(To conform with file uploading limits a reduced size PDF file containing the figures is attached).
Purpose
* To share hints and lessons learned from fixing my B790 turntable's speed fluctuations.
* This report complements the earlier report, "ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned" where I wrote, "Investigating the Motor Control Board 1.179.152 found its operation was unstable causing the motor to lose control and behave erratically - this I have fixed and will document and share in a separate post."
History
* My ReVox B790 Linatrack turntable serial number 004778 I purchased new in July 1978 from REW Audio Visual Co. Ltd, London, England.
Figure 1. Refurbished B790 Linatrack turntable - serial number 004778
Diagnosis and Repair - Motor Control Board
* With the fixed speed push button set to '33', the turntable speed fluctuated between 32 and 34 RPM on the digital display panel.
* By connecting an oscilloscope, it was possible to observe changes in turntable speed correlate with a change in the motor control board’s internal signals, the period of the waveforms shifting with the varying speed.
* For example, these internal signals could be monitored with an oscilloscope probe connected to:
a) The motor’s tachometer conditioned signal at output of RC4558 op-amp IC 19, pin 1.
b) The motor’s Hall effect conditioned signals at output of RC4558 op-amps IC 14 pin1 and IC 19 pin 7.
* While I could clearly observe on the oscilloscope waveforms the effect of the speed changes, the cause was not obvious. However, the following ReVox service note provided a recommendation.
ReVox Service note 67.17 for the B790, Op-amp RC 4558 unsatisfactory.
The op-amp RC 4558 (made by Raytheon) has shown to have an excessive failure rate. For this reason it will be replaced by the high performance type RC 4559 / 50.09.0107 (-----). This op-amp is used in the following circuits 1.179.150/152.
* On the Motor Control Board 1.179.152 all four RC4558 op-amps IC5, IC8, IC14 and IC19 were changed to the more reliable RC4559.
Figure 2. Motor Control board showing the four 4558 Op-Amps replaced by newer 4559 - photo taken August 2022
Diagnosis and Repair - Power Supply Board
* Another issue was the speed instability could be induced by cycling the power on/off switch, however checking the three regulated DC voltages on the Power Supply board showed all voltages to be stable and well within specification; +24V < +/- 1V, +6V < +/- 0.25V and -6V < +/- 0.25V.
* This led me to suspect there may have been a dynamic condition which couldn't be detected with a multimeter that averages the DC voltage, or that some of the components after +40 years had aged and become weak.
* Following components on the Power Supply board were replaced with new.
a) PTC resistors RT1, RT2 and RT3 (all 2.3R rating).
b) Electrolytic capacitors C2, C3 and C4.
c) Film capacitors C1, C5, C6, C7, C8 and C9.
d) Bridge rectifiers D4, D5 and D6 upgraded to higher current rated W04 (400V, 1.5A).
e) 6-volt regulators IC1 and IC2 upgraded to higher current rating 1-amp type MC7806CT.
f) 24-volt regulator IC3 replaced with new MC7824CT.
g) Renewed the TO-221 mica insulators and silicon grease.
Figure 3. Original Power Supply board - photo taken November 2021
Figure 4. Completed Power Supply board with upgraded components - photo taken August 2022
Operational Checks after Repairs
* With the B790's phono leads connected to input of preamp and the preamp's line outputs connected to the AnalogMagik test package showed:
a) Exceptionally stable speed of 33 1/3 rpm corresponding to measured test frequency of 3,150 Hz.
b) Measured Wow & Flutter under 0.1%.
* Considering this is a 45-year-old turntable I was very pleased with these results.
Figure 5. After repair, Wow & Flutter results - measured December 2022
What was wrong?
* To be honest it was not possible to pinpoint exactly what caused the speed instability, other than the following suspicions.
* In the Power Supply I wonder about IC2, the original +6V regulator rated 0.5 A as it supplies both the positive rail voltage for the op-amps and through a forward bias diode supplies +5.3V for the digital ICs - if this voltage fluctuates or has a glitch it might have triggered something untoward. As mentioned, this was replaced by newer series of +6V regulator rated 1.0 A.
* Another factor could be components in either the Motor Control Board or the Power Supply Board were borderline, or rated just sufficient when the product was manufactured, but after 45-years these components had become weak or worn down.
* Regret if these components had simply failed outright the troubleshooting would have been straightforward, but troubleshooting is never easy...
Final comments
* The reason for this writing is to share my experience repairing a ReVox B790 turntable erratic speed. At time of this writing the turntable has been operating without issue for 7 months now.
* Pleased to report my ReVox B790 turntable now tracks and plays LPs flawlessly, just like it used to!
