that's a pretty clever idea. 👍 for the same reason, I wish I had thought of that when I used to do repair work!
Thank you, anatech, I totally agree. My problem is: I'm on a limited budget and space (just a desk with a couple of component racks, a solder iron (Ersa 30), a basic DMM, a Chinese component tester and a super basic self-made power supply (no current regulation, no dual pwr etc.).
My plan is to start with this basic setup and see if I'm able to improve my knowledge and skill. If I succeed, I will invest more on the equipment and eventually expand my space also.
An analog scope is on the list, but you have to know those old machines, if not properly reconditioned and calibrated they can be a hell. For now, having a 3 in 1 DMM, AWG and Scope (when it gets back from the dealer), I can start doing some basic repair job.
I've seen people of your level doing miracles with just a basic MM, so I think it's better I improve my knowledge and understanding before investing further money. Does it make sense?
Hi m-tagg,
I came from where you are, so yes it makes sense. My bench is over-full. One reason I bought the MSOX3104T, it replaced three instruments (but not really as it turns out).
DSO's are terrible for looking at eye patterns. If you have limited funds, focus on the equipment that does the primary job very well. You don't need toys.
Generally speaking, a DSO is not a good service bench instrument. It has specialized uses and good for documenting. I used analogue scopes and a camera for documentation until I got this last scope. But I kid you not, this expensive piece of gear is just useable for analogue signals. I had an Agilent 54642D before this, excellent scope but useless for eye patterns (you could tell they were there at least). I still have it. I keep my analogue scopes because some times that is what you need and the expensive digital gear just won't cut it. Now, some really, really expensive scopes (digital) will do the job for me, but I can't afford those. My point is, the very inexpensive analogue scopes can outperform a digital without breaking a sweat.
I had to fix the Tek 2235, 2465B, Philips PM3070, sure. Also an excellent HP 1722A. Okay, they are old, but now still running. If I want new that will do as good a job, I'd be looking at over $50K. Maybe a bit less today as DSOs are improving.
All I can say is, spend your dollars where it counts.
You cannot do competent repairs without a good oscilloscope. You cannot do decent audio service work without a good THD meter. You actually need those things, an oscilloscope can only show gross distortion, like over 1% or higher (and you need experience to determine that).
I came from where you are, so yes it makes sense. My bench is over-full. One reason I bought the MSOX3104T, it replaced three instruments (but not really as it turns out).
DSO's are terrible for looking at eye patterns. If you have limited funds, focus on the equipment that does the primary job very well. You don't need toys.
Generally speaking, a DSO is not a good service bench instrument. It has specialized uses and good for documenting. I used analogue scopes and a camera for documentation until I got this last scope. But I kid you not, this expensive piece of gear is just useable for analogue signals. I had an Agilent 54642D before this, excellent scope but useless for eye patterns (you could tell they were there at least). I still have it. I keep my analogue scopes because some times that is what you need and the expensive digital gear just won't cut it. Now, some really, really expensive scopes (digital) will do the job for me, but I can't afford those. My point is, the very inexpensive analogue scopes can outperform a digital without breaking a sweat.
I had to fix the Tek 2235, 2465B, Philips PM3070, sure. Also an excellent HP 1722A. Okay, they are old, but now still running. If I want new that will do as good a job, I'd be looking at over $50K. Maybe a bit less today as DSOs are improving.
All I can say is, spend your dollars where it counts.
You cannot do competent repairs without a good oscilloscope. You cannot do decent audio service work without a good THD meter. You actually need those things, an oscilloscope can only show gross distortion, like over 1% or higher (and you need experience to determine that).
Thanks anatech, I’ll look for better tools. 100MHz analog scopes have raised their prices in the last times (at least in Italy), but I’ll keep searching.
I have a little update about the CDP. I decided to replace some of the power supply section transistors, regardless what the china tester said. It turned out that Q207 and Q201 were bad. I replaced Q207 with the same C2478 and Q201 (A1048) with a 2SA933, just because I had it available. I know component substitution is a tricky and potentially dangerous matter, but Alltransistor.com shows main values and characteristics of a given component and the ones of possible replacements, so it’s easy to compare each feature.
Well, by replacing the two, the display came back to life. Of course not the right way: all the symbols and digits are lit up, and pushing buttons generate random numbers and symbols. But it lights up at least.
I then found that the ICP PS201 was open again, and decided to replace it with the same type. No more light. After unsoldering it, the display was lit again.
It should indicate a short somewhere near, I have to go back to the schematics and follow the path to hopefully find out the culprit. It's on the negative rail, so I'll start measuring voltages on that line. Also checking involved components.I think is a better situation now, cause at least one of the problems can be narrowed to a specific part of the circuit.
In the end I learned something: the china tester is not 100% reliable. MM in diode mode is more accurate (which confirms what you said).
I have a little update about the CDP. I decided to replace some of the power supply section transistors, regardless what the china tester said. It turned out that Q207 and Q201 were bad. I replaced Q207 with the same C2478 and Q201 (A1048) with a 2SA933, just because I had it available. I know component substitution is a tricky and potentially dangerous matter, but Alltransistor.com shows main values and characteristics of a given component and the ones of possible replacements, so it’s easy to compare each feature.
