regulation mod
Posted: 22 Sep 2016, 03:27
From a .pdf by electronics manufacturer, Analog Devices:
"IS IT ALRIGHT TO OPERATE MY AMPLIFIER SLIGHTLY ABOVE THE ABSOLUTE MAXIMUM RATINGS?
Absolutely not! It is critical to adhere to the absolute maximum ratings in a data sheet to avoid damaging the part permanently. The absolute maximum ratings indicate the limits that a device can tolerate, but not operate at..."
While cloning the ADC-89A8B, by DATEL, I noticed that it uses a comparator, LM710CH, which takes an uneven bi-polar supply. The positive power pin takes +14 V, and the negative power pin takes -7 V. I also noticed that The DATEL 8 bit A/D converter datasheet indicates that its V+ power input should be +15 V (+/- 0.25V). V- is -15 V (+/- 0.25V). Their other converters also only want (uni-, or bi-, polar) 15 V...
That looks tight, compared to +/- 18 V, which is offered to it by THE LATHE... What's more, the V+ pin of the 8 bit A/D converter "host" is directly connected to the V+ pin of the LM710CH "guest" on the printed circuit board, although that part reportedly wants no more than +14 V on that pin. Clearly, components can tolerate more voltage than the datasheets want us to know... But it seems better not to throw more power at them than is necessary, since their decrepitude is accelerated, otherwise.
So, the stock LS-76 psu and circuit design is sending the DATEL converters a +/- 18 V supply, which is regulated by two 7818s, in the TO-3 metal can package. Their outputs also go to various other cards in the cage, under the lathe, to power op amps, such as TL081, which have 18 Volts +/- as Absolute Maximum ratings... ...even µA748C, as shown in the datasheet detail, below, (contrary to many µA748 datasheets) may not actually be built quite as impervious to high supplies as the µA748M version... (?)
...also, this hybrid op amp (HC2000H), below, is powered by the same regulated supplies:
The Depth coil is driven by this hybrid op amp. The power inputs of this part can take bi-polar 37+1/2 VDC for its V+ and V- inputs - a total of 75 V @ 7 Amps.
Since the DATEL converters only want +/- 15V, and the TL081 and µA748 op amps all do well below +/- 18V, I decided to modify the power supply by removing the 7818 regulators and replacing them with 7815s. Updated drawing:
______________________________________________________________________________________________________
Incidentally, I noticed that the LS-76 manual drawing for the Variable Depth card has an incorrect legend for the power pin of the DATEL Systems, Inc. DAC-98BI module. The legend shown is the symbol for +5V, but the circuit is bringing it in contact with the +"18" V supply. As mentioned, above, it needs to be only + or - 15 V on all of those power pins, however, so, a mod seems in order.
______________________________________________________________________________________________________
After replacing the two regulators in the psu, I did some tests and found that everything works fine this way, but a recalibration was required to fine tune the fixed pitch, and also the expansions seen under microscope, to agree with the readout on the LPI Display. To confirm that the slightly lower rails were not somehow attenuating the impressing action of the Fixed, or Variable, Depth signals, I also did a test cut of 200 Hz, but with the Left and Right channels 100% out of phase, producing 0 VU signal intensity (ref. +4 dBm @ 1 kHz = 5 cm/sec/channel/diagonal/peak modulation velocity) from the pickup stylus. Indeed, the depth increases by just as much as I told it to, after the regulation mod. The 15 V bi-polar supply is still much bigger than the out of phase automation audio signals going to the hybrid op amp's inputs, so, the attenuated rails are not being exceeded. Therefore, the automation audio would not be using the extra 6 dB of headroom supplied by the original, bi-polar 18 V supply, anyway. Furthermore, the entire Feed and Depth behavior of THE LATHE seems more reliable now. I think it's happy to be less stressed out by the op amp rails... I'll try to keep it this way from now on so that the parts will possibly enjoy a longer life not being under as much pressure.
The Scully family were very smart and made a great lathe, but they were sometimes apparently bored with the mundane concerns of engineering and embraced mirth in their work, as evidenced by a depiction of Kilroy in the drawing for the Display Processor circuit board.
