Sir samuel phillips . . . . .
Glad to hear that you speeded up the tracing process, when you unexpectedly happened to touch a keyboard key, while having the BA5417 audio input pin # 12 connected to the Output pin 1 of the left channel of the IC502 dual op amp preamplifier.
And
THEN the strength of that keyboard note, left no doubt about the BA5417 working and being at a decent power level of output.
Of course you had no volume control effect, since you were bypassing it .
That full volume and normal operation due to C514 was a definite surprise to me, as my suspicion was certainly to be with IC502 at fault.
PLUS . . . . what are the
SUPER-PHENOMENAL . . . odds, of its companion on the other channel,
ALSO being bad.
Those capacitors are not subjected to taxing applications . . . just merely audio coupling.
They must have failed waaaaay down into the picofarad range to give that feeble, tinny and bass-less sound reproduction.
I am sure that you can see that the choice of test coupling capacitor that you happened to have used, even gave a somewhat decent audio quality . . . . . was it probably a 0.1 ufd or so ?
At the mere 1 ufd that they are using, I think that in the future, any of the
SAME brand of electrolytic capacitor that they used in the unit, I would be leery of.
Now, in simultaneously referring to the special , Unitized, marked up and prepped up schematic for this unit.
The initial game plan was to have the test capacitor connected from BLUE triangle B over to RED triangle A and . . . .pray for rain . . . . and a positive feedback phasing to have your output IC then acting as a power audio oscillator.
Praise be, the number of internal stages agreed, and there was audio range oscillation.
Thus . . . . your confirmation that the power output stage
WAS “ power outputting”
Next logic would be to solder tack one lead of the test cap to a safe position . . .somewhere . . . onto the
BLUE buss, instead of chancing a short at the close spaced pins right at the IC proper.
Then you take a flexible test lead and solder tack to the other lead of the test coupling capacitor and make it a “flying lead” .configuration.
In that manner, it can be repositioned to some intermediate test points, to
AS FAR WAY AS the
RED G and H triangle test points at IC502’s inputs.
Your initial test point would have been at the
RED triangle test points C to see the tonal shift being imparted due to passing through added circuitry of C555-C551-R553 and R551.
Then you would have moved to
RED triangle test point D and found that with the volume at max, there would be the same tone.
As you then diminished the volume control level, the tone should shift, and volume drop and then a final chirp as it dropped out of feedback capability for further oscillation.
You would then move the volume control
BACK to its max volume position , to restore oscillation, and then move over to
RED triangle test point E, thus confirming all intermediate plug connections were being made and the unit oscillating to make a tone at that point.
AND THEN . . . the next move would have been to move to
RED triangle test point F to see if oscillation was being created there also.
If so, the final move would have been to have moved to either
RED triangle test point G or H, to see if a strong oscillation was possible to be created from those points . . . all being in accordance as to which phase relationship occurred.
The output should now be strong, since either op amp introduced in the order of ~20 db of extra gain (10X) into the total feedback loop circuitry.
If by chance the unit the unit worked from G or H, that would be bad as the next problem causing link should be the IC501
Digital Analog Converter chip.
You can see how I partitioned it up into its two sections, minimal testing of that unit would be relegated to checking for its supply power presence and then you could probe the cap to the pins 1-5 to listen for the audio clicks, whines and whirs that it will emit when and
IF any digital signal is being processed.
BUT you just totally lucked out in hitting that keyboard note, and then your isolation down to having
SOME audio before C514, but then finding
VERY LITTLE audio after it ? . . . found
OPEN C514 electrolytic coupling cap . .
.KUDOS !
Do you realize . . .that you now have a future spare parts repair cache of (1) BA5417 and (1) plug in Master Volume Control Board.
NOW . . .let there be music !
You say . . .
Need an esr meter would anyone like to donate lol.
ESR meterless analysis consists of shunting the suspect unit with another like value of known good unit.
UNITS . . . . .SPECIFICALLY CONDENSED AND UNITIZED REFERENCE SCHEMATIC
73’s de Edd
