Really valuable advice. Thank you very much Advis.
What i was doing so far was feeding a signal of about 30-40hz (the lowest i could hear) and reducing the noise around it so it could become very audible.
What i was doing so far was feeding a signal of about 30-40hz (the lowest i could hear) and reducing the noise around it so it could become very audible.
Just keep in mind that correct bias adjustments are more a matter of art than of science. 😉
Could somebody, please, help me where to trace (or measure) with oscilloscope or voltmeter on the record pcb (page 103 and 104 of 153 on the service manual) and the oscillator board (page 110 of 153 on the service manual)?
The whole block diagram for the audio path is on page 89 of 153 on the service manual.
Can’t really figure out easily what input from the record head is and how the oscilator of the erase head and the bias signal intervenes with the signal that come from the recording head.
Here’s a link to my service manual (too big to attach).
https://drive.google.com/file/d/1NcNTxCWY_5d1nJkhT-5oHixo4q1G1aCD/view?usp=drivesdk
The whole block diagram for the audio path is on page 89 of 153 on the service manual.
Can’t really figure out easily what input from the record head is and how the oscilator of the erase head and the bias signal intervenes with the signal that come from the recording head.
Here’s a link to my service manual (too big to attach).
https://drive.google.com/file/d/1NcNTxCWY_5d1nJkhT-5oHixo4q1G1aCD/view?usp=drivesdk
Maybe begin with page 100, where pins 2 and 1 have the equalized audio just before bias is added. Bias will be very large compared to signal, so you might need to back up behind the bias traps to see it well. The relay is an obvious failure source.
All good fortune,
Chris
All good fortune,
Chris
I'm a bit rusty (it's been about 50 years since I worked on that stuff) but I'd suggest you start simple. I take it the machine worked before you replaced the heads, prime suspect therefore are the connectors. Apparently you do have bias on the recording head (you may want to check again, 20kHz is really low, I'd expect more like 150kHz) so the last bit seems OK but I'd start by inspecting and reseating connectors and boards (and cleaning contacts with isopropyl alcohol -- Do Not Use Contact Spray!!). Any gold PCB edge connectors you can clean with a pencil eraser but if they look OK they are OK. Make sure everything is seated correctly. Visually inspect solder pads. Also look at the contact fingers of the edge connectors.
But before all that, you mentioned a wiring problem with the playback head, check that you don't have the exact same problem on the recording head (a wild guess since I have no idea what the wiring problem on the playback head was).
But before all that, you mentioned a wiring problem with the playback head, check that you don't have the exact same problem on the recording head (a wild guess since I have no idea what the wiring problem on the playback head was).
At some level we all are "unreliable narrators" but it's especially true in troubleshooting via Internet. Fortunately the OP is solid and not too many commenters' assumptions have proven wrong (well, a few). A couple more actual scope probes should tell the tale.
All good fortune,
Chris
All good fortune,
Chris
Thanks both of you gentlemen for the input.
First of all i have to make a correction on the data i’ve give so far.
What i trace on the recording head has nothing to do with the input signal (same trace either with or without signal) That’s a discovery for me but it may be expected for you. The recording head is the last on the record path.
Tape monitoring shows that what is written on the left channel sounds grea, but there is either silence or the ocassional hum crack noise on the right channel.
The bias trap]s are tajen to the lowest voltage pissible (120mv for the left channel- 80mv for the right).
I changed the relays between them, so the left relay of yesterday today is right and vice versa. Same results. So no relay problem.
What i trace on the head may be 150khz.
The playback wiring issue was with the main pcb header connectors. The recording head connects only on the oscillator biard.
The oscillator board is different to the one i had, because it’s a 2 track one. As i said the relays are checked. The tra nsformer hopefully is fine. The rest seems good (after studying the sevice manual to see what is what).
Maybe a bit of resoldering on the head (there mayve some residues on there) and cleaning and rechecking the traces and connectors.
First of all i have to make a correction on the data i’ve give so far.
What i trace on the recording head has nothing to do with the input signal (same trace either with or without signal) That’s a discovery for me but it may be expected for you. The recording head is the last on the record path.
Tape monitoring shows that what is written on the left channel sounds grea, but there is either silence or the ocassional hum crack noise on the right channel.
The bias trap]s are tajen to the lowest voltage pissible (120mv for the left channel- 80mv for the right).
I changed the relays between them, so the left relay of yesterday today is right and vice versa. Same results. So no relay problem.
What i trace on the head may be 150khz.
