This is topic DeVry XD sound problem in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=002197
Posted by Sean Weitzel (Member # 244) on 04-25-2001, 11:40 AM:
I am having an issue with my DeVry XD. It is completly original i.e., the sound is a mono Photoelectric cell and is amplified by this toolbox sized DeVry tube amplifier. The problem is twofold and has been worsening over time.When the amp is first fired up, the fidelity is very good and loud, however as the heat in the amplifier builds, the output volume will decrease - not by much, but I will end up haveing to bump the volume up another 15% or so.
While this is not too severe of an issue, the 2nd issue has become very apparent recently and is more important. That is, the fidelity of the audio really goes to crap after 2 or 3 reels. The best way to describe it would be if a noise reduction circuit was too strong. only loud passages are amplified and discernable. Dialogue gets lost.. what comes out of the speaker is only the loud part of a performer's speech. the soft portions fall off completly into the noise floor. Another symptom would be that the audio that does make it sounds 'fuzzy' or 'snowy' I have taken compressed air and cleaned the sound drum area and exciter lens very well. I don't think it is an alignment issue because this happens with both variable density and variable area soundtracks. Yesterday I tried switching out some tubes with ones I happened to have in a box, but the problem did not change at all. I am trying to determine if I have a failing PE cell or a bad tube or cap in the amplifier. I don't know much about vacuum tube electronics so any help would be appreciated.
some of the tubes in the amp that I can remember off the top of my head are:
5U6 (tried changing with NOS)
5881 (two of them.. haven't tried changing)
6N7 tried changing with NOS
6J7 havent changed
I am sorry that I don't have the amp model number. I can get that if it would help someone help me.. as well as verify all of the tubes.
I guess what I am asking is if anyone has seen this symptom and knows right off what it might be.
I appreciate it
-sean
Posted by Jeff Taylor (Member # 443) on 04-25-2001, 12:15 PM:
I would have to suspect that your problem is not a tube. More likely you've got a resistor or cap that's leaking as the temperature of the unit rises. Tube electronics are becoming a lost art, but it will probably be a trial and error exercise of checking resistance and capacitance values through the relatively simple circuit.
Posted by John Schulien (Member # 215) on 04-25-2001, 12:23 PM:
If you can find an old-time tube radio repair person, or someone who is qualified to repair/restore tube guitar amplifiers, or Hammond organs, then he or she should have no problem repairing your amplifier. With few exceptions, all tube amplifiers have nearly identical circuits and are analyzed/repaired the same way.
Posted by John Pytlak (Member # 331) on 04-25-2001, 12:45 PM:
My gut feel is that as the amplifier warms up, the values of some of the old passive components (i.e., resistors and capacitors) are changing, changing the bias voltages on the tubes, causing clipping and distortion. One way to diagnose the problem is to run the amplifier until the problem occurs, then use aerosol "component cooler" to cool the suspected parts and see if it affects performance:
Radio Shack Component Cooler Or use a voltmeter to monitor the bias voltages on the tubes as the amplifier heats up.
I suspect capacitors that are over 40 years old are pretty deteriorated, and should be replaced anyway.
------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion
Posted by Aaron Sisemore (Member # 145) on 04-25-2001, 01:23 PM:
The problem is most likely that there are several capacitors that are dying or pretty much totally dead. This is common with very old electronics, especially those with tubes, as they run at higher voltages and therefore take more stresses. Also these older capacitors do not age very well and will go bad just sitting there with no voltages at all on them. Thats why most if not all vintage tube equipment usually needs all the capacitors replaced before putting the units into operation. As for getting it repaired, I would seek out someone who does tube Hi-Fi repairs/rebuilding, and avoid the guitar/Hammond techs as they are more into building amplifiers that will sound good with guitars and tonewheel organs and may not have the 'ear' for what a film projection amp should sound like (even in mono). If you are interested in bringing the amp up north, I might be able to have a look at it myself.
Good Luck!
Aaron
Posted by Mark Gulbrandsen (Member # 72) on 04-25-2001, 02:24 PM:
I sort of agree with Aaron. This sort of thing should be re-capped and also any sleenium rectifiers in the power supplys should be replaced. I rebuild allot of old pre-1950 tube televisions and this is the first thing I do before and AC even hits the set. Also check your tubes on a good mutual conductance tube checker.
You may also want to consider replacing the amp with any decent solid state PA amp. 69.95 at Radio Shack. Or a cheapo KElmar unit that you can usually find on E-Bay
Mark @ GTS
Posted by Barry Hans (Member # 477) on 04-25-2001, 02:28 PM:
What about converting the photo cell to a solar cell? I think Wolk carried a solar cell replacement ( as I am sure others do ). Then you could use newer electronics, mic preamp/ amp or whatever. I am not sure what the cost of the cell is, but it may be a cheaper fix than your tube gear depending on whats wrong with it.
