- DC bias on heater winding ? - 2 Updates
- Black screen perforated with circular holes - 4 Updates
- SM for Philips 7951 receiver? - 2 Updates
| Phil Allison <pallison49@gmail.com>: Jul 27 06:51PM -0700 Hi, positive DC biasing the heater winding of a tube amp is occasionally used to eliminate H-K hum in the pre-amp - a clever alternative to using a well filtered DC supply for the same job as done in many guitar amps. However, it means the heater circuit is no longer firmly grounded and could acquire a high voltage under certain fault conditions. Marshall in their infinitesimal wisdom have done this on their latest models - the JVM 400 series. The 6.3V heater winding is floated to about +70V DC using a voltage divider off the HT. Now, output tubes like the EL34s used are prone to failures that send their cathode to a high voltage or short pin 3 to pin 2 ( anode to heater arc over). When that happens, the heater winding rises to meet the B+ supply. None of the other tubes will like that - max rated H-K voltage is about 100V. Maybe the idea is *too* clever. .... Phil |
| Jeff Urban <jurb6006@gmail.com>: Jul 28 04:44AM -0700 If the cathode is positive I can see why to put DC slightly higher on the filament. It will prevent any conduction I would think. However +70 is up there. What cathodes are at that high a positive voltage ? All you need is an equal or slightly higher positive voltage. I guess maybe for a common plate stage. (cathode follower) Perhaps a triode single phase splitter to drive push pull output tubes. As far as getting rid of hum, you have to rectify that 70 volts, so why not just rectify the damn filament voltage in the first place ? You never know what kind of drugs these engineers are on. I remember an RCA TV like a CTC211 or something, projection. They had a string of like seven transistors just to turn on the convergence subsystem. Why ? And it wasn't like there were a bunch of protection lines going to it, just a voltage and that's it. It was also not in the feedback loop. I think they just want shit to be as hard to fix as possible. |
| bruce2bowser@gmail.com: Jul 27 10:00AM -0700 What is this type of screen called? Where can you get it from? |
| Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>: Jul 27 01:43PM -0400 > What is this type of screen called? Where can you get it from? McMaster-Carr, if you're prepared to paint it yourself. Very roughly $10 per square foot in aluminum, $20 in brass. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com |
| whit3rd <whit3rd@gmail.com>: Jul 27 11:34PM -0700 > What is this type of screen called? Where can you get it from? Sometimes called 'shadow mask' and there's some behind the glass in every color CRT. |
| Jeff Urban <jurb6006@gmail.com>: Jul 28 04:29AM -0700 >Sometimes called 'shadow mask' and there's some behind the glass in >every color CRT. Only the older ones, the inline gun ones had an aperture grill which is only vertical. Slits not holes. |
| John-Del <ohger1s@gmail.com>: Jul 27 02:01PM -0700 Came in "dead", but only had really really bad tape monitor/source switch contacts. Cleaned all controls and switches and runs fine, except the FM seems to have weak sensitivity. AM seems OK. Might be normal but I'd like to eyeball the FM section anyway. |
| dplatt@coop.radagast.org (Dave Platt): Jul 27 05:55PM -0700 >switch contacts. Cleaned all controls and switches and runs fine, >except the FM seems to have weak sensitivity. AM seems OK. Might be >normal but I'd like to eyeball the FM section anyway. One net article suggests that the 7951 is a cosmetic (black) variation of the 795. HiFiEngine.com doesn't have pages for the AH795/AH7951, but has a page for the AH794, and the service-manual for that model implies that it might apply to the 90, 92, 95, and 99 models as well. Philips seems to have produced families of receivers which varied cosmetically and in their audio power outputs, but it seems plausible that the basic schematic may be the same (or at least quite similar) across the family. So, go register at HiFiEngine.com (it's free, and I've not gotten any spam or other nonsense from them after registering years ago) and download the AH794 service manual. I'd guess it'll be close enough to let you debug the FM section. If it is, I don't think you should expect miracles. If I'm reading the schematic for U1001 (the FM tuner board) rightly... well, it appears to have _one_ varactor-tuned RF amplifier transistor (a BF324 run common-base) and the varactor-tuned local oscillator, feeding a one-transistor BF494 mixer/amp, which then feeds a 10.7 MHz crystal filter, a BF494 IF amplifier, and another crystal filter. There's more IF gain in the TBA570A AM/FM chip. There's a double-tuned transformer in front of all of this, which probably gives it some selectivity against out-of-band signals, but that's it. So, its selectivity isn't going to be wonderful. Sensitivity might be something you can address, perhaps... RF and IF alignment, maybe changing out those BF324/BF494 transistors with something a bit less Neolithic :-) |
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