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Old 3rd January 2013, 04:08 AM   #31
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And I've just remembered-I think it's the zener diodes d57/58 - cheap horrible way of regulationg power, and they get noisy. I replaced them with 78/7915- cheap voltage regulators.

So either one of those may have failed, and be taking out the fuse. Measure across them with your meter on ohms or diode test.
Also test the output devices (the larger 3 pin devices on the heat sink). Meter check of a transistor : Bipolar Junction Transistors
Google is great for finding out how to do stuff :-)

Check between pins 4, 8 and ground on your chips. Likelyhood is you have some solder bridging something, or a zener is dead. Running solid state amps not connected to speakers is fine- most nowadays will run into a dead short as they are protected against such.
But let's be on the safe side and not try eh?
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Old 3rd January 2013, 04:10 AM   #32
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I think you have an additional fault that is blowing the fuse now ^ see above.
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Old 3rd January 2013, 09:50 AM   #33
Enzo is offline Enzo  United States
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Fix the fuse blowing first, do not worry about noise until the fuse issue is solved.
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Old 3rd January 2013, 05:55 PM   #34
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Are the output transistors shorted? When you removed the board and reassembled it. The outputs are still electrically isolated from the heatsink right? If not, you are shorting them to ground
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Old 3rd January 2013, 07:10 PM   #35
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Quote:
Originally Posted by badraven View Post
Nigel You said
For a start check to see if the output transistors read S/C or low resistance.
The power limiter works, I can turn the amp on and it doesn't blow the fuse.

So how do I check the output transistors?
You don't need a current limiter, you simply check them using the ohms range of your multimeter - with the power disconnected - otherwise you can't read resistance, and you would blow the meter as well.

S/C stands for 'short circuit' - basically zero ohms or near.

You should initially check between collector and emitter of the output transistors.
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Old 3rd January 2013, 09:48 PM   #36
JMFahey is offline JMFahey  Argentina
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Dear badraven, let's not mix 2 different troubleshooting avenues, or it will become confusing.

1) without load (speaker) connected, turn the amp on plugged into the current limiter, whether it blinks or stays brightly lit already is an indication, and the do some *voltage* measurements.
The first 3 are voltage at the +V and -V rails (+42V , -42V) and the DC voltage at the speaker output one (P1 spkr wht).
Post your results.

2) now with amp off, even better unplugged, you will take some resistance readings.
To be more precise, we will use the "Diode test" setting, which applies a couple mA to the probes and shows the voltage reading between them.
We will measure the big suspects, the output and driver transistors.
We *expect* to find around 0.65V (650mV) across BE and BC junctions, one way; open or much higher (over 2V) the other way, and also open CE, both ways.
If any reading in a transistor is close to "0", it's an immediate suspect.
Carefully remove it and remeasure "outside".
You must measure: Q18/19/20/21 ; also check (in the regular 200 ohms scale) that R109/112 are not open.
Your multimeter will probably show a higher value, say 1/2 or 1 ohm, but that's fine, regular multimeters also add their own internal resistance (wiring/cpnnectors/switches/probes) which never is "0", what we don't expect there is "open" or a *large* value , sy 10 ohms or more.

Post results.

PS: just as an extra test, measure resistance to ground from Q20/21 collectors.
We don't want to find them shorted to ground.
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File Type: gif FM212Lowell.gif (39.3 KB, 49 views)
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Old 3rd January 2013, 10:29 PM   #37
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Default Transistor ohms Check

The 4 transistors on the heat sink are from left to right:
Pin convention all four: (L - R) A B C
Q20 (chip marking C3263) A - B = 28.5 ohms, B - C = 0, A - C = 28.3
Q18 (C3298) A - B = 4.46, B - C = 28.5, A - C = 4.49
Q19 (A1306A) A - B = 4.46, B - C = 0.1, A - C = 4.46
Q21 (A1294) A - B = 0.1, B - C = 0, A - C = bounces 0.1 - 0.2

In the second pic there are two resistors R144 and R145
R145 has a deformation on top. It isn't burnt or discolored but it is deformed on top. It may be a casting flaw or something like that but I am bringing it to your attention just in case its something to worry about.

