Troubleshooting Hallicrafters HA-5 VFO

Discussion in '"Boat Anchor" & Classic Equipment' started by K1APJ, Jun 23, 2019.

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  1. K1APJ

    K1APJ XML Subscriber QRZ Page

    I picked up a cosmetically very nice HA-5 "in perfect working condition."

    Ahem.....

    Anyway, after replacing the completely open filter caps and a defective VR tube, and going through the usual cleaning and connection checks and so on, the thing sorta works, but the output from the 5 MHz variable oscillator wanders around a few dozen hertz. It also has more drift than I'd like, certainly OK for AM but not too good for CW.

    I powered this with external regulated power supplies, including regulating the heater supply, but the wandering continues. Rather than shot-gunning a bunch of parts replacements I figured there was undoubtedly someone who knows this product intimately and could give some guidance.

    I also have two different schematics for this, which one is the preferred version? One has the cathode of V1A directly grounded, the other has a 120 ohm cathode resistor, but there are other differences as well.

    Thanks in advance.
     
  2. K5UJ

    K5UJ Ham Member QRZ Page

    Wandering & poor stability sounds like it might be some sort of physical/mechanical problem: Dirty contacts, something not bolted down tight, dirty tube sockets, ... as always, do as much easy stuff as possible first. That's rule number one. Have you swapped out all the tubes?

    Sometimes, no guarantees, but sometimes, odd mysterious problems go away after you drill out tube socket rivets, one per socket, and replace with nut, star washer and bolt, and tighten them down good. Over time, the rivet can build up a resistance. If the circuit uses a socket lug as a grounding post and the path to the chassis is through the rivet, that can cause strange things to happen because of Vd across the rivet and differences in reference ground. The resistance doesn't need to be much. I've seen them at around 1 ohm. After the drill out operation, R drops down to around 0.4 ohm. Doesn't seem like much difference but the circuit starts working right. This has become standard practice for me when encountering riveted mounts in vintage gear, along with shotgunning out old paper and electrolytic caps and so on.
     
    KI4AX likes this.
  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    Be sure to clean the springy brass "wiper contacts" on the Variable capacitor shaft. They also can lose their springiness....there are some fixes for that, but they can be tedious.
     
    KP4SX likes this.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Rivet mounted ground lugs are definitely a source of all sorts of problems. Even with machine screw mounting, over the years they work loose and / or become corroded. Slightly loosening the machine screw / nut, and then re-tightening it, solves a LOT of screwy problems.

    I "fix" over 90% of the "screwy" problems in the boat anchor equipment that I repair for others just by doing this.

    One other possible problem is with the "duramica" capacitors. At least from the manual posted on BAMA, there are 6-each of this type of capacitor: C-4, C-5, C-6, C-21, C-30, and C-31.

    Also, make sure that the rotor of C-3 (dual section capacitor) is making good contact to the frame of the capacitor. You may need to flush out the bearings to get rid of "gunk", etc., that has built up over the years. If this doesn't work, solder a piece of narrow braid to the shaft and the other end to the frame. Make sure that there is enough "slack" so that the variable capacitor can go through the full 180-degrees of rotation. This problem is VERY common with the LMO (VFO) in the Heath SB-Line equipment and the frequency jumps around, warbles, and so forth.

    Glen, K9STH
     
  5. WD0GOF

    WD0GOF Ham Member QRZ Page

    Once you clean everything up and solve all the mechanical problems and corrosion there are several major offenders; C1, C2, C7 and C32. C1 is a 15uuf N750, it almost always needs to be replaced. If the drift in frequency is accompanied by an amplitude change check C32 and C7.
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    By the way, replace the 2-wire power cord with a 3-wire cord and add a fuse. The black wire needs to go to the fuse and then to the switch. The white wire connects directly to the power transformer primary and the green wire needs to be connected directly to the chassis.

    Glen, K9STH
     
  7. K1APJ

    K1APJ XML Subscriber QRZ Page

    Thanks to all for your replies. Here is what I found-

    I don't know if they were built this way originally or if mine has been modified, but all the "current-carrying rivets" have been soldered, the mounting ears on the tube sockets and terminal strips are soldered to the chassis. Looks to be original but I have only seen one HA-5 so I am not sure. I did however find that the three screws mounting the variable capacitor took about a 1/4 turn each, tightening those cleared up most of the wandering frequency issues. Once that was solved the unit had a pretty good "whoop." I found an old "Hints and Kinks" article that talks about that and said it was due to the heater voltage dropping when the VFO is keyed. I measured it and the heater voltage did in fact drop a few percent but I didn't think that small variation should cause that big of a problem. I picked through a few different 6U8 tubes and they were wildly different, I found one that is just about perfect so I put that one in. Other than the 6U8 the only other defective parts I found were the 0A2 and the filter capacitors. The VFO now drifts a couple kHz immediately after turn on but is pretty good 10 minutes later. Changed the power cord and I think we are good to go!
     
    N2EY and AD5HR like this.
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    Heater voltage variation can have significant effects.

    You will probably find that a 6U8A is significantly better.

    If you want to get fancy, add a 3 terminal regulator. 6 volt regulators exist, or, use a 5 volt regulator and a 5U8A.

    73 es GL de Jim, N2EY
     
    Last edited: Jul 7, 2019

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