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- May 14, 2014
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- Southwest Kootenays BC
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- No such things as bad days, just bad moments
Brought a Rolecor RTA-650 receiver home because it was old (circa 1968), heavy, cheap ($10) and the lights worked. DC offset is high 0.62 volts & 0.81 volts.
Tuner works through headphones and (expendable) speakers.
Wished to measure bias but found no specs, "how to" tips, manuals or schematic anywhere. Located (I think) the differential pair transistors (TO39 - 2SC497 / 2SA497), removed from circuit, tested with DMM diode function and are OK. There are some germanium transistors on the boards as well.
All 4 output transistors are the same number (2N3055) but from 3 makers and as yet untested.
Could these be responsible for creating the high offset voltage? Those transistors are available brand new locally and I'm tempted to replace all four.
Tempted to replace the electrolytic capacitors also - except the large single one.
And the oil ones.
There's lots of space so getting at them is easy - there's more on the underside.
But without knowing....
-where to check the bias voltage.
-what the bias voltage should be.
-which of the two trim pots (a large one and a small one per board) controls the bias.
....going further is likely pointless.
Anyone have any thoughts on how to do the above or what else might be responsible for causing close to 1 volt DC offset?
I've read in the past twenty-four hours that the brand Rolecor was exclusive or semi-exclusive to an Eastern US department store chain. But, the same product was branded differently and available elsewhere - Calrad was mentioned. If of any value to know, the actual maker was Roland Electronics in Japan. Claimed (or so it was written) to be a forerunner of Rotel in North America.
Tuner works through headphones and (expendable) speakers.
Wished to measure bias but found no specs, "how to" tips, manuals or schematic anywhere. Located (I think) the differential pair transistors (TO39 - 2SC497 / 2SA497), removed from circuit, tested with DMM diode function and are OK. There are some germanium transistors on the boards as well.
All 4 output transistors are the same number (2N3055) but from 3 makers and as yet untested.
Could these be responsible for creating the high offset voltage? Those transistors are available brand new locally and I'm tempted to replace all four.
Tempted to replace the electrolytic capacitors also - except the large single one.
And the oil ones.
There's lots of space so getting at them is easy - there's more on the underside.
But without knowing....
-where to check the bias voltage.
-what the bias voltage should be.
-which of the two trim pots (a large one and a small one per board) controls the bias.
....going further is likely pointless.
Anyone have any thoughts on how to do the above or what else might be responsible for causing close to 1 volt DC offset?
I've read in the past twenty-four hours that the brand Rolecor was exclusive or semi-exclusive to an Eastern US department store chain. But, the same product was branded differently and available elsewhere - Calrad was mentioned. If of any value to know, the actual maker was Roland Electronics in Japan. Claimed (or so it was written) to be a forerunner of Rotel in North America.
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