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   Home   Help Search  
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Author Topic: Heathkit SB400 versus 401  (Read 25219 times)
KD4SBY
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Posts: 269




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« Reply #30 on: October 14, 2011, 07:05:26 AM »

Hit the wrong button and everything got posted before I could go over it and spellcheck it.
Sorry about any misspelling of words that slipped by that way.
The sentence "I can hear the carrier freq" is almost complete. I meant to say here that I can hear all 3 carrier frequencies in the shortwave receiver.
In the last sentence the word "eact" was supposed to be "exact"  Sad
W5RKL- I hope that I have answered your posting. Thank you for your suggestions.
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W5RKL
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Posts: 1113




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« Reply #31 on: October 14, 2011, 11:05:15 AM »

Update - Indeed the VOX adjustment soved the problem of the relay. It is now working as it should. But that did not solve the other problem of no RF output.  I think that the problem is that the Xtal Osc. (V8A) 6AW6 is not working. I can not find an output on the plate with either an RF probe or Scope, but can hear the lower Xtal frequencies (12,15, and 22MHz)  on the shortwave receiver! Can't figure that out. I have ordered some spare tubes to be able to substitute
I can hear the Carrier freq
Also, I still can not get the meter to read any plate or grid current in any band. That might be a different problem, since I think that there is no RF needed to draw some grid current. Also the meter pins to the right when switched to ALC, I have not looked at that as yet either.
BTW I have all the usual test equipment, including several counters. But I am not sure if I can use them with the higher voltages present in tube equipment, and thus have not used them. In my case, I was not after eact RF frequencies anyway, just the presence of one.

The VOX "DELAY" adjustment was not meant to fix your loss of RF output. The purpose of the VOX DELAY adjustment was to determine whether the CW side tone oscillator V13B, CW side tone amplifier (V12C), VOX Amplifier V12A and VOX Relay amplifier (V12B) were all working. It appears they are working based on your recent response. Therefore, at this point, there is no need to focus on these circuits. You may have to refer back to these circuits when checking VOX operation in USB and LSB to ensure the audio from the speech amplifier V1A is being applied to VOX amplifier V12A and the VOX GAIN, VOX DELAY, and ANTI-VOX settings are set correctly (see the manual for VOX operation).

As for the lack of the HET RF signal from V8A, connect your frequency counter to pin 1 of HET MIXER V5 (6EW8). If the HET OSC is working properly you should see the individual band HET OSC crystal frequencies displayed on the frequency counter at each band switch setting. If you are worried about DC voltage on pin 1, you can check the voltage at pin 1 on V5 before connecting your frequency counter or simply connect your frequency counter to pin 1 of V5 through a .01ufd disc capacitor. The disc cap blocks the flow of DC voltage but allows the RF to pass through the cap to the frequency counter. Make sure the frequency counter's negative lead is connected to the chassis and the positive lead is connected to the circuit connection otherwise the counter may not display the correct frequency.

Next, with the SB-400 powered up and the Function switch set to TRANS, place the meter switch to HV. The meter pointer should rise to and rest pointing at "8" on the top scale. Multiply the top meter scale by 100. Therefore, if the meter points to "8" the meter is indicating "800VDC". This voltage will become less when current flows in the final tube, whether it be normal 50ma idling current or various levels of RF output in CW/TUNE (adjustable with the CW/MIC LEVEL control) or sideband (again adjustable with the CW/MIC LEVEL control).

Next, set the Mode switch to USB or LSB, the CW/MIC LEVEL control fully CCW then measure the final tube voltages as follows:

Pin 1 - transmitter not keyed / Key transmitter
Pin 3 - transmitter not keyed / Same as above
Pin 4 - transmitter not keyed / Same as above
Pin 5 - transmitter not keyed / Same as above
Pin 6 - transmitter not keyed / Same as above

Once the voltage measurements are made with the transmitter keyed, move the MODE switch from TUNE to either USB or LSB.

Next, leave the CW/MIC LEVEL control fully CCW and with your DC volt meter connected to pin 5 of V10/V11, place the Mode switch to TUNE. Final tube pin 5 voltage should become significantly less negative. Place the Mode switch to USB or LSB and the final tube pin 5 voltage should return to approximately -150VDC. Placing the Mode switch to TUNE should cause both relays to energize. Returning the Mode switch to USB and LSB should cause the relays to de-energize.

Perform these checks and report your results.

When you find and fix the problem(s) that is/are causing loss of RF output, when tuning the SB-400 transmitter into a 50 ohm dummy load, set the METER SWITCH to "GRID". The GRID meter position is much more accurate for peak grid drive (DRIVE control adjustment) to the final amplifier tubes than the PLATE setting is. Once you achieve a peak GRID meter display, switch the meter to PLATE for the DIP (PLATE control adjustment). If, at any time, the GRID meter position pegs fully scale to the right when adjusting the DRIVE control, reduce the drive by rotating the CW/MIC LEVEL control CCW so the meter indicates between 2 and 4 (top scale on the meter) then continue adjusting the DRIVE control for a peak GRID meter indication. Keep transmitter key times to a minimum. You can unkey the transmitter between steps without any problems.

