Hello users,
I have problems with my Stage Echo and would greatly appreciate any help / ideas for this one.
cheers
I have problems with my Stage Echo and would greatly appreciate any help / ideas for this one.
cheers
Here is the schematic,
http://i.imgur.com/uBUVsTw.gif
Found here,
https://www.google.com/search?q=Kor...ine-trimmer-capacitor-question.html;2160;1361
jer 🙂
http://i.imgur.com/uBUVsTw.gif
Found here,
https://www.google.com/search?q=Kor...ine-trimmer-capacitor-question.html;2160;1361
jer 🙂
Thanks for link Jer ! At the moment I'm injecting a 400Hz test signal and tracing it thru the Amplifier circuit A6 & A7 and on up thru to the feedback path thru switch A21 to the compander chip.
Meantime I was wondering what signal level should I expect back thru the playback heads ? Should it be the same level as the signal going into the recording head ?
Meantime I was wondering what signal level should I expect back thru the playback heads ? Should it be the same level as the signal going into the recording head ?
The signal coming out of the playback heads will be at a much lower level than what is going into the record head.
If it is not recording properly check to see that you have a high frequency bias signal that is imposed on the record signal going to the record head through the 100pf trimmer cap coming from the erase head.
If there isn't one, check that the bias oscillator is working properly.
Tape machines will not record without this bias signal or if they do it will be at a very very low level and most likely distorted as well.
There was a thread on this very same issue with the Korg 300-SE version a while ago here,
http://www.diyaudio.com/forums/anal...e-trimmer-capacitor-question.html#post3528565
jer 🙂
If it is not recording properly check to see that you have a high frequency bias signal that is imposed on the record signal going to the record head through the 100pf trimmer cap coming from the erase head.
If there isn't one, check that the bias oscillator is working properly.
Tape machines will not record without this bias signal or if they do it will be at a very very low level and most likely distorted as well.
There was a thread on this very same issue with the Korg 300-SE version a while ago here,
http://www.diyaudio.com/forums/anal...e-trimmer-capacitor-question.html#post3528565
jer 🙂
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Is that off of the record head or the erase head side?
Being that it is from a multivibrator type of oscillator it looks okay to me.
Exactly what issues are you having as you have not stated this yet.
Make sure that you have cleaned the tape path and heads good, and it is a good idea to demagnetize the heads as well.
One day I was setup to do some recording and my demagnetizer had stopped while I was demagnetizing them and it severely magnetized them to the point that none of the 16 tracks would record or playback at all!!!
I have lost highs before but never all of the tracks completely before!!
Luckily I was able do it again a few times and recover my machine!
I was quite worried as that had never happened to that extent in the last 20 years that I have owned the machine.
To much magnetism can permanently stay in the heads if it is strong enough and ruin the heads forever, I got lucky!!
jer 🙂
Being that it is from a multivibrator type of oscillator it looks okay to me.
Exactly what issues are you having as you have not stated this yet.
Make sure that you have cleaned the tape path and heads good, and it is a good idea to demagnetize the heads as well.
One day I was setup to do some recording and my demagnetizer had stopped while I was demagnetizing them and it severely magnetized them to the point that none of the 16 tracks would record or playback at all!!!
I have lost highs before but never all of the tracks completely before!!
Luckily I was able do it again a few times and recover my machine!
I was quite worried as that had never happened to that extent in the last 20 years that I have owned the machine.
To much magnetism can permanently stay in the heads if it is strong enough and ruin the heads forever, I got lucky!!
jer 🙂
Its off the record head side probing just before the 100p trim cap.
I found the problem finally!! or at least a solution.
The behavior was such that the playback signal faded away rapidly. I would only get 2 or 3 echos each one diminishing more in succession.
I followed my 400Hz 100mv test signal applied at the input of the playback amplifier circuit (A6/A7) all the way thru the analog switch (A21). The signal along this feedback path suddenly disappeared just after the 50k variable resistor, never reaching the compressor circuit (A14). So, I adjusted the trim pot while probing the test signal and behold ! I now see it going into the compressor circuit. The system now seems to be working as it should, but ....
The reason I stated I found "at least a solution" is that I have no idea how that trim pot was ever adjusted out of range in the first place ??? And although all seems to be working I really don't have any point of reference to measure / verify the circuit operation against specifications. The circuit board was taken out and handled which may have resulted in the trim pot setting to have changed accidentally, but I really don't know. So I remain somewhat skeptical and wonder if adjusting this resistor has just compensated for some other problem along this path i.e from the playback heads to the trim resistor.
It would be nice to know what signal levels to expect along this path. For example if I input 100mv test signal into the chan A input what level should I see at the playback head, the Amplifier input A6 and output at A7 and finally output of the expander A15 ??
