the test.

One of my shop rules is:
never think up reasons to not test something.
 
Enzo

Exactly. When you change some part and it works a minute than goes back to not working, it often is not the part you changed, but instead the act of working on it waqs teh issue. Whenever you do anything, you wind up flexing teh board, moving wires, etc.

With regard to the transistors, all I was saying was that Q21 being hot is only a clue. Q21 could be defective, or Q21 could simple be running hot because something else is not right. You can test transistors forever, and maybe will find a bad one and maybe not. Testing does not put real world stresses on transistors. This ciscuit puts what 30-40 volts across these parts at least, dooes your test meter do that? And we could have a resistor that is funny or a solder joint that looks wonderful but is not making good contact with a wire. SO that is all I was saying, Q21 AND the circuit it is in the middle of need closer attention.

You are correct, my meter isn't to inject 40 plus volts through these transistors. I guess if I really wanted to make a somewhat accurate test I would need to construct a test circuit that would place a similar load on them that they normally see in the amp circuit. Of course my lack of experience means I cannot construct a circuit like that and have confidence that it is accurate so I will see if someone has such a circuit online somewhere.

Until I find it though, I guess I'm not sure how to proceed other than to get my magnifier and eyeball the circuit tracks and look for cracks or something. and if I suspect an area to an ohms test between points that connect through that area.

The reason we plug a cord from preamp out to power amp in, is because they may not be connected together already. They may be SUPPOSED to be connected together, through the cutout contacts on teh jack. But what if those contacts are dirty of oxidized? Dirty patch jack contacts are one of THE most common problems in guitar amps. So we take 30 seconds and do the test.

One of my shop rules is:
never think up reasons to not test something.

I have to admit I have been ignoring the jacks because all five were changed first. I install new ones and the amp worked great for about a month.

As soon as everyone in the house it out of bed I will try connecting the Preout to the PA In and see what happens.

I really appreciate you input. thank you!

BR
 
I guess if I knew what voltage to expect at the B. C and E of each transistor I would just turn the amp on and test them in their native environment. Obviously this would be ideal but I am still confused on all the math involved here.

No math involved really (apart from 'perhaps' a little ohms law).

The voltages are pretty obvious if you understand how the circuit works, but unfortunately with DC coupled amps it's not really an option, as powering it up very often destroys something.
 
I plugged a cable from preamp out to power amp in but it made no difference.

As far as checking voltage at each transistor, on q9 and 10 we have r83 I understand using ohms law to calulate voltage through 470 ohms but d21, 22 & 23 should be .6v drop each? R84 is also 470 ohms then d24. How are these collectors getting the same voltage?

Same with the base terminal of each. U6 pin 7 feeds both. Yet q10 is fed via d17 which drops voltage. So u6-b amplifies the signal to compensate?
 
I plugged a cable from preamp out to power amp in but it made no difference.

As far as checking voltage at each transistor, on q9 and 10 we have r83 I understand using ohms law to calulate voltage through 470 ohms but d21, 22 & 23 should be .6v drop each? R84 is also 470 ohms then d24. How are these collectors getting the same voltage?

Same with the base terminal of each. U6 pin 7 feeds both. Yet q10 is fed via d17 which drops voltage. So u6-b amplifies the signal to compensate?

Q9 and Q10 are a long tailed pair, the entire power amplifier is essentially a huge opamp. It's not a static circuit, it's a dynamic one, the long tail pair compares the voltages on each of the bases and sends a correction voltage out of the collectors to make the bases the same.
 
Well I just got a lesson on transistors. So the collector is the output. No wonder this hasnt been making sense to me. I still dont understand how the collector receives power in from the +42 rail and is the output before the transistor.

The emitter has an arrow signaling an NPN OR PNP but has nothing to do with direction of power flow
 
Nigel I thought power flow is out the emitter? My understanding is the C is the input, E is output and the base voltage of .6 v opens the C/B gate allowing power to flow and B voltage above .6 is added to the output. Do I have a misunderstanding here?

Yes, thinking of 'power flow' just causes confusion.

On a transistor the base is an INPUT, the collector an OUTPUT, and the emitter can be either or both. In the case of a long tailed pair, the emitters are used as both input and output.
 
In terms of signal flow, input and output make some sense as terms, but as for transistors, that is not a good way to think of them. The majority of current flows between colletor and emitter. Doesn;t really matter which direction, that is a philosophical discussion.

The base is what controls the flow between them. More or less, the more current you draw through the base, the more can flow from E to C.

Try looking at an amp like this: You have a speaker. Put a positive voltage on it, and it moves forward. Put a negative voltage on it and it moves backwards. The amp has a positive and a negative power supply. We have a transistor from positive to the speaker, and we have one from negative to the speaker. By turning one or the other on, we can control the changing voltage at the speaker. And that moves the speaker to make sound.

So for signal, the base is the input and the emitter is output in your final stage. But for speaker current, the collectors are the inputs and the emitters the outputs.