Well, by replacing the two, the display came back to life. Of course not the right way: all the symbols and digits are lit up, and pushing buttons generate random numbers and symbols. But it lights up at least.
I then found that the ICP PS201 was open again, and decided to replace it with the same type. No more light. After unsoldering it, the display was lit again.
It should indicate a short somewhere near, I have to go back to the schematics and follow the path to hopefully find out the culprit. It's on the negative rail, so I'll start measuring voltages on that line. Also checking involved components.I think is a better situation now, cause at least one of the problems can be narrowed to a specific part of the circuit.
In the end I learned something: the china tester is not 100% reliable. MM in diode mode is more accurate (which confirms what you said).
Hi m-tagg,
The very best and most effective bench transistor tester is the Heathkit IT-18. They made other, more complicated models and they are also excellent. The critical test most testers are missing is the leakage test between elements. C-B and C-E. If a modern tester has it, it is not sensitive enough. I try various testers if they look promising, so far the Heathkits are the only affordable effective ones.
You can heat the transistor while watching leakage and beta. Watch for discontinuities or sudden rises in leakage current. For silicon transistors, the allowable leakage is zero. The first graduation on the IT-18 is 5 uA. I've caught many exxtremely odd transistor faults with these testers, I have worn out two over 50 years. One thing these testers do for you is that you get a feel for the part, and how transistors normally operate.
I don't think the parts you replaced are critical, so relax. If they run hot, I normally use a part with a larger tab like a TO-126 part in place of a TO-92 part. At times you even have to install a heat sink! In amplifiers they sure can be critical. Don't trust replacement lists, look at the specs, but you need to learn a lot. You also need to make certain the circuit is stable after changing part types.
Looks like your tester lied to you. Digital scopes can lie to you easily, they are sampling systems with a scope like display. Different beast entirely.
The very best and most effective bench transistor tester is the Heathkit IT-18. They made other, more complicated models and they are also excellent. The critical test most testers are missing is the leakage test between elements. C-B and C-E. If a modern tester has it, it is not sensitive enough. I try various testers if they look promising, so far the Heathkits are the only affordable effective ones.
You can heat the transistor while watching leakage and beta. Watch for discontinuities or sudden rises in leakage current. For silicon transistors, the allowable leakage is zero. The first graduation on the IT-18 is 5 uA. I've caught many exxtremely odd transistor faults with these testers, I have worn out two over 50 years. One thing these testers do for you is that you get a feel for the part, and how transistors normally operate.
I don't think the parts you replaced are critical, so relax. If they run hot, I normally use a part with a larger tab like a TO-126 part in place of a TO-92 part. At times you even have to install a heat sink! In amplifiers they sure can be critical. Don't trust replacement lists, look at the specs, but you need to learn a lot. You also need to make certain the circuit is stable after changing part types.
Looks like your tester lied to you. Digital scopes can lie to you easily, they are sampling systems with a scope like display. Different beast entirely.
Thanks for your advice, anatech.
Heathkit gear is nowhere to be found in Europe. The few sellers are based in the US and they don't ship to Italy. More, the IT-18 model is very old and I actually found just a couple of them on the net. Not expensive, but no ship to Italy (shipping costs would be huge anyway). All that said, we're talking about not tested units so it's quite a lottery. It doesn't look too complicated circuitry, and it was sold in kit, so if somebody has the original schematics that was sold with the kit, it could be replicated. If its specs are that good, it could have sense.
As far as the transistor substitutions, I'm aware it's a risky business but when I do that, and I do it very rarely, I replace them only with ones with same identical specs.
Heathkit gear is nowhere to be found in Europe. The few sellers are based in the US and they don't ship to Italy. More, the IT-18 model is very old and I actually found just a couple of them on the net. Not expensive, but no ship to Italy (shipping costs would be huge anyway). All that said, we're talking about not tested units so it's quite a lottery. It doesn't look too complicated circuitry, and it was sold in kit, so if somebody has the original schematics that was sold with the kit, it could be replicated. If its specs are that good, it could have sense.
As far as the transistor substitutions, I'm aware it's a risky business but when I do that, and I do it very rarely, I replace them only with ones with same identical specs.
Hi m-tagg,
European models will work as well. Look for the same characteristics.
As for old, these are extremely reliable and basic. Clean controls, switches and battery contact (replace if batteries rotted). They are very basic instruments and work on basic principles. The better ones will have sensitive meters to show leakage current, or you can rig up your own. You are reading micro-amperes. You want the functions the same, brand doesn't matter (unless it isn't good/cheaply made).
European models will work as well. Look for the same characteristics.
As for old, these are extremely reliable and basic. Clean controls, switches and battery contact (replace if batteries rotted). They are very basic instruments and work on basic principles. The better ones will have sensitive meters to show leakage current, or you can rig up your own. You are reading micro-amperes. You want the functions the same, brand doesn't matter (unless it isn't good/cheaply made).