I'll add to the debate over using protection.
In some cases, I choose the procedure which I determine to be better depending on the test and equipment involved.
A variac, a dim bulb(s), or both.
That's why I made my own "safety supply" which incorporates a variac AND a switchable dual-range dim bulb option.
This is the section I'm talking about, however there's more to the unit (edited for simplicity), voltage and amps meters, a variable DC supply, etc.
In some cases, I choose the procedure which I determine to be better depending on the test and equipment involved.
A variac, a dim bulb(s), or both.
That's why I made my own "safety supply" which incorporates a variac AND a switchable dual-range dim bulb option.
This is the section I'm talking about, however there's more to the unit (edited for simplicity), voltage and amps meters, a variable DC supply, etc.
No, they had low expectations for service depots. Any self respecting service depot used a variac on every bench. This has been a standard since at least the 1940's. They paid less for warranty work and just everything was very low budget with them.
Can you use a dim bulb? Sure, but it introduced a lot of variables. The equipment may not work the same as it does with a low impedance AC supply and it absolutely does not prevent damage to low energy circuits. Outputs? Yes, it should protect against those blowing as long as you don't allow it to run and overheat them. But then, so does a variac and then you do have exact knowledge of voltage levels (that remain stable).
I don't know, I've tried dim bulbs. I can't recommend that method.
Can you use a dim bulb? Sure, but it introduced a lot of variables. The equipment may not work the same as it does with a low impedance AC supply and it absolutely does not prevent damage to low energy circuits. Outputs? Yes, it should protect against those blowing as long as you don't allow it to run and overheat them. But then, so does a variac and then you do have exact knowledge of voltage levels (that remain stable).
I don't know, I've tried dim bulbs. I can't recommend that method.
Trying to test the other channel's equivalent component at the same time.
R457 is measuring 10 Ohms in circuit. Seems good. R458 is measuring 10 Ohms in circuit. Seems good. R456 is measuring 2.6 kOhms in circuit. Faulted? R455 is proving difficult to locate on the board. Closest I can find is one right next to VR401. It is labeled R445 on the board. Typo? This one is measuring 15 kOhms. R460 is measuring 17 MOhms in circuit. Faulted? R459 is measuring 330 ohms, seems good.
I'm not familiar with board design, could someone point me to exactly what I am looking for? Are these the resistors feeding the VR's that are used for the offset adjustment?
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"I acquired a previously blown NAD 7240PE from ebay. This unit, when received, had one good channel (L) and one bad channel (R)."
In any stereo amplifier, they have Two Channels.
IF..... one channel is having issues, then Comparing it, component by component against the Good Channel's same component is often helpful.
In any stereo amplifier, they have Two Channels.
IF..... one channel is having issues, then Comparing it, component by component against the Good Channel's same component is often helpful.
When I was training for my certification decades ago, it was what you did.... logic.
That certainly works when you don't have the manual and the two channels are laid out the same way. I do the same thing when information isn't available and you do have one good channel to compare to.
Yes, logic! Use your head!!!!
Yes, logic! Use your head!!!!
Unfortunate that the physical layout is different. Hard to know which two component do the same thing without cross referencing the schematic and board layout diagrams.
So have I..... many times.
Yes, it's a pain sometimes when you don't have a 'mirror image' type of layout.
Tedious work is part of the job.
Those look like the 15k's feeding the offset preset. Just unsolder and lif one end of any suspect resistor to get a true reading.
I'm not sure where you are up to with this and what fault/syptom you are looking at. Is the bulb lit brightly?
Last time I used the bulb it was not lighting and a few seconds later after slowly increasing voltage on the Variac I blew up the "good" channel. I won't be returning to the bulb based on others' comments.
The affirmation that others are doing the same certainly makes me feel better.
For the mean while I'm testing every single component from input to output, can you guys offer some tips for finding alternative transistors? Needing to find an alternative BD911 and BD912. Exact replacements on eBay are getting expensive.
Thank you sir. Could you walk me through your process for determining this? I've tried using this tool but im not 100% on it's accuracy.
https://alltransistors.com/crsearch...&ft=3&cc=0&hfe=15&caps=TO220&r1=0&r2=0&r1r2=0
You know what? Replacement device's are terrible. The NTE / ECG books (NTE copied ECG) are known as "the book of lies". Do not use those to cross reference whatever you do. If I see a replacement brand device in any repair, it's on the list of required parts to be replaced or I won't do the job. I used to sel lthat junk as an early tech at a place I worked.
