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Need help with HT-40 MK 1 troubleshooting
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Hi all,
New member here and recently retired, have been a ham since 1967 and started to fix/refurb old ham radio equipment in my leisure time now.
So far successful with some old receivers..Knight Kit Star Roamer, Heathkit 303 and most recently a Hammarlund HQ-100A. ?
So now I decided to try a transmitter to go along with the HQ-100A and picked up a beautiful Hallicrafters HT-40 Mark 1 in great physical condition but known problem.....only can get 2 watts out. I thought this would be a great project but now I'm not so sure. I think I'm reaching the point where I could use some help.
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I've attached the schematic I've been working from that closely matches my rig. When I received it I brought it up slowly on a variac and sure enough.,.Only 2 watts out.
I figured I'd do the basics since it appears all original...I replaced all electrolytics and any HV paper caps,? also replaced any resistors or caps that were more than 10% off, (except all disc ceramics) and cleaned all switch contacts.
Fired it up again...only 2 watts out.
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Long story short... I replaced each tube individually and then tried all new tubes..... still 2 watts out.
The biggest issues are.
1) cannot get any grid drive in tune position 2) I have only -5 vdc to +1.0 vdc on the Grid of the final V2 depending on position of Drive cap instead of -50vdc according to schematic. 3) Again, the Final V2 Pin 4 and 8 should have 130vdc in CW and 30vdc in AM. I have 26vdc in CW and 12 vdc in AM?? 4) Tube V1 Pin 8 should have 210vdc but I'm getting 350vdc and I've replaced R3, R6 and R7. ?
I've resoldered almost all connections, wiggled each tube socket terminal during transmit...nothing changes.
The only thing I found different was L2 is a 5uh choke instead of 100uh, don't know if this was a modification or mistake. I put a 100 uh in its place....still 2 watts out. I'm really stuck at this point and tested most components except the .001 and .005 ceramic disc caps, any of those I did pull and tested were good.
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I'm hoping someone could lead me in the proper direction.
Thanks,
Larry WA2DGD |
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Sounds like you don't have any RF at V1.? I'm assuming you're using xtal
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control.? S1 should be in XTAL.? I have a jumper across? TA1 3 and 4 and a key plugged into the key jack.? You should be able to hear the crystal on a nearby receiver when in tune without keying the radio. See what happens at V1 pin 6 when you key the transmitter in tune. My schematic shows 2.5 mh for L1 and 190 volts thru to the plate V1 pin 3.? L2 100 uh and 350 volts thru to the plate V1 pin 9. It's 210 volts on pin 8 thru R6 (12K 2watt). Make sure the two neon lamps are working. One is under the chassis.? The other is the right lamp on front panel.? Hope it's not a wafer switch issue. On 4/3/2025 12:56 PM, Larry - WA2DGD via groups.io wrote: Joe Hutchens AJ8MH - Radio Marquette, MI |
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Hi Joe,?
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Thanks for the reply.
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I left a few things out in my first post.?
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As you suggest, I have RF at V1, same as the crystal frequency, 7.125. I can trace the rf signal all the way from the oscillator, to the buffer, thru the bandswitch and to the grid of V2 pins 1,5. I can see the 7.125 signal at the output of V2 and the SO239 connector. The biggest issue the grid drive, looks like V2 pin 1,5 should be -50vdc and I only have -5.00vdc. I'm not sure where the -50vdc originates.
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The power supply is giving me over 585 vdc on a VTVM in AM and the same in CW, the schematic shows 430 vdc CW and 500 VDC AM. I'm not sure these are off due to the problems with the Grid Drive. The 210 vdc at V1 pin 8 is much higher, almost at supply voltage, 575vdc and the 12k measures good. Also pin 9 is at same voltage as pin 8. ?
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As suggested in a previous post, I can trace the -5vdc Grid from V2 back thru the switched inductors and bandswitch, but I can't find -50vdc anywhere.
Both neon lamps are working, the original under chassis one was blown and I replaced it, I can see both flash when I turn the rig on or off.
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I thought possibly C19 shorted and was pulling down the voltage, I disconnected it, voltage rose to -15vdc but it tested at 9pf on my capacitance meter. I replaced it.
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Thanks for the help. If you think of anything else, please let me know.
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Regards,
Larry |
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The -50 vdc on the grid is the effect?of grid leak bias. It is developed across R9 and R10. It will only reach -50vdc with proper drive and normal power output.
Walt Cates, WD0GOF
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From: [email protected] <[email protected]> on behalf of Larry - WA2DGD via groups.io <wa2dgd@...>
Sent: Thursday, April 3, 2025 6:09 PM To: [email protected] <[email protected]> Subject: Re: [HallicraftersRadios] Need help with HT-40 MK 1 troubleshooting ?
