Drake TR-4CW no receive then plate current oscillates then rises and blows fuse #TR4-c
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I recently acquired a Drake TR-4CW from Ebay. Not knowing the condition of the tubes I also bought a complete set of tubes that are for the TR-4C. Only thing I didn't like is that the 6JB6's are Zenith and not Sylvania because inside the rig it tells you to only use Sylvania otherwise you must do some extra work. I read somewhere that you have to neutralize the output tubes and a fast way of doing it is to tune up the rig on 10 meters, shut it off, disconnect the screen voltage then connect a T connector to the output and the straight thru port to a dummy load. Then you connect the T part to an oscilloscope and adjust the neutralization cap for minimal signal. I did that and it worked out perfectly!
I was listening on 40 meters for about 15 minutes and everything sounded well! I did not transmit because I just wanted to listen to that nice Drake sound for a little bit.
I switch it off and turn it back on 2 days later. I'm waiting for the audio to come up but I hear NOTHING. I then see the plate current on the meter start swinging back and forth then the needle stops then starts to move up scale. I managed to shut it off about midscale. I turn it back on and notice that it blew the 5 amp fuse in the MS-4 power supply.
I'm thinking the finals are oscillating? But why are they oscillating if I just neutralized them, also I was NOT transmiiting. I'm not in X-CW and my microphone is not even attached.
I'm also thinking maybe one of the NOS tubes that I just put in has a short? Not certain what a shorted tube would do but I am just trying to figure out this problem.
I then started thinking, "Why am I not getting any sound?" So....I removed all of the NOS tubes that I just put in AND pulled out the 3 final tubes and taped up the resistor plate wires (or whatever they are called) and put back in the original tubes (that worked without any issues) and see if I can at least get sound. Turn it on, no sound. Thinking, do the finals have to be put back in to get the receive to work (I know most likely not but let's just try it) So I put the Sylvania's back in instead of the Zenith's and turn it on. I turn it on and the plate current meter starts swinging back and forth! I turn it off and here I am writing to our group hoping that someone can help me figure this out because I don't even have a clue on what to do next?
Mark (N2DMI) |
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Hey thanks for the reply! The rig already came with a new Hayseed multi capacitor in it, so someone already did it. I was just looking at my rig to see if I can see any damage and found that the 27 ohm resistor that goes to the K1 relay? is cracked in half. So that is telling me something. It's the (R192) 27 ohm resistor (If you know the wattage I would like to know it) that goes to the coil winding on the K1 and that comes (I believe?) comes from my +250 supply line. Wonder what this is telling me? I don't want to attempt to replace the resistor because it might blow again? Again if someone knows the wattage, it would be appreciated.
Mark (N2DMI) |
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Oh also on the power supply, I bought that from Ebay and it was a rebuild. Seller said "Unit was brought up slow on a Variac for about 3 hours., then plugged into the wall for 2 hours and let run. The power supply has updated power boards with new caps, resistors and diodes. Speaker works good also no rattles. So, I kind of trust the power supply, but who knows?
Mark (N2DMI) |
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Maybe I'm reading this schematic wrong? The cracked 27 ohm resistor is on the relay board. From looking at where it is going, it looks to me like it goes to the solenoid of K1 relay and on the schematic I do see a 27 ohm resistor 1/2 watt (R192). It looks to me like from the back Jones connector (I think it's a Jones, I'm still trying to get my terminology right with Drake) (It's the 12 pin connector) any how you got the (R192) 27 ohm resistor to an arrow pointing UP that says +250. So I'm guessing it goes TO the rig. And I'm guessing 250V is the screen voltage for my finals? On the other side of that same resistor it goes to an RF choke, a cap in parallel and then to pin 11 of the Jones connector. So what would make the resistor crack? Too much current going through the resistor? If so, why?
Mark (N2DMI) |
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Good idea. Before checking, at least remove the finals or disconnect the screen supply.? Steve Wedge, W1ES Time flies like an arrow. Fruit flies like a banana. Sent from for iOS On Wed, Feb 26, 2025 at 10:33, ken KW1NG via groups.io <kwing54@...> wrote:
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Hi Barry, I replaced all the tubes because the previous owner said that it is only putting out about 210 watts when it should be putting out 260 watts according to the manual.
The only 3 things that I see so far is the cracked resistor, blown fuse and the power plug needed a good cleaning. Possible that the power plug wasn't making good enough contact? Possible but who knows? Everything in the rig looks to be alright. I pull the final tubes out and taped the 3 plate holders and replaced the fuse. Awaiting on the resistor but I know from experience that I.E. I change out a fuse that the fuse will only blow again until you find what caused the fuse to blow. I just got this rig about 2 or 3 weeks ago and now this. |
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Hi Steve, my AC-4 power supply doesn't have an on/off switch on it. Should it? I pulled the 3 final tubes completely out of the rig and put tape around the plate connectors. So you're saying to also remove the +250V screen wire off of the terminal (on the bottom of the unit) and tape that up too?
Mark |
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Hello Mark,
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First, I commend you for not working on the radio when you¡¯re not feeling good; this is a wise choice because you can end up making mistakes that could be much more than just irritating. ?Plus, it takes all the fun out of working on these vintage radios. ?Yes, wait till you feel good, and enjoy the work. ?I understand you have business matters to care for having bought from eBay; I expect you¡¯ll get all problems sorted out in short order, and report to the seller before 30 days expires. ?Also, ask God to heal you, believe and be patient; He will do it.
