sorry to be unclear the replacement JFET has now created a drop as per the circuit diagram of 398 on the +ve and 374 on the other side (next to terminal 3 on the TL071)
If is not damaged , obviously is not original or equivalent
Use a normal diode until you find the right part , although I doubt you could hear / measure a difference in noise
Everybody uses a normal diode in direct coupled cathode follower without noise issues as it is at the output where the signal is highest
Use a normal diode until you find the right part , although I doubt you could hear / measure a difference in noise
Everybody uses a normal diode in direct coupled cathode follower without noise issues as it is at the output where the signal is highest
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I wrote
"sorry to be unclear the replacement JFET has now created a drop as per the circuit diagram of 398 on the +ve and 374 on the other side (next to terminal 3 on the TL071)"
I was writing in a rush and I meant the Zener Diode not the JFET this refers to ZD17 I have replaced which was shorted.
The JFETs on the signal path act as a diode and are I believe functioning - they are NOS 2N5462 as specified.
These are the voltages on U1 the TL071CP pins as below
1=373V
2=374V
3=383V
4=374V
5=374V
6=374V
7=399V
8=20mV
"sorry to be unclear the replacement JFET has now created a drop as per the circuit diagram of 398 on the +ve and 374 on the other side (next to terminal 3 on the TL071)"
I was writing in a rush and I meant the Zener Diode not the JFET this refers to ZD17 I have replaced which was shorted.
The JFETs on the signal path act as a diode and are I believe functioning - they are NOS 2N5462 as specified.
These are the voltages on U1 the TL071CP pins as below
1=373V
2=374V
3=383V
4=374V
5=374V
6=374V
7=399V
8=20mV
OMG use then for B+ the other output voltage 402V used for the 3 triodes after the volume control
A bit lower B+ is not a reason for the big differencies you measured, use 402V and see
A bit lower B+ is not a reason for the big differencies you measured, use 402V and see
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My original logic was that if the B+ Voltage was designed to be +390V and I am getting + 374V the voltages at the valve plates would be slightly lower, but not as low as they are on the plate of V2 and the cathode of the follower V3. Especially in relation to the voltages on V1 being near the design with B+ at 374.
Hence my question as to what else am I missing (rather than 16V on the B+)
I just feel like I must be missing something 😕
Hence my question as to what else am I missing (rather than 16V on the B+)
I just feel like I must be missing something 😕
Why would you care if it's low noise? It's only on during startup. Its purpose in life is to limit the Vhk on V6 during startup.
Odd with the diode strapped JFET. That sort of stuff is common in IC designs (except modern IC processes often don't have JFETs) but rarely used in the discrete world.
Tom
IN+ >> IN- so the output (pin 6) should be near VCC (pin 7). It isn't so I wonder if the opamp is dead.
Tom
Thanks for this thinking.
Is there any way to determine if the TL071CP is at fault, It's quite a lot of work and fiddly soldering 😉
Is there any way to determine if the TL071CP is at fault, It's quite a lot of work and fiddly soldering 😉
A company (whose name escapes me) makes a special pair of cutters just for cutting the legs on dip chips. Cut all the legs and you can desolder them individually much easier. If you’re looking for a serious upgrade then Hakko makes a handheld desoldering gun that works amazingly well. The vacuum is powerful because it doesn’t have to go through flexible rubber tubing. https://hakkousa.com/products/desoldering/fr-301-portable-desoldering-tool.html
Thanks and I guess I would need a new tip on my iron that's a bit smaller, and maybe turn the heat down a tad.
But I still don't yet have way of confirming it is at fault do I?
But I still don't yet have way of confirming it is at fault do I?
There are many TL071 out there. The one by TI (data sheet fig. 6-28) shows no phase reversal. So if V(IN+) > V(IN-) you should have VCC less a little bit on the output. You have VEE. So the TL071 is dead as far as I can figure.
Snip the pins individually and de-solder. You can also (carefully!) cut through the pins along the IC package with a utility knife. They're likely just tinned copper so pretty soft. Just be careful that you don't damage the board (or cut your self).
Tom
Snip the pins individually and de-solder. You can also (carefully!) cut through the pins along the IC package with a utility knife. They're likely just tinned copper so pretty soft. Just be careful that you don't damage the board (or cut your self).
Tom
Do you believe the TL071 failing creates the drop from 390 to 372V (5% below) AND this then creates the drop on V2 from 150 to 100V on the plate?
That's where I struggle to see how, when by comparison at 372V V1 is not really much less different the expected design 170V.
