I'm trying to get my 1979 Korg Stage Echo back up & running. It was passing dry signal, but the tape repeats disappeared one day after I demagged the heads. I finally got the wet signal back by cleaning the adjustable bias capacitor with Deoxit, but it's thin, distorted, and doesn't seem loud enough. I had rotated the pot on the cap to test it before I cleaned it but I got no signal anywhere on the rotation. Bad trimmer cap?
I'd never even heard of a trimmer cap before this, and I'm unsure of how to read the schematic. It's old and fuzzy, but it looks like it's supposed be a 470uf trimmer cap. That doesn't seem right since all the trimmers I've found are in the picofarad range. Assuming it says 470pf, what does that mean? Should the cap have a range of 0-470pf?
Also, any other tips from you tape guys on other possible solutions would be most welcome!
I'd never even heard of a trimmer cap before this, and I'm unsure of how to read the schematic. It's old and fuzzy, but it looks like it's supposed be a 470uf trimmer cap. That doesn't seem right since all the trimmers I've found are in the picofarad range. Assuming it says 470pf, what does that mean? Should the cap have a range of 0-470pf?
Also, any other tips from you tape guys on other possible solutions would be most welcome!
Check to see if the Bias Erase oscillator is working.
I hope that you remembered what position the bias control was set at before you moved it.
You should have NEVER sprayed Deoxit into a trimmer cap!
It will change its value or could even short it out if it has any plastic dielectric film between the plates as the solvent will stick to everything.
The oils change the dielectric constant if it gets on the plates thus changing its value range.
Wash it out with some denatured alcohol to get the oily solvents out of it.
Schematics and a Model number will greatly help for us to help you as well.
The schematic that I have found shows a 4700pf capacitor feeding the erase head from the Bias Oscillator, And then from the erase head, the record head is fed though a 100pf trimmer capacitor as this is the bias adjustment.
jer 🙂
I hope that you remembered what position the bias control was set at before you moved it.
You should have NEVER sprayed Deoxit into a trimmer cap!
It will change its value or could even short it out if it has any plastic dielectric film between the plates as the solvent will stick to everything.
The oils change the dielectric constant if it gets on the plates thus changing its value range.
Wash it out with some denatured alcohol to get the oily solvents out of it.
Schematics and a Model number will greatly help for us to help you as well.
The schematic that I have found shows a 4700pf capacitor feeding the erase head from the Bias Oscillator, And then from the erase head, the record head is fed though a 100pf trimmer capacitor as this is the bias adjustment.
jer 🙂
Last edited:
Cool, thanks guys. My first instinct was to NOT spray it, but I got desperate. BTW the model is Korg Stage Echo SE-300. I've been unable to find a schematic for the SE300, so I'm working off a schem for the SE-500. They seem to match up pretty well. Here's the URL: http://i.imgur.com/uBUVsTw.gif. If you have a schem for the SE-300, would you share?
I'm clearly not very knowledgeable about tape stuff, so I need to ask some really basic questions:
1. Can you show me where the bias oscillator is on the schematic? I know what a oscillator symbol looks like, but I don't see one on my copy.
2. I see a 470pf trimmer and a 10pf trimmer in the heads part of the schematic. I don't see a 4700pf. Different schematics?
3. Is setting the bias with a trim cap something that requires lots of gear and skills? I have a multimeter but that's it.
I'm clearly not very knowledgeable about tape stuff, so I need to ask some really basic questions:
1. Can you show me where the bias oscillator is on the schematic? I know what a oscillator symbol looks like, but I don't see one on my copy.
2. I see a 470pf trimmer and a 10pf trimmer in the heads part of the schematic. I don't see a 4700pf. Different schematics?
3. Is setting the bias with a trim cap something that requires lots of gear and skills? I have a multimeter but that's it.
Oh lordy, I am an idiot. I just printed out the schematic nice and big and realized I was looking in the wrong place. Now I see the 100pf adjustable cap and the 4700pf cap, duh. I'm still confused about the oscillator. I see something that looks to me like a transformer to the right of the 4700pf cap. Is that the oscillator? I was looking for circle with a wavy line!
Yes, that is the Bias oscillator.
The SE-500 was the schematic that I had found as well.
You may need a scope in order to see if it is functioning or not.
If you turned off the demanetizer while it is still in close proximity of the head then they maybe magnetized still.
I made a low power demagnetizer that I use on my Tascam MSR-16 (because I lost the one I had) and some times I have to do it a few times in order to get my highs back.
I need to make it more powerful as it sometimes does't get to the lowest tracks on the head.
Also a clean tape path makes a huge difference as well sometimes giving me a false reading of the test tape.
How old is the tape that you have in the unit?
Many of the tapes from that era have shedding problems as they absorb moisture and get sticky clog up the tape path, My $200 test tape has this problem as it is now 20years old.
It sticks so bad that it will stop the machine from being able to spool it until a pull it off of the tape guides with a snap like glue.
Sometimes after a few runs of the test tape the signal will dropout and cleaning the heads seems to fix it every time!
Being that it is an echo unit that runs constantly how do the heads look?
I have had bad tape before and it does wear out, In one instance I had gotten some of the highly rate Maxell UD 1/4" tape it some of the reels would not record at all on my Fostex R8 machine and some would, But as soon as I put on a reel of some Ampex457 or 3M 996 it worked perfectly.
The maxell UD reels worked great on one of my old Teac's but not the Fostex!!!!!?
I even have a Sony MA Metal cassette the has a dead spot in it, and, still too this day nothing will record in a particular spot and it is not the fault of the machine.
jer 🙂
The SE-500 was the schematic that I had found as well.