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Second, Zenith tubes were made by Sylvania, as were other brands, so no worries there.
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With all that you have reported, I expect the negative bias power supply to be the problem, as Ken mentioned; in your upgraded AC-4, the electrolytic capacitors¡¯ polarities of C5a and C5b may have been installed backwards, that is, the positive terminal of the 20 uF caps, either one or both, was not connected to ground. ?Possible the negative bias is simply adjusted way too low, (low magnitude). ?As Steve mentioned, the 1/2 watt 27 ohm resistor is useful for a fuse to protect the finals, while it primarily serves as an RC filter on the +250 VDC line entering the TR-4Cw. ?Explaining, if you have no or insufficient negative bias at the grid, the tubes will conduct quite hard for an instant, causing R192 to burn and open, and causing the AC-4 protection fuse to blow; that fuse protects the transformer. ?The electron cloud in each tube finds the positive 250 volts first at the screen grid along the path, first. ?The designed protections did their job. ?You had no sound because there was no plate voltage supplied to the receiver tubes. ?I suggest you use the divide and conquer process and first separate the problem between the AC-4 and the TR-4Cw (congratulations on finding this), starting with checking the power supply. ?Pull the PS plug at the TR-4Cw. ?For convenience and safety, I have a socket for the AC-4 plug that I use to short pins 1 and 2 with a jumper, an ON/OFF switch for the AC-4, and is convenient for measuring voltages (see photo). ?You may want to find one of those later, but for now not having one, open up the power supply bottom cover. ?Now orient yourself to a component node for each output voltage on the schematic, before turning on the AC-4; you may need to do a little tracing around to make sure your looking at the correct node.
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I¡¯ve seen an AC-4 upgraded with the Heathkit version, but no other manufacturers. ?You may need to take the top cover off to determine a convenient node to check the output potentials, and better yet, a schematic may have been supplied showing were the components are on the new PCB(s). ?I expect in all cases the terminal strips were reused to connect the output cable¡¯s conductors. ?Before turning on power, check C5a and C5b for correct polarity installation, positive terminal to ground; if they are wrong, stop and correct. ?Before starting this work, make sure to discharge all capacitors by shorting each capacitor + and - terminals with a insulated (capable of at least 750 volts isolation) wire in series with a ~10 kohm resistor in series for no arcs and sparks (each discharge, count to ten). ?Now with your DCVM handy, measure across each capacitor terminal pair, best starting with the DC volts highest scale to ensure each one is close to zero. ?After all corrections are made, measure each PS output potential. ?Unloaded, they will be a little high with this unregulated PS; no worries. ?Take a moment to see if the negative bias supply output varies smoothly when adjusting the potentiometer; this is important. ?If the value jumps around, clean the potentiometer with some Fader 5 solvent (spray can); there needs to be really good voltage resolution for your idle current setting using the bias control. ?You can see the rotation orientation for the most negative value, around -90 VDC. ?Leave it there as a starting point for adjusting the idle bias current setting on the TR-4Cw.
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If you find everything correct with the AC-4, leave it disconnected, discharge the electrolytic capacitors in the TR-4Cw, get your handy ohmmeter and the schematic, and start ground-short isolation of the + 250 VDC distribution at R192: there are lots of loads, but begin just beyond R192 on the schematic, pulling V20, an 0A2 regulator. ?I¡¯ve had these regulators fail, and they can draw lots of current. ?When you find the ground fault, repair it, and replace R192, 27 ohm 1/2 watt (a carbon composition or carbon film). ?Check the resistors¡¯ values supporting the ground fault component. ?I would check C145, each capacitor element for correct capacitance and charging to open circuit with an ohmmeter. ?You might as well do this now for peace of mind while you have the radio covers off. ?This will require isolating each capacitor¡¯s positive terminal by unsoldering the wires on those terminals, then making your measurements with a capacitance meter and ohmmeter. ?Again, make sure there is no potential from each positive to ground terminals before making any measurements
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As a side note, I¡¯m expecting electrolytic capacitors were used for for AC-4 C5a,b in your upgraded unit, but if these were installed wrong, I would would isolate them and check for correct value and no observable leakage current using an ohmmeter (charge to high impedance) before reuse. ?If they need replacing, a better choice would be to use polypropylene dielectric metalized film capacitors with long-life high-reliability; they heal themselves. ?The DC working voltage should be 2-3 times the actual DC voltage, here a 200 or 300 VDC capacitor. ?As an added bonus, they are not polarized ¡ª no installation worries. ?I found a 200 VDC polypropylene 20 uF at Mouser available for a reasonable price. ?See attachments. ?The same goes for replacements of C145, if needed, using a terminal strip and round fender washers and center screw as a mount and to cover the hole. ?Enjoy your nice SSB, CW transceiver.
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73,
Michael Smith, N4KZO
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Thanks for those kind words Michael. The person I purchased the power supply wrote this about it, "Unit was brought up very slow on a variac for about 3 hours, and then plugged into the wall plug for 2 hours and let run. The unit has updated power boards with new caps resistors and diodes."? I also let it run on receive for about an hour and made 1 very reliable QSO with it. So I'm just a little surprised about the caps. I got to go to bed now, but I will definitely do what you told me to do (when I find a little time (Hi Hi). 73's fer now and I will keep you posted. If anybody else can also think of some ideas please keep them coming because in the beginning I was ready to toss this thing in the trash. Kind of glad I didn't now!
Mark |
Mark N2DMI
