Since there is little mentioned on the 'net re Sound Technology gear, perhaps it would be a good idea for those who have successfully repaired their gear to list the LDRs used along with any mods they have made to the circuit(s). Both in the Oscillator & Analyzer sections.
As a folllow up, they can comment on their findings. In addition, it would be helpful to establish a listing of results using different LDRs in different locations on the analyzer board.
ANALYZER BOARD
U205
U206
U207
U208
U7 (OSCILLATOR board)
The 17xx models share similar LDR circuits.
As a folllow up, they can comment on their findings. In addition, it would be helpful to establish a listing of results using different LDRs in different locations on the analyzer board.
ANALYZER BOARD
U205
U206
U207
U208
U7 (OSCILLATOR board)
The 17xx models share similar LDR circuits.
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LDR sources
This is copied from another reply by me. I'm adding it here so I can find it myself later:
When it switches from fast to low distortion it engages the LDR. Those are the real weak point in the analyzer (aside from the many switches). ST didn't reveal the actual part number. The original vendor doesn't exist. Vactec does : Welcome | PerkinElmer so if we can figure out which part it was we can cross check- Silonex Inc.: Products: Audiohm Optocouplers . My schematic shows a CL614m note next to the part. That seems to lead to a black hole. I have a very foggy memory of 200 Ohms on resistance. The coupler has a 10K parallel resistance so that may be a real target value. I believe a VTL5C10 will work as a replacement so the Audiohm http://www.silonex.com/datasheets/sp...pdf/103465.pdf NSL-32B-101 might be a good replacement.
This is copied from another reply by me. I'm adding it here so I can find it myself later:
When it switches from fast to low distortion it engages the LDR. Those are the real weak point in the analyzer (aside from the many switches). ST didn't reveal the actual part number. The original vendor doesn't exist. Vactec does : Welcome | PerkinElmer so if we can figure out which part it was we can cross check- Silonex Inc.: Products: Audiohm Optocouplers . My schematic shows a CL614m note next to the part. That seems to lead to a black hole. I have a very foggy memory of 200 Ohms on resistance. The coupler has a 10K parallel resistance so that may be a real target value. I believe a VTL5C10 will work as a replacement so the Audiohm http://www.silonex.com/datasheets/sp...pdf/103465.pdf NSL-32B-101 might be a good replacement.
LDR parts
It has been stated on the net that various vintage LDR's often are found to age badly, often becoming defective in ways that totally FUBAR the operation of vintage THD Analyzers, whether HP, Amber, Sound Technology, Tektronix, or others that use this technology.
I recently bought some 'VTL5C' Linear Optocouplers (LDR's) on eBay as there are not many choices to choose from circa late September, 2019. On some forums, these parts have been recommended for use in ST, HP, and other THD Analyzers if the vintage parts seem to be going 'loony'. I found that unlike vintage parts, that the outer case of these VTL5C LDR's are made of a black plastic. Upon testing, I noticed that the resistance readings on these items changed drastically when I leaned over the part to grab something else. I then shielded it with my hand - from the ambient room light, and the readings changed even more!
If ordinary room light affected these parts more than the internal LED does - that is a serious problem. Adjust a circuit with the device cover off and then put the cover on and it's a whole different set of full-off to full-on resistances? Yowza!!!
I thought of wrapping the VTL5C LDR body in copper foil, but I am not sure where the light is leaking in, between the leads where they exit the body of the device, or from all over...
MAJOR FAILURE MODE!
I BOUGHT SEVERAL DOZEN OF THESE LDR's... And the seller did not understand my complaint of their defective design... Arrrrrgh!
Anyone have any better ideas?????
I have not tried to use any yet.
Steven
It has been stated on the net that various vintage LDR's often are found to age badly, often becoming defective in ways that totally FUBAR the operation of vintage THD Analyzers, whether HP, Amber, Sound Technology, Tektronix, or others that use this technology.
I recently bought some 'VTL5C' Linear Optocouplers (LDR's) on eBay as there are not many choices to choose from circa late September, 2019. On some forums, these parts have been recommended for use in ST, HP, and other THD Analyzers if the vintage parts seem to be going 'loony'. I found that unlike vintage parts, that the outer case of these VTL5C LDR's are made of a black plastic. Upon testing, I noticed that the resistance readings on these items changed drastically when I leaned over the part to grab something else. I then shielded it with my hand - from the ambient room light, and the readings changed even more!
If ordinary room light affected these parts more than the internal LED does - that is a serious problem. Adjust a circuit with the device cover off and then put the cover on and it's a whole different set of full-off to full-on resistances? Yowza!!!
I thought of wrapping the VTL5C LDR body in copper foil, but I am not sure where the light is leaking in, between the leads where they exit the body of the device, or from all over...
MAJOR FAILURE MODE!
I BOUGHT SEVERAL DOZEN OF THESE LDR's... And the seller did not understand my complaint of their defective design... Arrrrrgh!
Anyone have any better ideas?????
I have not tried to use any yet.
Steven
That is a new one on me. ST mentions that heat will affect the LDR for a while so don't adjust right after installing. I'll see if the old samples I have here are light sensitive. That would be unfortunate. Maybe you can find a small metal can to put them in??
