RA7502 Input Board Biasing

Am new to dyiAudio forum but have enjoyed technical links and knowledge provided here. A recent death of my Soundcraftsmen RA7502 Class H power amplifier earnestly brings me for your advice.
While I have lost my RA75xx manual/schematics, deriving schematics from the single layered input signal preamplifier board's layout (figures 1,2) is easy.
Have isolated the symptom to the input board's TL072CP dual opamp positive(+) bias at pin 8 - it measures 0Vdc and ohms open to the a +61.0Vdc supply input due to two 6.8K, 0.5W resistors in parallel open (yes, both are blown open - will explain shortly). The opamp's negative(-) bias at pin 4, measures -23.8Vdc. Am sure this too is incorrect due to a second component (mystery resistor) failure. Input power to board is +/-61.0Vdc. Opamp TL072CP is spec'd at +/-18Vdc maximum, typically biased at +/-15Vdc, with 2.5mA quiescent current at no load. Left/Right channel topology is inverting amps with 13.2dB of gain.
For reasons unknown to me the board designer used resistor dividers to drop the +/-61.0Vdc supply input to some value within the opamp's range (for pins 4, 8) which seems shotty given opamp current loads will vary the biasing voltage rails through the resistor dividers. A description of the divider follows before I ask a few questions.
Top of divider is +61.0Vdc, then two parallel 6.8K (equivalent 3.4K) 0.5W resistors to pin 8. Both resistors are open for the positive rail - I removed them and ohmed them separately. Negative rail 6.8K's are still
good. The bottom of the divide is a parallel combination of 2.2K, 0.25W and what looks like 1K, only this mystery resistor is open - removed it from both +/- biasing circuits, ohmmed separately: both open! In
Figure 2 I cannot identify this large resistor encapsulated in a white plastic, tombstone style thru-hole package whose leads are spaced apart 0.400" (10.2mm). The top of it reads,"dot" then "1K 100*" where "K" is
underlined and "*" looks like "P" underlined or "2" superimposed over "P". It does not make sense that this mystery resistor would be 10K or 100K. It would have to be smaller, say, 1K in parallel with 2.2K (an
equivalent 690-Ohm resistor divider bottom) to knock down the 61.0Vdc to 10.3Vdc, acceptable to the TL072CP.
1) Does anyone know more details about this mystery component? Is is a fusible resistor? Maybe that's why it ohms out open? ...and explains its size? Too, the size could be explained if it is some kind of power
resistor but dunno? The -23.8Vdc seen on pin 4 is explained by the open in the mystery resistor since (-61.0V * 2.2K / (2.2K + 3.4K)) ~ -23.8Vdc, given 5% resistor variations.
2) Depending on the orientation of a jumper on the rear panel of the RA7502 either stereo or bridged mono output is provided. Without the jumper in place, though, the TL075CP opamp has an open input and output. Was the voltage divider uses [then] to protect the opamp by providing a current path for biasing supplies if the user happened to have the RA7502 power with the preamp's output open? Why wasn't a voltage reference diode or regulator used instead of a resistor divider to knock down the +/-61.0Vdc supply inputs?
Any advice would be appreciated. My background is predominently digital EE, not analog audio.
Thanks,
--chris
 

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mixerjv

Member
2010-11-24 9:56 pm
I'm new to this forum as well, I have 22 RA75xx units (for an indoor cinema)- just lost my service tech so I bought the schematics and starting to service them myself (I was an electronics technician in the military years ago- but a little rusty).

From the schematics two capacitors are in parallel with the 2.2K resistor, see the image attached.
 

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