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Been working on an Echoplex Sireko unit.Got it working good,but its not completely erasing.You can hear a low repeat of anything previously recorded.It is very low and can only be clearly heard when you stop playing with the unit running.I am assuming it is related to the modern tape bias.Anybody got any ideas?Thanks.
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Several thoughts, that may or may not be relevant.
Is the tape fresh? Enough time on a tape and normal variations in mechanicals and signal can get spread across the tape. Overly used tape can get harder to erase too. (and record)
Slight misalignment of the heads. The erase head should line up with the playback head stripe along the tape. If they are off a little, the erase might wipe off most of a track, but leave a little remnant along the edge of the stripe, enough for the repro head to detect.
As it plays back running but you not playing, so you can hear this, use a fingernail to tweak the tape a little higher, then a little lower as it crosses the playback head. Does this cause the unwanted sounds to increase or decrease? In other words, if moving the tape a little with respect to the head increases the sound, that lends some credence to my theory in the previous paragraph.
If any head alignment has been changed, remnants recorded at the old alignment may still remain and be close to the current stripe, and be picked up.
Is the ghost sound the very notes you recently played into it? Or could they be something recorded 5 years ago peeking out? I can envision some sort of print-through if the tape sat there wound for extended periods. I think it was the first Led Zeppelin album - something of that era - you can hear some print through clearly, and the notes can be heard faintly a half second before playing.
And yes, you do need to set the bias levels properly and also the bias trap.Education is what you're left with after you have forgotten what you have learned. -
The heads are out of alignment, the bias generator is not generating?Comment
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New tape,the record and erase heads are on one unit so no way to align between the 2 of them.I assume the playback head has a broader area on the face since any head alignment is done by bending the mounting bracket,a very crude system,but it is what it is.Its not old recording,its what has just been played.If I leave it running without playing it does erase a bit more with each pass,so I do suspect it has to do with the modern tape bias not matching the voltage at the erase head?If that makes any sense.Comment
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I'm not so sure that the modern tape is causing it, but a bad head that is not erasing, worn out, or the tape head does not have the proper wrap around.
If the level on the tape as recorded, is too high and saturating the tape, or the record bias is too high, or the erase voltage is too low... it just about needs to be one of those things.
did you measure an AC voltage on the erase head? the erase head needs power.
The "sound on sound" switch shuts the erase off, as I recall. I think those things used to have a foot switch to turn sound on sound on. If the jack for the foot switch has a bad contact- like corroded, that would explain why the sound on sound does not turn off, and that would explain no erase.
The foot switch jack contains a contact, which closes when the foot switch is unplugged. Make sure that the contact is closing and making continuity.
It's been a while, like a decade or two, since I really tore a echoplex apart. But now that you mention it, some of that is coming back to me. I think if you check the above stuff you are going to find a reason for no erase.Comment
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No sound on sound on this one.The Sireko is the bottom of the line as far as Echoplex goes.It is erasing,just not fully.The record function is fine,not overloading or distorting,clean playback.I'll have to try adjusting the voltage to the erase head and see what goes.Comment
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DO that with a scope, your meter won;t read much at that bias freq.
All tape recorders work about the same, so if you lack adjustment documentation for this unit, look over the more complete manuals for the Echoplex and adapt the service approach to your unit.
The record and erase may be one head, but bend or not, have you fiddled with the playback head vs record head alignment?Education is what you're left with after you have forgotten what you have learned.Comment
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The common use of modern tape formulations in old tape echo units is generally NOT a problem. However, keep in mind that oxide = money and professional tape has thicker oxide (and backing) than those supplied with tape echos, and might have a tough time erasing everything in one pass. There are also some pro tapes that were harder to erase than others (Scotch 250 is a good example; even on pro machines, you had to turn up the erase current to accomodate it). To complicate matters, most tape echos have no independent erase current adjustment, only bias, which also feeds the erase head. However, as you turn up the bias for better erase capabilities, your high frequency record response starts to fall off. Hey, no one ever accused these things of being high-fidelity, and ironically, THAT is what makes them sound so good.Comment
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Yes I think there is some type of erase head power supply. That's something to check out, it may not have the full voltage going.Originally posted by stokes View PostComment
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Is this the correct schematic?Comment
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"However, as you turn up the bias for better erase capabilities, your high frequency record response starts to fall off. Hey, no one ever accused these things of being high-fidelity, and ironically, THAT is what makes them sound so good."
Yeah john,thats a fact.Nothing prcise about these things at all.Align the heads by bending the bracket,close enough is good enough,eh.Actually started working on this last year,then I got a Dynachord tube unit and spent a long time on that one,which is a great unit,but the schems and any info I can find on it is in German.Got it working okay,but there are still a few issues with it with the repeat function,but my son is using it as it is,more to add some dynamics and a slight reverb,just to thicken up his sound.Took this Sireko out and started messing with it.It is "usable" but not perfect.I gotta open it up and try adjusting the bias.I think I have a NOS tape cartridge still in a box somewhere.I'll dig that out and see if the older tape works better.
Jazz,that is the schem,I have that already as well as instructions for adjusting things,thanks.Comment
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Following are my comments which I hope will help you troubleshoot the problem.
We know that all the functions operate just not up to spec. Good to know that you have the schematic and adjustment instructions. You have probably already cleaned the erase head but it’s worth checking again to make sure that it’s really clean. I once spent a couple of hours trying to get a tape deck working and it came down to a thin even film left on the head after I did the initial cleaning. The film came off with some extra scrubbing with a Q tip and alcohol.
Check that the tape guides and the record/erase head azimuth are such that the tape is in good contact with the erase slot portion of the head. You can experiment by guiding the tape with a thumbnail as Enzo suggested but guide it so it wraps around the head differently. You can also use a bit of soft foam or felt to temporarily push the tape against the erase portion of the head. Does that improve the erase function?Originally posted by stokes View Post
In your unit the signal to the erase head is not adjustable. They are slamming the erase head with a high level signal from the oscillator. The bias oscillator signal is fed to the erase head through a series .0012µF cap and a 270Ω resistor. The bias adjust only affects the bias signal level applied to the record head. Check that cap and resistor to make sure they are reasonably within spec. The only bias reading listed on the schematic is “54VAC” feeding the erase head. Note that it’s attenuated to 9.4V at the record head.(Just a reference point since it can be adjusted to optimize for the type of tape) If you have a good meter you can measure the signal feeding the erase head (~54Vrms). Otherwise scope it per Enzo’s suggestion and verify that you get ~150V peak to peak. The schematic doesn’t list the bias frequency but I’d expect it to be between 30kHz and 100kHz. Let us know what you measure.Originally posted by stokes View Post
It’s also possible that someone messed up the circuit during a previous repair or mod attempt. How’s the unit look inside?
Cheers,
TomComment
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Inside looked to be all original,except for the input cap.I replaced that as they had a series of caps to equal the right value,I believe it was 1uf.I replaced whatever electros where in there.I'll check that cr feed to the erase head,but I am pretty sure that was checked in the first round,but it was a long time since I had it opened.Comment
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I think that the next most interesting bit of information will be bias voltage readings at the erase head and the record head for reference. Might as well check the supply voltages in the bias oscillator circuit too.Comment
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Thats gonna be my next move,but its gonna be a couple days till I can get back to it.I'll post what I get.Comment
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