Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

[budwich]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

yes but did you do as suggested and disconnect the right side VU meter and then check for dc voltage on the "open" terminals to confirm or "disprove" whether DC is associated with an open terminal?

You might have to redo your measurements with the right meter disconnected to get an "equivalent" circuit setup to be able to compare between sides.

[UNITRAX]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

After disconnecting the left vu meter and measuring the voltage nothing changed.
I don't have DC current on the cables on the left.

Voltages on IC3 and IC4 unchanged.

I checked C230, C232, D103 and D203 again.

[budwich]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

what are you using to test the caps with... meter or checker (ie. transistor / cap tester)? Do you have a scope?
check c220?

[UNITRAX]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

ATmega328 Transistor tester lcr-t4.
I have an old analog oscilloscope, and I've only used it a few times.
C220 had a slightly overestimated capacity 14uF instead of 10uF, changed for new one.

[budwich]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

well... 14 from 10 is 40%... that's quite a bit considering most caps are spec'ed at 10% or so.

a while back you indicated a "meter problem" with the "touching of probes" to the ic1 causing that side meter to full scale... etc. Is that still the case. Do you have another meter. It seems that perhaps the impedance of the meter is not very high... should be greater than 10mohms.

another question when you say you have dc voltage on the meter terminal of the right.... is that + or - (are you metering the right polarity)?. Further, are you measuring to each terminal to the same ground reference point? and NOT just putting the probes across the terminals. Thus, you measure from a common ground point, the terminal + and then measure the - also. IF things are good, one measurement should produce a "0" while the other should produce a + or - value depending on what voltage condition exists. Just a check about the system.

[budwich]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

you need to redo your measurements on the IC's correctly as there is too much "misinformation" to provide a proper analysis around those chips.

[UNITRAX]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

Both vu meters share common plus from pin 106 on pcb.

Pin 107 feeds the negative of the left vu meter.
Pin 108 feeds the right vu meter negative.
I took measurements between the pins on the pcb. For comparison, I measured against the chassis, the same results.
I hooked up an old pointer multimeter.
Meter positive probe to pin 106.

Probe negative to 107 = 0V

Probe negative to 108 = about -0.5V

Measurements taken in playback mode. On the pointer meter you can see that the voltage pulsates along with the sine wave of the sound.

I recently repaired a very similar rt 1157 model, now works fine.

I similarly measured the DC voltage in vu meters. There is 0V on the left and right.

I checked that in the playback mode in this model also touching the pream pins with the probe results in a large distortion of the sound and the vu meter hits the end of the scale.

Similar story when I measure dolby IC.
If necessary I can re-measure preams IC1,2
and Dolby IC3,4.
But is it safe and won't damage them?

[budwich]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

ok... I can't see any pin numbers in the posted SM. No matter.

Back up... in post 13 and 14, I don't think that the voltages and comments were ever reconciled. IF there is 0 voltage on both inputs, why? Either the IC has an issue or the subtending circuit has problems... or there is a measurement problem.

Another comment, have done any of the alignment / adjustments stated in the document on page 8... at least to compare voltages at tp102, tp202.

Further, redwire asked about voltages at the transistors in post 8 but never got an answer.

[UNITRAX]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

Tp 101 +8,26v
tp 202 +7,16v

q102
b-c +8,05v
b-e -0,64v
c-e -9v

q103
b-c +7,20v
b-e -0,70v
c-e -7,91v

q202
b-c +8,02v
b-e -0,63v
c-e -8,97v

q203
b-c +7,22v
b-e -0,70v
c-e -7,93

[budwich]

Re: Tape recorder Sharp RT 1155 - DC voltage in the VU Meter circuit

the transistor values are of no value. Look at the schematic... you need to follow and check for those values as printed and as NOTED.

have you done any of the adjustments indicated in the document?