Working my way back in time - Drake TR-4

Discussion in '"Boat Anchor" & Classic Equipment' started by K0OKS, Feb 16, 2018.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: K5AB-Elect-1
ad: Left-2
ad: MessiPaoloni-1
ad: Left-3
ad: Subscribe
  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    FWIW, I made the following measurements tonight on my Drake TR4-CW:

    All made in SSB mode either with rig in Rx mode or mic keyed but mic gain at minimum to check bias voltage and idle current.

    Final tubes numbered from back of rig towards front of rig 1 through 3.
    Tx measurements made after adjusting idle current per manual while viewing front panel meter for 100 mA.

    Bias voltage measured from pin 2 of center tube (anywhere along flat copper strip connecting grids) to chassis ground
    Rx: -47 volts
    Tx: -46.5 volts

    Cathode current measured across 15 ohm cathode resistors with mic keyed (resistors checked individually with zero'd low resistance ohm meter and all new as of last month):
    Tube1: 398 mV ~26.5mA
    Tube2: 580 mV ~38.7mA
    Tube3: 508 mV ~33.9mA

    Total idle current ~99.1mA

    So it looks like Tube 2 (middle tube) is carrying the lion's share of the current but together the three carrying close to the desired 100mA of idle current.

    Screen voltage measured on each tube pin 1 to ground:
    Tube1 Rx: 246.6V
    Tube1 Tx: 236.2V

    Tube2 Rx: 246V
    Tube2 Tx: 237.8V

    Tube3 Rx: 246.5V
    Tube4 Tx: 237.9V

    Hope that yields some clues.
  2. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Your bias voltage is interesting. Recall that my power supply can only put out a maximum (least negative/most positive value) of -48V (no load). So perhaps I really do have a power supply issue...

    Would you mind checking your no load bias voltage (as set) out of your power supply?
  3. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Actually nevermind. I measured mine at the tube and it was -35V and I still had the issues with low current. I adjusted it to the -47V like yours and it was of course even lower output.
    K7TRF likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    My no load power supply voltages (rig completely disconnected) are as follows:

    HV = 727V
    B+ = 299.9 V
    Bias = -66.9 V
    Bias adjustment range no load = -55.8 t0 -96.2 V

    So clearly the rig alters the bias voltage substantially based on the Rx and Tx mode bias voltage measurements posted previously.

    My AC-4 is rebuilt with one of the AC4R (Mike Bryce) kits from The Heathkit Shop.

    That bias line coming in on pin 9 of the Jones connector and labeled -60V on the schematic runs through some passive components like RF chokes and some resistors and feeds other tubes besides the finals. For instance look around the cathode of V13B.

    I'd check bias voltage on the tube pins themselves and if they're much more negative than what I'm seeing for the loaded bias then look at the other circuits that use that bias for something like a carbon comp resistor way out of spec allowing the bias voltage to float more negative than expected.
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Thanks. I snuck a post in above yours I think...

    It is indeed not the bias voltage. Getting some new final tubes.

    Thanks for all the help. Will know more once I get the tubes, but it might be a week or so before I can work on it again.
    K7TRF likes this.
  6. K0OKS

    K0OKS Premium Subscriber QRZ Page

    So I got the new tubes and that solved all the major issues-- bias current is now correct, transmitting is good, and the crashing sound seems to have gone away as well. I have been on the air quite a bit with it, and it is generally doing well. I think the coils need aligning. I aligned the receive IF, which did help some, but still could get stronger I think (compared to other rigs here on the same antenna system).

    I had to clean the VFO U-Bar assembly as the frequency was jumping around a bit. That is good now.

    There are some other issues, but I want to attempt a full alignment at this point. I did neutralize the tubes per the manual, and they are good.

    Where is the best place (physically) to disconnect the plate power lead into the finals section? I don't have it in front of me right now, but I didn't see a similar pass through capacitor for the plate unless it is perhaps C60? This may become obvious when I open it up again, but any pointers would be helpful. (Manual says disconnect the plate and screen voltage before doing the alignment.)

    I was also having trouble peaking the Crystal Oscillator circuits for the pre-mixer. I got a peak on 40m, but the 15m and 10m adjustments show no peak when I adjust the slugs stop to stop, and on 10m the TP is reading positive. I am going to take a close look at the bandswitch in that part of the circuit, because that could certainly mess up the oscillators. If I have further troubles I will loosely couple (gimmick) a scope probe to it and see if it is oscillating at all on those bands.

  7. K0OKS

    K0OKS Premium Subscriber QRZ Page

    See other stuff above with which I need assistance, but I did attach a scope to the TP right behind the S-meter. I do see oscillation on 40m (which seems to work OK TX and RX). I also saw oscillation on 15 and 10(1) ans 10(2), but nothing on 10(3). So I do want to take a look at the bandswitch. The DC offset on the 10m bands is positive. The lack of oscillation on 10(3) (full clockwise) seems to point to the bandswitch or the crystal itself, as the other components seem to be shared with 10(1) and 10(2). Though perhaps R4 way out of spec could affect it.

    FWIW, I have swapped out V1 (premixer).

    Why do the instructions have you trying to get that TP as negative as possible? I realize it is after the mixer (EDIT: the TP is NOT after the mixer), and we are pre-mixing two frequencies to get the desired frequency, but I guess I don't follow the effects on the DC voltage. (EDIT: does it have to do with getting the phases correct to keep the ocsilattor going??)

    Would a modern alignment method be to check the frequency here on a frequency counter? The counter I have sitting here (Tek DC503A TM500 module) didn't want to count with such low input, but I can amplify it or use a different counter.

    Thoughts? Thanks.
    Last edited: Mar 8, 2018
  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Please note questions in the previous two posts as well...

    However, how far should the Carrier Balance pot turn? I think mine must be broken, as I turned it about 20 times one way or the other and it never stopped. So I need to replace that. I think I have to get one. Don't think I have any 200 Ohm pots in stock here.

    EDIT: Perhaps it is OK. Not sure how many turns it has... I was able to null out the carrience using the control. So it must be working.
    Last edited: Mar 9, 2018

Share This Page