I've come across enough NAD equipment in the shop that of course turned me off to that brand.
That's why my main home system is Panasonic/Technics stuff.
Reliable!
🙂 Yep, they are what they are I'm afraid. We (I lol) used to call a lot of stuff 'all glue and Sellotape'. I'm sure you know the sort of stuff.
Well, some Technics product is okay, but the vast majority sounds very grainy or rough. The later receivers have a small fan blowing perpendicular on a tiny heat sink. Solder connections always broken on heavier or hot parts. Somehow that doesn't instil confidence in the brand.
Panasonic makes everything else great!
Panasonic makes everything else great!
Thanks for the comments guys. Going to try to summarize the suggestions in one place before I move on, just to ensure I'm not mixing any conflicting advice.
Please quote this post with edits to the list if you see something I need to adjust.
1. Remove light bulb in series from variac to amp
4. Check R456 (1K0) and R460 (330R) for issues
6. Check the two 15k's feeding the offset adjust (need to acquire board number)
7. Check Q414 and Q416 for faults
Please quote this post with edits to the list if you see something I need to adjust.
1. Remove light bulb in series from variac to amp
a. instead start variac at 0v and slowly increase
2. Purchase AC ammeter and place in series between the variac output and the ampa. watch ammeter current draw closely when slowly powering up the variac to ensure the amp doesn't draw large amounts of current which can indicate a fault
3. Pull Q 432 and Q434 to isolate problems and prevent further damages4. Check R456 (1K0) and R460 (330R) for issues
a. I am expecting to need to remove these from the circuit to fully test them, accurate?
5. Remove and check Q418 for any faults6. Check the two 15k's feeding the offset adjust (need to acquire board number)
7. Check Q414 and Q416 for faults
a. if faulted, check low value output stage resistors
b. replace with 2W MOX
8. Test carefully
4a - no need to remove them to test. A fault condition is high resistance.
6 - R448, R450 (indicated as 15K , 1W) They should be dissipating 290 mW, so really a good 1W resistor mounted high should be fine. If a 2W part is too large, install 1W again. Metal film or Metal oxides are fine, Metal Film has a lot lower drift.
I think you have it.
6 - R448, R450 (indicated as 15K , 1W) They should be dissipating 290 mW, so really a good 1W resistor mounted high should be fine. If a 2W part is too large, install 1W again. Metal film or Metal oxides are fine, Metal Film has a lot lower drift.
I think you have it.
At the risk of not seeing eye to eye with anatech on this 🙂 I'm going to say just use a bulb, it's easier and self-limiting on current. Overide the vbe multipliers as I outlined earlier, and it should power up just fine on a bulb.
If they happen to read OK in circuit, then that is good enough usually. If they read low because of interaction with other parts and if you have doubts, then lift one end and measure. Same if they read high in circuit. Slight residual charge from caps (so very slight voltage present on the board) can confuse some DVM's and give weird inaccurate readings in circuit.
If they happen to read OK in circuit, then that is good enough usually. If they read low because of interaction with other parts and if you have doubts, then lift one end and measure. Same if they read high in circuit. Slight residual charge from caps (so very slight voltage present on the board) can confuse some DVM's and give weird inaccurate readings in circuit.
Could one of you provide a recommendation for an ammeter? Finding a very wide variety of ranges and designs and not sure which one would work best.
All you need is a basic AC ammeter. You can buy a bare meter and install it in a box, that is the least expensive way to do it. Get a current rating maybe 2x your rating on the variac, or the same if it is a high power variac.
Variac is 120v 20amp. I'm having trouble finding ammeters rated for 40amp. Is 20amp considered "high" for this type of equipment?
Yes, it is. A 20 Ampere ammeter would be fine.
For most jobs on the bench, a 2 amp model works well. I use a 4 amp model mostly, only because it had more features. I do have the heavy 20 ampere (American) model. The Asian 20 amp models are considerably lighter.
-Chris
For most jobs on the bench, a 2 amp model works well. I use a 4 amp model mostly, only because it had more features. I do have the heavy 20 ampere (American) model. The Asian 20 amp models are considerably lighter.
-Chris
Remember the currents you are dealing with in a fault scenario can be quite low. Even something low like 200ma @120 is 24 watt in theory (yes there are efficiency losses but it's just to illustrate) and that is plenty to let the magic smoke free. You need a meter capable of resolving that while being able to withstand a true short circuit fault current where the meter might see many amps.
This is why I like the bulb, if the current starts heading skyward the filament suddenly rises in resistance and self-limits the current.
This is why I like the bulb, if the current starts heading skyward the filament suddenly rises in resistance and self-limits the current.
Fully agree Mooly. I still hold the idea regarding lamps as current limiting devices. Low voltages at low impedance are still dangerous.
It's all about being aware. I have never had such a situation where further damage has occured because I pay attention while bringing something up. To be honest, you have to be aware of how much energy you are supplying and the equipment is expecting a low impedance AC source. When there is a bulb in series you are not aware of the applied AC voltage, you only get a sense after becoming very familiar with that wattage bulb.
To my way of thinking, you are introducing a variable that you have little control over. I have seen damage occur using a bulb. Normally because the person doing that procedure relies on the lamp to protect (lazy). With the AC ammeter on the 2 A scale at 120 VAC, you can easily pick up small changes in current. If you aren't paying close attention when you are powering an item in unknown condition, that's on you.
Anyway, use of the lamp is not recommended by any company I am aware of and for good reason. I'm just stating facts. I think it is an ill advised short cut.
To my way of thinking, you are introducing a variable that you have little control over. I have seen damage occur using a bulb. Normally because the person doing that procedure relies on the lamp to protect (lazy). With the AC ammeter on the 2 A scale at 120 VAC, you can easily pick up small changes in current. If you aren't paying close attention when you are powering an item in unknown condition, that's on you.
Anyway, use of the lamp is not recommended by any company I am aware of and for good reason. I'm just stating facts. I think it is an ill advised short cut.
I’ve seen damage (and smoke) using a bulb limiter too. But it doesn’t usually take out another set of output transistors, and the resulting DC applied to the speaker is manageable even if it heads for the rail. Some amps come up “better” on a dim bulb than on a variac (PL will snap to the rail and stay there if they don’t come up fast enough).
Working on a tube amp now, using all unknown-condition TV tubes. I’ve got my variac, 300 watts worth of switchable bulb limiting, and AC voltage/current meters hooked up. As well as monitoring the cathode currents. And working very slowly. Can never have enough DMMs.
Working on a tube amp now, using all unknown-condition TV tubes. I’ve got my variac, 300 watts worth of switchable bulb limiting, and AC voltage/current meters hooked up. As well as monitoring the cathode currents. And working very slowly. Can never have enough DMMs.
Last edited:
I watch the bias current. I have never once had an output stage fail on a variac.
Other power supply types have special soft start procedures. That requires some instruction / training and variable DC supplies as a rule.
Other power supply types have special soft start procedures. That requires some instruction / training and variable DC supplies as a rule.
If “all you have” is a bulb limiter you won’t take out a set of (real) output transistors either. Little stuff that’s cheap - quite possibly. Won’t protect a quarter watt resistor dissipating 10 watts.
Interestingly the NAD service manual requested a 40-80 watt bulb wired in series after amplifier repair. Possible that they're expecting everything broken to be replaced and that the bulb wouldn't be used to troubleshoot if other components had failed?