Great! It seems strange that the drivers can fail in ways that are not a complete failure. Been there, done that. I agree that 600 mA is too much. These transistors are typically biased around 70 or 80 mA for a single transistor in CB final service. Garey K4OAH (SK) origninally brought this up and I always add a second 47 Ohm resistor in series with R2303 per the article by Floyd K8AC. That should bring it down to 200-ish mA.
For stability, ground the upper right rear corner of the High Pass Filter module as shown in the image attached here:
/g/DRAKE-RADIO/message/58910Your power output sounds about typical. I usually go for 140-ish watts on 20m and see 100 to 110 out on 17m and above.
73
-Jim
NU0C
On Mon, 19 Feb 2024 15:14:44 -0800
"atlasstuff" <g4fph@...> wrote:
Jim,
Further positive progress here. The two driver transistors that were installed had 160 mV DC on their bases when on receive, rather than zero, so suspect leaky. I measured their standing current at the 'test jumper'. On transmit with no drive, they were pulling around 900 mA, which seemed excessive, assuming they are operated in class AB.
I took both out and tested them with my multi-meter and component tester. Transistor #1 was leaky B-C (1.7 kOhm) and showed an hFE of only 4 at a few mA. Transistor #2 B-E and B-C junctions looked to be intact, but showed 300 kOhm between C and E. The transistor tester awarded it an hFE of 40 though. So both clearly faulty, but for different reasons.
Scratching my head regarding close-at-hand replacements, I suddenly remembered I had an old Cubic Astro 102 BXA in storage that I bought as a challenge. I did get it working, with the exception of the PA brick. The four flange mount Motorola devices in that had been removed by a previous owner and, in place of the final transistors, a pair of (yes!) 2SC1969 had been hacked in. With nothing to lose, I removed the 2SC1969, cleaned up their legs and tested them. Bingo! Two good, original manufacturer, devices and very closely matched, with hFE of 45-ish.
I fitted them to the TR-7 and checked the standing current. Down to 600 mA, which still seems like a hammering to me given their position. I would not expect the final devices to be biased that hard. What are others running a pair of these at?
RF waveform on the pre-driver link is much improved, although at frequencies less than 7 MHz, there's still quite a bit of distortion. Running CW, if I turn the carrier control way down, the distortion looks more like an HF oscillation superimposed on the sin wave, which I shall look into.
Power output-wise, with max. set to 120 W on 14 MHz, I can now get up to 80 W on 28 MHz, depending on the setting of the gain adjust pot on the pre-driver board. If it proves stable at that setting, I'll be happy.
I hope this saga has been interesting to group members'. I am expecting it to end soon and that I shall be able to put the covers on and maybe even have a contact, or two. Standing back and considering the stock faults the TR-7 suffers from, I don't think any new ground has been broken.
Regards,
Mark, G4FPH.
--
73
-Jim
NU0C
Jim Shorney