DX-100 saga ends... not giving up, just moving on.

[kf6pqt]

You know how you sometimes wind up with a project that just ticks you off to the point you are just done with it?

I once acquired a chevy truck cheaply that had scattered its three-speed transmission, and I spent a whole bunch of time rebuilding it, only to find out the thing wouldn't run right, once it had gears again. Was so sick of the damn thing, I sold it for less than it was worth just to not see it anymore.

I'm at the same place with the dx-100b I got back in May. Its getting moved off the bench, put back in its case, and I'll find a nice shelf for it in my garage, and perhaps I'll find someone locally who is retired and has lots of spare time who might like to "borrow" it for a while and sort its issues out for me. Basically the speech amp decides its going to be an oscillator, and I bet the modulator bias is wrong.  Either way, I'm sick of working on it, its holding me back from other projects, and now the mere sight of the thing irritates me. A bunch of cash tied up in an anchor I don't even have a boat for! (I wont get my money out of this thing, so I'll just stash it in the garage.)

I digress, I don't even want to talk about the damn thing unless said local person wants to take on the project! (email me if interested, perhaps we can work out a deal, must be within 30 miles of Burbank and promise to give it back!)

I could have built a grounded grid amp for one of my smaller transmitters in the mean time that I've been messing with this thing, or procrastinating  on messing with it.

Either way, the lesson learned is I'll never pay more than fifty bucks for a transmitter you haven't heard on the air!

Now, if its a basket case BC-610 we're talkin' about, this doesn't apply...

[The Slab Bacon]

Jason,
         What we have here is a lack of rizing up to the challenge. The dixie 100 is a fairly simple transmitter, as table top BAs go. This rig is a really good starting point to get your feet wet in the world of repairing and home brewing. If you give up on this rig, how do you ever expect to build or troubleshoot something from scratch??

Dont wimp out, grab yourself by the bootstraps and pull yourself up out of the quagmire!! Remember that quitters never win and winners never quit!!

there is no such thing as "unrepairable" if you really want to do it.
Do some reading and research, then drop back 5 and punt! All of us have dealt with some real "challenging" projects from time to time, the satisfaction of making them go is the real reward. Dont wimp out now!!
                                                The Slab Bacon

[kf6pqt]

I concur totally. However, sometimes a project needs to just be back-burnered. When you have to force yourself to give attention to the "problem child," you're not having fun. Patience and free time are in short supply here! THAT is all on me, but the guy who "modded" the hell out of this thing deserves some blame.

I've got a DX-60 that works and sounds great, and my real desire is to build up an amp to use behind it. This was the original plan, and this dx-100 showed up and got in the way. I feel I'll do a lot better with a project thats entirely mine, and not dealing with someone else's hackery.

I'm going to pack it up nice and safe in the garage, and spread the word that I'd like some serious help with it. If I get no takers for assistance, perhaps within a couple of years I'll be more up for the challenge of sorting it out.

[W1GFH]

Too bad, I'd volunteer for DX100 duty but I cannot take on any more heavy metal storage at this time. As it is, I need to divest of various effluvia. Your amp project is a good idea, altho I hate the concept of a buzzy DX60 being the exciter. Better to design your own 35 watt hi-fi AM xmtr to drive the amp. I have an A3808 60W Stancor mod tranny I can donate to your cause (OK for a pair of 807s, the primary is rated at 260 MA , 3.8K Ohms and 3.3K CT. Secondary rating is 170 MA with taps for 10K Ohms, 7.5K, 5K and 4K.) if you so choose.

[kf6pqt]

Thanks for the offer Joe, I've got a small pile of modulation transformers already.  The DX-60 has been recapped, so I don't think it sounds buzzy. I have a slew of other puny transmitters that with a little work would be excellent drivers.

Some even use carbon mics.

Speaking of, Joe, I got a little Stancor 10m mobile transmitter... Did you ever get that Ten'er recapped? I need to order some xtals from Bry for this thing.

[WBear2GCR]

Oscillating speech amp?

What frequency?

If it is LF, then add/replace the two caps that bypass the plate supply on each of the two front end triode sections with larger caps!! Done.

Bet you increased the grid resistor on the input right? 

                    _-_-bear (who just went through the same thing on an Apache)

[The Slab Bacon]

C'mon guys, its only 1 evening's (2 at the most) work to completely bulldoze out the audio section of a dixie 100 or 'patchy and build the ever popular 12AX7 / 6C4 circuit in its place. Taint no rocket science here. Itz often a lot less work to bulldoze it out clean and rebuild it from scratch then to try to figger out what the last guy wuz thinkin.
Especially if it haz been hammed up badly.
     
                                       the Slab Bacon                               

[W1GFH]

Speaking of, Joe, I got a little Stancor 10m mobile transmitter... Did you ever get that Ten'er recapped? I need to order some xtals from Bry for this thing.

I am way guilty of procrastination on the Ten'er, haven't even acquired the friggin' caps. If 10M starts roaring again tho, this could change.