Posted by Sean Weitzel (Member # 244) on 04-25-2001, 02:55 PM:
Thanks everyone. I think I'm going to check all of my resistors and caps and see if anything is going wonky. That leads me to another small request - can anyone point me to a good dealer for vintage gear capacitors? I don't trust that I'm going to be able to find these things at radio shack. Aaron, I'll contact you if I can't get anywhere with it, thanks for the offer!
-sean
Posted by Jeff Taylor (Member # 443) on 04-25-2001, 03:43 PM:
Antique Electronics in Phoenix will have any of the tubes or caps you might need. Their catalog is on the web. I can't find the exact website address at the moment, but perhaps Key Layton will notice this string and post it.
Posted by Paul G. Thompson (Member # 655) on 04-25-2001, 06:53 PM:
I agree with John and Aaron. There could be a number of things that is causing your problem. If you have a cathode resistor increasing its value, (which will increase the tube bias) it will cause the tube to operate on the non-linear portion of the EcIb curve. This, in turn, will cause the gain to decrease, as well as distortion. This distortion would be caused by assymetrics being introduced into the audio signal.
On the other hand, a leaky cathode bypass capacitor will cause the bias to decrease, which will give a slightly higher gain, but it can start clipping if the control grid is driven positive with respect to the cathode. It could also be you have a coupling capacitor that is leaking, which will drive the next stage nuts. If you know a little about tube theory, I would suggest checking the plate voltage, cathode bias voltage and control grid voltage when you fire the system up, and do the same when it starts deteriorating. You will probably notice a change somewhere.
Shorting or leaking of the screen by-pass capacitors will also cause some major problems with distortion and gain, so I suggest you check those voltages as well.
A significant change of screen grid voltage could be caused by the capacitor leaking, or the screen dropping resistor changing its value.
Unless the amplifier employs direct coupling, the DC voltage on the control grid in the quiescent state should be ZERO, but if the output is push-pull, the control grid of the phase inverter may ride at a positive voltage. It should never be more positive as the cathode in a class "A" condition.
In vacuum tube applications, bias is the DC difference in potential between the grid and the cathode, using the cathode as a reference.
Again, in class "A" applications, the control grid should never be positive or driven positive with respect to the cathode. If that condition occurs, it'll draw grid current and you'll have distortion up the gazoo.
If it is, you more than likely have a leaky coupling capacitor.
Many vacuum tube amps have a degenerative feedback loop that uses resistors and capacitors. Beware of those. If those capacitors start shorting out, they, too, can feed a positive voltage back to the control grid and cause some major problems.
I might add that your PE cell may be going south. Have you replaced it yet? They do get gassy.
Have fun....and good luck.
Posted by Aaron Sisemore (Member # 145) on 04-25-2001, 07:59 PM:
>>Antique Electronics in Phoenix will have any of the tubes or caps you might need. Their catalog is on the web. I can't find the exact website address at the moment<<
http://www.tubesandmore.com Get their catalog! it's GREAT!
Aaron
Posted by John Walsh (Member # 168) on 04-25-2001, 10:35 PM:
I don't think you need to use a place that sells "vintage" caps. Caps are still pretty common; Allied, Digikey, Mouser, Newark probably will have the cap(s) you need. They may be in a slightly different shape, but you should look a cheaper sources first.
Posted by John Eickhof (Member # 323) on 04-26-2001, 12:36 AM:
John, the latest Allied & Newark books are very sparse on HV caps! Remember, he needs caps with a rated WV of 350-500 volts! Not the norm in the suppliers inventories anymore, except for those damned "Orange Drops' that fail after about 100 hours of use! Antique Electronic Supply is the best source for reliable parts. I place my bet on cathode & coupling caps!! REPLECA ALL OF 'EM!------------------
John Eickhof President, Chief Slave
Northwest Theatre Equipment Co., Inc.
P.O.Box 258
Wendell, ID. 83355-0258
208-536-5489
email: jeickhof@nteequip.com
Posted by John Walsh (Member # 168) on 04-26-2001, 11:41 AM:
Can't get some HV caps anymore? That's the kind of thing that make me feel old.I remember you used to just go buy a regular vacumn tube and be done.. now, some are sold as a "special" matched set in a fancy wooden case.
I'm stuck in a time warp... I can't get over having to pay more than $6 for a tube, or $50 for a beat-up Volkswagon bug.