"IS IT ALRIGHT TO OPERATE MY AMPLIFIER SLIGHTLY ABOVE THE ABSOLUTE MAXIMUM RATINGS?
Absolutely not! It is critical to adhere to the absolute maximum ratings in a data sheet to avoid damaging the part permanently. The absolute maximum ratings indicate the limits that a device can tolerate, but not operate at..."
While cloning the ADC-89A8B, by DATEL, I noticed that it uses a comparator, LM710CH, which takes an uneven bi-polar supply. The positive power pin takes +14 V, and the negative power pin takes -7 V. I also noticed that The DATEL 8 bit A/D converter datasheet indicates that its V+ power input should be +15 V (+/- 0.25V). V- is -15 V (+/- 0.25V). Their other converters also only want (uni-, or bi-, polar) 15 V...
That looks tight, compared to +/- 18 V, which is offered to it by THE LATHE... What's more, the V+ pin of the 8 bit A/D converter "host" is directly connected to the V+ pin of the LM710CH "guest" on the printed circuit board, although that part reportedly wants no more than +14 V on that pin. Clearly, components can tolerate more voltage than the datasheets want us to know... But it seems better not to throw more power at them than is necessary, since their decrepitude is accelerated, otherwise.
So, the stock LS-76 psu and circuit design is sending the DATEL converters a +/- 18 V supply, which is regulated by two 7818s, in the TO-3 metal can package. Their outputs also go to various other cards in the cage, under the lathe, to power op amps, such as TL081, which have 18 Volts +/- as Absolute Maximum ratings... ...even µA748C, as shown in the datasheet detail, below, (contrary to many µA748 datasheets) may not actually be built quite as impervious to high supplies as the µA748M version... (?)
...also, this hybrid op amp (HC2000H), below, is powered by the same regulated supplies:
The Depth coil is driven by this hybrid op amp. The power inputs of this part can take bi-polar 37+1/2 VDC for its V+ and V- inputs - a total of 75 V @ 7 Amps.
Since the DATEL converters only want +/- 15V, and the TL081 and µA748 op amps all do well below +/- 18V, I decided to modify the power supply by removing the 7818 regulators and replacing them with 7815s. Updated drawing:
______________________________________________________________________________________________________
Incidentally, I noticed that the LS-76 manual drawing for the Variable Depth card has an incorrect legend for the power pin of the DATEL Systems, Inc. DAC-98BI module. The legend shown is the symbol for +5V, but the circuit is bringing it in contact with the +"18" V supply. As mentioned, above, it needs to be only + or - 15 V on all of those power pins, however, so, a mod seems in order.
______________________________________________________________________________________________________
After replacing the two regulators in the psu, I did some tests and found that everything works fine this way, but a recalibration was required to fine tune the fixed pitch, and also the expansions seen under microscope, to agree with the readout on the LPI Display. To confirm that the slightly lower rails were not somehow attenuating the impressing action of the Fixed, or Variable, Depth signals, I also did a test cut of 200 Hz, but with the Left and Right channels 100% out of phase, producing 0 VU signal intensity (ref. +4 dBm @ 1 kHz = 5 cm/sec/channel/diagonal/peak modulation velocity) from the pickup stylus. Indeed, the depth increases by just as much as I told it to, after the regulation mod. The 15 V bi-polar supply is still much bigger than the out of phase automation audio signals going to the hybrid op amp's inputs, so, the attenuated rails are not being exceeded. Therefore, the automation audio would not be using the extra 6 dB of headroom supplied by the original, bi-polar 18 V supply, anyway. Furthermore, the entire Feed and Depth behavior of THE LATHE seems more reliable now. I think it's happy to be less stressed out by the op amp rails... I'll try to keep it this way from now on so that the parts will possibly enjoy a longer life not being under as much pressure.
The Scully family were very smart and made a great lathe, but they were sometimes apparently bored with the mundane concerns of engineering and embraced mirth in their work, as evidenced by a depiction of Kilroy in the drawing for the Display Processor circuit board.