Drake L-75 Help

[W3SLK]

Having asked on the Drake list with little or no interest, I come with hat-in-hand where I get some of the best answers. Anyway, I'm restoring a Drake L-75 amp that suffered from an HV glitch. I was fortunate to get a new filter board since the one in the unit had evaporated the run. However, I am left with a dilemma. On the lower left-hand side of one of the filter boards, (the one with the bias and meter adjustments), there is a pin that a wire goes to. I cannot, for the love of money, find which wire goes there. This would be near the bottom left opposite of the adj. pots. Also, there is an eyelet hole which I suspect the other half of the (2) PC boards is connected. I just want to make sure before I dump the coals to it. Any help is greatly appreciated.

[W3SLK]

Okay I'm resurrecting this topic because I'm in definative need of technical advice. I got said amp up and running but found I had no power out on 160m, about 100W on 75, and maybe about 500w on 20m with a sharp dip. I found that C25 across the plate choke, (its a split choke for whatever reason) was shorted. It was a 50 pfd mica with no voltage rating. I went to the my parts jobber and obtained a like capacitor that was rated at 1kv, (my thought was that the voltage potential across the coil was minimal). Well I got the necessary power out on 75, (about 100w on 160m) until the new cap went up in smoke, (just cooked). So back to my parts jobber were I obtained a 56 pfd 6 KV ceramic disc. I was in the middle of tuning it on 75 when the fireworks came. I suffered an arc fault at the ceramic disc which took out the glitch resistor and 10A fuse. Can someone offer me some help here? I know the disc isn't as stable as the mica but across a plate choke I thought it wouldn't matter that much. I could be wrong though.
Here's the schematic:

[WQ9E]

Take a close look at your RF choke to see if it was damaged during one of the earlier glitches.  It sounds like your choke itself may have cooked some turns together or otherwise you have a serious parasitic in the final.  If you have a grid dip meter, check the choke for resonance within the bands of interest.

Also check L-4 for damage.

That capacitor does need to have a high voltage rating as it can see significant RF voltage.  Your ceramic cap may be a problem depending upon construction since it could have a bit of inductance but I suspect that there is another issue taking it out.

I have the L-7 but it uses the same style choke with a capacitor across part of it.  Mine has been glitch free.

[WQ9E]

This should shed further light on your capacitor issue:  http://www.wb4hfn.com/DRAKE/DrakeArticles/L7-L75_C41_Replacementl.htm

[KM1H]

Replace the choke with one from one of the big Ameritrons, they are all the same; buy direct from them. That Drake choke was a feeble attempt at designing one to cover all bands.

Add a .01 at 4kv or more to the base of it. Add a glitch resistor.

Carl
KM1H

[WQ9E]

Carl,

Good call on the glitch resistor.  Drake uses a .82 ohm resistor as a HV fuse but an additional glitch resistor is a smart addition.

I use the Ameritron choke in my homebrew triple 4CX800 amp and checking it with a dip meter it has a resonance near 12 Mhz. which is no problem.  MFJ is a good source of a number of RF parts and transformers since you can order (at a quite fair price) any of the components used in their amps and tuners.

Rodger WQ9E

[KM1H]

Yep, the only problem with Ameritron is trying to get thru on the phone. They have one live person answering and then transferring calls. Guess they havent got multi line technology in MS yet   It took me 3 days of intermittent trying last week.

Ameritron, Drake, Dentron, Heath and others use a way too small choke bypass especially for 160 and sometimes 80. It lets RF get into the filter caps which get rather upset and react accordingly.

Carl
KM1H

[WQ9E]

Yep, the only problem with Ameritron is trying to get thru on the phone. They have one live person answering and then transferring calls. Guess they havent got multi line technology in MS yet   It took me 3 days of intermittent trying last week.

Carl
KM1H

Hey Carl,

Be gentle with my home state    Keep in mind that many of our top political leaders are the first generation in their family to walk upright and it holds the rest of the state back.

When I accepted my position at Illinois State University I found the faculty still had party line phones (in 1989).  I asked a lot of questions during my trips to various universities but I never thought to ask whether they had this new individual user modern phone technology.  They did upgrade a couple of years later but that was an odd experience while it lasted.

Rodger WQ9E

[W3SLK]

Thanks guys, I really appreciate the assistance. I used to belong to the AMPs reflectors but my email box became too full of back-n-forth crap over crap. I was thinking about the choke in and of itself but I never heard (or read) anything contrary about it. I'll check it with the dip meter.

I thought that the 0.82 ohm resistor WAS the glitch. Hmmmm. I'll look further into this but I think for the time being, based on the comments, the choke is going to be choked. I'll post later to hopefully bring a conclusion to this saga.
 

[Jim, W5JO]

With Ameritron or any of the MFJ companies email from the website seems to work best for me.  Getting through on the phone is hard to do.  They ususally answer their email queries and send order confirmations within a day.

[n2bc]

RFPARTS sells that plate choke.

http://www.rfparts.com/choke.html#hightrans

$25

[W3SLK]

Bill said:

RFPARTS sells that plate choke.

I just purchased it via email from Ameritron for $5 less. 

[KM1H]

Yep, they buy it as well as other parts from Ameritron and mark up to what the traffic will bear.  At least the choke is a decent one for all 160-10M bands but it still needs a .01 uF at the tail end to keep the lowband RF out of the electrolytics. Ive had no problem with it at close to 4500W out in a LK-800TNY with 3CPX800's that I modified for a customer.

Roger, I can feel your pain   It must be hard on the spine when the knuckles cant be used for stability

Carl
KM1H

[W3SLK]

Carl said:

Yep, they buy it as well as other parts from Ameritron and mark up to what the traffic will bear.  At least the choke is a decent one for all 160-10M bands but it still needs a .01 uF at the tail end to keep the lowband RF out of the electrolytics. Ive had no problem with it at close to 4500W out in a LK-800TNY with 3CPX800's that I modified for a customer.

Now is that in addition to the 2 750 pfd's (1500 pfd total), between it and RFC#3

[W3SLK]

Just as an update, my plate choke from Ameritron was $20+ $13.17 shipping, for a total of $33.17. I don't think that is too awfully bad for an off the shelf part.

[KM1H]

Now is that in addition to the 2 750 pfd's (1500 pfd total), between it and RFC#3

.

Yes, its so-so for 80 but not 160. If you cant find a .01, Mouser has .0047 6KV discs pretty cheap.

Carl
KM1H

[Gito]

hi

I see that the 50pf capacitor is in parallel with a part of RFC 3,so at a certain frequency it can became a parallel tuned circuit at that frequency,and depended on the Q factor of this parallel circuit ,it can developed a high voltage at the ends of this 50 pf capacitor.
I agree that there may be a high parasitic going on Your transmitter, in an article I read,it says that in modern tubes,it has much higher gain and more  sensitive than the older one even if the name is the same.(3-500Z).
So changing the tube can cause Parasitic oscillation ,that did not happen before.
Try to change Your Parasitic coil with nichrome wire or stainless wire.
making the Q of this output  circuit lower (plate--Parasitic choke---RFC)
A ceramic disk has a limited Frequency rating ,if we used with HF/UHF frequency (parasitic frequency),maybe it cannot stand .

Gito

[W3SLK]

Well, I got the new plate choke on Tue. and attempted to install it tonight. Appeantly, it is about 1/2 to 3/4 of an inch taller than the original. So now I have to shave some of the material off. I'm assuming the form is made from steatite? I have access to a belt sander at work. Anyone have any "Do's & Don't's" when trimming this down?

Happy New Year!!!!!