Drew up some equivalent circuit diagrams to see what it would take to get down to 83 ohms. Assumed the DC case: all inductors/coils were shorts, caps were opens, drew in transistors as essentially switches that are either totally open or totally shorted (I realize this is not realistic, but this is back of the envelope).
The "perfect" circuit, where all transistors are open, gave 8.8k ohm to ground.
When I assumed the oscillator was shorted, the equivalent resistance ended up ~3.5k ohm.
If FET103 was completely shorted source-to-drain, the resistance got down to 213 ohms, and FET101 or FET102 shorted gave approximately 120 ohms.
The only way to get down to the 83 ohm range was to assume two FETs were shorted: that got down to 74 ohm. Could be any combination of the three, and since that got us below the 83 ohm point we can assume the more realistic, physical case where the short isn't complete and instead merely a low resistance, which would of course bump us up a little into the 80 ohm range.
I'm probably overthinking it, I guess, and there are other possibilities (a shorted or even just leaky ceramic cap throws most of my assumptions out the window as it might provide an alternate low- resistance path to ground which could drive down the equivalent resistance quicker).
Would checking the source-drain resistance of the FETs be of value, or would the other circuitry in the tuner make the values so distorted as to be useless?
Or just bite the bullet and order some FETs?
The "perfect" circuit, where all transistors are open, gave 8.8k ohm to ground.
When I assumed the oscillator was shorted, the equivalent resistance ended up ~3.5k ohm.
If FET103 was completely shorted source-to-drain, the resistance got down to 213 ohms, and FET101 or FET102 shorted gave approximately 120 ohms.
The only way to get down to the 83 ohm range was to assume two FETs were shorted: that got down to 74 ohm. Could be any combination of the three, and since that got us below the 83 ohm point we can assume the more realistic, physical case where the short isn't complete and instead merely a low resistance, which would of course bump us up a little into the 80 ohm range.
I'm probably overthinking it, I guess, and there are other possibilities (a shorted or even just leaky ceramic cap throws most of my assumptions out the window as it might provide an alternate low- resistance path to ground which could drive down the equivalent resistance quicker).
Would checking the source-drain resistance of the FETs be of value, or would the other circuitry in the tuner make the values so distorted as to be useless?
Or just bite the bullet and order some FETs?
dr*audio
Fish fingers and custard!
This was excellent troubleshooting! I basically did the same thing, going through the circuit, following the power supply path through resistors to see what could be shorted. Yes, measure the D-S resistance of each fet. If more than one reads low, something bad happened to cause more than one to be damaged, Probably not a lightning strike, because many components in the rest of the receiver would have been affected. Maybe a powerline surge or ESD hit.
Will check source-drain resistance later.
Picking up the kids from summer camp and got a line on a 5000A and a pair of SP2000s in the area. Guy's asking $300 and I'm debating because I'm sure those 1040 boards are gonna be ready to fail and I don't know that I need another project right now.
Also, it causes me pain to pay that much for something I could have picked up for like eighty bucks and change ten years ago before The Great Hipster Stereo Inflation took hold.
The 800, however, I will likely take without a question. It's a sickness, I tell you.
Picking up the kids from summer camp and got a line on a 5000A and a pair of SP2000s in the area. Guy's asking $300 and I'm debating because I'm sure those 1040 boards are gonna be ready to fail and I don't know that I need another project right now.
Also, it causes me pain to pay that much for something I could have picked up for like eighty bucks and change ten years ago before The Great Hipster Stereo Inflation took hold.The 800, however, I will likely take without a question. It's a sickness, I tell you.
Source-drain measurements of FETs:
FET101: 400 ohms
FET102: 83 ohms
FET103: 3 ohms
FET103 is way low unless the gates are stuck on, I guess, and FET102 looks suspiciously like the power to ground resistance of the entire tuner, though perhaps that's just coincidence.
Thoughts? I'd open another one to see how they compare, but I'd rather not desolder another tuner box. Like, ever.
FET101: 400 ohms
FET102: 83 ohms
FET103: 3 ohms
FET103 is way low unless the gates are stuck on, I guess, and FET102 looks suspiciously like the power to ground resistance of the entire tuner, though perhaps that's just coincidence.
Thoughts? I'd open another one to see how they compare, but I'd rather not desolder another tuner box. Like, ever.
dr*audio
Fish fingers and custard!