Hi Joe,?
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Thanks for the reply.
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I left a few things out in my first post.?
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As you suggest, I have RF at V1, same as the crystal frequency, 7.125. I can trace the rf signal all the way from the oscillator, to the buffer, thru the bandswitch and to the grid of V2 pins 1,5. I can see the 7.125 signal at the output of V2 and the
SO239 connector. The biggest issue the grid drive, looks like V2 pin 1,5 should be -50vdc and I only have -5.00vdc. I'm not sure where the -50vdc originates.
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The power supply is giving me over 585 vdc on a VTVM in AM and the same in CW, the schematic shows 430 vdc CW and 500 VDC AM. I'm not sure these are off due to the problems with the Grid Drive. The 210 vdc at V1 pin 8 is much higher, almost at supply voltage,
575vdc and the 12k measures good. Also pin 9 is at same voltage as pin 8. ?
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As suggested in a previous post, I can trace the -5vdc Grid from V2 back thru the switched inductors and bandswitch, but I can't find -50vdc anywhere.
Both neon lamps are working, the original under chassis one was blown and I replaced it, I can see both flash when I turn the rig on or off.
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I thought possibly C19 shorted and was pulling down the voltage, I disconnected it, voltage rose to -15vdc but it tested at 9pf on my capacitance meter. I replaced it.
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Thanks for the help. If you think of anything else, please let me know.
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Regards,
Larry
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I still feel like V1 isn't doing much.? Have you tried a new 6CX8? Have
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you checked the heater voltage?? How about any tube socket grounds to the chassis?? If all the voltages are high, current isn't being pulled from the power supply...I think.? What is pin 6 at keyed and un-keyed?? What about the V1 pin 3...200 v? I would measure that at L1 and C8. I believe both neon lamps are lit in AM or CW.? Maybe, in tune also.? I haven't check my transmitter.? The 6DE7 acts as a series regulator when in CW. On 4/3/2025 7:09 PM, Larry - WA2DGD via groups.io wrote: Joe Hutchens AJ8MH - Radio Marquette, MI |
开云体育No lamp in tune or AM.? Only CW.? Just checked. On 4/4/2025 11:57 AM, Joe Hutchens
wrote:
I still feel like V1 isn't doing much.? Have you tried a new 6CX8? Have -- Joe Hutchens AJ8MH - Radio Marquette, MI |
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I tend to agree with your feeling that the voltages are high because no current is being drawn.
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Here are the measurements you requested. These were done with the original 6CX8 and 6DE7. I also repeated the measurements with?
2 new 6CX8's and 2 new 6DE7...all measurements were within 1 volt of each other. I checked all the grounds on V1 and did notice that C7, a .01?
had the ground attached but not soldered, I resoldered it...no joy.
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V1 Pin 6? Keyed = 14 vdc,? Unkeyed = 40 vdc
V1 Pin 3 at L1-C8 junction? Keyed = 107 vdc,? Unkeyed = 125 vdc?
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As I said both neons flash turning function switch but front panel neon does not light during cw.
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I also noticed the 6DE7 acts a series regulator..replaced it also...no change.
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Another voltage that is way off is V1 Pin 8 should be around 210 vdc, I have full B+ at Pin 8 >585 vdc.?
R3 measures 22k and R7 measures 2500 ohms but gets very hot. Also the seondary of the transformer should be around 195 vac, I have 225 vac. My line voltage is usually around 123 vac.
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I appreciate your help, this is almost driving me to go back to work...it was easier.
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Thanks,
Larry |
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I wouldn't worry abut the secondary voltage.? However, I have bucking
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transformers on my outlets to lower the commercial AC down from 120v. Question...? You wouldn't happen to have something connected to pins 1 and 2 on TA1.? You should only have a short across pins 3 and 4. V1 pin 6 and pin 3 (L1 and C8) voltages look close enough.? I don't understand why...with such a high voltage on pins 9 and 8 of V1. It looks to me like the junction of L1 and C8 gets its voltage thru the HOT R7 back thru R3, thru the S3 front function wafer pins 1 and 2.? But, only in tune.? In CW, that voltage appears to come from R20 and R21 thru front function wafer pins 3 and 5. That's in the same circuit as R29 and the neon lamps, which should be on in CW (I think) and flicker in AM with modulation.? My eyes have gone double vision trying to figure it out. I wish someone else would tell us if their neon lamp is on during CW...? Well, that's it from me.? Someone else needs to chime in. On 4/4/2025 1:44 PM, Larry - WA2DGD via groups.io wrote: Joe Hutchens AJ8MH - Radio Marquette, MI |
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TA1 Terminals 1&2 have nothing connected to them and 3&4 are shorted together.
