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Discussion in 'Amplifiers' started by youngest man, Jun 14, 2006.
www.cbtricks.com Hope this helps,Mike.
Looks like what's posted at CB Tricks is only the later 10-tube versions. They are a bit easier to service, since they used a 12-VOlt relay and transistor keying circuit.
The relay between the coax sockets on the rear panel, and the high-voltage relay alongside the four driver tube sockets are in all versions. The third relay is the one directly under the tubes, between the drivers and finals. ALL your RF power goes through this one, and the tuneup is a bit bizarre. Two knobs on the front function ONLY on low side. This is because that third relay bypasses the eight final tubes on Low side. The far-left lower knob peaks the driver tubes ONLY on high. The main Load control , the upper large left-most knob, is active on low side, too. You have to tune it up on High, to get that knob where it belongs, and then switch to Low side. Then, those two knobs on the bottom will work.
The High/Low relay always fails. Always. Since High side goes through the Normally-Closed contact points, those tend to fail first. Never hurts to clean them off before replacing that relay, but wear and tear will have pitted the contact surfaces so much that cleaning almost never helps.
And if either the HV or Antenna relay turns out to be only a 35-year item, they'll have to go, as well.
The tube-type keying circuit tends to require a lot of carrier drive from the radio to make it key. The SSB delay almost never works right. Some units were made to use a 12AQ5 keying tube. Yep, has a 12.6-Volt heater. Others were built to take the more-common 6AQ5 tube. The heater is on pins 3 and 4. One of those pins will be grounded, usually pin 3. The other one of those two has a wire on it, that is either solid green or green with a yellow stripe. As a rule, the yellow stripe indicates the 6.3-Volt 6AQ5, and the solid green wire for a 12AQ5. Not a big deal to change it from one to the other, if you don't have the version of the keying tube it's built to take. And if you plug a 6.3-Volt tube into a socket wired for 12.6 Volts, it will key as soon as it warms up, and destroy the keying tube. But usually not before the rest of the tubes cherry up and fail. Always wise to keep a close eye on one of these while it warms up.
I have a 350k Jpeg of that diagram, but I need to find someplace to host it.
If the filter capacitors are original, they are a time bomb. Or, maybe like a grenade with the pin pulled? We always add a bleeder resistor across EACH of the replacement filters. This version has only two sets of three in series, six total. The factory used a single 1-Meg 2 Watt resistor as a bleeder, but it's not effective. A separate bleeder across each filter also serves to divide the 950 Volts evenly across all three capacitors in series. Without adding them, you can't count on that. Just because the factory got away with leaving them off, doesn't mean that you can. The value isn't critical. We use 240k 2 Watt, but up to 470k is alright, just so long as they are ALL the same resistance value. Makes it marginally less suicidal to reach you hand into later, besides.
Or, you could choose a cheaper, simpler restoration project, like a 1970 Plymouth Road Runner.
Well, maybe not cheaper, but there's a resemblance. And if you have to buy tubes, that makes the car look a little cheaper in comparison.
Uh, okay. I uploaded this to imageshack. Rather than make a dial-up user wait for this thing to display, try this hyperlink and see if you can't access the file that way.
Right-click on the link above and "Save As" is probably better than just left-clicking on it. Tried it with "preview", and it worked for me.
Walk away from it for a day or two.
Look at the schematic that Chris posted and on day three, your first goal is to make the thing safe to work on (make sure it is unplugged and discharged).
You can do this by disconnecting the red wires coming out of the transformer and going to the diodes in the high voltage full-wave bridge section; put RED tape on the ends of the individual wires.
The six volt tubes are strung in series 2 at a time across the 12 volt line; if one tube loses filament voltage the other tube in series with it will not light either. With the amp completely safe (no B+ voltage and the high voltage AC input isolated) take yout VOM and check pins 4 and 5 of all the final tubes and see if you are losing the 12 volt source to part of your tube bank.
You might have bad tube sockets or tubes with open filaments.
After you have found the source of your filament voltage loss and made repairs, reconnect the high voltage input wires to the diodes and you should be good to go.
I have made the assumption that you are technically sharp enough to trouble-shoot this problem however, if you don't feel confortable inside with 900 volts running around, find someone who is.
D & A 12 Tube amp
Dulaney used to wire his sockets in series parallel.
I've run across MANY D & A amps that had banks of tubes that didn't light up. Problem? Bad sockets!
You can try to "fix" the 9 pin sockets, but if you have some spares around, it may be better to try that.
Test pins 1 and 9. If memory serves me well enough, they should be wired series with 2 sockets, then that pair wired in parallel to the rest.
If you REALLY need a schematic, I might have one, but it's packed, and I'm moving this week. Message me in about 2 weeks if your still having problems with it, and I <<should>> finally (3 years after they where packed) have access to the books and such.
Good amp.. Monster... Monster on 11 meters, tv's, anything with transistors. Drive it with a modulator type amp and they get into toasters and bedsprings very well, too
Every D&A base amplifier was built with transformers that had a 12.6-Volt heater winding. Some versions grounded one pin on each socket, feeding two sockets from one transformer wire, the other two from the other wire, with no connection to the center tap. Later transformers didn't have a CT on the heater winding. Other years they would ground one pin only of each series-connected pair, with a jumper wire between each pair.
One transformer for each four tubes was the formula used in all the base amplifiers.
A bum socket tends to make only two at a time go dark. If this one is particulary dirty, that's not helpful. A dirty surface on the socket's contact spring makes it run hot, become oxidized and lose its spring temper. Once they get loose, there's no point to squeezing them back to restore contact with the tubes' pins. Scum on the tube pins should be removed, if you see any. "Terminal Crud" syndrome may be what's wrong with your specimen.
Not sure what year they stopped making the 12-tube version, somewhere around 1972 or 1973, I think. Ten tubes is more manageable, on the whole. The pi-network coil on a 8-tube final section can be tricky to tune. Taking two tubes out of the final stage removes a lot of capacitance from the output circuit. Makes the output coil's adjustment far more forgiving when tubes are changed.
Like Toll says, you can 'borrow' an unused pin contact (like pin 9) from a socket to fix one that snaps off, or gets loose. Takes a steady hand, and luck. Nobody sells "just" those spring contacts.
Good to see you back, Toll. Glad to discover that rumors of your premature demise were, well, premature.
Your right on the pin configuration. In the middle of a move, new baby and custody battle... Things tend to get a bit muddled up lol.
All I can say if they stopped in 72, then there isn't a 12 tuber around as old as I.... I REALLY feel old now.... HA
And yeah, he used AC for the fils, grounding one side of the XFORMER and grounding one side of the socket in the dual socket string.
I've worked on TONS of Maverick 250s, and the reason I had brought up the sockets was I had one that the rear-most bank of tubes didn't light, and the front-most bank didn't light, but the two banks in the middle did! Was a bad socket on each string.
Dropping two tubes from the output network dropped the total capacitance more than 15 percent. Lots better Q from that network now that, as you said, it's more managable. Just tube capacitance alone dropped it 15 percent!
Anywho... Time to get ready to move. Take care all, see ya in a couple days... And what where the stories of my early demise? I know someone filed me for dead in Texas (legally, by God, Im actually dead there), but I didn't know I died in DX, too