Hi Mooly,
Hi Glen,
The transistors you will be checking are normally good for about 25 VDC and low current, 50 mA being common. So they are not as robust as the normal audio transistors. Meters like a Simpson 260 and other older pointer type meters can blow them up just by testing. Modern digital meters should be okay as they normally run the transistor at approx. 1 mA.
-Chris
Remember that we don`t want people quoting the entire previous posts, just the section they are responding to. I had just showed him how to do a selective quote.
No, the wave form will be down approximately 3 dB, that`s all.
LM7000 failure is rare. Often the actual failure is one of those coupling capacitors which shorts the bias to ground. So, it could be but I don`t see any point in throwing other things into the mix until we look at what we have on our plate so we don`t confuse the issues. Lets keep focused on what we are doing and let it run it`s course before introducing any other ideas.
Hi Glen,
The transistors you will be checking are normally good for about 25 VDC and low current, 50 mA being common. So they are not as robust as the normal audio transistors. Meters like a Simpson 260 and other older pointer type meters can blow them up just by testing. Modern digital meters should be okay as they normally run the transistor at approx. 1 mA.
-Chris
I just double checked my measurements on the transistor I removed from the tuner (the one in the wax). It is a PNP (base is negative- middle pin). I also checked Q2 to make sure I was not making a mistake in my measurements. Q2 is a PNP on the schematic and tests as one. So, the transistor I have removed cannot be TR4. The schematic is hard to read but it looks like TR6 and TR7 may be PNPs so it may be one of these. Or am I going crazy? Glenn
Chris, I'm using a Uni-T UT61E which is fairly new digital type. I have a Fluke and a Brymen as well. Glenn
Hi Chris, I located the capacitors connected to AMin (pin 13). I have only measured them in circuit at this stage. C8 should be 100pF but measures 380pF. It is a small ceramic axial cap. Many of these on this board. C104 should be 47pF (I think) but is measuring 280pF. It is a brown disc ceramic. So on that basis, both way out of spec, assuming of course that schematic for tx550 is same as for tx540. I will pull one leg on each and remeasure capacitance along with ESR. Glenn
Hi Chris,
I'm not familiar with that particular scope and was just trying to cover all bases 🙂 I don't know if it has any kind of user selectable bandwidth to deliberately limit HF response, for example my 100Mhz scope has a 30Mhz setting which gives a super clear trace... but with restricted bandwidth of course.
Checking for problem caps (little compressed disc ceramics) is sound.
Glenn... if you are finding multiple caps that seem out of tolerance then its much more likely to be either a measurement error or just that they are not what you think they are.
Mooly, you are of course correct. Was not accounting for zero error. Changed over to Wavetech cap meter. So, C8 is OK. C104 is way out of spec, reading 1400pF when should be 47. I'll double check this by lifting a pin tomorrow but it looks bad. Glenn
Hi Mooly,
-Chris
That's certainly fair enough. If it has a bandwidth limit setting, I've never used it.
-Chris
Hi Glenn,
Some RF transistors have the emitter in the center. Best look it up and confirm the pin-out.
C104 is shot. Your sky high capacitance reading is leakage. In situations like this, I change the other capacitors related to this circuit as well if they are the same value. You already have to order one, so may as well make it two.
-Chris
Some RF transistors have the emitter in the center. Best look it up and confirm the pin-out.
C104 is shot. Your sky high capacitance reading is leakage. In situations like this, I change the other capacitors related to this circuit as well if they are the same value. You already have to order one, so may as well make it two.
-Chris
That is very unusual and probably incorrect that they use any pnp's in the FM FE pack. If so, the schematic does not match the actual assembly, since it shows all npn's. Some how you are going to have to figure this out.
Chris we do not have a BOM for the RF FE pack, the numbers he read off do not make any sense to me unless they are Yamaha numbers instead of EIAJ.
