IT'S ALIVE!! (Well, sort of.) - DX60 help needed please

[WB2YGF]

So... I re-capped my rusty old DX60B (sat in a shed for 20 years), hooked it up to a Variac with an Amprobe measuring the input current, ran it up, and low and behold, it didn't smoke.

I don't have very many crystals (always ran VFO as a novice) but I did find a 3885 and a 7036 in my junkbox.  Good enough for testing.  Connected up a wattmeter and dummy load and it tunes up fine on all bands, however, the output is only 50W on 80, 40 & 20 and about 40W on 15 & 10 so I suspect the 6146 has seen better days. 

First mod I made was changing that Amphenal mike jack to a 1/4 inch phono jack like I had on my last DX60. Plugged in my trusty Electro-Voice 915 crystal mike, and.... sadly, no modulation. 

Oh well.  It's good progress for one afternoon. 

[WB2YGF]

Can someone please help a tube n00b?

I checked all the relevant DC bias voltages in the audio section.  Most are pretty close. V5-6 & V5-1 are a little lower than specified.

I applied a 1kc signal to the mike input, and made the following observations using a scope:
I have 300mV P-P on V5-7
I have 2V p-p on V5-2  The R19 pot adjusts the signal as expected.
I have 80V p-p on V4-7
When I look at V4-2 there is 20V of DC bias with no AC signal.  I would expect to see a big signal here.  If not, where should I look?

I have 170mA of plate current on CW with about 50W out, but only 10mA of plate current on AM and practically no power out.  The amount of audio has no effect on the plate current or output power.

http://www.nostalgickitscentral.com/heath/schematics/heathkit_schema_dx60b-1.gif

[w4bfs]

is v4 the 6de7 ?  if so try a new one ...73...John

[Steve - WB3HUZ]

50 watts is plenty for a single 6146 but 170 mA is too much plate current.

[WB2YGF]

is v4 the 6de7 ?  if so try a new one ...73...John

Thanks John.  Toob is deader than a doornail - new one on order.  When am I going to learn to check the obvious first.

[WB2YGF]

50 watts is plenty for a single 6146 but 170 mA is too much plate current.

The manual says typical 150mA.  It doesn't give a maximum, but i'll take your word for it.

[Steve - WB3HUZ]

Straight from the 6146 spec sheet.


Maximum Ratings (Design Center Values)

RF Power Amplifier & Oscillator - Class C (CCS)
Plate Voltage ................................. 600 V
Grid No. 2 Voltage ............................ 250 V
Negative Grid No. 1 Voltage ................... -150 V
Plate Input ................................... 67.5 W
Plate Dissipation ............................. 20 W
Grid No. 2 Dissipation ........................ 3 W
Plate Current ................................. 140 mA
Grid No. 1 Circuit Resistance
  Fixed Bias .................................. 30K Ω
Bulb Temperature (At Hottest Point) ........... 220 °C

[WB2YGF]

I see that's the CCS rating.  The ICAS rating is like 180 mA.  I suppose CCS is more appropriate for old buzzard QSO's. 

[KX5JT]

I see that's the CCS rating.  The ICAS rating is like 180 mA.  I suppose CCS is more appropriate for old buzzard QSO's. 

But the DX-60 is controlled carrier right?  So will it not have a lower "resting" keyup current in AM, then swing up with modulation?  I'm paying close attention to this as I'm intereted in a DX-60 myself.

[Steve - WB3HUZ]

I think the ICAS is 150 mA. It doesn't matter that much since on AM, as the previous poster noted, the plate current should be at a very low value and swing up with modulation.

[WB2YGF]

Well I put in the new modulator tube.  Thought I heard a pop when I turned on the rig but it could be unrelated.  Resting output of about 10 W on AM.  No perceptable increase when speaking into the mike.  Injected a 1kc signal into the mike input.  Seemed like output increased non linearly with increasing input (jumped up).  Started taking measurments for about 10 minutes.  Seems like there was too much AC mixed with the signal. Put unit into standby.  Sound of something cooling down?  Turned AM on again and noticed no output.  6DE7 shield seemed VERY hot.  No filament.  Guess I blew it.    (Ordered FOUR 6DE7's.  Shipping is a killer one tube at a time.) 

[W3NP]

Have you measured the DC on pin 7 of V4? - should be a negative voltage. You could have a leaking or shorted coupling cap from the plate of V5. I would check ALL of the components around V4. Sounds like the tube is going into heavy conduction. Not sure why the filament is opening up though - unless the internal elements of the tube are sagging and melting together.