[Blaine N1GTU]

I agree with Frank,
strip out the audio stage all the way up to the mod tubes.
throw in a 12ax7/au7 or maybe a few op amps instead.
make it sound the way you want it to, pull out any interstage transformers too while your at it

[Steve - WB3HUZ]

Then put a couple of 813s in the final and a pair of 810s in the modulators. Outboard the HV supply and run it at 1500 volts. Outboard the mod iron too. Then you will have a KNX-100!!!

I agree with Frank,
strip out the audio stage all the way up to the mod tubes.
throw in a 12ax7/au7 or maybe a few op amps instead.
make it sound the way you want it to, pull out any interstage transformers too while your at it

[flintstone mop]

Yea A SUPER Dixie.
I gave up on a Apache and outboarded the mic audio driver with a 10 watt amplifier and an audio output transformer from a Bogen PA amp(a pair of 6L6's) and connected the tranny backwards Primary facing the mod pubes (center tap brinngs in the biass) Secondary facing the 10 watt audio amp and the problems went awaaay. Plug your favorite mic in the 10watt amp with EQ and processing and you are cookin'.
05.5 cents worth
Fred

[w5omr]

I don't even want to talk about the damn thing unless said local person wants to take on the project! (email me if interested, perhaps we can work out a deal, must be within 30 miles of Burbank and promise to give it back!)

OK Jason, I'm going to offer assistance! Pack that 'problem child' for shipping and I'll have UPS pick it up at your place.

Mack

no no no.. there's no such thing as a 'free' "Dos Eckes" a'hunnert..
ya gotta be within 30 miles of Burbank, -and- promise to give it back!

[The Slab Bacon]

Jason (and others).
                           this post is not meant to be inflammatory, but to make you do a little headscratching. The good old DX-ahunderd is a relatively simple transmitter to repair and modify. The circuitry is pretty straightforward and simple,and there is ample room inside the chassis to work. (once you remove the speech amp shield). Not to mention that there is tons of documentation available for these units. If you are unable to repair this rig and get it working, your skill level is most likely not up to the level needed for home brewing and repairing larger projects. Not to mention the danger levels of working with high voltages..

My suggestion to you and others is to hit the books. Do some research and self teaching. Early ARRL handbooks, Bill Orr's west coast handbooks, The Radiotron designers handbook, just to mention a few are fantastic sources for information, theory reinforcement and a lot of the material that you NEED to know to do these jobs.

Do yourselves a very large favor: Resist the temptation to drool over the neat looking pieces and massive wattage ratings, and spend more of your time learning how this stuff works. You will never live to regret it!! And that which once seemed so difficult will become child's play.

Learn how things work, how tubes work, how circuits work, etc. And you will be able to troubleshoot things easily for the rest of your life!
This knowledge is absolutely priceless, and you can teach yourself!

                                                    the Slab Bacon

[flintstone mop]

WOW
Talk about kicking the gift horse in the mouth?
Jason, my man, a fellow Ham op is offering to pay for UPS shipping (am I ASSuming??) and fix your Dixie a'hundred, prolly for nothing.
Why would he want to keep something that can be gotten so easily anywhere in perfect working condition??
Fred

[WA1GFZ]

I agree with Frank and suggest you avoid anything bigger until you understand how to fix the rig you have. You might consider getting a scope and learning how to use it. The bigger the rig the bigger the problems. You can't run till you learn to walk first.

[flintstone mop]

If You want to get into outboarding components:
I has a AF-67 with a lot of outboarded stuff. Timtron would be proud of me............
The audio driver for the 5881's is a Radio Shack PA amp with the famous output transformer connected backwards. 22.5 volt battery on the center tap to biass the modulators in the AF-67. The mod tranformer is outboarded using a power transformer from an old heavy duty tube stereo amplifier, using the turbo connections Timtron mentions in an old posting from "The AM Window" and then we have a Heising choke outboarded, about 30HY, and the 2 or 3 MF coupling cap connected to the mod tranformer. TREmendous audio, and plenty of it. The AF-67 drives an Ameritron AL811, three 811A's for 600 PEP. That's  the "small" station for 80M and up.
G'day
Fred

Fred

[kf6pqt]

I've had the speech amp in and out twice. Tried outboarding, with two different interstage transformers, being driven by all sorts of things, guitar amp, PA amp, this exact combination, too:
>The audio driver for the (1625)'s is a Radio Shack PA amp with the famous output transformer connected backwards.

All sounding like crap.

So, yeah, it got a bit frustrating. If I'm sick of working on this damn thing, hey, its my radio, If I want to stash it in the garage and forget about it for a year or so, that is my prerogative.

No other commentary necessary.

Thank you. I'll be sure to only post about my successful projects from now on.

[Jim, W5JO]

Let it sit for a while Jason.  Sometimes when I am frustrated by a problem like that, I walk away until something happens in my brain.  Just keep it until you feel like you want a project to solve.

[WBear2GCR]

Hang on, hang on there!

Wait a segundo, mon ami!

Now, before we make any rash decisions here, just tell me what the problem actually is/was? Was it that the speech amp simply did not work at all? Or that it didn't work right? How so? Would it modulate to 100% and sound like poo-poo, or would not modulate to 100% and still sound like poop?