R145 = 328 ohms
R144 = 341 ohms
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File Type: jpg R144 and R145.jpg (729.6 KB, 51 views)
File Type: jpg Transistors.jpg (813.1 KB, 50 views)
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Old 3rd January 2013, 10:49 PM   #38
JMFahey is offline JMFahey  Argentina
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Well, Q20/21 are definitely shorted (it's what was expected anyway ), must be replaced.
Q19 is also shorted and Q18, although it might be alive, should be replaced anyway because it probably overheated anyway.
No problem but next time let's call transistor pins B-C-E (as shown, left to right) to avoid a possible extra source of confusion.
Please check R109/112 are not open.
R144/145 are not shown on the cropped schematic I posted, but I think they are the voltage droppers to get +/-15V rails .
The measured value is consistent with the 470 ohms? original value, so they look fine.
That small imperfection looks like a casting drop, nothing dangerous.
Ok, keep testing and order replacement transistors + micas/grease, whatever you need to mount the new ones.
The FM212 is a nice good sounding little amp, quite well made , typical Fender product, its only problem is that its production cost in a robot factory is so low, that Fender is not too keen on repairing it, after warranty time.
Delights of globalisation.
Good luck.
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Old 3rd January 2013, 11:41 PM   #39
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Default JMFahey Tests

I have a request for everyone. People keep using abbreviations and technical terms.
I understand we try to type as little and precise as possible but a lot of what you all are posting is "Greek" to me. I'm not stupid but I am playing catch up here. If you all could please be specific on what you want done. If you want me to take a voltage or resistance measurement please tell me the pin number(s) or the item designation as shown on the schematic. That way I know I am checking the correct item and am not wasting your time giving you wrong information. Thanks!.


When I run the amp through the limiter the fuse does not blow. The 75watt light comes on and it fairly bright. I don't think its as bright as normal but its not dim and it does not blink, its on steady.

Speaker output (speaker disconnected) reads .009 volts. (DC)
I am not sure which points you mean when you wrote "+V and -V rails" I think that means the power coming into the amp. On the schematic, power goes through the switch and then to P7 and P8
P7 = 114.5v
P8 = 120.4v

1) without load (speaker) connected, turn the amp on plugged into the current limiter, whether it blinks or stays brightly lit already is an indication, and the do some *voltage* measurements.
The first 3 are voltage at the +V and -V rails (+42V , -42V) and the DC voltage at the speaker output one (P1 spkr wht).
Post your results.

R109 = 0 ohms
R112 = .1 (20 scale)

The items listed below are my next tests.

I will order the resistors. Is there a recommended vendor or brand I should get?

Thanks!

BR

2) now with amp off, even better unplugged, you will take some resistance readings.
To be more precise, we will use the "Diode test" setting, which applies a couple mA to the probes and shows the voltage reading between them.
We will measure the big suspects, the output and driver transistors.
We *expect* to find around 0.65V (650mV) across BE and BC junctions, one way; open or much higher (over 2V) the other way, and also open CE, both ways.
If any reading in a transistor is close to "0", it's an immediate suspect.
Carefully remove it and remeasure "outside".
You must measure: Q18/19/20/21 ; also check (in the regular 200 ohms scale) that R109/112 are not open.
Your multimeter will probably show a higher value, say 1/2 or 1 ohm, but that's fine, regular multimeters also add their own internal resistance (wiring/cpnnectors/switches/probes) which never is "0", what we don't expect there is "open" or a *large* value , sy 10 ohms or more.

Post results.

PS: just as an extra test, measure resistance to ground from Q20/21 collectors.
We don't want to find them shorted to ground.[/QUOTE]
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Old 3rd January 2013, 11:46 PM   #40
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If your lamp is coming on then something is talking serious current.
Points to a short somewhere like transformer, smoothing capacitors, output transistors.
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