DX-60A Bias issue?

[w2rik]

Greetings!

I will try to get all pertinent info into this note hoping that someone can assist me. I have a DX-60A that I completely restored by replacing nearly all of the caps except for C3 (100pf on V1), C21 (68pf on Band Switch) and C19 (.001mf on RF Choke) and C42/C43 (.005mf on AC Line). I replaced all resistors except for R28 (10k 2w on Function Switch) I forgot to order that one and it measures 13k ohms. All electrolytic caps have been replaced, as well as the diodes with 1N4007s.

The only mod I did was a snubber found at VE6KQ.com, see attached images. The problem pre-existed this mod!

1- Using a Heathkit HG10 VFO, I get ZERO, nothing on 10 meters. No meter indication for Grid or Plate voltage at all. All other bands load fine, save 15 which loads poorly.

2- Using 80m as my example, the Plate voltage will never load over 98ma.

3- The radio puts out about 10w on AM.

My  Bias voltage is -80 in standby mode, here are the readings with a crystal and VFO with Key Down. Reference is what is published on the schematic.

VFO V1 Pin 6 = 259v        XTAL V1 Pin 6 = 300v           REF V1 Pin 6 = 310v
VFO V1 Pin 8 = 127v        XTAL V1 Pin 8 = 186v           REF V1 Pin 8 = 215v
VFO V1 Pin 9 = -0.46v      XTAL V1 Pin 9 = -24v           REF V1 Pin 9 = -25v

VFO V2 Pin 6 = 276v        XTAL V2 Pin 6 = 309v           REF V2 Pin 6 = 340v
VFO V2 Pin 8 = 137v        XTAL V2 Pin 8 = 9.78v          REF V2 Pin 8 = 12v
VFO V2 Pin 9 = -3.3v       XTAL V2 Pin 9 = -33v           REF V2 Pin 9 = -115v

VFO V3 Pin 1 = 0.5v        XTAL V3 Pin 1 = .5v              REF V3 Pin 1 = 1.2v
VFO V3 Pin 3 = 84v         XTAL V3 Pin 3 = 78.8v           REF V3 Pin 3 = 65v
VFO V3 Pin 5 = -28v        XTAL V3 Pin 5 = -27.7v         REF V3 Pin 5 = -38v

VFO V4 Pin 1 = 710v        XTAL V4 Pin 1 = 714v           REF V4 Pin 1 = 720v
VFO V4 Pin 2 = 50v         XTAL V4 Pin 2 = 49v             REF V4 Pin 2 = 25v
VFO V4 Pin 6 = 50v         XTAL V4 Pin 6 = 49v             REF V4 Pin 6 = 25v
VFO V4 Pin 7 = -2.2v       XTAL V4 Pin 7 = -2.2v          REF V4 Pin 7 = -1.0v

VFO V5 Pin 1 = 122v        XTAL V5 Pin 1 = 133v           REF V5 Pin 1 = 135v
VFO V5 Pin 2 = 0.1mv      XTAL V5 Pin 2 = 0.2mv          REF V5 Pin 2 = 0v
VFO V5 Pin 3 = 1.3v        XTAL V5 Pin 3 = 1.4v            REF V5 Pin 3 = 1.3v
VFO V5 Pin 6 = 80v         XTAL V5 Pin 6 = 82v             REF V5 Pin 6 = 80v
VFO V5 Pin 7 = -0.7v       XTAL V5 Pin 7 = -0.7v          REF V5 Pin 7 = -0.7v

I have attached an image of the voltage readings in case the text format gets all SNAFU.

Can anyone help me address these discrepancies?
Is the radio OKAY to use in this state?

Tnx and 73

Rik
w2rik

[DMOD]

What kind of probe did you use to determine the bias on Pin 5 V3 and the voltages of the RF amps during AM keydown?

Were all of these readings on 10 meters?

My first inclination is to check L1 for proper adjustment and connections, then make sure C2 is 100 pF in value.

Also, make sure R13 = 8.2 ohms. If it is bad and you don't have an 8.2 ohms, use a 10 ohm 1W and a 47 ohm 1/2 W in parallel.

