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Author Topic: Dead record head or faulty driver circuit?  (Read 14260 times)

Offline t-dogg

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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


« Last Edit: September 03, 2009, 08:09:30 AM by t-dogg »

Offline steveidosound

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Re: Dead record head or faulty driver circuit?
« Reply #1 on: September 01, 2009, 10:31:48 AM »
There are others here FAR more qualified to answer, but the first thing I would check is the bias oscillator circuit with the 6C4 tube. If that oscillator circuit doesn't work for whatever reason you will have no record or erase function. Heads do go bad but that is not as common. Do make sure that the heads are clean and you have good tape to head contact also. Especially important with a tape loop that goes round and round. The audio drive for the record is both halves of one of the 6EU7 tubes, so it supplies the audio drive to the record head. When looking at tube circuits PLEASE NOTE that there can be at least a couple of hundred volts or more on the plates of the tubes. BE CAREFUL ! Especially if you are not used to working on tube equipment.
I presume you have checked the tubes themselves to make sure those are OK? The other 6EU7 provides playback, so if that works probably the power supply works.  If the power supply works the neon bulb should light. With a scope you should see a high frequency signal across the erase head and that plus the audio across the record head. good luck...good luck...good luck
Steve Williams

you don't want to know what equipment I listen to...

Offline Tim

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Re: Dead record head or faulty driver circuit?
« Reply #2 on: September 01, 2009, 10:34:45 AM »
-first check the resistance of each head winding with an ohm meter.  Heads windings can open up.  160 ohm record and 28 ohm erase.  Anything close is good.
-then, with an oscilloscope, check for the presence of bias signal on both windings. Perhaps 50-60 peak to peak volts AC on the erase head and 10-20 on the record.  AC frequency could be 80 to 150 kHz.  All these values are pure guesses, BTW.
-If bias is good, then pull the bias tube out (to kill the oscillator) and check for an audio signal on the record head with the 'scope.  If none, then trace back to the input to see where the signal quits.   Perhaps a defect cap or level pot or tube.   If a bad cap, then you may consider replacing all the caps for reliability sake.  They do get flaky with age.
 
Tim L.
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Offline t-dogg

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Re: Dead record head or faulty driver circuit?
« Reply #3 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!)

Offline t-dogg

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Re: Dead record head or faulty driver circuit?
« Reply #4 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...
« Last Edit: September 02, 2009, 10:10:38 AM by t-dogg »

Offline Tim

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Re: Dead record head or faulty driver circuit?
« Reply #5 on: September 02, 2009, 10:23:32 AM »
-I think 7 ohms is ok.   Just to be sure, power up the unit with the erase hd disconnected, and see if the bias osc works.

-check for 125 volts (DC) on pin 5 of V3

-check resistance of osc transformer (42 ohms and 25 ohms).  No need to desolder. 

-check R36 and R27 (100k and 560 ohm)  No need to desolder.

-C16 (.002mf)  See if shorted (with ohm meter).  Try a new cap.
Tim Leinbaugh
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with RTR specialty.

Offline t-dogg

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Re: Dead record head or faulty driver circuit?
« Reply #6 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)...

Offline steveidosound

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Re: Dead record head or faulty driver circuit?
« Reply #7 on: September 02, 2009, 10:54:16 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)...

If anything makes it to tape at all without the bias oscillator running correctly it would be at very low level and highly distorted.
There would be no erase function.
Perhaps if enough gain were available to overdrive the input with no bias you might hear some of what you are describing.
That peak on the 1KHz sine wave is another oscillation in the circuit - perhaps from the faulty bias oscillator? 
Do you see the AC  bias waveform frequency at pin 5 of the 6C4 tube with your scope on AC  (DC decoupling mode)? DC mode would show it riding on the 125VDC plate voltage of the tube (would displace the trace center line far up the screen or off if you don't have the scope set to account for the 125V measurement). Just look at it in  decoupled mode. That is the job  in the circuit of C15 - but it might be bad. There should be NO DC across the record or erase head.
« Last Edit: September 02, 2009, 11:55:23 AM by steveidosound »
Steve Williams

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Offline t-dogg

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Re: Dead record head or faulty driver circuit?
« Reply #8 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

Offline t-dogg

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Re: Dead record head or faulty driver circuit?
« Reply #9 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... 
« Last Edit: September 03, 2009, 11:28:34 AM by t-dogg »

Offline steveidosound

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Re: Dead record head or faulty driver circuit?
« Reply #10 on: September 03, 2009, 01:18:28 PM »
Did you see note 3 in the triangle on your schematic about a bias trap addition? Perhaps that is part of what you are seeing.
Look in the lower left corner for the added components.
I would make yours like the schematic unless you are a qualified design engineer. Though it is a very simple circuit, perhaps as simple as any tube tape recording/playback system I have seen, it should nominally work OK the way it was drawn - I think - (I'm no engineer either). You never know what experimental hack or repair has been done by someone who knew even LESS than you about how it worked and was just guessing/trying things.
BTW, in your trials It would be good at first to isolate the record and play section by turning down the pot that regenerates the signal for repeat echo intensity. The other pot called echo balance should be set to give full tape sound and not the "dry" input. Operate it and get it working so the output happens with tape delay one time with the best fidelity you can make it have. All sort of weird things start to happen with multiple regenerations of the signal (which is sort of the point of the effect).
Steve Williams

you don't want to know what equipment I listen to...

Offline ironbut

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Re: Dead record head or faulty driver circuit?
« Reply #11 on: September 03, 2009, 03:54:01 PM »
You never know what experimental hack or repair has been done by someone who knew even LESS than you about how it worked and was just guessing/trying things.

Unfortunately, that's more likely than not! A friend of mine used to work at a local guitar/music shop (I won't name names here but it was in the early 80's) and since he thought I knew a little about tape he had me come over to take a look at a Melotron that was in their shop. Well, at the time I knew how to use magnetic tape but I didn't know the first thing about how it worked. I spent the better part of a day looking at the guts of the thing but never did figure out what was wrong (those monsters were a constant headache from what I hear). While I was in their shop I did learn never to take anything to one of those guys. All day long it was "do you think this might work?" or "this kinda looks like xxx so I'll try this". This was punctuated by sparks and smoke. I had to leave because I knew I'd start laughing the next time it happened.
 Real scary!
steve koto
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Offline t-dogg

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Re: Dead record head or faulty driver circuit?
« Reply #12 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?






Offline Tim

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Re: Dead record head or faulty driver circuit?
« Reply #13 on: September 04, 2009, 01:36:38 AM »
1. Are all the tubes heated (glowing or feeling hot)? 

2. C16 might be open or way off value.  Try a new one.

3. It's possible that one of your three power supply caps is open: that might give higher PS voltage readings.

Tim Leinbaugh
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Offline ironbut

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Re: Dead record head or faulty driver circuit?
« Reply #14 on: September 04, 2009, 10:41:59 AM »
Be sure that the tubes that were provided with the unit are the ones specified in the schematic. It could be that someone at some point has changed the tube compliment and that these voltages are now correct.
steve koto
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