Purpose
* To share hints and lessons learned from fixing my B790 turntable's speed fluctuations.
* This report complements the earlier report, "ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned" where I wrote, "Investigating the Motor Control Board 1.179.152 found its operation was unstable causing the motor to lose control and behave erratically - this I have fixed and will document and share in a separate post."
History
* My ReVox B790 Linatrack turntable serial number 004778 I purchased new in July 1978 from REW Audio Visual Co. Ltd, London, England.
Figure 1. Refurbished B790 Linatrack turntable - serial number 004778
Diagnosis and Repair - Motor Control Board
* With the fixed speed push button set to '33', the turntable speed fluctuated between 32 and 34 RPM on the digital display panel.
* By connecting an oscilloscope, it was possible to observe changes in turntable speed correlate with a change in the motor control board’s internal signals, the period of the waveforms shifting with the varying speed.
* For example, these internal signals could be monitored with an oscilloscope probe connected to:
a) The motor’s tachometer conditioned signal at output of RC4558 op-amp IC 19, pin 1.
b) The motor’s Hall effect conditioned signals at output of RC4558 op-amps IC 14 pin1 and IC 19 pin 7.
* While I could clearly observe on the oscilloscope waveforms the effect of the speed changes, the cause was not obvious. However, the following ReVox service note provided a recommendation.
ReVox Service note 67.17 for the B790, Op-amp RC 4558 unsatisfactory.
The op-amp RC 4558 (made by Raytheon) has shown to have an excessive failure rate. For this reason it will be replaced by the high performance type RC 4559 / 50.09.0107 (-----). This op-amp is used in the following circuits 1.179.150/152.
* On the Motor Control Board 1.179.152 all four RC4558 op-amps IC5, IC8, IC14 and IC19 were changed to the more reliable RC4559.
Figure 2. Motor Control board showing the four 4558 Op-Amps replaced by newer 4559 - photo taken August 2022
Diagnosis and Repair - Power Supply Board
* Another issue was the speed instability could be induced by cycling the power on/off switch, however checking the three regulated DC voltages on the Power Supply board showed all voltages to be stable and well within specification; +24V < +/- 1V, +6V < +/- 0.25V and -6V < +/- 0.25V.
* This led me to suspect there may have been a dynamic condition which couldn't be detected with a multimeter that averages the DC voltage, or that some of the components after +40 years had aged and become weak.
* Following components on the Power Supply board were replaced with new.
a) PTC resistors RT1, RT2 and RT3 (all 2.3R rating).
b) Electrolytic capacitors C2, C3 and C4.
c) Film capacitors C1, C5, C6, C7, C8 and C9.
d) Bridge rectifiers D4, D5 and D6 upgraded to higher current rated W04 (400V, 1.5A).
e) 6-volt regulators IC1 and IC2 upgraded to higher current rating 1-amp type MC7806CT.
f) 24-volt regulator IC3 replaced with new MC7824CT.
g) Renewed the TO-221 mica insulators and silicon grease.
Figure 3. Original Power Supply board - photo taken November 2021
Figure 4. Completed Power Supply board with upgraded components - photo taken August 2022
Operational Checks after Repairs
* With the B790's phono leads connected to input of preamp and the preamp's line outputs connected to the AnalogMagik test package showed:
a) Exceptionally stable speed of 33 1/3 rpm corresponding to measured test frequency of 3,150 Hz.
b) Measured Wow & Flutter under 0.1%.
* Considering this is a 45-year-old turntable I was very pleased with these results.
Figure 5. After repair, Wow & Flutter results - measured December 2022
What was wrong?
* To be honest it was not possible to pinpoint exactly what caused the speed instability, other than the following suspicions.
* In the Power Supply I wonder about IC2, the original +6V regulator rated 0.5 A as it supplies both the positive rail voltage for the op-amps and through a forward bias diode supplies +5.3V for the digital ICs - if this voltage fluctuates or has a glitch it might have triggered something untoward. As mentioned, this was replaced by newer series of +6V regulator rated 1.0 A.
* Another factor could be components in either the Motor Control Board or the Power Supply Board were borderline, or rated just sufficient when the product was manufactured, but after 45-years these components had become weak or worn down.
* Regret if these components had simply failed outright the troubleshooting would have been straightforward, but troubleshooting is never easy...
Final comments
* The reason for this writing is to share my experience repairing a ReVox B790 turntable erratic speed. At time of this writing the turntable has been operating without issue for 7 months now.
* Pleased to report my ReVox B790 turntable now tracks and plays LPs flawlessly, just like it used to!