I did a quick check of the Vactec and the NSL LDRs I have on hand. No evidence of sensitivity to ambient light even with a 5W white LED 1" away (lots of light) I still got over 100 Meg resistanc and they dropped to around 15 Ohms with 20 mA or so. I think you may have fakes.
How many do you need and what for?
How many do you need and what for?
I believe Demian is likely correct in thinking counterfeits. I know from hands-on experience that molded "black" plastic isn't necessarily opaque--- an old battle scar.
If you want to try using the LDRs, I suggest characterizing them for acceptable control range using an ohmmeter and variable bias source with the LDR thourghly shielded from ambient light. i've used a wad of black cloth with cardboard or a thick magazine on top to cover a clip-lead experiment. The ohmmeter can confirm adequate isolation from ambient light. You've probably already done this.
If the modules seem worthy of rescue, I imagine heat-shrink tubing is likely sufficiently opaque as I've read accounts of using it to hand craft couplers. Samples of electrical tape and duct tape I had on hand weren't enough. Trying to see through the candidate material at a bright point-source light is a good, insightful experiment.
If curiosity compels you discover the failure mechanism, you could try biasing the LED to about 20mA. In a dark room, you maybe able to see light leaking through the package.
Good luck!
If you want to try using the LDRs, I suggest characterizing them for acceptable control range using an ohmmeter and variable bias source with the LDR thourghly shielded from ambient light. i've used a wad of black cloth with cardboard or a thick magazine on top to cover a clip-lead experiment. The ohmmeter can confirm adequate isolation from ambient light. You've probably already done this.
If the modules seem worthy of rescue, I imagine heat-shrink tubing is likely sufficiently opaque as I've read accounts of using it to hand craft couplers. Samples of electrical tape and duct tape I had on hand weren't enough. Trying to see through the candidate material at a bright point-source light is a good, insightful experiment.
If curiosity compels you discover the failure mechanism, you could try biasing the LED to about 20mA. In a dark room, you maybe able to see light leaking through the package.
Good luck!
I am reviving this thread as I just recently reserviced my older ST1710A. It is very good machine however the photocouplers need replacement. The THD reading is sometimes unstable. I ran the generator onto my AP and the distortion is very good and stable, therefore m the problem resides in the analyzer.
It seems it uses is CL601. I looked on Google but I can’t even find anything. Do you know if this part can still be bad somehow or if there is a suitable replacement I can use?
Btw in the picture of my unit, it can be seen that U206 was replaced already at one point. So maybe the other one now is faulty.
Attachments
After more measurements and tuning I measured distortion at 1KHz 6dB about 0.00075% while AP shows 0.00058%.
Also interesting, I used the AP generator which has 0.00022% the ST shows 0.00035%.
However at 100KHz distortion rises to 0.04%. Unfortunately I cannot find a spec that specifies distortion at that point, all it says it less than 0.35% with no filters in.
This is a good machine, given I also have autolevel and IMD options, granted it won’t replace the AP, but at least for quick checks it’s very good and unlike the AP it doesn’t have a fan constantly running lol.
Things I have noticed are, distortion is not completely stable but has a little wiggle action at low readings. I would be curious to know how it performed when all the LDRs where new, also when I am in THD mode and I switch frequency, the needle jumps to the right then the low distortion clicks and it jumps (is this normal?).
Other than that great unit highly recommended if you can find a functional one.
Also interesting, I used the AP generator which has 0.00022% the ST shows 0.00035%.
However at 100KHz distortion rises to 0.04%. Unfortunately I cannot find a spec that specifies distortion at that point, all it says it less than 0.35% with no filters in.
This is a good machine, given I also have autolevel and IMD options, granted it won’t replace the AP, but at least for quick checks it’s very good and unlike the AP it doesn’t have a fan constantly running lol.
Things I have noticed are, distortion is not completely stable but has a little wiggle action at low readings. I would be curious to know how it performed when all the LDRs where new, also when I am in THD mode and I switch frequency, the needle jumps to the right then the low distortion clicks and it jumps (is this normal?).
Other than that great unit highly recommended if you can find a functional one.
I don't think you will improve it with newer LDR's and they are really hard to find. I would socket them. I did that on the one I had many years ago. It made selection much easier. You can use socket pins from an IC socket.
I posted info on a fan upgrade for AP system 1 using a thermally controlled fan. Works great.
I posted info on a fan upgrade for AP system 1 using a thermally controlled fan. Works great.
Yes it’s a great idea, it would be necessarily to socket U206 and the associated resistor R359 if I am hunting for a replacement.
Since you know how a fresh new unit performs, is the distortion measurement supposed to be perfectly stable or it also wiggles?
Also it is normal when I switch from fast to low distortion the needle jumps all to the right banging the needle against the end stop as the unit clicks waiting for the integrator and nulling circuit to stabilize? Or when I change frequency while in THD mode, same thing?
I usually avoid it by starting on VOLTS mode, wait for the unit to reach low distortion, then switch to THD.
Since you know how a fresh new unit performs, is the distortion measurement supposed to be perfectly stable or it also wiggles?
Also it is normal when I switch from fast to low distortion the needle jumps all to the right banging the needle against the end stop as the unit clicks waiting for the integrator and nulling circuit to stabilize? Or when I change frequency while in THD mode, same thing?
I usually avoid it by starting on VOLTS mode, wait for the unit to reach low distortion, then switch to THD.