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QRO HF-2000 schematics wanted

Discussion in 'Amateur Radio Amplifiers' started by K2CB, Oct 15, 2016.

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  1. K2CB

    K2CB Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Need schematics for the QRO HF-2000. The LV / Bias board in particular. The one I have here is an early productions unit, and the schematics on the QRO website do not match the revision boards in this amp. Anyone have the earlier revision versions?

    The current version on the website shows the key line as 12vdc. I am measuring 160vdc at the amp relay key jack on this earlier version. Everything looks good on the board, and I suspect it may be ok, but I'd like to verify it..

  2. W1QJ

    W1QJ Ham Member QRZ Page

    Eric, Unfortunately I do not believe the updates to the board were documented. I just went through this with a QRO 2500. After much hassle we got hold of Ray and he verified that he never updated the changes to the low voltage boards. This is typical as well for Command Technology amps. Ray appears to be retired and doesn't support his equipment anymore. He says he has no replacement parts either. I actually have a QRO 2000 here right now that I fixed up. To my knowledge there can't be any voltage near 160vdc in that amp except for the HV which is about 3100 volts. You say you are getting 160vdc at the key jack in the rear of the amp? I'll check the schematic that is on line for you. Lou
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Eric, pardon me, yes, there is a 160vdc supply labeled "key line". When the circuit is activated the voltage drop across those big 25 watt resistors gives the lower voltages for the relays. So it is quite possible you will read a static voltage of 160vdc but when the circuit is activated the drops across the relays is proper. A similar thing is done in the 2500 as a safety measure should the bias voltage disappear. Probably not needed in this amp but done that way anyhow. I would ground the jack and then take a measurement. Lou
  4. K2CB

    K2CB Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi Lou, thanks for the reply. My only concern is the open static voltage of 160vdc affecting the radio used to key the amp. Most newer radios like Icom only like to see 12v or so so. I'm curious how that open circuit voltage is going to come into play. Maybe I should just build a buffer circuit with an opto IC and MOSFET transistor while I have it open.

    I'll probably look at the amp some more later this afternoon.

    When I first got it, it had a bad tube, which took out the TIP33 bias control transistor and a 1.5KE100CA TVS diode in the metering circuit. The version control board I have here has a TVS bidirectional diode parallel to a 47K resistor to ground. Actually tack soldered on top of the resistor, so must've been an afterthought. Amp was showing negative grid current and high idle plate current until I found this not-so-obvious TVS diode. It also had a bad NPN transistor in the power output metering circuit on the ALC board.

    As you mentioned in the past, Lou, it really is frustrating the way this thing is constructed. At first glance it looks well-built, but after trying to disassemble various subassemblies, it is a real nightmare. The vertical stack of the three boards up front behind the two meters is especially frustrating, as the cable harnesses were made so short that you cant easily move the boards around to repair them. I had to work some pretty crazy angles to get in there with the desoldering tool to replace the bias control transistor. Not to mention a few new tricks to get probes down into the right positions for test measurements!

    Very time-consuming amplifiers to work on, even for the simplest problems!

  5. W1QJ

    W1QJ Ham Member QRZ Page

    You are right Eric, the 160vdc is a carry over from the 2500 amp. With the 2500 there are several relays that have to close on transmit so they are all in series should one fail they all do. That circuit isn't nec
  6. W1QJ

    W1QJ Ham Member QRZ Page

    Necessary because the triode doesn't work the same way. The 2000 I just worked on, I had to chaout all the filter caps and bleeders and replace three shorted diodes in the rectifier. I had to work at those weird angles you speak of. I know! They ate great amps when they are working. I'm still working on the 2500 amp. It's been one problem after another with that one. The zener diodes for the idle current bias are shorted so when keyed the idle current is 800 ma. Should only be 400-500. So those diodes are on order. I may have screen voltage zenerss shorted too. But in all honesty when I got the 2000 all fixed up and bench tested it, I find it to be the best amp for making extraordinary power from a pair of 3-500 tubes. I thought the AL- 82 was impressive but the 2000 is more so. I wouldn't actually mind owning one and would buy one if I could find one. I don't think they made that many. If you look at the Harbach soft key for the SB-220 it uses a big MOSFET for switching the 120 vdc in that amp. You know you have an alternative method of keying that amp? You can provide 12 volts on transmit using the other key jack provided. If your radio offers that, you can use that method. Lou
  7. K2CB

    K2CB Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Just a quick follow-up, Lou. Per the schematic of the newer version relay control board on the QRO website, I added the MPSA42 transistor and associated components, and updated this particular HF-2000 amplifier to the newer, lower voltage circuit design. It now measures 12v open circuit, and 82ma when pulled to ground.

  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Jim W7RY (link) uses a TIP147 Darlington to regulate & switch the bias. He uses a MPSA42 bipolar to drive vacuum & reed relays. Unlike the Harbach setup, hot switching, dry switching, and noise are eliminated. He has similar boards for other popular amplifiers.
  9. KI7AY

    KI7AY Premium Subscriber QRZ Page

    Are you still looking for a QRO hf2000? I have one that I would be willing to sell, but it does have a problem that I haven't run down. Tubes are new, and were tested recently by RF Parts. I replaced all the HV caps and bleeder resistors, plus put in a new cooling fan. When I try to bring it up on a variac, once the step start kicks in, it starts popping, and it sounds like it's coming from the board stack behind the meters. I SUSPECT that it's the HV Diode stack, but I can't tell for sure.
    73 Jim KI7AY
  10. W1QJ

    W1QJ Ham Member QRZ Page

    Thanks for the offer Jim but I am not looking to buy any QRO amps. After the last debacle I had with the 3 hole 2500 I don't want to own any. I'll fix them but not own them. You should easily be able to check the PS didoes with a n ohm meter on diode check. Good Luck

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