| View previous topic :: View next topic |
| Author |
Message |
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Tue 03, 2009 5:55 pm |
 |
|
Hey Guys,
Well i picked my 6888 back up again and this thing is pretty beat! Basically when i built it, i slacked off a bit and i suffered the consequences with some intermittent problems. And recently while in a rush i had to "borrow" a couple parts out of there.
So today inspired by new radio find i decided i would rebuild it. Ive taken it totally apart.
What im looking for is the schematic that included a power supply for the oscillator chip. I didnt use the original frequency that the original schematic had so i had to use a different osc with its own power supply. I tried the forum search but i cant find the post that used to be up with different versions of it. I did find one but it does not have this same version im looking for.
Im also trying to find a voltage doubler schematic which i believe was designed by Mike Toon. It used two capacitors and a couple diodes.
Any help appreciated and thanks in advance!
-Carlos
_________________
 |
|
|
|
 |
Kevin Kuehn
Member
Joined: 01 Jan 1970
Posts: 1742
Location: N.W. Wisconsin
|
|
|
|
|
35Z5
Member
Joined: 01 Jan 1970
Posts: 5728
Location: Chesapeake VA
|
| Posted: Nov Tue 03, 2009 6:54 pm |
|
|
You need the additional 5v P/S to power the osc, your IC draws more current than the tube cathode can supply...
If you increase the B+ past 140v or so, you'll need to be sure the screen supply resistor is up around 10-12K @ 1W, the 1/2w 4.7K will burn(been there)...
The more B+ you have the greater the RF out, but remember it will require additional audio input for full modulation... A 6888 is rated for 250v max, I'd shoot for B+ around 180v... There are doubler circuits on the net, I'll see what I can dig up...
Here ya go... I've used the full wave doubler circuit to give 375v on my now long extinct 6BM8 Tx...
http://www.tubecad.com/january2000/page14.html
Tom |
|
|
|
|
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Tue 03, 2009 8:33 pm |
|
|
Tom ive already figured the voltage doubler i used before.
What is the best way too hook up the audio transformer? I have an RS transformer one side has three wires the other two. The way i used to have it was the three hooked up to a pair of RCA jacks. With one of the wires acting as the ground.
However the problem i had with this was that when both RCA jacks were being used the audio wasnt as good. Connecting only one made it sound better, mainly when only the left one was used.
_________________
|
|
|
|
|
gadget73
Member
Joined: 04 Mar 2008
Posts: 2003
Location: New Jersey
|
| Posted: Nov Wed 04, 2009 12:48 am |
|
|
I wasn't able to get enough audio drive from mine using the transformers. I used a 6SN7 twin triode wired as a mixer and used that as my audio input. The mixer schematic I adopted from the back of the RCA tube manual, but its just two very basic triode amps, each grid driven from a seperate audio input, and both outputs capacitor coupled to the second grid of the 6888 tube.
_________________
Saving vintage electronics, one dumpster at a time. |
|
|
|
|
Kevin Kuehn
Member
Joined: 01 Jan 1970
Posts: 1742
Location: N.W. Wisconsin
|
| Posted: Nov Wed 04, 2009 2:05 am |
|
|
| tube42 wrote: |
What is the best way too hook up the audio transformer? I have an RS transformer one side has three wires the other two. The way i used to have it was the three hooked up to a pair of RCA jacks. With one of the wires acting as the ground.
However the problem i had with this was that when both RCA jacks were being used the audio wasnt as good. Connecting only one made it sound better, mainly when only the left one was used. |
The original Radio Shack transformer in question is a transistor push-pull output: 1kct to 8ohm.
The way you had it hooked up, the two stereo (left & right) signals were canceling out common mode signals. You want to use only the two outside wires on the 1k side of that transformer,one end to ground, the other is your audio signal to the 6888 grid. You should sum the left and right signals first by connecting them in parallel or through two summing resistors into a single signal. Norm’s original design showed the 8-ohm side on the input for a voltage gain. Unfortunately the 8 ohm input is too low of an impedance for anything other than being directly driven from a speaker output. This is the reason 35z5 and others came up with a 2nd tube gain stage to replace the transformer input. A transformer actually works fine for a line level input stage, but you need something more like an interstage transformer with at least a 1:3 turns ratio. I used an old UTC 15k to 135K interstage transformer and I have all the gain I can use comming directly out of the line level jacks of my CD player. I ended up using a volume control at the grid of the 6888 to pad down the signal.
Kevin
Last edited by Kevin Kuehn on Nov Wed 04, 2009 2:36 pm |
|
|
|
|
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Wed 04, 2009 9:42 am |
|
|
Kevin can you please tell me which side is which 1k/ 8 ohm? Im a bit confused.
What i did to have more modulation was i took an old circuit board from a PC speaker system and i put it in a project box adapted for use on the transmitter. It worked pretty great so i dont mind using this little transformer.