Look at Q20. The collector is at +42, and the emitter is to the speaker. The more we draw through its base, the harder Q20 conducts, pulling the speaker terminals closer and closer to +42. If you scope the parts of Q20, the collector will sit there at +42. You will see your signal at the base, and also at the emitter. You will find a fairly steady .6v between base and emitter, but not from base to ground. Q18, Q20 are current amplifiers, not voltage amplifiers.
 
Does anyone have a line on the data sheet for the 2N4001? I keep doing searches so I can get the pinout and all I seem to come up with is:
2N4001 - Bipolar NPN Device in a Hermetically sealed TO39 Metal Package. - Seme LAB

I don't have any metal canister transistors on my board.

I'm trying to get the pinout on these transistors leading up to Q18,19, 20 and 21.


Thanks!


BR
 
Pick one. Follow the trace from each pin over to some other component on the board, and check the schematic to see which transistor element that component connects to. You can thus identify the base, emitter, and collector of the part. All the 2N400x in your amp will have the same pinout. PNP and NP won't change the pinout.

Look at Q23, The base conects only to R132, and the emitter only to D48. That should be simple enough to find on the board. And once you know which pin is base and which emitter on Q23, you will know the pinout for the bunch of them.

Do you have reason to believe they are defective?


Contact Fender and ask them. Ask them if the part is available through their dealers, and if so, what is the part number. Ask them if there is a domestically available substitute part if not.

Q16,17 are just limiters. You have several of these 4000 series parts in the footswitch circuits. Do all the switching functions happen? If you suspect Q16,17 and do not have the new parts, just swap Q16,17 for Q22,23. It is unlikely both sets are defective. If moving them makes a difference, then they are involved in your problem. If not, they aren't.

2N4001 LOOKS like a JEDEC number, and back 30-40 years ago there were 2N4001, 4003 types, in the metal TO39 even. That is the old data sheet you are finding. I suspect that the parts in this Fender ar not JEDEC 2N4001, but someone is making transistors and has recycled that number. Fender has absolutely no need to invoke 40 year old transistor types for simple switching circuits, and I am sure they have not.
 
Transistors

Enzo

I am thinking a transistor must be bad or like you offered, may be turning one of the last ones on too hard. I've tested all the resistors, made corrections after you showed me why I was getting weird readings. I replaced the couple of resistors that were out of spec. I have been through all the diodes and haven't found any out of spec. I can't find any broken tracks on the underside.
The couple of connections I suspected, I desoldered and resoldered, just to make sure.

Unless I am missing something, transistors are what's left. Unless of course you or someone else here has a different idea. I haven't actually asked anyone if they know what's wrong or think they know what's wrong. Is there someone here that does? I'm not pridefully, if someone wants to shed some light I won't complain... lol

I'm reading everything I find on SS guitar amps and how transistors are made up, wired up, adjusted, biased, manipulated etc... Apparently transistors are generally used in a voltage divider circuit, the transistor being the bottom variable resistor. Which is frickin' cool that someone figured out how to do that, I might add. I can not seem to get my head wrapped around biasing. I read it over and over in every article I read. But I just don't get it. I understand that two supplies are being compared and the circuit is self adjusting based upon the balancing of this arrangement. But I don't get how its doing it. Its like there is a missing piece or something. And until I get the understanding of how this is working I don't think I will ever figure this amp problem out unless I accidentally change the right part. Which I hate to imagine but I fully admit would be just fine with me at this point.

That probably sounds like a cop out but I'm stumped man. Getting frustrated and tired of messing with this thing. But I don't want to let it beat me either.

Maybe I'll just take a couple days off and give my brain a rest. Its not like I haven't being trying. Maybe I just need to step back.

BR
 
The bias circuit is also called a Vbe multiplier.
It basically takes the transistor BE voltage and multiplies it to across CE.

This means the voltage always stays the same unless the temperature of the transistor changes and this will make the CE voltage go up ore down slightly. If temp goes up the voltage drops and vice versa.

If the bias setting is wrong and has too many volts on it then the output transistors can pass to much current and get hot.
 
Bias current

The bias circuit is also called a Vbe multiplier.
It basically takes the transistor BE voltage and multiplies it to across CE.

This means the voltage always stays the same unless the temperature of the transistor changes and this will make the CE voltage go up ore down slightly. If temp goes up the voltage drops and vice versa.

If the bias setting is wrong and has too many volts on it then the output transistors can pass to much current and get hot.

Ok, so how do I tell if a given transistor bias is too high?
 
Ok, so how do I tell if a given transistor bias is too high?

There's actually two Vbe multipliers in the circuit (Q14 and Q15), it's a really weird and over-complicated circuit.

As I recall it was suggested much earlier to disable the bias network, standard practice on transistor amp repairs. Sort the bases of Q18 and Q19 together, this sets the bias to zero.
 
Nigel

There's actually two Vbe multipliers in the circuit (Q14 and Q15), it's a really weird and over-complicated circuit.

As I recall it was suggested much earlier to disable the bias network, standard practice on transistor amp repairs. Sort the bases of Q18 and Q19 together, this sets the bias to zero.

When I short the base of 18 to 19 what am I looking to find?