Phil - AC0OB

[w2rik]

I just used a multi-meter. I took my readings on 80 where I loaded up. C2 is listed as .005, should I change it to a 100pf? R13 is fine.

I did work 80m tonight and was told I had a good signal and audio. Then I tried 40m. The rig loaded up but it only put out 1 or 2 watts?!

I will double check L1 tomorrow. Now I don't have a 40m crystal as the manual suggests on pg 36 (see attachment). Will the VFO give me the same result? Maybe L1 being out of whack is why 40m didn't work. 

 Tnx Phil!

[DMOD]

My bad, C3 should be the 100 pF cap. C2 is 0.005 uF. The mica C3 has been problematic on some units.

Without a special probe, RF will get into your multimeter and give you false readings. The input capacitance of most multimeters will reduce RF on the plate or grid pins.

Voltage readings should be taken at the bottom of any inductor where there is usually a 0.005 uF RF shunt capacitor to ground.

Another thing to check are the bandswitches and their contacts with a bright light and a magnifying glass. Make sure C21 is not getting switched in on any band but 80m and that the other L2 AND L3 coil taps are getting shorted as you go up in frequency.

Phil - AC0OB

[w2rik]

I will check this out tomorrow and get back to you. Tnx!

[W1RKW]

Is the VFO up to the task?

[w2rik]

Yes, the VFO is fine.

Maybe I need to be schooled on the DX-60 operation?

I adjusted the 40m drive coil and that seemed to make all the bands load up. Here is were I may need an education... When I load up using a DUMMY LOAD, I will obtain the 150 ma in the CW position. Once I switch over to my antenna, I get about 98 ma. Looking at the output, the radio is consistent with about 10 w on AM and about 50 w on CW.

[w2rik]

OK, so my QTH is NY about 35 miles from NYC as a crow flies. I was in a QSO with one station working his New Hampshire station remotely and another in S. Carolina. They both said I had a great signal esp. for a DX-60.

Then as I was talking, a fuse blue. I replaced it and the radio wouldn't load up. I put it on the bench and tried to load it and when I dipped the plate, the fuse (3AG) blew again.

What's going on??  

Is this pointing back to the bias issue I thought I had?

[KA2DZT]

What size fuse are you using??

[DMOD]

Yes, the VFO is fine.

Maybe I need to be schooled on the DX-60 operation?

I adjusted the 40m drive coil and that seemed to make all the bands load up. Here is were I may need an education... When I load up using a DUMMY LOAD, I will obtain the 150 ma in the CW position. Once I switch over to my antenna, I get about 98 ma. Looking at the output, the radio is consistent with about 10 w on AM and about 50 w on CW.

You never mentioned the grid drive or the Final grid current. Final grid current should be about 2.5-3 mA.

Pages 33-35 of the manual gives you the tune up procedure.

Phil - AC0OB

[w2rik]

I follow the load up as directed in the manual and set it to 2.5 ma.

The fuse is a 3 amp, 250 volt.

[KA2DZT]

I follow the load up as directed in the manual and set it to 2.5 ma.

The fuse is a 3 amp, 250 volt.

Does the manual show what size fuse should be used.  Reason I ask is you referred to the fuse as 3AG.  3AG is the type of fuse (same as type AGC).  3 amp is probably about the lowest value to use for that xmtr.  You may need a slo blo fuse or a higher current fuse.

Fred

[w2rik]

That's a good point Fred. The original fuse was a neon bulb that blew out years ago. At that time I drilled a hole in the chassis and installed a fuse holder. When I google searched at that time, the most popular answer was just a 3 amp and no mention of a slo-blow.

Maybe Phil can comment on what he uses.

But my concern is now the fuse is blowing out on load up, it didn't do that before and as stated, I was in a QSO when it happens tonight.

[w2rik]

The attached image is the driver plate coil at lug 5. Did the contact break off the rivet? I took this pic earlier today before I went on the air.

[KA2DZT]

The last time I worked on a Heath DX xmtr was over 50 years ago so I don't remember.  3 amp seems a little marginal for a xmtr, especially with today's higher line voltages.  The 3 amp fuse may be running on the edge and with normal operating surges the quick acting fuse may be just blowing.