There are only so many popular EIAJ that they used. For instance a RX-900/U that I have, the SM says for the FM osc they use either, 2sc2724,2839.461,930. Mixer 2sc535,2999,2668,2786
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Hi all, the scope does not have a function to limit HF response as far as I am aware. I did manage to find the actual schematic for the TX540 from one of the specialist radio sites. However, you can only download one page at a time. I have the pages that show the LM7000N and the tuner packs. I can't see any difference between the TX540 and TX550 schematics at this stage. The page for the tuner packs is easier to read and does show that all the transistors in the pack are NPNs. I can post relevant images if you so wish. I'll remove C104 and test properly. Hard to test these little ceramics with small values! I'm still wondering about C1 which attaches to Vt. Can't find it on the board. I wonder if it's inside the tuner pack. Glenn
Hi Glenn,
If Vt wasn't shorted to ground, that capacitor is probably okay.
Look up the transistor you have out.
-Chris
If Vt wasn't shorted to ground, that capacitor is probably okay.
Look up the transistor you have out.
-Chris
Sometimes it helps to look at problems from a different angle... sorry as I know I keep banging on about this 🙂
To me, everything changed when Glenn mentioned it would tune AM stations manually with a separate tuning voltage (Vt) applied.
Forget FM for the moment and concentrate on why AM doesn't work. Attack the problem from this angle and you are not looking at a local FM oscillator issue.
Right, all said now 🙂 carry on 😉
To me, everything changed when Glenn mentioned it would tune AM stations manually with a separate tuning voltage (Vt) applied.
Forget FM for the moment and concentrate on why AM doesn't work. Attack the problem from this angle and you are not looking at a local FM oscillator issue.
Right, all said now 🙂 carry on 😉
And well said Mooly. I'll keep plugging away checking transistors in the front end and check that suspect cap properly. Glenn
Hi all, after a bit of research, it looks like the transistor (oscillator) is a Sanyo SVC211SPA which is a Varactor Diode for FM low voltage electronic tuning. Middle pin is cathode and outers are anodes. So, my understanding is that current will only flow from cathode to anode (plus lead on cathode and negative lead on anode). That explains why it looked like a PNP. Also explains why I was getting identical measurements on both sides. I am not going crazy after all! Have I got that right? Symbol on schematic is incorrect if so. Glenn
A varactor or varicap is actually a diode rather than a transistor and it is one that has the property of showing a different junction capacitance that is related to the reverse bias voltage... which simple means it is used as a variable capacitor, the value of which depends on the applied (reverse) voltage.
This variable capacitance is used to tune the oscillator frequency. In the old days we would have used a true variable capacitor (with vanes), now we use varicap diodes.
This variable capacitance is used to tune the oscillator frequency. In the old days we would have used a true variable capacitor (with vanes), now we use varicap diodes.
Hi Glenn
Boy, you are learning a lot. Vari-cap diodes are normally two lead devices. It would be located close to the oscillator transistor. Do keep in mind that at high frequencies, the transistors used will be different to what we are used to seeing. I was tricked once by a non-standard pin-out. It was a lesson I learned well. They moved the emitter to the center to reduce the capacitance between the base and collector (very important) which put the collector and base at opposite sides of the device. Pretty smart actually.
Now, in that circuit with the vari-cap diode and the transistor you should also find a trimmer capacitor and adjustable transformer and coil. The trimmer capacitor has got a slot screw in the top, try not to disturb it or it will be alignment time. Now this is important. The coil is just a loose coil with maybe 4 or 5 turns of heavy wire. Resist the urge to touch this! It is a serious PITA to adjust these damn things. You don't have the equipment for that.
Anyway, I mentioned the capacitor to help you get your bearings in that tuner pack. The buffer and mixer transistors will not have a trimmer capacitor or vari-cap diode associated with them. The buffer will lead to the osc. out pin. The mixer would normally drive a transformer that then goes to the IF out pin. I don't know if that tuner pack has an RF gain stage in it or not, but that would be another transistor in case you have "spares" that you don't know what they do. It will also have a vari-cap diode and a tune-able transformer or coil.
