Luminance amp fault won't give a grainy picture....only lack of contrast perhaps or poor definition. If it's grainy, it will be RF or AGC......that's where I would look next.
Thank you, of course that makes sense. I have ordered a variety of transistors that are used in that area and will report back when I have replaced them.
Well, the transistors arrived, but the BF196 are different from the description in the order. They are clearly marked BF196, but the package is not TO92, but a round plastic housing with gold leads. The base and emitter are also reversed, so I‘m not sure I received the right parts. I have not been able to find a datasheet that shows this package option, so I’m not sure I should give them a try. Any insights?
thanks!
That sounds like the Fairchild epoxy package- are they dark brown with a domed top and short, round wire leads, quite stiff as if they are steel or hard-drawn copper under the gold-colour plating? I've come across quite a few different transistors in this package including the famous 2N3819 FET, and yes, different pin-out to the "book" packaging. I gather that there's a long-term question over the environmental sealing and device reliability of this package type, but not as notoriously bad as Lockfit!
Thanks for your insight! That's exactly the package that they come in. The domed top and the stiff leads. A bit annoying that the pinout is different, there's not a ton of space, so I will have to see how to get the wires to go to the right locations. I'm a bit annoyed, because the shop specified the package as TO92 but then crossed it out on the delivery note.
A quick update:
I replaced all the transistors in the HF and IF stages. The transistor in the last stage was a BC199 instead of a BC197, as indicated in the circuit diagram. Unfortunately, this has not brought an improvement of the signal. Maybe the issue is even before the tuner. I will have a look at the splitter and maybe make a direct connection between the RF signal and the tuner. If all fails, I really may have to resort to replacing the front end with parts from my other N1500...
What is the tuner module used in your N1500? I may have a spare somewhere.
Wow, that would be wonderful!
As the issue is present in both VHF and UHF channels, I‘m not sure it’s really a fault in the tuner. I suspect an issue in either the IF circuitry or maybe even the RF signal splitter. I am attaching a couple of pictures of the tuner module.
Hopefully, I will have some time over the holidays to continue the project.
Thanks a lot for everyone‘s help!
Might still not be the tuner. Could be AGC.
Have you tried over-riding the tuner AGC with a pot?
I have tried two things, adjusting the pots of the AGC does make a difference, but the image can’t be improved beyond the current state. I also have applied an adjustable external voltage, the results were similar. Thinking about this again, I may only have tested one part of the AGC with the external voltage…
Also worth checking that you have some emitter volts on the BF200.
It’s been a while, and there’s not much progress, but I still want to report what has happened since my last post. As none of the previous measures have made a real difference, I looked at the last part in HF chain that I hadn‘t looked at all. The amp/splitter. I first connected the output of my signal generator directly to the UHF tuner, this does seem to bring a tiny bit of improvement. However, all I really did was hold the center of the antenna cable to the solder connector on the splitter that goes to the UHF tuner while also trying to contact the the metal box with the outer part. The change is not huge, but I would say noticeable, despite this connection being less than ideal. I opened up the splitter and found two BFW92 inside. Any chance those might be faulty? The image I am seeing does look a lot like the signal is a bit weak, but I also noticed that the best picture does not coincide with the best sound. Maybe the IF stages do need a bit of a realignment?
I ended up replacing the two BFW92s in the splitter/amplifier. Didn't really make any difference. The picture is not terrible at this point, but also not as good as it should be. As the best sound is obtained with a slightly different tuner setting than the best picture, I am very tempted to try messing with the alignment. Terrible idea? It's not like I can break too much, if all fails, I can still replace the front end with the one from the other N1500 with the defective head...
