Messages

1

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 09, 2009, 02:24:09 PM »

Yeah, I definately learned alot, and one day hope to get back into tape.

Sadly, I've owned a Teac 30-4, a Tascam .32, a Tascam .38, and an Otari MTR-10 at various points in my life...  The Otari was the last to go.  I'm in a townhouse nowadays and can't really justify an r2r, but once I'm settled into a house and can reclaim some studio space I plan on joining the fold again!!!  

To be honest, I found myself on ebay checking the going prices right after I came across this forum -- wound up leaving the browser window up on the communal laptop my wife and I use to surf the internet. She sees it and says, "...ummm, what are you doing looking at another one of those big 300-pound thingys you got rid of right before we moved?!?!"  Lol...  consider that foreshadowing -- of me not getting another one just yet! 

~tim

2

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 09, 2009, 12:59:41 PM »

Hey guys,

Just to check in...  From the resources I've been able to track down, it seems that this Echoplex has been completely and thoroughly reworked by someone, I'm afraid in ways that are simply incorrect.  The transformer is in fact not original, as are many of the component s-- C16 for example is an electrolytic .001, as oppose to a non-electrolytic .002... I keep on finding more variances the deeper I look, to the point it would almost be easier to DIY the circuit from scratch --at least I'd know what I have to work with!!!  Either parts were just arbitrarily thrown in, or someone with an engineering degree re-worked their own version of it! 

Its gotten to the point I'm considering cutting the cord, though I feel bad considering all the time and effort folks have contributed to my cause!   If it were mine I'd keep at it over time, but...  I dunno.  I might try one or two more things, but given the incorrect (or undocumented) transformer voltages, wiring, parts -- it may just be a losing battle.

In either case, I really want to thank everyone for their contribution here!  I really did learn alot stepping through everyone's suggestions, and maybe, just maybe...  I will actually get the damn thing to work after all!

Thanks,
Tim

3

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 04, 2009, 11:33:36 AM »

Guys, thank you for the continued support.  Now I'm starting to feel guilty, relying on you guys to fix my problems. I'll be away for the weekend, so probably won't make much headway until next week.  I'll hit the electronics store then to test the tubes on their tester (if it does actually work) and pick up some new components to swap in.

Couple things I can say:

--The included tubes do match the schematic -- 2 6EU7's, 1 6C4...

--I have to look at it again, but something made me suspect the transformer may not be original.  No markings on it, but it seems to have been remounted, or perhaps extra holes were just drilled into the chassis for one reason or another.  Pure speculation.
 
--The tubes do glow and pass a visual inspection -- no redplating or anything out of the ordinary...

Quick question on testing to see if the caps are open -- It's as simple as desoldering one end and measuring resistance across the leads?  Also, for the cap can...  I'll have to desolder the rats nest of wires connected to the negative terminals, right?

Thanks so much, and all you US folks, enjoy the Labor Day weekend...

4

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 04, 2009, 12:27:36 AM »

Ha ha...  I can't say I'm in much better shape over here as far as my fixin' technique goes.  but at least I've sourced a good braintrust for (much appreciated) advice!

I haven't tested every component, it'd be alot to un/resolder, but I think its now mostly in check with the schematic.  I did note the bias trap on the schematic, though this one in particular doesn't have it installed. All my testing is as steve suggested -- regeneration down, balance set to full playback head/no direct signal.  here's what I checked specifically (keep in mind I found a better matchnig schematic,so some of the expected values from previous posts have changed:)

SUPERBAD--I'm measuring 312VDC after the diodes!  (schematic references 140VDC)  Is this a transformer issue, operating waaaaay out of spec? Could this be the whole problem?  Wrong transformer altogether?

BAD--no bias signal on pin 5 of V3; oscillator definately isn't working... 180VDC on the meter with respect to ground (schematic suggests 125VDC)

GOOD--resistors R36 and R27 are within spec.

GOOD--impedance across both windings in the oscillator transformer checks out...

GOOD--No DC across erase head or record head. (Out of curiousity, is this the sort of thing where you don't want to see any DC whatsoever no matter how small?  If I did it would imply the c14/c15 were bad?)

MAYBE--C16 isn't shorted, but I haven't tried a new one yet either... will pick one up...


Transformer, right?

5

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 03, 2009, 11:07:24 AM »

Frustration setting in...

As I traced the circuit to test some of the components referenced above, I began to realize things are not in agreement with the schematic  -- different resistors here and there, lamp assembly wired completely differently...  There are several revisions of the EP-2 Echoplex referenced on the site I linked to in my original post...  I've edited the link to a schematic that more closely reflects what I'm seeing, but even that doesn't correspond with my ep2 entirely.  I suspect someone has attempted repair in the past, perhaps using a different reference schematic...  who knows -- these were handwired point-to-point over 40 years ago, so no telling if they made smaller undocumented design revisions along the way.

