I have a Texas Ranger TR 696F SSB that's just driving me crazy. Someone has done countless mods to this unit (e.g. dimmer and channel 9 switches change frequency bands and SWR cal control changes AM RF power). I can't find a schematic for the unit but it closely resembles a Galaxy DX2547 except it uses a TDA6130 TX mixer rather than the S042. AM produces a strong 7-8 Watts albeit you have to play with the mic gain to keep the modulation and RF envelop from distortion. But in SSB it's extremely weak (2 Watts at best). The problem seems to be in the TX mixer but there was no change after replacing the IC. Using a spectrum analyzer I checked the mixer output and it seems OK relative to AM level (within 2dB). by accident I touched L40 with a probe tip and the output shot up to over 20W - but was off frequency so, I'm guessing this would indicate the RF driver and final are ok? I sure would appreciate any input from you folks out there with much more knowledge than me. I'm really discouraged. Thanks so much for all your help.
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Texas Ranger TR696F SSB RX ok, AM TX ok, SBB TX very weak
- Thread starter David Springmire
- Start date
Circuit board is type EPT069611Z most likely. Used in more than one FCC-legal 40-channel radio. The Siemens "S042P" transmit-mixer chip has a new name. Now made by ST Micro, it's TDA6130. Same part new name only.
A probe on an unshielded wire becomes a feedback antenna. The change in counter reading probably reflects the unstable frequency you get when an amplifier circuit becomes an RF oscillator.
Seeing that L40, L42, 43 and 44 are all properly peaked would be the next step. They will affect sideband more than AM if someone has randomly turned one or more slugs away from the proper resonant peak. You'll need a steady tone into the mike with the mike gain turned down to a sideband power of between 1 and 2 Watts. This serves to 'sharpen' the peak response of those four tuning slugs. I'll be surprised if all four of them are already at the peak position. Might turn out to be the whole problem.
73
What kind of probe? A unshielded wire from a DMM? Or the tip of a 'scope probe with shielded wire attached?
A probe on an unshielded wire becomes a feedback antenna. The change in counter reading probably reflects the unstable frequency you get when an amplifier circuit becomes an RF oscillator.
Seeing that L40, L42, 43 and 44 are all properly peaked would be the next step. They will affect sideband more than AM if someone has randomly turned one or more slugs away from the proper resonant peak. You'll need a steady tone into the mike with the mike gain turned down to a sideband power of between 1 and 2 Watts. This serves to 'sharpen' the peak response of those four tuning slugs. I'll be surprised if all four of them are already at the peak position. Might turn out to be the whole problem.
73
Thank you so much for your help!
Q: What kind of probe? A unshielded wire from a DMM? Or the tip of a 'scope probe with shielded wire attached?
A: I was using a typical x1/x10 scope probe with the SA. So, your point is well taken and I appreciate it. I'm sure you're right that it does oscillate - the 20W output jump would tell me the RF amp is Ok along with the AM output being normal.
I followed your direction, L40, L42, L43 and L44 are all peaked with no difference. AM is near 6W and SSB around 2W. In the photo there is one difference I see that L44 is near the top of the slug travel (that may or may not be typical?) I'm using a single tone 1KHz (per the service manual 30mv) but I used 50mv. still no difference in output. I did go ahead and set the bias for the driver and final but RF output remains the same AM is OK SSB very low. Would this tell you the problem must be with the mixer or potentially the mic amp circuit? Thanks again I really appreciate it!
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If AM is ok, RF is making it through the mixer and cans just fine. Could this point to an ALC issue? You did mention playing with the mic gain to prevent distortion, so something is amiss there.
Next question. Do any of the slugs in the four transmit coils show a peak with the top of the slug DEAD EVEN with the rim of the hole? A coil that's behaving properly and giving you a proper resonant peak will have one peak with the slug below the rim, and a second peak with the slug protruding above the rim. The "dead-even" position appears to be a peak but it's not. It's the max-inductance position of the tuning slug. When you see this, it means the internal capacitor in that can has died. Reducing the circuit's capacitance increases the inductance needed to reach the desired resonant frequency. With that capacitor out of the picture, the slug goes to the functional end of its travel without reaching resonance. Looks like a peak as you turn the slug but it's not.
73
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Thank you for your help. The ALC (VR13) has little to no effect (2W max) The APC should be 12.5VDC by the manual I'm using the best it gets to is 12VDC but I wouldn't think 0.5V would make a 10 to 12W difference?
Thank you very much for that information! the L44 slug is almost dead even with the top of the can (please see picture) and that point is peak RF. The other slugs as you mention are near center of travel. By LCR meter it looks like most of these if not all in the transmit chain seem to be about 2uH. L44 is stamped "12255" would you know where I could find one? Maybe L44 will get it if I can get one I'll sure let you know the results. Thanks again to all very much appreciated!
My apologies L44 is about 1.5 turns from dead even with the top. So this may or may not be normal? But all other coils in the TX chain you mention are very near center of travel.
Thank you for explaining. I used your earlier explanation to check the 4 coils in the transmit chain. They each behave as you explained having 2 peaks one above the can and another "true" peak closer to center of travel. I set the bias according to a manual I have to 50mA and 100mA both adjustments have range and the values are steady. The ALC control has little to no affect. One thing I notice, this may or may not be relevant, someone removed the on board AM power pot and used the SWR adjust. I put this condition back to original and adjusted the pot to near 5W. It was giving near 8W in the modified condition. The mic gain has a huge effect on AM power, at minimum and maximum the AM power varies from less than 2W to almost 8W and at just above the 9 o'clock position in AM the envelope becomes extremely distorted. It seems very sensitive, but the output of the mic amp looks clean regardless of setting. Of course the mic gain must be set to maximum to get the 2W in SSB. The output of the balanced modulator is just over 1Vp-p. I really appreciate all your ideas, and open to all your suggestions, sure hope I can fix this silly thing and learn something in the process.
The ALC pot will have no effect until the power level is brought back up to normal.. When I commented on that circuit possibly misbehaving, I was referring to the audio side of that circuit. However, with 2w swinging 8 in AM, I am inclined to believe you have plenty audio, even if the limiter is not working properly.
12v on TP7 is a bit low, but it should be good for more than 2w in SSB, I gotta get ready for work and think about this more. If needed, I can pull out my 2547 this weekend and we can do some signal level comparisons.
12v on TP7 is a bit low, but it should be good for more than 2w in SSB, I gotta get ready for work and think about this more. If needed, I can pull out my 2547 this weekend and we can do some signal level comparisons.
Wait a minute. You said you set AM power to about 5w, but get only 2w with mic gain down. That 4w setting is the dead key without modulation.
Thank you! Yes before I removed the mod where they used the SWR cal control for the RF power, in dead key (no modulation) I was getting about 6W. Then I removed the mod, re-installed the 5K ohm on board pot and adjusted to about 4W dead key. The huge swing is still present when adjusting the mic gain. With mic gain at minimum I get near 2W and mic gain maximum I get well over 6W.
Thats not right. It should be doing 4w with mic gain minimum and no audio to peaking 16w with 100% modulation.