Fender FM212R Problems

[badraven]

Alright. So its not critical to the operation of the am. Thanks!

BR

[JMFahey]

Those seem to be FUsible resistors, which behave like standard resistors except when overheated, they crack open.
I suppose the maker leaves an air bubble or something inside the ceramic tube so when heated .... BOOOOM!
The idea is that in the case of a gross short they do not catch fire or burn a hole in the PCB.
Living in Argentina they are not really available for me, so in that case, I replace them with a regular 47 ohms 1/4W resistors but mount them with longer than usual leads, say 1/2" away from the PCB.
In practice, works about the same.
But if yours are cracked open and you can get the real stuff at some supplier, by all means use the originals.

So, what's the last "health bulletin" about your amp?

[badraven]

As I stated in last nights post, I have been testing the Test Points to see if I am getting the values listed on the schematic. Per note 6 TP7 thru 19 are performed with a 3mV signal at the first input. I don't have a signal input yet so I have not performed those points yet. The last remaining one to check was TP24 which does not indicate a value. So I determined which end of R130 and R133 were facing each other by ohming the legs of each. U7 leg 2 and the collector leg of Q23 are both zero resistance to the resistors. The resistor ends facing TP24 show 11.4 v, Q23 C leg shows 11.35v but leg 2 of U7 shows 2.333 v. I don't get that. (Data sheet attached)

I connected the speakers and guitar and made a short video of the meter set to mV which I strummed the guitar on the clean channel. In the video you can hear what it sounds like. Its fuzzy and tinny sounding. The amp was on for about 1.5 to 2 minutes and shut down from overheating just as I turned the recorder off. I walked outside to my carport and grabbed my laser temp meter. The heat sink was 140 deg. The following link is to my server for you to download the video if you wish to see it and listen. The amp shows .5 mV when sitting idle and jumps around when I play the guitar.

This one.mp4 23-Jan-2013 21:29 2.2M So I guess now I purchase or make a signal generator and test the other TP's?


fu47 does not appear to be blown so I will leave it alone.

BR

Originally Posted by JMFahey

Those seem to be FUsible resistors, which behave like standard resistors except when overheated, they crack open.
I suppose the maker leaves an air bubble or something inside the ceramic tube so when heated .... BOOOOM!
The idea is that in the case of a gross short they do not catch fire or burn a hole in the PCB.
Living in Argentina they are not really available for me, so in that case, I replace them with a regular 47 ohms 1/4W resistors but mount them with longer than usual leads, say 1/2" away from the PCB.
In practice, works about the same.
But if yours are cracked open and you can get the real stuff at some supplier, by all means use the originals.

So, what's the last "health bulletin" about your amp?

[badraven]

Just in case it means something. U7 leg 1 = -15.34v / leg 2 = -2.333v / leg 3 = -11.46

[Nigel Goodwin]

Originally Posted by JMFahey

Living in Argentina they are not really available for me, so in that case, I replace them with a regular 47 ohms 1/4W resistors but mount them with longer than usual leads, say 1/2" away from the PCB.
In practice, works about the same.

I don't live in Argentina - but that's generally what I do as well

[badraven]

So that's it? Nothing? So insights of what might be the problem here or suggestions on a direction to proceed in? I thought the idea of this board is to solve problems? I am pretty sure I have accomplished all tasks assigned and posted the results each time. Am I missing something here?
I'm reading as fast as I can take in information on the subject of SS guitar amps. ???

[ouimetnick]

People on this forum don't work here. They do this as a hobby, and on their own time. Yes sometimes threads are abandoned, but I'm sure someone will chime in continuing the thread, and getting you to the end of the tunnel.

[nigelwright7557]

A blunderbuss way of fixing it is to remove all semiconductors from the pcb.
Test them all on a DMM for Hfe.

While the transistors are out check all values of discrete components (R's and C's)

Replace all potentiometers. Replace all IC's.

Then put back together and set the pots as per spec sheet.

I had an amp that I used this approach with and by sods law it was the last transistor unsoldered that had an Hfe of 1.

++++++++++++++++++++++++++++++++++++++++++++++++++ +++

What I sometimes do is remove the output transistors and feedback the output into the LTP feedback. Wit hthis it is possible to power up the pcb without blowing fuses.

[badraven]

Nigel

I'd like to avoid throwing parts at it hoping to accidentally cure the problem. But as a last resort I will probably do something like that.

When you say pull and test the transistors, are you saying to pull all of them one by one and test them? Or are you talking specific ones? Q18 thru 21 have been replaced and all show correct values until the heatsink overheats.

The issue with the fuse blowing went away when I changed Q18 thru 21.

Originally Posted by nigelwright7557

What I sometimes do is remove the output transistors and feedback the output into the LTP feedback. Wit this it is possible to power up the pcb without blowing fuses.

Can you explain this further please. I don't understand all the lingo here.

Thanks!

BR

Originally Posted by nigelwright7557

A blunderbuss way of fixing it is to remove all semiconductors from the pcb.
Test them all on a DMM for Hfe.

While the transistors are out check all values of discrete components (R's and C's)

Replace all potentiometers. Replace all IC's.

Then put back together and set the pots as per spec sheet.

++++++++++++++++++++++++++++++++++++++++++++++++++ +++

What I sometimes do is remove the output transistors and feedback the output into the LTP feedback. Wit hthis it is possible to power up the pcb without blowing fuses.

[nigelwright7557]

Originally Posted by badraven

Nigel

I'd like to avoid throwing parts at it hoping to accidentally cure the problem. But as a last resort I will probably do something like that.

When you say pull and test the transistors, are you saying to pull all of them one by one and test them? Or are you talking specific ones? Q18 thru 21 have been replaced and all show correct values until the heatsink overheats.

The issue with the fuse blowing went away when I changed Q18 thru 21.


Can you explain this further please. I don't understand all the lingo here.

Thanks!

BR

I wasn't suggesting throwing all the transistors away just remove them one by one testing as you go until they are all out. Who knows you might have multiple failures. This leaves few parallel paths for testing RC components.

The LTP is the long tailed pair on the front end but if you are not blowing fuses then you probably wont need to do that.

Its a pig of a job doing it this way but that is usually my last resort method.

I fixed an old Maplin amp that way, turned out someone had put in a wrong transistor !!! They used an npn instead of a pnp.