How about with the outboard drive? Driving the mod iron direct? Driving the grids of the 1625s? Did we try any/all of these??

Have we looked at any waveforms using a scope? Is one available??

This is important because the problem, as it now seems to be emerging from the story, isn't in the speech amp at all??

Why would I say this? Well, simply stated, if the rig is being driven from a variety or sources or means, and it still doesn't produce either 100% modulation and/or it still sounds wrong/bad in the same way in all cases, we can conclude my dear Doctor Watson that the problem lies elsewhere, other than the areas that you have thus far considered in this project! (unless of course, there is a common element in all of your tests so far - such as a defective microphone, etc...)

Or, to put it another way, IF this is the case, you have eliminated what it is NOT, leaving us with only what it is!! You're almost at the point of finding the cause of the problem(s)!!

 

[kf6pqt]

Allright, I'll take the bait.

I'm sure I screwed the speech amp up, as it used to work. I think the 2uf 50v cathode bypass electrolytics I replaced the old papers with were simply too puny.

The problem that sticks in my mind is this. Even when substituting a backwards audio output transformer as the interstage, (this having spent its previous life following p/p 6v6's) and using the RS public address amp, fully wound out, on the 70v tap, with my d-104 plugged into it, I'd say the modulation is probably only 20%, if that much.

(d104 to pa amp to junk speaker sounds very nice, and can lead to plenty of feedback.)

Here's what I think is going on. Recall that when I got this thing, someone had done a poor job of modifying the modulator section, using 6146's instead of 1625's.

My hunch is that the voltage divider, which provides the mod tube grid bias, (which connects to the center tap of the secondary of the interstage transformer) has been altered.

(disconnecting the center tap leads to even less modulation.)

6146's would require a different grid bias, correct? What should the voltage be at the center tap of the interstage?

I have a decent scope, but no, I have not dragged it over. I've been using a receiver, headphones, dummyload, wattmeter, and my ears only.

Should I be comparing what I would see on the scope from either side of whichever interstage transformer I'm using?

[flintstone mop]

I don't know if this is the same saga/thread of someone using 6146 tubes as modulators in a dixie a'hundred, but your biggest goal is to back track this process and put the 1625's back or the famous 6CA7's, 5881's, 6L6's. Those would be a better impedance match and better audio choice.
The backward output transformer and external amplifier with a mic matching the input of that "speech amp" shudda produced fantastic audio with plenty of modulation. Look thru your Junque box for the stock modulator tubes and recheck your screen volts and screen resistors and the bias volts for the original design and you should be good to go. Try not to re-invent the wheel using the 6146's......not goodness
The DX100 is so easy to get along with, with minimal mods to get nice smoooth audio.

Fred

[kf6pqt]

Fred, I DID put the 1625's back.


I didn't do anything to change the grid bias when I did.

I think this is the source of my trouble.

The help I think I need, should I decide to pursue this, is how to determine what the grid bias should be... then making it so.


Yes, you recall correctly, some one fuXX0r3d this thing really hard, which has been, and continues to be the source of my frustration.

I have been working on an undocumented Frankenstein mystery transmitter, that just happens to resemble a very nice looking DX-100b.

Thanks,
Jason kf6pqt

[WA1GFZ]

Measure the bias voltage at the CT of the driver transformer. It should be around -36 volts and the resting current around 50 ma. report your reading and get plenty of advice. Also check the screen voltage and report. remove the driver tube before you read the numbers to make sure nothing is driving the modulators. Do not give up

[WBear2GCR]

The other thing you want, besides the -36vdc bias on the grids (we can skip the discussion on the 6146s, since they're gone, right?) and the resting current about 50ma., is to look for symmetrical AC drive voltage on both legs of that secondary. The drive voltage needs to be on the order of the bias voltage, and then some extra, we're talking peak voltage, peak-to-peak would have be be 2x that. That voltage will permit the grids to be driven to "0" volts on the 1625s - more drive is good, as that ought to permit grid current to be drawn, which will provide more peak modulation.

Now, unless the tubes were cutoff, with extreme minus voltage, running that Ratshack amp off the 70v tap into the primary of the interstage *ought* to have produced a significant amount of swing on the secondary. That you can check without any tubes in place.

I'd be suspicious of both that interstage transformer - although you say you ran a backwards output iron to drive the grids, so that ought to be off the table - and the actual mod iron! Assuming you can get the bias at the right voltage, get the resting current ok, and then drive it with the *requisite* signal voltage on the grids - the only thing left is the mod iron. It ought to produce full modulation - how it sounds in terms of tonal quality is another matter, the first step is to get 100% (or nearly so) modulation.

So, if the drive is right and the B+ is on the 1625s, unless there is a short that the secondary is driving (possible, check the bypass cap(s) off the RF choke!) then the problem is isolated to the mod iron.

Fwiw, at this point it would make sense to post anything more you do on this in the Technical Forum...

                           _-_-bear

[W1GFH]

Jason, have you tried getting an exorcist to come over and throw holy water on the DX-100?