At any rate, that sidetracked me most of last night...  did some rewiring, but didn't get to test the oscillator, which I still suspect isn't working...  hopefully tonight I'll make out better... 

did I mention I was checking it out as a favor for a friend, and that I'll be giving it back if I get it working???  It's my azz-backwards way of learning electronics -- no matter how much I read about it, I learn 10x more doing these little repair projects for folks...  Of course, that involves full disclosure that I'm in no way a qualified pro, but so far I've had enough success/dumb luck that people keep asking me to look at other stuff for them... 

6

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 02, 2009, 12:34:50 PM »

Thanks Steve.  That does really make it sound like bias issues could be the big stumbling block... And I hadn't mentioned it, but I *think* I hear a bit of the test tone behind the wierd oscillations...  I do have the record level pot maxxed out -- as you mentioned, maybe enough gain there to drive a bit of it to tape at a low distorted level, if thats's what I'm hearing... 

I'll fire it up tonight and report back! 

Thanks,
TimF

7

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 02, 2009, 10:37:18 AM »

Tim -- THANK YOU!  Above and beyond the call of duty to help a random newbie on the forum!

I'll dive back in later tonight, and will do my due diligence to understand the rationale behind what you're suggesting I test.

As an aside...  Shouldn't something make it to tape (or existing audio on the tape be erased) even without any bias signal?  On playback the tape loop contains a bunch of wierd oscillations and pops (granted the tape is in somewhat terrible condition and will invariably need to be rewound)...

8

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 02, 2009, 08:15:36 AM »

Ok... I desoldered the record/erase head and checked impedance:

--record head 165ohms... (right where you said it'd be!)
--erase head 7ohms... (whatcha think -- indicative of a partial short?)

Hooked the head back up and probed for a bias signal with the scope, but found nothing...  As I mentioned, I'm a novice here, so let me recap in case of error:  All I did was plug the probe into channel 1, set it for AC and the proper timing scale, and probe each head's "positive" lead (the other side of each head is tied to ground...)  Saw nothing.  Just to verify my scope, as it is largely a piece'o'shite, I connected it to a signal generator set to 100khz and did see a signal, although it was a PITA to stabilize the trace.

The above, assuming my methodology was correct, leads me to believe there is no bias signal.  I was told the tubes are good, but I'm gonna run to a local electronics store today and test them just to be sure they're convincingly in spec (the store actually has an old public tube tester that supposedly works!)

Barring the possibility of bum tubes, I guess I need to probe around the oscillator circuit and find my missing bias signal?  I'll study the schematic and see if I can figure whats goin on in there, but as always, any assistance is gratefully accepted!

In either case, thanks for getting me this far guys...  learning every day with these little projects...

EDIT:  forgot to mention...  I did see audio signal across the record head, but not exactly what I expected.  I fed the unit a 1Khz sine wave, then probed the record head lead expecting to see a 1k sine on the scope...  What I saw instead was a distorted 1K signal -- looked like a 1Khz sine with an additional narrow peak riding atop the peak and trough of the wave...  Not sure what to make of that, obviously something is getting injected into or distorting the signal...

9

Tape Tech / Re: Dead record head or faulty driver circuit?

« on: September 01, 2009, 10:58:24 AM »

Wow -- thank you both for your prompt responses and input!  I will give your suggestions a try tonight and report back!

FWIW, up to this point I was able to verify:

--both 6EU7 are working suitably...
--power supply checks out...
--tape transport is sufficiently setup (though just enough for testing, certainly will require major overhaul and calibration once the electrical issues are sorted out)
--playback head is working, at least to some degree... (Lol, I actually ripped apart a prerecorded cassette and manually ran the tape across the PB head as a quick and dirty test to see if it produced reasonable output, which it did!)

10

Tape Tech / Dead record head or faulty driver circuit?

« on: September 01, 2009, 09:31:17 AM »

Hi all,

First post here; stumbled across this forum while attempting to fix an old tube echoplex tape delay...  Seems the record/erase head is not working, first thing I want to sort out is whether the head assembly itself is shot or the circuit driving the head is to blame.  I'm hoping one of you technical gurus might be able to give me some very general tips to help out.  My ee experience is limited to DIY'in some guitar amps and preamplifiers from schematics and so forth, but I don't necessarily understand how all aspects of the circuits work -- consider me an electronics novice who knows enough to be dangerous:-) 

Specifically... 

**My record/erase head has four wires coming off it -- undoubtedly two for record, two for erase...  My thought was to test the solder points these leads are connected to with a scope and see if I have signal...  Ie, if I run a 1K test tone into the input, I should see some sort of corresponding signal across the record head leads, correct? (if so, am I looking for a proportional 1K signal?)  I'd assume no or little signal here tells me the driver circuit is to blame?

**what about the erase head?  what sort of signal should I see across it's leads, how to best test?

Any other ways to quickly determine whether the head or supporting electronics are to blame?  If anyone is feeling particularly saucy, here is a schematic:

http://www.schematicheaven.com/effects/ep2_2750-5499.pdf


Thanks in advance,
Tim