Meantime its nice to have echos back 🙂
Thanks for your input so far, Jer !
I found the problem finally!! or at least a solution.
The behavior was such that the playback signal faded away rapidly. I would only get 2 or 3 echos each one diminishing more in succession.
I followed my 400Hz 100mv test signal applied at the input of the playback amplifier circuit (A6/A7) all the way thru the analog switch (A21). The signal along this feedback path suddenly disappeared just after the 50k variable resistor, never reaching the compressor circuit (A14). So, I adjusted the trim pot while probing the test signal and behold ! I now see it going into the compressor circuit. The system now seems to be working as it should, but ....
The reason I stated I found "at least a solution" is that I have no idea how that trim pot was ever adjusted out of range in the first place ??? And although all seems to be working I really don't have any point of reference to measure / verify the circuit operation against specifications. The circuit board was taken out and handled which may have resulted in the trim pot setting to have changed accidentally, but I really don't know. So I remain somewhat skeptical and wonder if adjusting this resistor has just compensated for some other problem along this path i.e from the playback heads to the trim resistor.
It would be nice to know what signal levels to expect along this path. For example if I input 100mv test signal into the chan A input what level should I see at the playback head, the Amplifier input A6 and output at A7 and finally output of the expander A15 ??
Meantime its nice to have echos back 🙂
Thanks for your input so far, Jer !
It is good that you found the problem. 🙂
The trimmer could have gotten moved or it could have just gotten dirty and stopped conducting just like another pot would.
Sometimes on older units like that, a cold or cracked solder joints can become an issue.
I see that a lot.
Typically you can maybe use one of the other playback channels to calibrate the mis-adjusted channel so that they are the same with the same volume settings with a particular input level.
There should be point where there is exactly same level coming out of the unit that is the same as that is going in to the unit (hence 0db line level). Then you should be able to have a + or - gain adjust from that point.
Line level - Wikipedia, the free encyclopedia
Since it is an unbalanced line unit I would expect it to be set at the standard -10db reference for 0db.
The voltage you see at the Playback or Record head will not be anything close to as far as referenced to any level coming into the unit.
That voltage is completely independent on the record heads needed drive to produce the dynamic range that it has to produce.
The same goes for the Playback heads.
This is why there are such trimmers and adjustments through out the signal chain.
Just by reading the schematic you can determine how much gain each opamp stage has.
But, I couldn't tell you how calculate the gain for the JFET preamp stages.
You can measure the input and output voltages of a particular stage in question with a scope and calculate the gain of that stage that way
jer 🙂
The trimmer could have gotten moved or it could have just gotten dirty and stopped conducting just like another pot would.
Sometimes on older units like that, a cold or cracked solder joints can become an issue.
I see that a lot.
Typically you can maybe use one of the other playback channels to calibrate the mis-adjusted channel so that they are the same with the same volume settings with a particular input level.
There should be point where there is exactly same level coming out of the unit that is the same as that is going in to the unit (hence 0db line level). Then you should be able to have a + or - gain adjust from that point.
Line level - Wikipedia, the free encyclopedia
Since it is an unbalanced line unit I would expect it to be set at the standard -10db reference for 0db.
The voltage you see at the Playback or Record head will not be anything close to as far as referenced to any level coming into the unit.
That voltage is completely independent on the record heads needed drive to produce the dynamic range that it has to produce.
The same goes for the Playback heads.
This is why there are such trimmers and adjustments through out the signal chain.
Just by reading the schematic you can determine how much gain each opamp stage has.
But, I couldn't tell you how calculate the gain for the JFET preamp stages.
You can measure the input and output voltages of a particular stage in question with a scope and calculate the gain of that stage that way
jer 🙂
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Thanks so much for your input on this jer ! It was nice to get to know the system better and learn some things with your support !
cheers.
cheers.
Output levels of SE500 Stage Echo
Hi,
I'm working on an SE500 at my shop right now with some output issues. Was wondering if anybody has any specs as to the actual numbers I should be seeing at various points in the circuit.
A signal can be traced through the unit with a scope, but with some obvious issues. Thing is, without any sort of specs, it's hard to know if any low-output issues are normal or not.
For example, with a 0dbu 1kHz sine wave going in, I'm only seeing 60mV P-P on the output (-20dB setting)
any ideas?
Hi,
I'm working on an SE500 at my shop right now with some output issues. Was wondering if anybody has any specs as to the actual numbers I should be seeing at various points in the circuit.
A signal can be traced through the unit with a scope, but with some obvious issues. Thing is, without any sort of specs, it's hard to know if any low-output issues are normal or not.
For example, with a 0dbu 1kHz sine wave going in, I'm only seeing 60mV P-P on the output (-20dB setting)
any ideas?