-Carlos
_________________
|
|
|
|
|
Kevin Kuehn
Member
Joined: 01 Jan 1970
Posts: 1742
Location: N.W. Wisconsin
|
| Posted: Nov Wed 04, 2009 2:29 pm |
|
|
The center tapped side is 1k, use the two outside wires. No connection to the center tap.
Kevin |
|
|
|
|
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Wed 04, 2009 2:41 pm |
|
|
Thank you Kevin!
_________________
|
|
|
|
|
Kevin Kuehn
Member
Joined: 01 Jan 1970
Posts: 1742
Location: N.W. Wisconsin
|
|
|
|
|
35Z5
Member
Joined: 01 Jan 1970
Posts: 5728
Location: Chesapeake VA
|
|
|
|
|
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Thu 05, 2009 11:41 pm |
|
|
Tom thank you that last one was just what i was looking for.
_________________
|
|
|
|
|
gadget73
Member
Joined: 04 Mar 2008
Posts: 2003
Location: New Jersey
|
| Posted: Nov Fri 06, 2009 2:01 am |
|
|
You wouldn't happen to have a pinout for those 1500 kc oscs would you? You sent me one a while ago and I've finally gotten around to making use of it, but I can't figure out how the heck to hook it up lol.
_________________
Saving vintage electronics, one dumpster at a time. |
|
|
|
|
35Z5
Member
Joined: 01 Jan 1970
Posts: 5728
Location: Chesapeake VA
|
| Posted: Nov Fri 06, 2009 2:07 am |
|
|
LOL here you go...
I numbered the pins when I drew the 9KC6 Tx schematic...
http://mysite.verizon.net/a35z5gta/9kc6finalb.jpg
BTW, I believe the pin view is from the top of chip as it's inserted... Pinout is std, same as the current 1Mc units...
Tom |
|
|
|
|
gadget73
Member
Joined: 04 Mar 2008
Posts: 2003
Location: New Jersey
|
| Posted: Nov Sat 07, 2009 2:49 am |
|
|
well, the thing is I'm not sure how to determine which end pin 1 is on. There isn't a dot. Looks like the white ceramic bit overhangs more on one end than the other, but thats the only really identifying marking I see. Is the overhang end where pin 1 is ?
_________________
Saving vintage electronics, one dumpster at a time. |
|
|
|
|
35Z5
Member
Joined: 01 Jan 1970
Posts: 5728
Location: Chesapeake VA
|
| Posted: Nov Sat 07, 2009 3:26 am |
|
|
OK check this out, maybe yours is like this one...
http://mysite.verizon.net/a35z5gta/9kc6osc.jpg
Pin #1 is on right front, I believe some of those chips have a dot for the indicator...
I'll check the remaining ones I have tomorrow...
Tom |
|
|
|
|
35Z5
Member
Joined: 01 Jan 1970
Posts: 5728
Location: Chesapeake VA
|
| Posted: Nov Sat 07, 2009 5:22 pm |
|
|
All the osc I have are like the one pictured... The 1 near right front signifies pin #1(it's nearest to where #2 would be)... The other pins are per schematic, looking down from top...
Tom |
|
|
|
|
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Sat 07, 2009 8:01 pm |
|
|
Tom im going to be getting a different oscillator for the rebuild. The frequency i was using gets bombarded by a new religious station right next door to it.
Would there be any differences in what i would need for a power supply for it?
Also i checked and this doesnt look like the same one i had built before.
The one i had built before used a 1000uf@16V cap on the osc power supply. But anyway i guess itl work the same.
_________________
|
|
|
|
|
35Z5
Member
Joined: 01 Jan 1970
Posts: 5728
Location: Chesapeake VA
|
| Posted: Nov Sat 07, 2009 8:33 pm |
|
|
Carlos, if the osc is a newer type that only draws a couple milliamps of current, you can build it to Norms original 6888 specs(do note what I said about the screen resistor if you raise B+).. The osc P/S was necessary to power the older chips that require approx 20 ma... For any xtal osc, would still be necessary on a 6GY6 Tx as those tubes require 3v bias vs 5v for the 6888...
Tom |
|
|
|
|
tube42
Member
Joined: 23 Jan 2007
Posts: 2839
Location: Chicago Il
|
| Posted: Nov Sun 08, 2009 1:41 pm |
|
|
Hi Tom,
I still need to figure out what kind oscillator i will get. Im not really sure at what kind frequencies these come in. I checked around a dial for some clean spots and this is what ive found:
Some range from one spot to another. And some are just one frequency i found empty. I used a digital tuner for this,
531-540
585-612
639-648
693
801
864-873
909
1224
1574
So i doubt oscillators actually come in these exact frequencies so i dont know what frequencies they do and do not come in so i know what to buy. For example on the single frequencies on the list, they dont have to be exactly that, but something near in a frequency that is actually available on an oscillator.
Any help?
Im also trying to figure out what the original vari cap is on norms first schematic.
http://i121.photobucket.com/al ... posite.jpg
Thanks in advance
-Carlos
_________________
|
|
|
|
|
|
