Hello !
I'm trying to give life to a Quad FM2 tuner. I have a problem (for me, it looks like) in the second half of V2 (the 12AT). I find only 2 volts at the plate of V2b, pin 6 of the tube. My HT line is at 165v, the anode of the first half of the tube is 96v.
It's not easy to look into the RF section, there's not a lot of place. And I don'T exactly know what I can test or change, in the RF section, without needing an RF realignment after... I would like to be sure that I missed nothing before...
For the moment I can hear just a very few stations, with a really weak signal. The voltages of the others tubes plates is ok, the voltages of the cathodes too.
Some advice ? I'm not technician, but I'm full of goodwill !
.Thank you !!
I'm trying to give life to a Quad FM2 tuner. I have a problem (for me, it looks like) in the second half of V2 (the 12AT). I find only 2 volts at the plate of V2b, pin 6 of the tube. My HT line is at 165v, the anode of the first half of the tube is 96v.
It's not easy to look into the RF section, there's not a lot of place. And I don'T exactly know what I can test or change, in the RF section, without needing an RF realignment after... I would like to be sure that I missed nothing before...
For the moment I can hear just a very few stations, with a really weak signal. The voltages of the others tubes plates is ok, the voltages of the cathodes too.
Some advice ? I'm not technician, but I'm full of goodwill !
.Thank you !!An externally hosted image should be here but it was not working when we last tested it.
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I am not familiar with this unit, but V2b appears to be the local oscillator. Low anode voltage may indicate no oscillation, or it may be that your meter is being confused by the high RF voltage which could be here if it is oscillating. Try measuring the voltage via a 1M resistor - this will reduce the RF getting to the meter. 2V sounds implausibly low.
What voltage do you have,
Each side of C9? measured to Gnd!
Looks like 82pF (meter set to volts DC one lead on the cap terminals the other on Gnd)
With HT discharged what resistance do you have across C10? (This is the IFT)
As said don't move any IFT settings!
C9 might be inside the IFT if you can't find it!
So check voltage each side of R5 and then each side of the IFT. the connections from R5 and Anode of v2b.
Regards
M. Gregg
Each side of C9? measured to Gnd!
Looks like 82pF (meter set to volts DC one lead on the cap terminals the other on Gnd)
With HT discharged what resistance do you have across C10? (This is the IFT)
As said don't move any IFT settings!
C9 might be inside the IFT if you can't find it!
So check voltage each side of R5 and then each side of the IFT. the connections from R5 and Anode of v2b.
Regards
M. Gregg
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Thank you !
For R5, 165 V between each side, and 1,2 V between the transformer-side and the ground.
I'm looking for C9 and C10... I'm looking for C9 and C10, it's not easy, the RF case is deep and really not large, no way to read the component values, it's a 3D construction with several components levels... I try...
For R5, 165 V between each side, and 1,2 V between the transformer-side and the ground.
I'm looking for C9 and C10... I'm looking for C9 and C10, it's not easy, the RF case is deep and really not large, no way to read the component values, it's a 3D construction with several components levels... I try...
Mr Gregg, it works !!
But don't ask me why, I don't know....
Perhaps because it is hot, I passed a good time to look after these capacitors, or perhaps because with my touch point I moved a bad contact...
I found voltage around the T3 transformer I didn't find before, so I measured again the voltage AFTER R5, and there was... For the first time, I see the stereo indicator lighting, and not just during the "power on", and I see the balance of the 2 tuning indicators (Before, there were only lighting all the time...
I think I touched a bad contact, in fact. And I hope that I will find it, because if not it can happen again...
Thank you for wanting to help me (And you DID !)
But don't ask me why, I don't know....
Perhaps because it is hot, I passed a good time to look after these capacitors, or perhaps because with my touch point I moved a bad contact...
I found voltage around the T3 transformer I didn't find before, so I measured again the voltage AFTER R5, and there was... For the first time, I see the stereo indicator lighting, and not just during the "power on", and I see the balance of the 2 tuning indicators (Before, there were only lighting all the time...
I think I touched a bad contact, in fact. And I hope that I will find it, because if not it can happen again...
Thank you for wanting to help me (And you DID !)
Yes, it worked just the time for me to put it in its case...
Do the same tests again,
It sounds like a dry joint. Ie bad soldered connection!
You need to know if the fault is after R5 and if there is a connection across the IFT.
Do the tests very carefully so as not to disturb anything.
You need to find the bad connection. Don't get excited remember to make sure the High voltage has discharged before you touch anything!
Ie always check the anode voltage on a known point that is normally high or direct on the power supply.
You need to find the bad connection and re solder it very carefully, so you don't damage anything else while trying to fix it!
Take your time be careful don't rush and you will find it!
The last thing you want is for it to work before you find the problem this fault will reoccur unless you can make a permanent fix!
Again don't alter any IFT settings. It worked before the fault so the settings are OK don't adjust anything just find the fault.
Regards
M. Gregg
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I did the tests after, And I found again a very low voltage after R5, 4 or 5 volts.
I suppose the dry joint is after R5, in the RF cage. All the IF part is very easy to control, all the components are readable and present an easy access.
But an RF part for me is VERY mysterious. In T3, a found a connection between one pin at the base and one pin at the top of the transformer, but no connection between the two others, I don't know if this is normal. I didn't solder, or desolder anything in the RF section. In every page I could read about this tuner, it is advised to replace all the red "Hunt" capacitor. I did it in the IF, but there's one at the bottom of the RF section. Perhaps its replacement is needed, but it's difficult to access it... And the service manual of the tuner I found on the net is a bad B&W scan, not very readable for the confused RF section ! That's why I have to admit that I see 2 capacitor around T3, one between the 2 pins on the top, and one between the 2 pins told before where I found a contact, and to know witch one is C9, I don't really know...