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I'll try and find a youtube video demonstrating the HT40 and see if the neon bulb lights on CW.
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Next week if no one else chimes in I'll try getting in touch with Walt Cates for help.
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Have a nice weekend and thanks again.
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PS..checked youtube. The modulation lamp should be on in the CW position.
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Larry
WA2DGD |
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I found this info on the web.? It's from the document "HT-40 Preferred
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Upgrades.pdf".? You can search for it.? I think it's for the plain HT40 and not the Mark 1.? (I've still been trying to figure out the function switch without any luck.? Clean it and make sure it's switching properly.) >>>>>The Buffer-Multiplier-Driver Stage The Buffer-Multiplier-Driver stage V1B uses the same 6CX8/6HF8 as described above except the pentode section is used as the Buffer- Multiplier-Driver. It buffers the oscillator from the Final, multiplies the frequency when necessary, and also drives the grid of the 6DQ5 Final. See Page 2 of the manual for the frequency multiplication process. Without the buffering function of this stage, impedance changes at the output of the transmitter would be reflected back to the oscillator, changing the crystal's oscillation frequency and V1A's stage gain. Some Turkey-Bird owner had replaced R4 with a 1 Megaohm resistor. Undoubtedly this contributed to the demise of R5 since this stage was operating in saturation rather than in class C. Furthermore, with the 1 Megaohm resistor value, I doubt any frequency multiplication could occur. You know, if 100k is good, then 1 Megaohm has to be 10X better! After the necessary component changes and power-on, there was no RF output even though the filament was lit. I checked this original tube once again in the tube checker and wolla, it was failing on all counts, most likely due to the R4 component change and subsequent suffering. So a NOS tube went in and power output was restored. Always measure the cathode resistor R5 and the screen resistor R6. I found R5 was 715 ohms so it was replaced with a metal film resistor of the stated value of 470 ohms. R6 was right on and miraculously, I have found this 12K resistor to be stable in most HT-40's. In this case, and due to the failure of the original V1B, I replaced both R6 and L2. Another Turkey-Bird moment was found when the expected stage gain wasn't as high as it should be. If this was originally a Kit, then Turkey-Bird forgot to solder in C11 from the cathode Pin 6 and to ground. With C11 now in circuit the stage gain was higher, with less RF on the Keybias rail. This stage feeds the 6DQ5 Final's grid via a pi-net input circuit. RF feedback from the Final is fed to the junction of C14 and C15 to curb any parasitic oscillations and is a Neutralizing circuit. I have never experienced any neutralization problems in an HT-40 with or without a stub at the 2 lug terminal strip next to the Final. The Cathode Bias Rail: This transmitter uses cathode bias to shut off the buffer-multiplier-driver and Final tubes on Keyup. In Keyup a high voltage is on the V1B and V2 plates but very little current is flowing through the tubes, but it is enough current (0.7 mA) to pull the cathodes up to a voltage of 32V across R2. This voltage reverse biases the tubes and keeps them from amplifying RF.? On Keydown, the “Keybias” voltage goes to zero allowing the V1B and V2 tubes to conduct. On 4/4/2025 3:51 PM, AJ8MH Radio wrote:
I wouldn't worry abut the secondary voltage.? However, I have bucking --
Joe Hutchens AJ8MH - Radio Marquette, MI |
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Joe,
Thanks for the info from the mods. Had a break through early this morning.
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I kept staring at the function switch and something didn't look right, so I enlarged the photo of the function switch for the Mark 1, which is different than the standard HT40.
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I thought this was a factory assembled unit, all the tube sockets and terminal strips are riveted with lock washers. But in it's 60 year life someone else was in there. The function switch had one wire about 3 terminals off in the wrong direction. At 2 am this morning I fixed that wire and all the voltages fell into place.? I went to sleep.
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Just got back from days activites. I put a new 6CX8 and new 6DQ5 in the rig. Oddly it still shows no grid current in tune but when I went to AM or CW I have power out. On CW I get about 55 watts out and can modulate the rig and hear myself on AM. The Drive control does function to peak the output, just have to figure out why it's not showing on the meter.
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Thank you for all your help.?
Larry
WA2DGD
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PS:? found a ground missing on the meter switch, now I can read Grid Current. Now to get it on the air. |
开云体育Unbelievable!!! ?Good work. It could be just a bad output/drive meter switch… with the VFO, I only get one bar on the drive meter.Joe (AJ8MH-Radio) Sent by Wireless! On Apr 6, 2025, at 11:40?PM, Larry - WA2DGD via groups.io <wa2dgd@...> wrote:
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waltcates
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