I will check whether the scope is ok but I think so. I adjusted for 27.5mV over the 0.1 ohm with the amp open and after closing it for 55mV over the two 0.22 ohm resistors
Can your scope handle 220 Volt, or is that beyond its sensitivity reach?
When your ground lug is attached to Protective Earth, the probe can measure respectively the voltages on the two mains wires.
One is also zero and the other is 220V.
Hans
When your ground lug is attached to Protective Earth, the probe can measure respectively the voltages on the two mains wires.
One is also zero and the other is 220V.
Hans
I'm not sure, but my scope goes to max 20V per division so I wouldn't be able to se if it's accurate with 220V
Anyway I just checked again the bias, and for some reason it was 400mA, don't know how it god there. the amp wasn't well warmed up so it probably would have dropped a bit but 400mA seems too much. Will measure again the caps with the bias correctly setup
Anyway I just checked again the bias, and for some reason it was 400mA, don't know how it god there. the amp wasn't well warmed up so it probably would have dropped a bit but 400mA seems too much. Will measure again the caps with the bias correctly setup
O.k. here we go with testing the input stage.
There are four 1n914 diodes protecting the input and directly connected to both bases of the LM394.
1) Touch one of the green circles with a wire that is connected on the other side to gnd while measuring the other green circle.
In one case the other side will still show ca 1Volt, then you have connected the wrong side to gnd.
When having connected the right green circle to gnd, the other green circle will also have to show gnd.
When this is not the case, LM394 is no good.
2) when the above was successful, measure the voltages over the four resistors encircled in red with the gnd wire still connected to the right green circle.
Hans
.
There are four 1n914 diodes protecting the input and directly connected to both bases of the LM394.
1) Touch one of the green circles with a wire that is connected on the other side to gnd while measuring the other green circle.
In one case the other side will still show ca 1Volt, then you have connected the wrong side to gnd.
When having connected the right green circle to gnd, the other green circle will also have to show gnd.
When this is not the case, LM394 is no good.
2) when the above was successful, measure the voltages over the four resistors encircled in red with the gnd wire still connected to the right green circle.
Hans
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Yes the 14K7 voltage is still 15V.
Voltage over the two 100 ohm resistors is -25mV on one and -180mV on the other. At switch on it is possible to try and balance them with the pot but after 2-3 minutes they drift and stabilize at the values I wrote.
Voltage over the 4.32k resistors is ca -600mV and -700mV, respectively.
Voltage over the two 100 ohm resistors is -25mV on one and -180mV on the other. At switch on it is possible to try and balance them with the pot but after 2-3 minutes they drift and stabilize at the values I wrote.
Voltage over the 4.32k resistors is ca -600mV and -700mV, respectively.
You know what's wrong, don't you ?
The LM394 has to be taken out, a lot of additional work.
Hans
The LM394 has to be taken out, a lot of additional work.
Hans
Yes I imagined that. I already ordered a NOS replacement (by the way is LM394CH the same as LM394BH right?).
Are the 15 V on 14K7 explained by the faulty LM394 or there could be other faulty components? Although I already replaced all the other transistors in the inverting input section.
Are the 15 V on 14K7 explained by the faulty LM394 or there could be other faulty components? Although I already replaced all the other transistors in the inverting input section.
Stefano,
From whom did you order the replacement? I've been told of fakes on the big auction site.
If you lack confidence in the part you've ordered or the supplier, this has come up in other discussions in the past:
https://www.ericasynths.lv/media/AS394CH.pdf
You might also review:
LM194 LM394
and starting at post 101
MAT02, 03 Elektor preamp schematic
Best,
Greg
From whom did you order the replacement? I've been told of fakes on the big auction site.
If you lack confidence in the part you've ordered or the supplier, this has come up in other discussions in the past:
https://www.ericasynths.lv/media/AS394CH.pdf
You might also review:
LM194 LM394
and starting at post 101
MAT02, 03 Elektor preamp schematic
Best,
Greg
Hi Greg, thanks for the warning. I ordered this Lm394ch DUAL amplifier TRANSISTOR ORIGINALE National Semiconductors TESTED NOS | eBay
Of course there’s always the risk of fakes…any tips on how to know if it’s authentic?
I wouldn’t know where else to get the replacement, or of alternative currently produced parts
Of course there’s always the risk of fakes…any tips on how to know if it’s authentic?
I wouldn’t know where else to get the replacement, or of alternative currently produced parts
Hi Stefano,
I'm not knowledgeable about how to spot a fake.
I think the AS394 is still in production. Here's what a Google search came up with:
Cabintech Global LLC
ALFA RPAR AS394H Ultra-matched NPN transistor pair (TO5-8) - LM394 equiv | eBay
Infinite Machinery. AS394
I'm not knowledgeable about how to spot a fake.
I think the AS394 is still in production. Here's what a Google search came up with:
Cabintech Global LLC
ALFA RPAR AS394H Ultra-matched NPN transistor pair (TO5-8) - LM394 equiv | eBay
Infinite Machinery. AS394
When you haven’t taken everything apart yet, I’ll give you another test tomorrow to be 100% sure.
Now I’m at 99% 😂
Hans
Also, I recall being instructed that a pair of 2SK170 can be used if matched and in contact with each other (e.g. paste and copper tape).
Thanks. No it's still assembled, exactly because I'd like to avoid disassembling it more than twice 😀
Thanks, I'll check the other options you posted as well.
I'd like to keep it original if possible, especially for symmetry between the two channels, but if needed, or if the risk of a fake LM394 is high, I'll consider alternatives.
I've always thought that a new fake device looks different than a NOS one because of the visible effects of age on the materials (dirt, discoloration etc), but maybe they can fake that two.
Depends on how the part is stored. I have thousands of genuine NOS out of production parts that look brand new. Sadly, no LM394! My first choice would be to locate the new production AS394 if the original can't be verified as non-counterfeit.
I have several LM394 as well as the identical Mat02, but all used.
When you can’t get a proper one, I can send you one of mine for free.
But let’s first wait the outcome of the final 100% tests.
Hans
P.s. how do you connect your preamp to the 23.5, via RCA or XLR?
In case of RCA the inverting amp is not used.
When you can’t get a proper one, I can send you one of mine for free.
But let’s first wait the outcome of the final 100% tests.
Hans
P.s. how do you connect your preamp to the 23.5, via RCA or XLR?
In case of RCA the inverting amp is not used.
Understood. Yes the AS394 is available here in Europe from an online store.
Thank you Hans, it's amazing the time and effort you are putting to help me in this repair. I really appreciate it.
Yes I use balanced connection since I have a ML28 preamp and never wanted to buy the expensive Camac adaptors (I only have a pair to connect the turntable).