Webster Chicago Model 80-1 Not Erasing (and Recording Distortion)

[BetaMaster64]

Hello all! TL;DR at the bottom if this. This isn't exactly a "tape" question, but it's related enough, I think. Please bear with me, as I'm pretty new to in-depth troubleshooting; most of my previous restorations have just been blind capacitor replacement, and then hoping everything gets fixed, so this one has caused me to step a bit outside of my comfort zone.

I have a Webster-Chicago model 80-1 that plays perfectly, but does not record very well (it's very distorted), and does not erase. I replaced all capacitors, and any resistors that were out of tolerance.

My troubleshooting, referencing the service manual, has led me down the following path:
- Verified C4 and C7 are fine
- Verified 6V6 tube is fine (tested with an old tube tester that hasn't been calibrated in probably 50+ years. I assume it's probably good enough to give me an estimate)
- Similar to the point above, I swapped out other spare 6V6s I had, and they all had the same behavior

Now, where I did see some results, was when I tested the oscillator circuit using the "home made device," suggested on page 22 of the service manual (for those interested, this involves powering a 6.3v incandescent light via pin 3 of the record head); my unit failed this test. However, why it failed this test is what I'm currently stuck on.

While doing some out-of-circuit testing, I noticed the following:
- The erase & bias coils (pins 3 and 4 of the record-playback head) measure at 0.1 ohms (practically dead short)
- The oscillator's secondary coil (presumably, the one connected to the safety switch and pin 3 of the record-playback head) measures at 0 ohms (legitimately a dead short!)
- The oscillator's primary coil measures at 4.9 ohms, which also seems pretty close to a short, but not near as close as the others
- One side of the center tap (switch to C7) measures at 4.1 ohms
- The other side of the center tap (between C4 and C7) measures at 0.7 ohms

With the above values, I'm led to believe that I have bad erase coils, and a bad oscillator coil. However, I'm not totally certain, because the service manual makes no mention of what these coils are supposed to read on an ohmmeter, and I'm not experienced enough with electronics to say for certain they're toast.

If these coils are actually the problem (or, at least, a problem), I'd like to try to re-create them. They were hand-assembled in a factory, after all, so they have to be possible to make, given the proper resistance.

Also, if anyone has a dead record-playback head they're willing to sell, I'd love to buy it to experiment on. I'm a bit hesitant to rip mine apart, because it is at least partially working.

My main question, and TL;DR: Webster Chicago 80-1 wire recorder plays fine, but doesn't record well, or erase at all. Troubleshooting led me to coils. Does anybody know the resistance of the erase/bias coils (pins 3 and 4) of the record-playback head, and/or the resistances of the oscillator coil?

Thanks in advance!

 

[dr*audio]

You need an oscilloscope to troubleshoot this properly. Buy yourself a Christmas / Chanukah present.

5 MHz Handheld Mini DSO Oscilloscope with 1-10x Probe

Full function, high performance handheld mini oscilloscope with high resolution 2.4 inch 320x240 LCD screen. 3hr. run time on charged battery One button AUTO, the waveform can be displayed without complicated adjustments. Waveform file manager supports thumbnail browsing, viewing, detail view...

www.mpja.com

 

[BetaMaster64]

You need an oscilloscope to troubleshoot this properly. Buy yourself a Christmas / Chanukah present.

5 MHz Handheld Mini DSO Oscilloscope with 1-10x Probe

Full function, high performance handheld mini oscilloscope with high resolution 2.4 inch 320x240 LCD screen. 3hr. run time on charged battery One button AUTO, the waveform can be displayed without complicated adjustments. Waveform file manager supports thumbnail browsing, viewing, detail view...

www.mpja.com

Actually, I do have an oscilloscope! It's a Tektronix 2225; nothing crazy, but it should get the job done for this project.

There is actually some other behavior I left out of my original post, but after I took some readings with the scope just now, I think it's actually very relevant.

Every 10-20 seconds, or sometimes as soon as every 5 seconds, there is a "pop" coming from the speaker in record mode.

I took these readings right before the safety switch, with the record-play head removed (to eliminate that variable).

Typically, the output from the secondary coil looks like this. Basically, no activity; just some background noise (zoomed ALL the way in):


However, when I hear that "pop" from the speaker that I described earlier, the scope very briefly looks like this (zoomed ALL the way out):


I think this is telling me that my oscillator coil is likely fine (or, at least, not completely dead). So, the problem I'm looking for is likely somewhere else! That's progress, I think.

Originally, I thought this occasional "pop" may be leakage from a weak rectifier tube. Now I'm starting to think it may be related to my issues with recording/erasing.

 

[dr*audio]

Yep, every 5 seconds the oscillator briefly starts up. I'll see if I can locate a schematic.

 

[dr*audio]

Check the voltage at the output transformer first, in Record mode. Check it with your meter and then with a scope. I think what you will see is the voltage slowly rises, then the oscillator fires and it crashes the voltage. If so, the electrolytic caps are probably bad.

 

[BetaMaster64]

Check the voltage at the output transformer first, in Record mode. Check it with your meter and then with a scope. I think what you will see is the voltage slowly rises, then the oscillator fires and it crashes the voltage. If so, the electrolytic caps are probably bad.

After lots of troubleshooting, I ended up getting it working! TL;DR at the bottom, but this ended up being a very simple mistake on my part, with lots of red herrings.

I hooked my multimeter up to pin 8 of the 6v6, and noticed that it starts at 0v, then builds up to 0.8v after several seconds, speaker pops, and then it immediately goes back down to 0v.

My initial instinct was maybe BOTH tubes I tried were bad, since the voltage on pin 8 would have to be coming from the 6v6, so I swapped out that tube with another spare I had lying around, and....I saw an oscillation on the scope! However, this was a red herring, because eventually that tube stopped working too. (I thought for a moment that maybe it was killing my tubes, though I put this theory to rest by trying them in my guitar amp, and they still worked fine)

So, what went wrong?
When I was replacing the R12 resistor, my solder joint was bad, so the process of inserting the tube flexed the pins, and broke the joint. But, sometimes, it would connect just enough to appear that it was working, which explains why it seemed to work when I put another 6v6 in. Lesson learned; I need to make better solder joints and check my work again, after inserting the tubes!

When I fixed that connection, pin 8 of the 6v6 went between 8-10.5v (depending on the tube I used), there were no more pops, and I saw a nice pair of sine waves on the scope. Now it's playing, recording, and erasing absolutely perfectly. (attached picture is showing the voltage coming off of pin 8 of the 6v6, and the scope is showing the oscillation right before the safety switch)


Thanks so much for the help! Your suggestions caused me to double-check some parts I hadn't really paid attention to. Next step will be to send off the idlers to Terry Witt to be rebuilt, and this thing will be all the way there!

TL;DR: Bad solder joint on R12 that broke when inserting the 6v6 tube.

 

[dr*audio]

Glad I could help. My older brother had one of these back in the 1960's that he found in the garbage and repaired. He sold it to a friend before I could get my hands on it.

 

[rojoknox]

Greetings from RojoLand!

Glad, too, to hear you've found the problem with your W/C model 80. A quick, rather unrelated question, if I may: What does "TL;DR" mean?

Take care,
—
J. E. Knox "The Victor Freak" (Webcor Freak too)

 

[dhnash]

TL;DR is an abbreviation for "too long; didn't read".