Dead record head or faulty driver circuit?

[t-dogg]

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...

[steveidosound]

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?

That is pretty odd. Does the transformer look like it has been replaced? (new mounting holes etc.) transformers don't typically fail by putting out MORE voltage. with the right combination of shorted turns it MIGHT be possible but not a normal failure mode at all.
There are DC voltages shown on the schematic at the takeoff points on the filter caps in the power supply. I would look there and at the resistors in the power supply to make sure they are on value as well as the other advice about power supply caps - but if one were open I would think you would be hearing lots of AC hum in the audio. If all the B+voltages on the tubes read much higher than the schematic shows and nothing looks tampered with or re-wired, perhaps in later units they just re-designed for higher voltages.

Just had an inspiration looking at the schematic. Were you trying to measure DC voltage at the other ends of the 2 diodes before the filter caps? That is, one lead on the far side of each diode from the transformer? That will give a false reading. You will note the center tap of the transformer is grounded. And although the power has been rectified at the diodes, it is not really smooth DC till the filter caps.  I think that 140V on the schematic might reference either diode to ground voltage measurement. Not across both diodes.
the AC voltage off either leg of the transformer before the diodes with respect to ground should be in the same ballpark. Across one leg to the other, about double that.

Yes to your question about DC on the heads. If C13, 14, or15 were leaky it would put DC across the record or erase head. BTW, DC on the erase head, though bad, would tend to erase the tape.

Did your 6C4 oscillator tube check out O.K.? How hard would it be to get another and sub?

[steveidosound]

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?

All the negatives of the filter caps are common ground. I presume you question implies a 3 section metal can with a common ground. The rats nest should be coming off of the positive terminals. The negative tabs on the can may or may not be chassis grounded.
I think I would use a scope and look at the power supply. The high DC B+ will cause the flat trace to move up. If you decouple the DC by switching your scope to AC and downranging you will be able to see the residual AC hum after each stage of filtering. It should be less and less. If that is true and the resistors in the power supply section are within tolerance, then the power supply is normal.
I wouldn't de-solder anything unless you get a false reading with the meter. Trying to check caps, except for gross shorts or opens, with a meter is difficult. If you make sure there is no voltage across the cap - power off and cap discharged - the resistance will change on the ohms scale as the cap charges. (easier to see with an old school analog ohmmeter) a false reading could be caused by a low value resistor across the cap for example.

BTW, these are all good tips for troubleshooting circuits for any tape recorder or other electronic device - tube or solid state.
So it is a good thread for other DIY Tape Project members to follow if they don't know this stuff already.

[t-dogg]

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

[steveidosound]

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

And perhaps if you have had a look around the forum, we have opened your eyes a bit as a musician to the wonders of archaic reel to reel technology.

[ironbut]

You're certainly welcome Tim.
Many times these kinds of projects, even when it fails to secure the outcome you hoped for, turn into great learning experiences. I know they do for me.

Unfortunately, these old units tend to get pretty hacked up over the years. All it takes is an article in Guitar Player or Keyboard magazine and all of the sudden you end up with tons of gear that was working fine or had completely unrelated problems (sometimes just needing adjustments) half finished and/or completely useless.

It's always great to be able to tell the owner that you brought something back from the dead, but sometimes it just isn't in the cards and you shouldn't feel bad about it.

[t-dogg]

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