[W3NP]

One other thought - Do you know if this TX ever worked properly on AM? Could it have been incorrectly wired by the original owner?

[WB2YGF]

I don't know if I was getting -1V.  I do know that with the tube out, the average is 0V.  The thing that puzzled me was seeing the 60hz component mixed in with 1khz.  I had not seen that anywhere last weekend.

I don't know if the unit ever worked on AM.  In fact, I got this unit so long ago I have no idea who I got it from, or why I would have wanted it. 

I will see if I can find anything wrong with the wiring or components.

I won the Dowkey relay I was looking for on eBay, so I am pretty close to getting on the air if I can figure out this modulator issue.

Thanks for the help Dave

Edit:  Just noticed that the tube cracked at the base just like the original.

[W3NP]

Edit:  Just noticed that the tube cracked at the base just like the original.

Well, at least that explains why the filament went out in both tubes. I don't have the schematic to the 60 in front of me now but since the grid of the first half of the tube has proper voltage readings, check the cathode circuit components of the 2nd half of that tube. Something is causing it to conduct hard and self destruct. If you haven't found anything wrong by the time you get your new tubes, power it up at reduced voltage with the variac and keep the tube shield off while probing around. Maybe the cathode is not positive enough for the 22v on the grid.

[K4TLJ]

I suggest that you check the tube base wiring carefully. Sometimes if a pin is mis wired to an internal connection  there is current inside the tube where it is not supposed to be and heating of the glass where the wires pass through can break the glass. Don't ask how I know.

[WB2YGF]

I made some progress this weekend.  First I put in the new modulator tube.  Then I carefully checked all the bias voltages with no modulation.  Nothing seemed totally out of whack.  The 300V DC is a bit low and this is kinda reflected proportionally in some of the other voltage readings.    I tuned the rig in the CW position for max power and used the loading to back Ip down to 150 mA per the tuning instructions.  This yielded about 50W out.  In the AM position, I was getting a plate current of about 50 mA. and about 9 W output.

Next I hooked up my new (to me) SB-614 monitor scope. I used my Icom 706 connected to a 19" mag mount antenna to monitor my audio. 

I applied a 1kc signal.  As a reference point, I noted that .4V p-p on the wiper of the mic gain pot yielded approx. 100% modulation.  According to the instructions, the mic gain should be set for a peak of 75 mA Ip but I was not getting anywhere near that amount at 100% modulation.

I noticed a LOT of hum.  At first I thought the speech amp circuit might be introducing it. Perhaps the 33K/20uF RC was not decoupling the AC ripple from the 300V supply.  I noticed that if the mic gain was set to zero, the hum went away.  Finally, I grounded the mic input and the hum went away and did not increase when the mic gain was increased, so the speech amp was not causing the hum. I could decrease the hum by lowering the mike gain control and injecting a bigger 1kc tone.

Next I tried the EV 915 microphone.  I knew I needed a .4V p-p signal at the wiper of the gain control. It became clear thet the mic was not providing nearly enough signal even wilth the gain all the way up (and lots of hum).  My voice was barely audible on the IC 706.

Earlier in the day, I discovered that the desk mic from my FT101ZD (YD-844A) had an impedance switch on the bottom to set for 50K or 600 ohms.    Comparing the mic signals (unconnected) on the scope, it was clear that the Yaesu mic had a HUGE signal compared to the EV 915.  I made an adaptor so I could connect the Yaesu mic to the DX60.  As soon as I connected the Yaesu mic, the hum went away and I had plenty of audio.  I could kick the Ip up to 60mA or so.  My voice was loud and clear on the IC 706.

Bottom line, I guess I was fighting a bad microphone all along

W4NP:  BTW, Dave, you had an S4 - S6 signal in my basement this afternoon on a 19" whip. 

[WB2YGF]

Next step is to connect the Dow Key relay.  One set of contacts I can use for the RLY input on the leenyar.  Does anyone know if there is any way to mute the Pro II receiver when the DX 60 is in transmit?  I reviewed the manual but could not find anything that looked like a mute input.