73s
Mike


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KD4SBY
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Posts: 269




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« Reply #32 on: October 20, 2011, 06:41:59 AM »

Update - I checked all the voltages on the sockets in the STBY position and found them all within 10% of the values stated.
I then pulled all the Xtals of the SB-400 and connected the SB-301 to the 400 to provide the frequencies for the BFO, LMO and HET OSC. Now the 300 controls all the frequencies for the 400 in the TRCV. position. ( I left the other control leads off to reduce the clutter.)

Problem: In the CW position the carrier osc. V2C can not work, for  the carrier from the SB-300 is switched off by the MODE switch in that position.
Plugging the CW Xtal back in will not help since the FUNCTION switch will switch off the plate voltage for the carrier Osc V2A+V2B in the TRCV position.
The question now is:  where is the Carrier coming from when the MODE is in the CW position while the FUNCTION switch is in TRCV?
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W5RKL
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Posts: 1113




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« Reply #33 on: October 20, 2011, 10:06:31 AM »

The Carrier Oscillator signal for TRCV operating with the SB-400/401 transmitter is supplied by the receiver's "BFO" oscillator V9B through buffer amplifier V9C (6AS11) compactron tube.
The BFO buffer/amplifier transformer must also be adjusted for peak signal to the transmitter. This is discussed in the SB-300 manual. You can also see the signal path, including the
BFO output transformer, on the SB-300 schematic which is at the rear of the SB-300 manual.

73s
Mike
W5RKL
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KD4SBY
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Posts: 269




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« Reply #34 on: October 22, 2011, 09:14:03 AM »

W5RL - The can see the RF signal coming from the SB-301 into the HET OSC jack of the SB400 allright, it is from there, in the SB-400, where I get confused, and there is where my question is directed to. I just can not trace the RF further than from the input jack to the  MODE switch contact on MS4R, there it dead ends when it is in the CW position. It never gets to V2C, the carrier generator/cathode follower amplifier tube. I must be missing something, for I assume that the 401 should be able to work CW in the TRCV mode, where the carrier signal is provided by the 301.
Maybe you can enlighten me?
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W5RKL
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Posts: 1113




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« Reply #35 on: October 22, 2011, 11:53:18 AM »

I recommend tracing the circuit from the SB-401's BFO socket through the wafer switch to V2C's grid. It's possible a previous owner/builder made an error in construction.
The BFO signal should appear at V2C's grid in CW mode. I suspect if you made a jumper from the rear BFO socket's center terminal directly to V2C's grid with the Mode switch
in CW, you would see RF output from the SB-401 in CW mode.

73s
Mike
W5RKL
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KD4SBY
Member

Posts: 269




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« Reply #36 on: October 22, 2011, 01:18:09 PM »

W5RKL - That is the whole problem! It does not appear at the grid of V2C.
According to the circuit description (page 8 of the manual), the BFO signal is only connected to the grid of V2C in the LSB-USB Mode. In the CW-TUNE Mode, it is disconnected. And that is verified by my findings. The RF signal is disconnected by the switch (MS4R) in the CW-TUNE Mode, and never reaches the grid, hence my original question: where does it come from?
One possibility I can think off is that the drawing is wrong, and it should not be as it is shown.
But that does not explain my findings.
Also, since I have to assume that the unit worked before, and there are no signs that somebody did any modifications, I have to conclude that the previous owner never used the 400 in the TRCV mode. (All the Xtals were present)
In that way, he never noticed this problem.
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W5RKL
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Posts: 1113




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« Reply #37 on: October 23, 2011, 03:28:04 PM »

I pulled my SB-400, removed V2, injected an RF signal into the rear panel's RCVR BFO socket then checked pin 11 on V2 for the signal in CW mode. I can see the signal in LSB and USB but, as you have said, not in CW or TUNE modes.

I did study the SB-400's schematic and it's clear to me the CAR OSC crystals must be in their sockets for both TRCV and TRANS Function switch settings for all modes. I would remove the RCVR BFO connection and only use the HET and VFO (LMO) connections to see what you get.

I have never operated TRCV with an SB-400/SB-401 and an SB-300 or SB-303 receiver. Therefore, I do not have any first hand experience with this feature. I do know there are some changes that
are made which are outlined in both the SB-400, SB-401, SB-300, SB-301, and SB-303 manuals to operate TRCV.

