farfisa fr40 - need help

[Yvan44]

Hi,

I've open a new topic for my amp rebuilt, here's the one of Racing which is very interesting:
viewtopic.php?f=10&t=90637

So I've just replaced all the eCaps, and .... non sound

None of the preamp tubes are heating, and it looks like the transformer is heating : the transformer is making some "bubble" noises after about one minute : then I switch off.

Well, I used to be electronics engineer when I was young, but no experience with tubes.
I've check the secondaries of the transformer :
- first : 345 AC -> feed the EZ81
- second : 18v AC -> 25,7V DC
- Tubes heating : 5,4 V AC

Are these value correct or the transformer is KO?

And here is (at least) one problem one may opinion : output of the EZ81 is about (tension is not very steady) 42,6V DC only (taken on the 32µF eCaps), it should be much more, isn't it

Is there anything else I should check?

Should I change that EZ81 tube first, or adjust the Bias (which is comon to other tubes) to a higher value?

Here is the chematics :


http://iloapp.elegic.com/data/_gallery/ ... height=966


Thanks in advance for your help

Yvan

[oid]

Pull the power tube and the ECL86, disconnect the speakers and the reverb tank. Measure the resistance of the windings of the output and reverb transformer (amp off and caps discharged) and check the resistance of the speakers if you have not.

Something is sucking down a good amount of current in the amp.

[oid]

Forgot to ask, did you ever power the amp up before replacing the caps? If yes, how did it function?

[Yvan44]

Hi Oid,

Forgot to ask, did you ever power the amp up before replacing the caps? If yes, how did it function?

I did about one minute : about the same, some tubes get warm, no sound and some noise coming from the transformer. And the transformer getting really warm : more than 50°C.

The Ecaps really seem to be the original ones : they were really out of age and use. I do believe no one try to repair it badly.
The Ecaps of the power part were like le red ones (including the blue ones on the picture of Racing :

[Yvan44]

Hi Oid,

Thanks for your reply and advices.

disconnect the speakers and the reverb tank. Measure the resistance of the windings of the output and reverb transformer (amp off and caps discharged) and check the resistance of the speakers if you have not.

output windwings : 0.6 ohm
each speaker : 3.1 ohm, so 6.2 Ohm together

Pull the power tube and the ECL86.

Can you detail what should I checked? Power tubes = EL34?

Aslo, some additionnal info : when I power up the amp, the ground get power slowly, meaning feeling a small tension if I touch it, and going higher and higher. Even after power, I've to wait few minutes before tuching the ground.

Anyway, I continue investigations on the output transformer and I remove the EL34 tubes to check it :
impedance is not symetrical on the primary : one half is 142 ohm and the other is 192 ohm - 335 ohm in total



In addition, I've disconnected the preamp part : same result : some tubes get warm, no sound and some noises coming from the transformer.

[Yvan44]

OK, I really wonder about the EZ 81 after about 30 secndes:



This one is getting really hot before one minute and I can touch the glass as it will burn my fingers.

Also the other EL34 look better behin.


Don't you think?

Regards

[oid]

The EZ81 is glowing like that because something in the amp is trying to draw way more current then it or the transformer can provide, you should stop powering up them amp until this is figured out or you may end up with a dead power transformer. We also know that the fuse is the incorrect value, it should have blown at this amount of current draw. The schematic does not indicate what the proper value is, should say on the amp by the fuse, get some proper fuses first of all, get a dozen, you are most likely going to blow a few and it never hurts to have extras anyways.

The EL34s are the output tubes, pull them and measure resistance from the center tap to each of the outer leads and from outer lead to outer lead. Measure each lead to ground and finally measure each lead of the primary to each lead of the secondary.

Do the same with the reverb transformer.

[Yvan44]

Hi Oid,

Again, may thanks for your help and advices, I really appreciate.

I'm french : I'm not so familiar with that english terms, not sure I really understand what you mean.

measure resistance from the center tap to each of the outer leads and from outer lead to outer lead

Is that about the transformer? or the EL 34 tubes or slot?

Transformer :
output secondary windwings : 0.6 ohm
primary windwings : one half is 142 ohm and the other is 192 ohm - 335 ohm in total

Regarding the tubes if that can help :
When pulled off : only a significant resistance between pin 2 & 7 : about 0.7 ohm each.