Finally if the TL071 is dead can I measure any in circuit resistances to confirm this, I have a couple on order and can swap it out, but is it also possible that Q6 and Q7 might also be at fault?
That's where I struggle to see how, when by comparison at 372V V1 is not really much less different the expected design 170V.
Finally if the TL071 is dead can I measure any in circuit resistances to confirm this, I have a couple on order and can swap it out, but is it also possible that Q6 and Q7 might also be at fault?
How are you measuring V2's anode voltage? Remember that it's at a very high impedance point and that your voltmeter probably loads that point significantly.
Semi-cons tend to fail catastrophically, usually to short, and tend to damage other connected semi-cons. Replace all of the semi-cons in that regulator. (Also any electrolytics if there were any). Parts are cheap and potential unsoldering/soldering damage to the board is more important than a few cheap parts - fix it once. You really don't care if it costs 5 or 50. Doesn't matter.
All good fortune,
Chris
Semi-cons tend to fail catastrophically, usually to short, and tend to damage other connected semi-cons. Replace all of the semi-cons in that regulator. (Also any electrolytics if there were any). Parts are cheap and potential unsoldering/soldering damage to the board is more important than a few cheap parts - fix it once. You really don't care if it costs 5 or 50. Doesn't matter.
All good fortune,
Chris
Well that is an interesting question "How are you measuring V2's anode voltage? Remember that it's at a very high impedance point and that your voltmeter probably loads that point significantly." as the measurement takes a considerable time to stabilise and starts off at a higher voltage, near what is expected, and then over 30-50 seconds gradually falls to a stable lower value.
I use a DMM and set to DC, with negative onto the ground rail, and the positive pushed into the track that feeds the FET and the anode.
I will look for replacement parts for the TL071 and Q6 and Q7, and maybe the two diodes as well?
I use a DMM and set to DC, with negative onto the ground rail, and the positive pushed into the track that feeds the FET and the anode.
I will look for replacement parts for the TL071 and Q6 and Q7, and maybe the two diodes as well?
Attachments
A double sided board, and you're understandably worried about the unsoldering/soldering process for the opamp. So, I'll change my recommendation: the opamp probably can't hurt the other semi-cons, so maybe leave it for a separate, last step, only if needed. But be prepared to need it - luck favors the prepared.
The completely unrelated issue of measuring V2's anode voltage can be eased by making the measurement at V3's cathode (a lower impedance point and, if nothing's badly broken, only a volt or couple higher than V3's grid). DMMs vary all over; some are 1M Ohm and some are 10M Ohm, so some are more of a load than others.
The 30 to 50 s time constant is interesting. Too long to be RC, but could be misbehavior inside V3. I'm trying to guess if the JFET "diode" would survive a test with V3 pulled out. Very probably, so this could be an approach somewhere down the road if nothing else rears its ugly head as a culprit.
All good fortune,
Chris
The completely unrelated issue of measuring V2's anode voltage can be eased by making the measurement at V3's cathode (a lower impedance point and, if nothing's badly broken, only a volt or couple higher than V3's grid). DMMs vary all over; some are 1M Ohm and some are 10M Ohm, so some are more of a load than others.
The 30 to 50 s time constant is interesting. Too long to be RC, but could be misbehavior inside V3. I'm trying to guess if the JFET "diode" would survive a test with V3 pulled out. Very probably, so this could be an approach somewhere down the road if nothing else rears its ugly head as a culprit.
All good fortune,
Chris
any ideas if the D40K1 transistor (not available) can replaces with a D40K2 transistor (which is available)
I have ordered some replacement diodes and transistors and the TL071, and will share if this is resolved for anyone in the future having a similar rebuild.
I also wondered about R52 and R52 which the manual says can be adjusted to trim the voltage, any idea if higher resistance = increase voltage B+ or will it decrease?
I also wondered about R52 and R52 which the manual says can be adjusted to trim the voltage, any idea if higher resistance = increase voltage B+ or will it decrease?
Yeah ok - for 402, I have 398 so if it was easy I might tweak with increasing R51 or R52.
for 390V I will try the some of the components to see if anything is faulty, and if this fails then adjust the 100K // 1.82M+1.82M.
So here can I assume to increase from 374 to 390 I decrease the 100K a little or increase the 1.82M?
for 390V I will try the some of the components to see if anything is faulty, and if this fails then adjust the 100K // 1.82M+1.82M.
So here can I assume to increase from 374 to 390 I decrease the 100K a little or increase the 1.82M?
Yes it is like that , but you should try to adjust it first with some resistors in parallel to see if it works or not , of course if the opamp or other component is bad it won't change the ouput voltage