The playback wiring issue was with the main pcb header connectors. The recording head connects only on the oscillator biard.
The oscillator board is different to the one i had, because it’s a 2 track one. As i said the relays are checked. The tra nsformer hopefully is fine. The rest seems good (after studying the sevice manual to see what is what).
Maybe a bit of resoldering on the head (there mayve some residues on there) and cleaning and rechecking the traces and connectors.
Referring to pg. 104, do you have audio signal at C12 and C23? Let's walk this problem step by step.
That was excellent. Even if the signal (very small) is super imposed on another similar trace it’s there for the left. For the right what i get is some kind of noisey shape and no signal.
So the problem is before that point.
Could we find a possible other side border, please, now that we have the limit (just before the bias trap)?
So the problem is before that point.
Could we find a possible other side border, please, now that we have the limit (just before the bias trap)?
I guess c9 and c8 and r1 and r2 on the muting circuits may be good points. I use 2 jumper wires soldered on the measured components. Very narrow space between boards.
C9 and c 8 negative side signals are both good. So the problem with the right channel may lie around c19.
C9 and c 8 negative side signals are both good. So the problem with the right channel may lie around c19.
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Around c19 the right channel waveform shape seems very “fat”comparing to the left on c6 which seems normal.
Yes there is signal on c19 but it seems like it’s very distorted.
Another observation is that the signal is equal on r1 and r2 but on c9 (left channel) and c8 (right channel) the right channel is about 50% higher (we’re talking about small signals in general around 5-10 mv). Of course this may be due to a bit of unequal input calibration, cabling etc.
Yes there is signal on c19 but it seems like it’s very distorted.
Another observation is that the signal is equal on r1 and r2 but on c9 (left channel) and c8 (right channel) the right channel is about 50% higher (we’re talking about small signals in general around 5-10 mv). Of course this may be due to a bit of unequal input calibration, cabling etc.
Very happy. As i said before my rec pcb was heavily modified so there was a risk and i found two broken junctions. Very old and fragile pcb. So i went point to point with jumpers.
Signal back on right channel. Equal on both sides.
Now i can start callibrating the recording path.
Thank you all for the input so far.
Signal back on right channel. Equal on both sides.
Now i can start callibrating the recording path.
Thank you all for the input so far.
All is done for the time being. I think the 2 track calibration (if you ‘ve done it 3-4 times in the past like me) is much easier than the 4 track one. The high speed really make things sound better.
As advis said bias and eq is more art (i would also add and good “ears”) than science (especially for music use).
For example you can make the slower speed a bit darker more appropriate for bass instruments or the high speed a bit more brilliant for treble instruments, or depending on the quality of the tape that is more likely to be used (older tape tends to lack high freq). Reference numbers are great of course, but a bit of experimentation doesn’t harm, especially if it’s for music making.
Broadcasting, or commercial recorded music listening and mastering are different things.
Thanks everyone for the input and advice. Really really appreciate it, that’s why i like this forum the best . Knowledge and great “normal people” vibes and attitude.
Ps Just in case somebody comes across the odd issue with shorts and loud hiss and noise from used older heads , check with a wood stick or plastic screwdriver if the 4 pins where the cables are soldered are shorting- touching on any of the metalic (mu) surface. Also check if they need some heatshrink or even the cable is properly soldered or about to break. They are not so very fragile and quick good soldering skills will not damage anything.
As advis said bias and eq is more art (i would also add and good “ears”) than science (especially for music use).
For example you can make the slower speed a bit darker more appropriate for bass instruments or the high speed a bit more brilliant for treble instruments, or depending on the quality of the tape that is more likely to be used (older tape tends to lack high freq). Reference numbers are great of course, but a bit of experimentation doesn’t harm, especially if it’s for music making.
Broadcasting, or commercial recorded music listening and mastering are different things.
Thanks everyone for the input and advice. Really really appreciate it, that’s why i like this forum the best . Knowledge and great “normal people” vibes and attitude.
Ps Just in case somebody comes across the odd issue with shorts and loud hiss and noise from used older heads , check with a wood stick or plastic screwdriver if the 4 pins where the cables are soldered are shorting- touching on any of the metalic (mu) surface. Also check if they need some heatshrink or even the cable is properly soldered or about to break. They are not so very fragile and quick good soldering skills will not damage anything.
This article was of help for me:
http://blackmerdesign.com/resources/how-to-bias-analog-tape-recorders/
http://blackmerdesign.com/resources/how-to-bias-analog-tape-recorders/