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Post by paradisecs on Jun 25, 2018 19:12:49 GMT
I have two SHP-3 pre-amp boards, both 70-319269-A. Why would one have the trace cut out like in this picture and the other does not? I'm assuming it was a way to stop energy flowing up to the vacant slide switches not used on this board but I don't see any info on this. Does it matter that the other board traces are still intact?  |
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Post by robnyc on Jun 25, 2018 20:18:13 GMT
That drilled out trace is typical of how Seeburg (or their service rep's) modded various boards. I've seen that done dozens of times now. It appears to sever a connection to the deleted equalization curve switch. I don't know why they bothered here since simply omitting the switch defaulted to a sort-of RIAA curve.
It should be fine as-is.
RobNYC
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Post by Ron Rich on Jun 25, 2018 23:35:57 GMT
On acounta acause that trace acted as an antenna--picking up local b'dcast radio !
Ron Rich
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Post by paradisecs on Jun 26, 2018 4:42:33 GMT
Interesting. How did they finally determine it was acting as an antenna? I figured it wasn't needed but I like knowing everything I can, even the undocumented mods.
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Post by robnyc on Jun 26, 2018 10:40:40 GMT
Steve, ( I knew Ron would answer this one) this sort of thing usually only shows up in the field reports. In the case of SHP's in their first cabinets they were well shielded by a metal backing plate that was (hopefully still) grounded. In some later models I didn't see as much shielding.
There is also the potential of oscillation due to the proximity of the two cards -one with high gain, the other a power stage. This is not how a high gain amp would normally be laid out, but jukeboxes seem to be in their own orbit.
The later TSA series have their own problems of oscillation and RF pickup. Pin "A" on the AGC daughter card is at very high impedance and I've had to neutralize it by wrapping a thin, grounded wire around the first few pins on the card for each channel. this was due to my boosting the high frequency & AGC gain, as original I doubt it would ever occur.
RobNYC
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Post by Ron Rich on Jun 26, 2018 13:18:26 GMT
Yep-- the SMC models had no rear metal shield--less expensive (especially since the switch was eliminated), to open the trace on the board--later boards eliminated the trace too- and this was "documented", by a bulletin. Ron Rich
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