You may need a scope in order to see if it is functioning or not.
If you turned off the demanetizer while it is still in close proximity of the head then they maybe magnetized still.
I made a low power demagnetizer that I use on my Tascam MSR-16 (because I lost the one I had) and some times I have to do it a few times in order to get my highs back.
I need to make it more powerful as it sometimes does't get to the lowest tracks on the head.
Also a clean tape path makes a huge difference as well sometimes giving me a false reading of the test tape.
How old is the tape that you have in the unit?
Many of the tapes from that era have shedding problems as they absorb moisture and get sticky clog up the tape path, My $200 test tape has this problem as it is now 20years old.
It sticks so bad that it will stop the machine from being able to spool it until a pull it off of the tape guides with a snap like glue.
Sometimes after a few runs of the test tape the signal will dropout and cleaning the heads seems to fix it every time!
Being that it is an echo unit that runs constantly how do the heads look?
I have had bad tape before and it does wear out, In one instance I had gotten some of the highly rate Maxell UD 1/4" tape it some of the reels would not record at all on my Fostex R8 machine and some would, But as soon as I put on a reel of some Ampex457 or 3M 996 it worked perfectly.
The maxell UD reels worked great on one of my old Teac's but not the Fostex!!!!!?
I even have a Sony MA Metal cassette the has a dead spot in it, and, still too this day nothing will record in a particular spot and it is not the fault of the machine.
jer 🙂
Last edited:
I just replaced the 100p bias cap and I just slowly rotated it until I got the best signal. Until/unless I take this thing in to a real repair guy, it will have to do. It sounds nearly right but it's still a little bit soft and warbly and distorted. Much better than before I replaced the bias cap but not perfect.
I looked over the schematic and examined the the oscillator under a loupe -- no markings on the unit and nothing I can make out on the schematic either. I even took it to the local electronics store (we have an old school electronics store) and they didn't recognize it either. Is there any way to identify it without a parts list? There are a few parts listed on the far right of the schematic, but I can't make them out. The rest are ICs and transistors.
I bought a few tape loops from a guy here in California a few years back. They're correct with the self-lubricating tape. The one on the deck now is new. The heads look decent to my eyes. I bought the unit broken and I think it had sat in a box for decades. I do also have a R2R and I had the sticky tape syndrome hit me a few times. Yech. I only buy new tape on pancakes now.
I'll try the demag again too now that I know how to do it correctly. I've redone the record head but not the rest. Thanks again for your help.
I looked over the schematic and examined the the oscillator under a loupe -- no markings on the unit and nothing I can make out on the schematic either. I even took it to the local electronics store (we have an old school electronics store) and they didn't recognize it either. Is there any way to identify it without a parts list? There are a few parts listed on the far right of the schematic, but I can't make them out. The rest are ICs and transistors.
I bought a few tape loops from a guy here in California a few years back. They're correct with the self-lubricating tape. The one on the deck now is new. The heads look decent to my eyes. I bought the unit broken and I think it had sat in a box for decades. I do also have a R2R and I had the sticky tape syndrome hit me a few times. Yech. I only buy new tape on pancakes now.
I'll try the demag again too now that I know how to do it correctly. I've redone the record head but not the rest. Thanks again for your help.
I am very glad to here you got it working again!!!
The Bias oscillator is of a simple transistor Multivibirator type with a transformer output, pretty standard.
The transistors are Q16 & Q17 in the schematic 2SD438's.
There are only to company's world wide making audio tape now.
I just Started using ATR and it is incredible sounding stuff!!
ATR Magnetics
My favorite in the past has been the 3M 996 but sadly it can rarely be found.
You can find lots of info on the net on how to set the bias all you really need is a signal generator and a volt meter (that is accurate for wide frequency range) or a scope (would be the best).
This will help you set it up for its widest frequency range and lowest noise and distortion.
Any warbling you hear may go away with some light machine oil were ever you can get to the capstan bearing and any moving parts.
I also use some good ole' Prolong it is great stuff!!
I few of my machines have a hard time for a while if they have been sitting too long until the bearings warm up and loosen up a bit.
Good Luck!!!
jer 🙂
The Bias oscillator is of a simple transistor Multivibirator type with a transformer output, pretty standard.
The transistors are Q16 & Q17 in the schematic 2SD438's.
There are only to company's world wide making audio tape now.
I just Started using ATR and it is incredible sounding stuff!!
ATR Magnetics
My favorite in the past has been the 3M 996 but sadly it can rarely be found.
You can find lots of info on the net on how to set the bias all you really need is a signal generator and a volt meter (that is accurate for wide frequency range) or a scope (would be the best).
This will help you set it up for its widest frequency range and lowest noise and distortion.
Any warbling you hear may go away with some light machine oil were ever you can get to the capstan bearing and any moving parts.
I also use some good ole' Prolong it is great stuff!!
I few of my machines have a hard time for a while if they have been sitting too long until the bearings warm up and loosen up a bit.
Good Luck!!!
jer 🙂
Last edited:
So glad to find this information online - I have the exact symptoms as described by neddyboy. A low output on the echo side of things, and adjusting the trim, i only got distortion and a bit more output. There are several trimpots on the mainboard - i guess some are for adjusting feedback and so on- I do find the lack of service manual disturbing. Have worked on Roland Space Echoes and their manuals are top notch
And btw i do have a frequency counter and oscilloscope at disposal - what is the frequency of the oscillator - would be nice to have some values.
So I measured the frequency of the delete head - it was 50khz do anyone have any values I can compare with?