I can understand that you want to learn how to repair electronics - and analog gear in this case. What you do not yet have is an understanding of electronics theory to be able to do the analysis. That's what you rely on the AK armchair bench techs (ACBT's) for. You are, essentially, the eyes, ears and measurement taker. The more accurate those functions are, the better the ACBT's can help you.
On Q101, no need to replace it right now. That's called 'shotgunning' and it usually back fires on you in that you introduce more problems rather than solve the one you have now.
The key to understanding what's happening is the analysis of the voltages throughout the power supply. As you can see from the schematic and the board layout, there are specific expected voltages.
Since you're still in diagnostic mode - you don't know what the root cause is yet - then finding what voltages are in spec and what ones are not is the top priority. As the measuring dude, your job is to record them accurately and present them back to the ACBT's who can figure out from them what's working and what isn't.
Rather than to do the 'check this, check that', perhaps it is better that you go through the entire power supply circuit, take the measurements it calls for and record them on a copy of the schematic. Take a pic of that marked up schematic and post it back here. That will speed up the process.
Cheers,
David
Hi Grant,
Here's what you need to do next:
- Connect the black lead to the common ground. It gets left there since all of the voltages called for in the service manual will reference Ground.
- Then, use the schematic and the excellent board layout pictures to locate the actual place to take a measurement. Some of these will be difficult to get to unless the board is loosened or raised from its fixed position. That's where your skills in patience will come in.
A word of caution: If you do try to remove the circuit boards or even just move them enough to get underneath, please check each of the wires that are connected to the board. They can be easily broken if flexed - they're solid core wires, I believe. Heavy-handedness working with wire wrapped boards is well known to cause breaks that just add to your trouble.
- Lastly, record the measurements adjacent to the spec'd voltages on the schematic. This is the thing that the arm chair benchtechs will want to see - not the circuit board.
Hope that gives you enough direction.
Cheers,
David
Yes, that's correct. The only additional item the bench techs need are the voltages put on the schematic - not on the board layout. Bench techs need to understand the relationship of the voltages compared to what they _should_ be and what they _are_ from your measurements. This means you have to compare the board layout to the schematic and identify the components to make the jump to the schematic.
When that is understood, then they can use that data to figure out where the problem is. The schematic tells them what devices are being used and how they're wired. Take those 2 items together and they can figure out from the comfort of their own home where to start looking for the problem.
The ability to analyze the circuit is the key thing they can do. You are their tech who is doing the hands-on work for them.
Cheers,
David
David,Yep, those frayed, while still connected, are the result of the board being pulled away or having the wires pushed around. This was always a problem with wirewrapped connections. Yes, it is best to solder those connections back. As long as they're still connected, however, that's not the root cause.
Cheers,
David
Great job on recording the measurements! So, what they're saying is:
- Q101 is not conducting. This means that the 14 volts called for is not likely going to be there if you could find the test point.
David
David,The black probe is correct. The red probe needs to go to the other side of R100. We already know that the voltage a D102 is 23.8 so that is one side of R100. The other side is what's missing.
Cheers,
David
Ok, using the board layout diagram, you want to measure at the Green and Blue points shown below:
View attachment 889757
Blue should read about 24 volts. This is the junction of R100 and the collector of Q101.
Green should read 14 volts when things are working correctly. I know it isn't right now but the Red test point didn't have a report from you on a reading. What do you read at those 2 points?
Cheers,
David
Check R100. It should read 3.9 ohms. I'll bet it doesn't. No appreciable voltage there will cause the other voltages that don't match to be that way.
Cheers,
David
Yep, that's the procedure.
Cheers,
David
Do you have 14V at the test point on R103? You circled it, but it doesn't have a measurement written next to it. I'd pull one leg of that resistor and check it, too.
The test point on RV101 should be at 6.5V, you're showing it at 1.0V. Can you adjust RV101 to reach 6.5V?