Never saw a neon light being used for a fuse so I'm not sure what that's all about.  Most of those xmtrs used a special AC plug that had two fuses in the plug.  Someone probably removed that AC plug or replaced the whole line cord.  That's why the rig had no AC fuse holder.

Fred

[KA2DZT]

The attached image is the driver plate coil at lug 5. Did the contact break off the rivet? I took this pic earlier today before I went on the air.

Not uncommon for a rotary switch to have unused dummy contacts.  The dummy contact is just there to be used as a circuit tie point.

[w2rik]

 Maybe it's not a neon bulb but it sure looked like one  

See attached.

[KA2DZT]

OK,  I have seen that type circuit breaker.  They do look like neon bulbs.  They were automatically resetting.  You did the right thing by replacing it with a real fuse.

Fred

[Pete, WA2CWA]

The schematic section you posted earlier is for a DX-60B. Are you using a DX-60 or a DX-60B because the voltages on the various tubes are somewhat different? There are also some circuit differences in the power supply area.
The DX-60 specification is 90 watts input power on CW. If you can achieve 70% efficiency that's roughly 63 watts output on CW given everything is working the way if should. On 10 and 15 meters, output would be lower. Switching to AM, the carrier output power will be even lower since the screen grid of the 6146 now is only at 65 volts rather then 150 volts (DX-60B schematic says 180 volts) for CW. When you modulate the rig, the screen grid voltage and plate current will "undulate" upward as you talk. Also, the parts list for the DX-60 and DX-60B show two different part numbers for the power transformer but no other info relative to ratings.

[K4RT]

Interesting thread & photos.  Not to say that the fuse rating shouldn't be higher, but if it the transmitter was working okay before, I would suspect that the fuse rating is not the problem, but rather an indication of a failure elsewhere.  I'm wondering if any components in the PS have failed or are deteriorating.

[w2rik]

Here's the thing... all components, i.e. resistors, caps, diodes have been replaced with modern parts except for the few that I mentioned in my opening comments.

Can someone tell me where to start looking because I can load the 2.5 ma grid current but as soon as I try to load the plate, *POP*, the fuse blows.

Another thing..., as I was checking voltages in the TUNE position with the 2.5 ma loaded but no Plate, maybe after about 20 -30 seconds the fuse blew.

No smoke or sparks and only two resistors were hot to touch, R4 (15k, 2w on V1 pin 8 ) and R1 (47k, 1/2w on pin 9). Both read OK.

Now this is a DX-60A but I used the manual for the DX-60B because the A is unavailable. I did install the power supply caps according to the B version. See attached.

[Pete, WA2CWA]

The power supply wiring and components differ between the DX-60A and DX-60B. Transformers are also different. The DX-60A using three electrolytic caps in the HV supply; DX-60B uses four. R4 should be warm (relative hot); it's a screen resistor. R1 shouldn't be hot unless it's passing a lot of current. You might try reversing the two 6CL6's and see if the "hotness" changes with those two resistors. You might also check to make sure there's nothing shorted in the crystal/VFO switch and wiring. Is the 6146 good?

[w2rik]

First, Tnx to everyone who replied. I really appreciate the feedback.

Do you think there is a problem created by adding the extra cap? The 6146 is good, it tests very well on my tube tester but I realize that's not under load. I will try swapping the 6CL6.

I have visually looked at the crystal/VFO but can't seem to see anything. Do you have any tips for that?

[w2rik]

Interesting, after switching the two 6CL6 tubes, R1 didn't get hot like before. However, the fuse did pop again while in tune mode after about 20 seconds.

[Pete, WA2CWA]

Now take all the tubes out of their sockets except for the 1st 6CL6, power it up; fuse OK yes/no
If OK, put 2nd 6CL6 in socket, power it up; fuse OK yes/no
If OK put 6146 in socket, power it up; fuse OK yes/no
If OK put 6DE7 in socket, power it up; fuse OK yes/no
Try tune, cw, am positions each time to see if the fuse goes.