I have a Rotel RTC-970 Tuner Preamplifier that has several issues that all seem to be related. The obvious operational issue manifests itself as the volume control turning itself down after the unit is on for about 20 minutes. At that point, to turn the unit off, you have to unplug the unit as the power off switch will not work. Unplugging the unit for about 15-20 minutes restores the unit to normal operation.
Opening the cover, I notice that after a few minutes U26 a voltage regulator on a large heatsink gets really hot. The transistor physically in front of the regulator, Q31 gets pretty warm too, but nothing like U26.
Checking some of the voltages in the power section, I noticed some differences from what the Tech Manual notes. A.C. voltages at pins on JP4 were as follows/specified: 1 was 15.4/20, 2 was .06/Gnd (0), 3 was 15.6/20, 4 was 2.8/3.5, 5 was 1.0/3.6. Notice that pin 5, the second leg of the 3.5 VAC pair was off significantly from the published voltage.
A couple of voltages at JP5 were farther out of spec. Pin 7 which should be 0 volts DC, actually measures -10.1V and pin 8 which should be +3.5 volts, measures a whopping -14.2V!
Given the age of the unit (almost 25 years), my first attempt to resolve the above issues was to replace the electrolytic capacitors in the power section of the main board. That did not help. Since then, I have replaced several of the diodes, transistors and even the optocoupler (was LTV-817, replaced with LTV-817A).
I sure would appreciate some help in trying to understand the voltage differences above. I tried tracing the improper -14.2 V back to a source, but could not understand how or why it is so out of whack to what it should be at+3.5 V, and pin 7 reading so wrong on JP5 (reading -10.1V instead of 0V).
Attached is the power portion of the schematic from the Tech Manual for reference. Moments ago, I also loaded the complete Technical Manual to this site, too. I could not attach it to this post as it is too large.
I would like to get this unit fixed as it has just enough features to suit my needs. Thanks in advance for your interest in this!
Opening the cover, I notice that after a few minutes U26 a voltage regulator on a large heatsink gets really hot. The transistor physically in front of the regulator, Q31 gets pretty warm too, but nothing like U26.
Checking some of the voltages in the power section, I noticed some differences from what the Tech Manual notes. A.C. voltages at pins on JP4 were as follows/specified: 1 was 15.4/20, 2 was .06/Gnd (0), 3 was 15.6/20, 4 was 2.8/3.5, 5 was 1.0/3.6. Notice that pin 5, the second leg of the 3.5 VAC pair was off significantly from the published voltage.
A couple of voltages at JP5 were farther out of spec. Pin 7 which should be 0 volts DC, actually measures -10.1V and pin 8 which should be +3.5 volts, measures a whopping -14.2V!
Given the age of the unit (almost 25 years), my first attempt to resolve the above issues was to replace the electrolytic capacitors in the power section of the main board. That did not help. Since then, I have replaced several of the diodes, transistors and even the optocoupler (was LTV-817, replaced with LTV-817A).
I sure would appreciate some help in trying to understand the voltage differences above. I tried tracing the improper -14.2 V back to a source, but could not understand how or why it is so out of whack to what it should be at+3.5 V, and pin 7 reading so wrong on JP5 (reading -10.1V instead of 0V).
Attached is the power portion of the schematic from the Tech Manual for reference. Moments ago, I also loaded the complete Technical Manual to this site, too. I could not attach it to this post as it is too large.
I would like to get this unit fixed as it has just enough features to suit my needs. Thanks in advance for your interest in this!