73s
Mike
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KD4SBY
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Posts: 269




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« Reply #38 on: October 25, 2011, 05:58:02 AM »

W5RKL - That's the way I see it and I put the all the Xtals back in. That way I can choose which configuration I want to use.
Did some checking again. Got good signal in CW MODE going into the Balanced Mod. at the wiper of the pot R31, and  at pin 5 of V3. From there on, I am not sure what I am getting. The problem is that I never troubleshot a rig before and do not really know what to expect. One thing is sure, the driver V9 is not getting any signal, and the ALC Meter is not doing anything. So the trouble must be somewhere around V3-V4-V5.
I also tried to put a 1000Hz into V1 through the microphone pin, and get good signals at the Balanced Mod. Again, I am not sure what I get out of it at Pin 1 of V3 is correct or not.
I decided to stop  trying to find out what is wrong with it, and ask around in my Radio Club if anyone can help me with it. I just do not have the expertise to work on this.
Thanks for the help, fellows.
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W5RKL
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Posts: 1113




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« Reply #39 on: October 25, 2011, 09:28:11 AM »

The SB-400 and SB-401 transmitter both have the same stages of frequency producing circuits.

The Car Oscillator crystal frequency is determined by the Mode switch setting. I will use the 3395.4Khz CW/TUNE Carrier Oscillator crystal frequency in the following discussion. The USB and LSB Carrier Oscillator crystal frequencies follow the same path and mixing scheme as the CW/TUNE Carrier Oscillator crystal frequency does.

The LMO (VFO) output frequency varies from 5500khz to 5000Khz, depending on the LMO dial setting. Setting the LMO to zero (bottom of the dial) results in an LMO output frequency of 5500Khz. Setting the dial to 500, the LMO output frequency is 5000Khz. The following discussion assumes the LMO dial is set to zero.

The output of a mixer circuit has 4 frequencies, both original input frequencies, a "sum", and a "difference" frequency. Depending on the desired frequency, either "sum" or the "difference" frequency is used. The other frequencies are rejected.

A "SUM" frequency is just what the word means, adding the 2 input frequencies together to produce a "sum". If the 2 input frequencies are 20Khz and 30Khz, the "sum" frequency will be "50Khz" (20 + 30 = 50).
A "difference" frequency is the opposite, the 2 frequencies are subtracted from one another. Using the same 20Khz and 30Khz frequencies input frequencies, the "difference" frequency will be "10Khz" 30 - 20 = 10).

Starting at the Car Oscillator, the 3395.4Khz signal is applied to the BM (Balance Modulator). The output of the BM is applied to the BM transformer T1. The 3395.4Khz signal is then applied to the Isolation Amplifier V3. V3 amplifies the 3395.4Khz signal then applies the signal to the Crystal Filter. The output frequency of the Crystal Filter, same as the input frequency, is then applied to the LMO Mixer, V4 where the 5500Khz LMO frequency mixes with the 3395.4Khz Carrier Oscillator frequency, producing an 8895.4Khz signal. The 8895.4Khz signal is then applied to the Band Pass transformer T2. T2 is designed to only pass frequencies between 8895Khz to 8395Khz while rejecting all other frequencies. Notice that T2 has a "500Khz" band width, same as the LMO (5500 - 5000 = 500).

The output of T2 is then fed to the HET OSC Mixer, V5. Depending on the band switch setting, determines what the "desired" output frequency of V5 will be. Let's assume the band switch is set to 80 meters. The HET OSC Crystal frequency for 80 meters is 12395Khz. The 8895Khz signal mixes with the 12395Khz frequency producing an output frequency the "desired" frequency of 3499.6Khz, bottom of the 80 meter band. V5's output tank circuits are tuned to 80 meters, allowing the "desired" signal to pass, rejecting the Difference and both input frequencies.

V5's desired frequency is then applied to the driver, V9, where it is amplified to a level that can be applied to the final amplifier where the desired frequency is further amplified and applied to the PI Network and ultimately to the antenna.

Tracing the RF signals through the transmitter is very simple once you understand how the circuits work, where the signals are applied, and what to expects at the output.

If you have the complete manual, I recommend you thoroughly study the "Circuit Description" section so you get a further understanding of how the signals are generated, converted (mixed), and ultimately applied to the driver and final amplifier.

73s
Mike
W5RKL
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KD4SBY
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Posts: 269




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« Reply #40 on: October 26, 2011, 12:15:43 PM »

W5RKL - I understand the workings of the 400, I just do not know what to expect at the various points of the circuit as far as real voltages is concerned. Heathkit is great in their instructions as far as assembly is concerned, not so detailed when things do not work as they should. Their checking points were geared towards assembly problems, not troubleshooting defective gear that worked before.
I just do not know how to interpet the signals that I see, or what I should expect to see at certain points. If I see a voltage, (RF or DCV) is that correct? Or should it not be there or is it too weak or too low? That is my problem. The only thing I know is that I do not have any RF drive at the grid of V9.
BTW I found out from an other part of the forum that the three signal leads from the 300 have to be made each out of 2 Ft of RG62U Coax.  That exact length and no other type of coax is to be used, if you want to use the 300 and the 400  as a Transceiver. It has to do something with the capacitance and loading of the cables. I did not know that and have to get myself some RG-62U cable to make the leads.
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