Look up the original specifications for the devices you need to replace.
Go to On Semi's site, or Digikey and select the same case styles and specs (or greater) and see what available devices pop up. You need to compare collector-emitter voltages, not collector-base! Look at Beta or hFE, max collector current and power dissipation. Also consider fT. There are other specs I look at but we're talking experience. Those basics should keep you out of trouble. One word, using a faster transistor because "it's better" often will get you into trouble. Use one close the the original speed if you can. The circuit was stable with the original devices and you don't want to redo the stability of the amplifier. You need an oscilloscope and oscillator minimum for that.
Using cross-reference manuals is risky as heck. Also, never get a cross in, then cross it again to an available part.Two parts that cross reference to an NTE 332 (for example) can be vastly different in ratings. Just because they cross to the same part has no bearing on how similar they are!
Look up the original specifications for the devices you need to replace.
Go to On Semi's site, or Digikey and select the same case styles and specs (or greater) and see what available devices pop up. You need to compare collector-emitter voltages, not collector-base! Look at Beta or hFE, max collector current and power dissipation. Also consider fT. There are other specs I look at but we're talking experience. Those basics should keep you out of trouble. One word, using a faster transistor because "it's better" often will get you into trouble. Use one close the the original speed if you can. The circuit was stable with the original devices and you don't want to redo the stability of the amplifier. You need an oscilloscope and oscillator minimum for that.
Using cross-reference manuals is risky as heck. Also, never get a cross in, then cross it again to an available part.Two parts that cross reference to an NTE 332 (for example) can be vastly different in ratings. Just because they cross to the same part has no bearing on how similar they are!
Ya know, every time I mention using an NTE replacement device, you tarnish and bash the brand and make me look like a slob idiot giving out bad information.
Perhaps you had some issues in the past using them, and that's why you're so biased.
BUT I NEVER DID!....... not in 30+ years of using them as replacements.
Well, then you haven't been testing them. In fact, you are completely unaware of what actual transistor part number you are installing.
I tested these device new in their bags. My failure / return rate dropped to next to zero when I started using actual, real devices. Thankfully when I was forced to use these, it was someone else's name on the repair and not mine.
I'm just stating facts from direct experience both selling these to the public and having to use them - plus seeing blown ones in repairs. Now, if you want to think I am attacking you personally, I'm not. But I know as fact these are not first rate parts. If you decide to use them - hey! That's on you. I know better.
I tested these device new in their bags. My failure / return rate dropped to next to zero when I started using actual, real devices. Thankfully when I was forced to use these, it was someone else's name on the repair and not mine.
I'm just stating facts from direct experience both selling these to the public and having to use them - plus seeing blown ones in repairs. Now, if you want to think I am attacking you personally, I'm not. But I know as fact these are not first rate parts. If you decide to use them - hey! That's on you. I know better.
Point taken, safe side would be to replace with the exact part. I'll let you two duke it out in DMs please keep my thread productive for the sake of others attempting similar repairs.
Too bad these BD911/912s are so expensive, but I'll just have to be more careful to ensure supporting components aren't defective.
Too bad these BD911/912s are so expensive, but I'll just have to be more careful to ensure supporting components aren't defective.
If they are current and available, use them. The expense of a part really shouldn't be considered if it is the original part required.
If you can't get them from a proper distributor, they are discontinued and the chances are much higher getting a fake. At that point it is realistic to find an equivalent part that is currently manufactured and use that. At least then you know the part is a real one and will conform to the specifications. A popular tactic is to buy parts from the same family but lower collector voltage rating, or lower gain (beta) and remark them as the premium device. They are very good these days and it is almost impossible to tell a fake from the real thing. Others are obvious. Just be careful.
If you can't get them from a proper distributor, they are discontinued and the chances are much higher getting a fake. At that point it is realistic to find an equivalent part that is currently manufactured and use that. At least then you know the part is a real one and will conform to the specifications. A popular tactic is to buy parts from the same family but lower collector voltage rating, or lower gain (beta) and remark them as the premium device. They are very good these days and it is almost impossible to tell a fake from the real thing. Others are obvious. Just be careful.