HRO-60 Mystery Capacitor on V16

Discussion in '"Boat Anchor" & Classic Equipment' started by KW4H, Jun 5, 2021.

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  1. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    I'm restoring an old HRO-60 (Engineering Run 345), and have run across a .015 uF capacitor between pin 5 of V16 (0B2 regulator) and ground. None of the schematics I've got call for this capacitor -- pin 5 shows as unused. According to the 0B2 data sheet, pin 5 is the same connection as pin 1 (anode) - a previous owner must have made the modification. Before I remove it as part of the restoration process -- any thoughts as to why that cap would be there? A documented modification somewhere? See photo below. Arrow points to the mystery cap.
    IMG_4898.jpeg

    Thanks, Steve, KW4H
     
  2. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Well, well -- the more I dig the more I find. Even that resistor between pins 1 and 6 of V16 is incorrect -- should be a 39k, not a 47k. Pin 6 on 0B2 is unused, so National used it as a connection point for that resistor -- so the position of the resistor is correct, but the value is not. Lots has obviously happened to this particular HRO-60 over the decades.

    So, with that in mind -- a question about the best way forward. I think what I should probably do is stay true to the schematic and, where I find it, remove any mods that appear incorrect or don't make sense. Trying to decipher why someone did what they did in this rig could be unfruitful. It appears on the surface that someone went through and, using whatever parts they had on hand, attempted to replace the caps and some resistors as well. Would that be a sensible way forward? I've worked with many boatanchors in the past, but only recently got into restoration work.

    Steve, KW4H
     
  3. WD0GOF

    WD0GOF Ham Member QRZ Page

    Pin 5 is not unused. It is internally connected to pin 1.
    upload_2021-6-6_16-38-34.png
    A decoupling capacitor on pin 5 is common.
     
  4. W1NB

    W1NB XML Subscriber QRZ Page

    As Walter points out, the application of a coupling cap is common. It is addressed in the tube data sheet with instructions to keep the value below .1uF to avoid oscillation.

    Regarding other modifications, I would personally deal with them on a case by case basis. You might try visiting the Antique Radio Forum and/or AM Fone forum. Both of those forums have participants who may be able to provide advice on modifications.
     
  5. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    I may not have explained what I'm seeing correctly. To explain better, here's a photo of a correctly wired 0B2 tube in an HRO-60. It's a little grainy because I had to zoom in a little. True to the schematic and the data sheet on a 0B2, pin 1 is connected (which internally in the tube includes pin 5), pin 7 is grounded (which internally in the tube includes pins 2 and 4), and a resistor is connected between pins 1 and 6. The resistor is R64 ( for V10A plate load). National appears to have used pin 6 as a connection point for that resistor because pin 6 isn't utilized on the 0B2 and it was a handy tie point. What's confusing me is the incorrect R64 resistor value and the undocumented capacitor from pin 5 of the 0B2 tube to ground in the radio I have.

    Just spent a couple more hours and went over the wiring for V1 and found some more undocumented differences that are puzzling and seem to be atypical for the radio. Other signs: most every pin of every tube has been re-soldered on and the interconnecting wires in many places are slightly melted/burned from overheating. The ground connection lugs show clipped factory wires and newer connections above them. The transformer was removed and another one installed. I'm beginning to wonder if this particular radio was almost totally disassembled and rebuilt for some reason -- maybe a learning experience -- I'm not sure. Something this old can have quite a history.

    This is actually an enjoyable puzzle. I'm trying to get this HRO-60 operational again and it's fun work for semi-retirement.

    73, Steve.

    OB2 - HRO60.png
     
    Last edited: Jun 7, 2021
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    I suspect it was an attempt at hash suppression.
     
    WA1GXC likes this.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    The HRO-60 schematic I have shows R26 (dropping resistor/current limit) for the 0B2 gas regulator as having a value of 5k 10W.

    Pheel
     
  8. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Pheel, that's correct for R26, which is physically located near the power transformer. I just checked and in the HRO-60 I have, however, it's a 6000 Ohm 10W wirewound, which is contrary to the schematic and the parts list. The soldering on the unit I have seems to indicate that it may have been replaced at some point. The resistor positioned directly under V16 is R64, which appears on the schematic over by V10-A.
     
  9. SM0AOM

    SM0AOM Ham Member QRZ Page

    Your HRO-60 most likely have had one of the most common problems around with this receiver; a burned-out power transformer.

    For some unexplained reason, a capacitor,C116, was connected between one HV winding and ground. When it ultimately shorted, it took the transformer with it. The lack of a fuse for the secondary windings did not make it better.

    This caused many replacements in HRO-50 and 60s, so many that when the Enköping receiving site was cleared out prior to its sale in the late-90s, a whole shelf filled with mid-50s after-market replacement transformers was discovered.

    An HRO-60 is a very nice receiver as long as its "quirks" are understood.
    One thing that may puzzle the unwary is the single/double-conversion switch. If it becomes worn or intermittent very strange things may happen.

    A mechanically worn switch that stayed in the double-conversion configuration despite pressing in the "D" coil box with all my force
    led to an hour or two of troubleshooting.

    73/
    Karl-Arne
    SM0AOM
     
    W3SLK likes this.
  10. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    I was able to find a reasonable high-resolution photo of a well-kept HRO-60 and will be using it to help reconstruct the one I have where needed. In the below comparison photo, the chassis on the left is mine, the chassis on the right is a factory original (mostly, anyway). The markings on the capacitors are mine -- as I'm going through and identifying what is what. If you play "Where's Waldo" with the photos you'll see many components that aren't in the expected mounting position -- this radio has been plowed through (lots of sloppy soldering as well).

    If anyone has a HRO-60 in working condition and wouldn't mind taking some high resolution photos of the chassis components and wiring underneath, I would be grateful! I can provide a secure link so that the full resolution photos can be transferred without degradation. At this point, some more photos like the one on the right (although a close-up at each tube would be helpful) would be a huge help. I'm determined to bring this HRO-60 back from the brink.

    Steve, KW4H

    HRO60-Comparison.jpg
     

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