| sdoom |
hi @ all.
I got a tape (Pioneer CT-W300) that wont record at all. This is a dual - tape. Tape A only can playback, Tape B can playback and record.
Both tapes play back correctly.
If I feed an input signal to the inputs both level meters work correctly , level can be adjusted with the record level shifter, too. But when finished recording, there is still the old music on the tape as it was before . Sometimes it is audible, that the recording is there in the background, but very silent and not all the time.
I have a good understanding of reparing amplifiers but I only have little knowledge about tapes. I checked the TAPE B tone-heads (fully metal type) with an ohmmeter and both coils read 220 Ohms, compared eith TAPE A I have the same readings there.
There is an additional black plastic type head in TAPE B. I assume that is a "delete head" ? It reads only 1,4 Ohms.
I tried to measure the voltage across this plastic head, and it is always 0.00V, regardless if in playback or recording mode. Is this section faulty ?
Am I right if I say, the delete head deletes the tape in record-mode and the fully metal head is used for both playback and recording ?
Thx,
Stephan |
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| Geek |
Sounds like the bias/erase oscillator is hosed.
Can you scope for an HF signal superimposed on the audio signal on record? |
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| sdoom |
Can I scope that signal directly on the "delete head" - leds ?
What frequency should that signal aprox. have ? |
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| Geek |
Yup! But have your scope on the highest range.
For something in that range, expect a bias frequency of ~85KHz. Not too critical. |
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| sdoom |
ok I will try to scope tonight at home. I ´ll let you know what I can figure out.
Thx again
Stephan |
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| sdoom |
Hi again,
I tried to measure the voltage across the plastc head when recording, there is nothing. I tried the scope and the DMM , both have no readings.
I fed a sine (10kHz) into the inputs and started recording. I can red the 10kHz sine on the metal head for each channel. The sine is clear and free from distortion.
BTW, is 1.4 Ohms a nomal value for the delete head ?
Thx again, Stephan |
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| sdoom |
| I resoldered the whole board to make sure its not a bad solder joint, but no luck :mad: |
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| dnsey |
| OK, you need to start checking around the bias oscillator. You'll recognise the area by the inductor - it's usually a pretty simple cct, so shouldn't take too much troubleshooting;) |
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| sdoom |
so if I understand correctly: the "delete" head is "powered" with A/C and prepares the tape, so that the normal heads can record the music signal ?
one section of the curcuit board is labeled with "bias " and there are two pods , one for left one for right channel..... There are not many parts, four transistors, couple of resistors and capacitors , an IC ( C1290 from NEC ) and a small coil assembly ( I´ve seen those only in TV sets and radios so far) .
Ok when I find the time I will check all the parts in this section. |
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| Geek |
The bias oscillator performs two important functions - to erase the previous material and remove any unwanted magnetic biases on the tape with the erase head, as well as sweep the tape across its linear B-H curve and allow recording of material without distortion.
No bias, no erase, no recording.
Try to not adjust the pots you found, they are critical. Test the transistor near the big inductor. It most likely failed, oe some electrolytic in the area. It's rare that the iductor itself fails.
Low DCR is OK, because its inductance will be high at the oscillator frequency. Be careful to keep DC out of those windings, even an ohm meter. You will need to demagnetize the heads (or have someone do it for you) after servicing. |
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| Enzo |
After mechanical problems in the transports, one of the most common electrical problems I have encountered in cassette decks is dirty R/P switches. Record/Play switches.
Is there a longish multipin switch in the center of the board on this model? it would be actuated by a link from the recording deck if so. Squirt some cleaner into it and actuate it back and forth a few times. Of course if it has an IC instead of an R/P switch, then cleaner won't help.
When you look for bias/erase oscillator on the scope, start with the scope down. You might have 20v or more of bias signal. If the scope is set for looking at the audio signal, the bias might be off the screen. |
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| sdoom |
well, had the scope on DC-setting so it should have read everything .... I hope :)
No I´m pretty much sure that there was nothing. Also my DMM is quite good (it´s not a $5.00 Home Depot meter) and it also read nothing on AC-volts , DC-volts and frequency.
I will try to check all the transsitors with the diode-tester , I hope I find the fault ..... |
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| Enzo |
My Flike meter won't read bias on AC either. At 100kHz, it is too high freq for the meter to detect.
By off the sdcreen on your scope, I meant it could be oscillating at 50v and if the screen is set to display 1 volt, then the trace will be way larger than teh screen and might be hard to see. The trigger light would be blinking though. |
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| gmphadte |
1)Do the tapes get erased? If yes, then the bias oscillator is ok if it is AC erase.
2)How was the last recording, before damage occured?
Gajanan |
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