[K4TLJ]

I recently refurbished a DX60. The 6DE7 was bad and a replacement shorted as well. I checked all the wiring, caps, etc. No problems. Tried another tube and all was well. I found that the rated heater to cathode voltage is exceeded in this circuit when modulation is applied. Resting is OK. It takes a good tube to work here.  I replaced the mike connector with a later Heath two pin jack so my 'Heath' unamplified D104 could be used. I made a few modifications to my DX60 to improve the audio. If interested here is the schematic.
http://home.comcast.net/~suptjud/DX60AudioChanges.jpg

[WB2YGF]

I recently refurbished a DX60. The 6DE7 was bad and a replacement shorted as well. I checked all the wiring, caps, etc. No problems. Tried another tube and all was well. I found that the rated heater to cathode voltage is exceeded in this circuit when modulation is applied. Resting is OK. It takes a good tube to work here.  I replaced the mike connector with a later Heath two pin jack so my 'Heath' unamplified D104 could be used. I made a few modifications to my DX60 to improve the audio. If interested here is the schematic.
http://home.comcast.net/~suptjud/DX60AudioChanges.jpg

Ahhhh.  Good to know that the 6DE7 self-destructing is "normal".  I was probably over-driving the heck out of the first replacement tube with too much 1kc.  Now, with the monitor scope, I could see how much signal was needed so as to only provide the minimum required.

I had considered using the 2 pin Amphenol audio jack, but I think it requires drilling out the chassis and front panel, while the 1/4 in phono jack I am using, fits in the existing hole.

Thanks for the circuit mods.  It would be interesting to know what the purpose of each change is.

[K4TLJ]

All of the changes were made with the goal of broadening the frequency response and reducing distortion. Lowering the value of the 12AX7 plate resistors reduces the effect of load capacitance on high frequency response. The one meg gain pot reduces distortion caused by AC loading and increases the gain. With the added gain I was able to put in some heavy feedback.  The negative feedback from cathode of the 6DE7 to cathode of the second section of the 12AX7 lowers the impedance of the driver and thus reduces the distortion caused by the extreme variation of load.

The load varies from no load at all when the screen is driven negative to  fairly large when the screen is driven to the positive peak. There is no rocket science involved. The back of an RCA receiving tube manual from the fifties or sixties or a 'Radio Amateur's Handbook' from the same period explains most of the theory. Heath engineers were designing a circuit to cut the lows and the highs to narrow the bandwidth of the transmitted signal. My voice has very little content below 100 Hz and above 3000 so there is little chance of splatter just from a too wide source. It just sounds better and more 'natural'.

I tried adding a 3300 ohm cathode resistor in the first 12AX7 cathode for 'proper' cathode bias. It doubled the AC hum probably due to the increased impedance from cathode to filament. The 'contact/grid leak' bias is fine for the signal levels involved anyway. The filament really needs to be raised positive in relation to the cathode for hum reduction but can't be done here due to one side of the filament supply being grounded.

The over modulation clamp just cuts off the drive at near cutoff of the 6146 leaving a residual RF signal. A capacitor across the zener might further reduce the sharp change in wave shape but the RF is already at minimum so the splatter and distortion caused by the sharp change in slope should be minimal. Just my thoughts anyway. I used a 22.5 volt zener but different 6146 characteristics might call for a lower or greater value. I arrived at that choice by monitoring the RF out with a scope.

Yes, I did enlarge the hole in the panel to accommodate the new mike jack. I used a chassis punch. I also drilled a small hole for the modulation LED.

Not shown on the schematic is the PTT. I just stuck a golf tee in the CW key jack to open the contact and connected a wire from the PTT on the mike jack to the key jack and use the normal CW keying for PTT.

Someone somewhere has probably done it all before. It's hard to to come up with original modifications on a nearly fifty year old piece of ham equipment.

[WB2YGF]

Thanks for taking the time to explain the mods.

I was also looking at the K4TAX mods.  http://www.amwindow.org/tech/htm/dx60k4tax/dx60k4tax.htm

Your mods look more straightforward.

I was wondering where I would get the 1M pot but I think I can cheat by adding 500K of combined resistance at both ends and limiting the adjustment to the most likely needed range.

Yesterday evening she made her maiden voyage on 3885 (worked KC8ZUL).  Audio was a bit muddy as expected.  I was disappointed that the idle carrier was only about 90W out of the linear according to the power meter on the AL-80B, but surprised that the output kicked up to 500W on voice peaks.

My Heathkit power meter only showed about 150W peaks.  Guess the meter on the linear responds faster or is peak reading?

[K4TLJ]

I have some 1 meg pots. Free if you want one. They are a direct fit in the old mounting hole. Send your address via my email in QRZ if you want one.

Yes my mods retain the controlled carrier. Some of the others pretty much convert to just screen modulation. Maybe that is better for driving a linear amplifier. Your results with the power kicking up on voice peaks indicates the controlled carrier is alive and well.