On the board itself without the tubes (values are the same and each EL34 slot, execept when mentionned with xx / xx) :
pin 1 - 2 : 54
pin 1 - 3 : 360 / 310
pin 1 - 4 : 2.7k
pin 1 - 5 : 240k
pin 1 - 6 : 240k
pin 1 - 7 : 53 /55
pin 1 - 8 : 0

pin 2 - 3 : 410 /360
pin 2 - 4 : 2.7k
pin 2 - 5 : 240k
pin 2 - 6 : 240k
pin 2 - 7 : 0
pin 2 - 8 : 53

pin 3 - 4 : 2.7k
pin 3 - 5 : 240k
pin 3 - 6 : 240k
pin 3 - 7 : 410 / 360
pin 3 - 8 : 360 / 310

pin 4 - 5 : 240k
pin 4 - 6 : 240k
pin 4 - 7 : 2.7k
pin 4 - 8 : 2.7k

pin 5 - 6 : 2.2k
pin 6 - 7 : 240k
pin 7 - 8 : 240k

pin 6 - 7 : 204k
pin 6 - 8 : 240k

pin 7 - 8 : 53

measure each lead of the primary to each lead of the secondary

REgarding the output transformer, there is no significant resistance to the ground.

I'ven't checked the reverb transformer yet, I'll do that.

I've a question : is it usefull and safe to power it without 1 or both EL34 in order to fix if one of them is faulty?

REgards

[oid]

schematic with numbers in red

We need these resistances, power tubes pulled and speakers disconnected.
1 to 2 142 ohms
2 to 3 192 ohms
1 to 3 335 ohms
4 to 5
5 to 6
4 to 6

Measure each of the primary leads, 1, 2 and 3, to each of secondary leads, 4, 5, and 6. If you get anything under a few k suggests a short between the winddings of the transformer

Measure all 6 leads each to the chassis, this should be the many hundreds of k to megaohm range, if not you have a winding that has shorted to the core..

report any and their resistance which are not in the ranges mentioned.

Do these same reading between the 4 leads of the reverb transformer with the tank disconnected and the ECL86 pulled.

You will need a properly functioning set of power tubes to test the amp, but you need those fuses before you even should turn it on again. There is a short somewhere in the amp and we need to find it and figure out why it shorted before there is any need for power tubes.

Regarding your measurements on the power tube sockets, they do not quite add up. Is the any scorching, black build up (carbon) between any of the pins on either side of the socket?

[oid]

Forgot to mention, the reason you can feel the tension (current) when you touch the amp is likely that it has an ungrounded power chord and the polarity of either the outlet or the amp is reversed making the chassis live. replacing the power chord with a proper grounded one is a must if it has not been done, if it is grounded make sure it is properly wired.

[jimboyogi]

Re. the output transformer, don't worry that the primaries have a different dc resistance, that's normal.

[Yvan44]

Forgot to mention, the reason you can feel the tension (current) when you touch the amp is likely that it has an ungrounded power chord and the polarity of either the outlet or the amp is reversed making the chassis live. replacing the power chord with a proper grounded one is a must if it has not been done, if it is grounded make sure it is properly wired.

Hi Oid,

You are right, the ground is isn't wired! I'll rebuild that.

Yvan

[Yvan44]

Hi,

I will mesure the resistance on wingings you asked this evening after work.

Still, I've check quickly the EZ81 and here the resistance - when not mentionned, resistance is very high (> 10M).
1 - 3 : 0.1 (!!!)
2 - 6 : 3.5 k
2 - 8 : 960
2 - 9 : 2k
4 - 5 : 1
6 - 8 : 3.6k
6 - 9 : 4.7k
8 - 9 : 2.2k

So there is a short circuit between 1 and 3. Is that really fine with this tube?

Also, when I remove the tube (EL34 * 2 + EZ81) and check for any loss on pin 3 (power supply for the EL34 if I'm not wrong), the resistance pin 3 to ground is above 2M. Meaning to me that there no particular issue. Is that OK?

Regards

Yvan

[Yvan44]

You will need a properly functioning set of power tubes to test the amp,

My goal was to get it working like that, meaning, replace only what should be replaced to get it up and running, then replace all tubes that need to be replaced because they getting to old. I'm not the kind of man who change all tubes every year, even if the amp has been used a couple of hours.

Maybe I can go directly to the second step. Should I have to replace all that tubes anyway :
1 EZ81
2 EL34
3 ECC 83
2 EF86
1 ECL86

but you need those fuses before you even should turn it on again. There is a short somewhere in the amp and we need to find it and figure out why it shorted before there is any need for power tubes.

There is no fuse right no on this amp, I will add it, is 1A is OK for this amp? Slow or fast?

Regards

Yvan

[Yvan44]

Re. the output transformer, don't worry that the primaries have a different dc resistance, that's normal.

Hi Jim,

Thanks for your answer - surprising for me anyway .

Yvan