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

[UNITRAX]

I have a faulty right VU meter.
At the poles, the DC voltage varies between 0.5 - 1.5 V
The indicator does not respond to the ohmmeter at all.

I checked
transistors Q202, Q203
capacitors
C230, C231, C412
D202 diode.
Variable resistor VR 204
T 201 coil

Anyone have an idea how to get rid of the DC voltage?

[budwich]

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

is that voltage being measured when no input is connected / outputting sound into the record. Rather strange that the meter / indicator shows no impedance or do you means the ohmmeter is showing "over" meaning open "circuit"? What impedance does the "working meter" show?

[UNITRAX]

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

Ohmmeter set to 600 ohms. The pointer in a working vu meter deflects about 75%.
I disconnected the faulty vu meter for measurements. The ohmmeter shows overload on all ranges. The indicator itself is defective, I would like to replace it. However, I don't want to connect the replacement to DC voltage. DC without specified sound source 0.5 V
In playback mode 1.5V

[budwich]

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

ok... so you are saying on the working meter (other side), there is no dc voltage at the poles of the meter. Are you getting the voltages on the upstream components as shown in the schematic... including IC2 and IC4?

[redwire]

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

Two relevant capacitors yet apparently they are checked, plus an output transformer for the headphones- so it doesn't make sense. OP measured something wrong.

[UNITRAX]

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

IC4:
1 6,89V
2 6,93V
3 6,80V
4 6,96V
5 6,71V
6 6,96V
7 6,99V
8 7,20V
9 14,18V
10 0,53V
11 0,54V
12 0,66V
13 0,66V
14 7,44V
15 7,26V
16 GND

IC3:
1 6,90V
2 6,94V
2 6,94V
3 6,69V
4 6,97V
5 6,88V
6 6,96V
7 7,85V
8 5,66V
9 14,27V
10 0,56V
11 0,56V
12 0,66V
13 0,69V
14 5,35V
15 6V
16 GND

IC2:
1 1,94V
2 0,58V
3 0V
4 GND
5 0,52V
6 0V
7 14,96V

when I connected multimeter to IC1, VU Meter hits the end of the scale, the sound crackles and I don't want to damage the working channel.

[budwich]

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

? not sure I follow what your list of voltages are... especially pin 16... why are you saying this is grd? I don't see that in the schematic that was posted by you.

[redwire]

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

Yup the voltage measurements and pin numbers are out to lunch. I guess it's Playback mode.
Why do you care about the IC voltages, VU meter is driven by Q102/Q103 and Q202/Q203 and where are those voltages.
Is the 19V rail OK at Q501?

[budwich]

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

I indicated to the op to check the "upstream voltages including the ics" basically to see if anything "jumps" out. They are given so it costs nothing to see if anything is out of whack.

[redwire]

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

I see many blocking capacitors and thought DC from any IC will not make it to the VU meters. There must be language difficulty.
Probing IC1 head-preamp and it starts picking up noise - I would make sure the REC-PLAY switch is clean and has contact treatment.

All of these old cassette decks, first thing is to clean the REC-PLAY switch and apply PPE-based contact cleaner. Don't get any spray on the belts or rubber!

[UNITRAX]

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

I had already cleaned the rec switch when assembling the cassette mechanism.
The voltage line of Q501 is 19.4V
In the pins of IC4 and IC3 I got the order of the second half wrong.
Gnd is at 9, not 16. I counted the pins from the wrong side.

[redwire]

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

Can you drink less Duch Puszczy when making voltage measurements

IC2 pin 6 you measure 0V but it should be 8.2V check again otherwise the IC is shorted pin 6 to pin 5/4. The schematic is wrong, it shows two pin 6's? RH-IX0437AGZZ is also known as ECG1462, NTE1462, SVITA7129P, TA7129, TA7129AP, TA7129FG, TA7129P.

Dolby IC3, pins 8, 10, 11 are lower than the schematic values. So C118 is getting a bit of reverse-polarity and is known to get damaged. RH-IX0431AGZZ is NE545.

[UNITRAX]

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

IC3
1 1.9V
20V
30V
4 GND
5 0.5V
6 5.9V
7 15V

When I come back from my third job, I still have 2 hours for audio equipment. Sometimes I don't think clearly, but that's due to fatigue and lack of sleep.

[redwire]

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

OK it's not the moonshine I think you mean those latest voltage readings are for IC2 (not IC3)?

For IC2, pin 2 is 0V not 0.7-1.0V which could be from shorted C233 10uF 16V, or open R206 270k or a bad IC. Diode-test pin 2-3 should have transistor E-B junction 0.6V and no short.
Sometimes if you demagnetize the tape head it can blow the head pre-amp IC. A strong demagnetizing coil near the head can do it.

So you get distorted audio, or is it no audio from IC2?

[UNITRAX]

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

Diode test pin 2 and 3 = 0.7V
I'll check C233 and R206 after work
Both channels sound clear. The crackling sound, and vu meter madness occurs when I touch the pins of IC1 with a multimeter and therefore cannot measure it.

[budwich]

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

resolder the ic. when you touch any pin on ic1 or a specific pin?

[UNITRAX]

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

It's probably the digital multimeter's fault. When I touch the pins with a screwdriver, IC1 behaves like IC2. Both resoldered.
I checked R206, C233 replaced with a new one.
I treated the rec switch with contact cleaner spray. I'll desolder it tomorrow and clean the sliding pads.

[redwire]

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

I'm lost what the problem is now. Is this correct:
The right VU meter coil is open-circuit and DC voltage is seen at the terminals - but this is not a problem, it is normal to have DCV there because the meter coil resistance is not loading/discharging the capacitor.

Touching probe to head preamp IC1 or IC2 input pins it is normal for crackling sound because the IC has so much voltage gain, it amplifies mV signal from tape head. Any mains hum or noise will get picked up on multimeter probes.

So is there actually a problem with the unit now?

The VU meter sometimes you can take apart and fix a broken wire inside, but it is like doing surgery.

[budwich]

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

check the other VU output condition (dc on the terminals) by unsoldering it.

[UNITRAX]

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

Slide fields in record switch cleared.

The right Vu meter is operational. Checked with an ohmmeter. I measured the voltage on its terminals several times. There is no DC = 0V.
Both channels play.
The left vu meter is out.
On the cables from the right vu meter I have 0.5 to 1.5VDC

In the voltage measurements of IC3 and IC4 you can see the difference on 2 pins.
IC 3
pin 10 = 6V
pin 11 = 5.35V

IC4
pin 10 = 7.26V
pin 11 = 7.44V