On FET 103 are you measuring D - S or D to ground? If D-S, FET103 is bad. The others are probably ok.Source-drain measurements of FETs:
FET101: 400 ohms
FET102: 83 ohms
FET103: 3 ohms
FET103 is way low unless the gates are stuck on, I guess, and FET102 looks suspiciously like the power to ground resistance of the entire tuner, though perhaps that's just coincidence.
Thoughts? I'd open another one to see how they compare, but I'd rather not desolder another tuner box. Like, ever.
Probing right across the D-S terminals. I can check again to make sure I'm not shorting one of the probes to an adjacent component, but it's been pretty repeatable.
Guess I'll order FETs. I'll order all three and then replace 103 and see if I recover. If not, replace the other two (I'm assuming I'll want to replace both 101 and 102 together for symmetry reasons... those circuits are extremely similar).
dr*audio
Fish fingers and custard!
While we are waiting for the parts, lets talk about ESD safe installation of them. You need a ESD wrist strap connected to ground, which can be a cold water pipe. If you don't have one, now is the time to buy one. You need a heat controlled grounded soldering iron.
You will wrap a wire around the leads of the fet to short them together during installation and then remove the wire after installation. Download a copy of the datasheet for the new fet so you know which lead is which. Solder the source first, then the drain then the gates. Do not leave the soldering iron tip on the joint longer than necessary to get a good joint. Use solder removal braid to remove the solder on the old part. Try to install the new transistor exactly the same position and lead length as the old one to minimize de-tuning.
You will wrap a wire around the leads of the fet to short them together during installation and then remove the wire after installation. Download a copy of the datasheet for the new fet so you know which lead is which. Solder the source first, then the drain then the gates. Do not leave the soldering iron tip on the joint longer than necessary to get a good joint. Use solder removal braid to remove the solder on the old part. Try to install the new transistor exactly the same position and lead length as the old one to minimize de-tuning.
Hmmm... so you would not recommend maxhifi's suggestion to snip the old transistor and J-hook the old leads and new leads together?
When i'm looking at the datasheets I can find for the OLD transistor, the diagram appears to be drawn from facing the leads ("dead bug", as we call it in the semiconductor industry), but I cannot be certain. Is that standard or do datasheet conventions differ?
When i'm looking at the datasheets I can find for the OLD transistor, the diagram appears to be drawn from facing the leads ("dead bug", as we call it in the semiconductor industry), but I cannot be certain. Is that standard or do datasheet conventions differ?
maxhifi
Addicted Member
The impact of de-tuning here, is going to be fairly simple.. you're just going to knock off the front end alignment a tiny bit. It would be easy enough to fix this, simply put a VTVM (or heck even a DMM) on the AVC line, tune in a strong station, and then peak the two trimmers for maximum AVC voltage. I don't remember if the 5000 has a signal strength meter, but if it does, it's already measuring the AVC voltage, so you can just peak the reading on the meter with a strong station and those two trimmers. I am not talking a big adjustment, just rock it a bit and see if 1/8 turn in each direction causes the signal strength to go up. Make sure to do this with a plastic screwdriver or alignment tool, any metal tools will of course affect the circuit operation. Of course, NOT touching the IF alignment or anything further downstream. I'd also leave the oscillator trimmer alone.
I personally would have absolutely no issues doing this with my non temp regulated Weller WP35 iron, but you know, to each his own, a temp controlled soldering station would guarantee better results, and the wrist strap is definitely a good measure to mitigate the effect of electrostatic discharge.
I suspect my methods are a little more cowboy than dr*audio, he's offering excellent advice.
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kevzep
Its all about the Music
Hakko FX888D is not a bad station, I had one for a few years, you don't need any more than that.
its a nice temp controlled iron, you have tip options etc...
You don't need a Pace pps85ae rework station on Shop Air for solder extraction like I have, but I do huge volumes of work and need it, the Hakko is a decent iron though and will see you right for what you are doing.
If you wanted to have something to do solder extraction the Hakko FR300 is quite good for small volumes of work, I have one but its retired as it wasn't coping with the amount of work I was doing with it.
But there's you'r options...
I used to use Wella, but find they are expensive for what they are these days, they used to be good...
its a nice temp controlled iron, you have tip options etc...
You don't need a Pace pps85ae rework station on Shop Air for solder extraction like I have, but I do huge volumes of work and need it, the Hakko is a decent iron though and will see you right for what you are doing.
If you wanted to have something to do solder extraction the Hakko FR300 is quite good for small volumes of work, I have one but its retired as it wasn't coping with the amount of work I was doing with it.
But there's you'r options...
I used to use Wella, but find they are expensive for what they are these days, they used to be good...