With a small piece of wood, I tried to touch the joints in the RF, no results... But it worked, so it can be only a bad contact, a bad joint !
I suppose the dry joint is after R5, in the RF cage. All the IF part is very easy to control, all the components are readable and present an easy access.
But an RF part for me is VERY mysterious. In T3, a found a connection between one pin at the base and one pin at the top of the transformer, but no connection between the two others, I don't know if this is normal. I didn't solder, or desolder anything in the RF section. In every page I could read about this tuner, it is advised to replace all the red "Hunt" capacitor. I did it in the IF, but there's one at the bottom of the RF section. Perhaps its replacement is needed, but it's difficult to access it... And the service manual of the tuner I found on the net is a bad B&W scan, not very readable for the confused RF section ! That's why I have to admit that I see 2 capacitor around T3, one between the 2 pins on the top, and one between the 2 pins told before where I found a contact, and to know witch one is C9, I don't really know...
With a small piece of wood, I tried to touch the joints in the RF, no results... But it worked, so it can be only a bad contact, a bad joint !
If you look at the circuit diagram,
You should have a supply via R5 down to the IFT and the capacitors.
The supply goes via the IFT to the anode of the tube.
So you need to see if the voltage is lost before R5,
After R5<<ie it might be coming out of R5 on its wire but not getting to the IFT so:
Is it open circuit connection on the R5 out wire,
Is it Open circuit when it connects to the IFT
Is it open circuit after the IFT
Its common for a wire to be wrapped around a connection but the connection under the wrapped wire to corrode and get a bad connection.
If you have a low voltage after R5 what is before it do you have voltage going into R5?
If you have voltage on one side of R5 and not on the other change R5.
Remember resistors have a working voltage!
Don't get involved with the RF idea of the circuit, if you have no voltage its a dry joint or damaged component.
Regards
M. Gregg
You should have a supply via R5 down to the IFT and the capacitors.
The supply goes via the IFT to the anode of the tube.
So you need to see if the voltage is lost before R5,
After R5<<ie it might be coming out of R5 on its wire but not getting to the IFT so:
Is it open circuit connection on the R5 out wire,
Is it Open circuit when it connects to the IFT
Is it open circuit after the IFT
Its common for a wire to be wrapped around a connection but the connection under the wrapped wire to corrode and get a bad connection.
If you have a low voltage after R5 what is before it do you have voltage going into R5?
If you have voltage on one side of R5 and not on the other change R5.
Remember resistors have a working voltage!
Don't get involved with the RF idea of the circuit, if you have no voltage its a dry joint or damaged component.
Regards
M. Gregg
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Remember the golden rule,
Never assume anything when fault finding.
Ie do you have voltage going into R5.
Do you have voltage going out of R5.
Do you have voltage going into the IFT.<<assumption.
Is the wire connected to the IFT actually connected to the IFT and not a bad joint between the wire and the IFT leg.
Regards
M. Gregg
Never assume anything when fault finding.
Ie do you have voltage going into R5.
Do you have voltage going out of R5.
Do you have voltage going into the IFT.<<assumption.
Is the wire connected to the IFT actually connected to the IFT and not a bad joint between the wire and the IFT leg.
Regards
M. Gregg
Thank you for helping !
So, from the groud :
- before R5, it's the HT line, I have 170 v
- at the R5 output, It goes down to 5v.
The output of R5 goes to one of the 2 pins at the base of T3. This pin is also at 5v. This pin is in contact with one of the 2 pins at the top of T3, and this pin at the top, also à 5v, is connected to the pin 6 of the 12AT7 (anode of V2b).
At the base of T3, the pin told is connected to a second at the base, with a styroflex capacitor. The second pin at the base is at 0 volt.
At the top of T3, the other pin (not connected to the 12AT7) has a voltage of around 90v, and is connected to V7 (12AX7).
There's a tubular capacitor between the pin at the base connected to R5, and the pin at the top connected to anode of V2b
An other tubular capacitor is between the two pins at the top of T3. One of these
2 capacitor must be C9.
In fact, there's a big drop of voltage in R5... is it possible that a resistor looks ok power off, but is out of tolerance under power ?
So, from the groud :
- before R5, it's the HT line, I have 170 v
- at the R5 output, It goes down to 5v.
The output of R5 goes to one of the 2 pins at the base of T3. This pin is also at 5v. This pin is in contact with one of the 2 pins at the top of T3, and this pin at the top, also à 5v, is connected to the pin 6 of the 12AT7 (anode of V2b).
At the base of T3, the pin told is connected to a second at the base, with a styroflex capacitor. The second pin at the base is at 0 volt.
At the top of T3, the other pin (not connected to the 12AT7) has a voltage of around 90v, and is connected to V7 (12AX7).
There's a tubular capacitor between the pin at the base connected to R5, and the pin at the top connected to anode of V2b
An other tubular capacitor is between the two pins at the top of T3. One of these
2 capacitor must be C9.
In fact, there's a big drop of voltage in R5... is it possible that a resistor looks ok power off, but is out of tolerance under power ?
Try re soldering R5 first,
See if it makes any difference.
If it has no effect replace it with the same value with correct working voltage!
Metal film resistors might not have enough working voltage ie 400V rated.
If the resistor flashes over it could damage the IFT/tube.
Regards
M. Gregg
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