Best, Chris
Boy, you are learning a lot. Vari-cap diodes are normally two lead devices. It would be located close to the oscillator transistor. Do keep in mind that at high frequencies, the transistors used will be different to what we are used to seeing. I was tricked once by a non-standard pin-out. It was a lesson I learned well. They moved the emitter to the center to reduce the capacitance between the base and collector (very important) which put the collector and base at opposite sides of the device. Pretty smart actually.
Now, in that circuit with the vari-cap diode and the transistor you should also find a trimmer capacitor and adjustable transformer and coil. The trimmer capacitor has got a slot screw in the top, try not to disturb it or it will be alignment time. Now this is important. The coil is just a loose coil with maybe 4 or 5 turns of heavy wire. Resist the urge to touch this! It is a serious PITA to adjust these damn things. You don't have the equipment for that.
Anyway, I mentioned the capacitor to help you get your bearings in that tuner pack. The buffer and mixer transistors will not have a trimmer capacitor or vari-cap diode associated with them. The buffer will lead to the osc. out pin. The mixer would normally drive a transformer that then goes to the IF out pin. I don't know if that tuner pack has an RF gain stage in it or not, but that would be another transistor in case you have "spares" that you don't know what they do. It will also have a vari-cap diode and a tune-able transformer or coil.
Best, Chris
Duh, I should have clued in that you pulled the varactor instead of the LO bjt.
I will attach the FM tuner pack schematic from the RX-900/1100 manual. It is a bit different design but shows the parts involved.
Look at the 2sc461,930 datasheet, both are ecb. Understanding the design involves researching the parts. The internet is great for this, just think what it would be like without this information like in the old days.
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Hi Glenn,
Referencing Rick's diagram of the tuner, let's identify the transistors.
TR1 - tuned RF amplifier (amplifies the station you are tuned to
TR2 - mixer stage (LO is mixed with the tuned signal)
TR3 - this is your LO, local oscillator
TR4 - the buffer for the LO out for your LM7000. In this case, it is a J-FET instead of a BJT
Now we'll look at the data sheet for the varactor diode. It is a dual device containing two varactor diodes. Cool, most are single packages. This can tune two different stages, or double the total capacitance you are using.
It's nice to see the two transistors have a normal pin-out, but keep on your guard for parts that have different pin-outs.
-Chris
Edit: By looking at the transistors, you can see they have a low collector voltage and current. Low powered devices and probably sensitive to ESD events. When handling these parts, you will want to touch the metal of the tuner, then the transistor. Either that or wear an anti-static bracelet that is grounded to your bench and tuner pack. Same for your soldering iron.
Referencing Rick's diagram of the tuner, let's identify the transistors.
TR1 - tuned RF amplifier (amplifies the station you are tuned to
TR2 - mixer stage (LO is mixed with the tuned signal)
TR3 - this is your LO, local oscillator
TR4 - the buffer for the LO out for your LM7000. In this case, it is a J-FET instead of a BJT
Now we'll look at the data sheet for the varactor diode. It is a dual device containing two varactor diodes. Cool, most are single packages. This can tune two different stages, or double the total capacitance you are using.
It's nice to see the two transistors have a normal pin-out, but keep on your guard for parts that have different pin-outs.
-Chris
Edit: By looking at the transistors, you can see they have a low collector voltage and current. Low powered devices and probably sensitive to ESD events. When handling these parts, you will want to touch the metal of the tuner, then the transistor. Either that or wear an anti-static bracelet that is grounded to your bench and tuner pack. Same for your soldering iron.
Hi all, thanks for the great information in last couple of posts. Have identified the other two varactors (at bottom of image in post 132). Have also located T3 and T4 which are on right side of image in post 132. Located these by tracing back from Osc pin (top right in image). The varactor I removed was the bottom right one which is associated with TR4 (in the wax). Assuming that TR4 is the Osc. Please confirm that next step is to remove TR4 and test. Chris, no variable cap on this board but there is an adjustable coil (clearly visible in post 132 image). I think I am getting the hang of this. It is completely logical. Glenn
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