4766	2010-06-10 03:49:16	lb3hc	Linear amplifier. Low output on 12m and 10m. Need advice.
Message	Date	From	Subject
I am working on a FT1000MP MKV that I am restoring and have noticed that in this group there are a lot of knowledgable people. (I have had no luck on the FT1000MP group with this question). The linear amp on this rig outputs approx 150+ watts (200w version of rig) on all bands except on 10 and 12 meters. I have checked the drive level into the linear amp with my spectrum analyzer and it looks good on 10 and 12 meters. This leads me to think that there is some kind of gain problem in the linear amp. itself. I have replaced the pre driver, driver and final transistors with new ones after the previous owner had problems with the linear. I have noticed that there are several negative feedback circuits in the linear to prevent oscillation and to keep the amp. broadbanded. However, I suspect that the negative feedback circuits also make the gain go down on 10 and 12 meters. Alternatively the new transistors (that I bought on eBay) may have lower gain on high frequencies. I have posted the schematic of the linear on the photo album section ( http://groups.yahoo.com/group/emrfd/photos/album/535254555/pic/597076725/view?picmode=&mode=list&order=ordinal&start=1&count=20&dir=asc ) The yellow markings on the schematic are the feedback paths in question. I tried to increase the resistor values in the driver (BLF145 transistor) to reduce feedback. This helped a bit but with too low value the BLF145 transistor started to oscillate. I also see that there is a capacitor in this feedback chain that. This I have not changed since at 28MHz it presents a very low reactance. In the pre driver circuit, the Yaesu engineers have used a 2SC2166 with a feedback circuit consisting of a series CLR circuit. I wonder if this circuit may have something to do with the problems I see. Question: what would you do to make this amp work well on 10 and 12 meters? The drive level is fine. All DC voltages also look ok. The bias currents are according to spec. Any hints? 73 LB3HC
4767	2010-06-10 04:56:13	David Stone	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
Really you do not have a problem. Your output is about 1 dB low, which is imperceptible the other end. If it were me, I would leave well alone, as the risk of an oscillation breaking something is quite significant. Do you have a proper power meter BTW, a 60 dB calibrated power attenuator and HP 435 or similar. Most other types, even the Bird thrulines are often 0.5 dB wrong, then you really could be chasing a non problem. Regards David GI8FNR ________________________________
4768	2010-06-10 04:59:40	Thomas S. Knutsen	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
Do you have the option to try to measure the output power after the pre-driver? It may help to find if the problem is in the driver or in the PA. I think I would try to measure the different amplifier blocks in order to see if they funtion as they should. Also I would consider looking at the low pass filter What output power level do you have on 10 and 12M? 73's de Thomas LA3PNA 2010/6/10 lb3hc <rezycle.bin@gmail.com> > > > I am working on a FT1000MP MKV that I am restoring and have noticed that in > this group there are a lot of knowledgable people. (I have had no luck on > the FT1000MP group with this question). > > The linear amp on this rig outputs approx 150+ watts (200w version of rig) > on all bands except on 10 and 12 meters. I have checked the drive level into > the linear amp with my spectrum analyzer and it looks good on 10 and 12 > meters. This leads me to think that there is some kind of gain problem in > the linear amp. itself. > > I have replaced the pre driver, driver and final transistors with new ones > after the previous owner had problems with the linear. > > I have noticed that there are several negative feedback circuits in the > linear to prevent oscillation and to keep the amp. broadbanded. However, I > suspect that the negative feedback circuits also make the gain go down on 10 > and 12 meters. Alternatively the new transistors (that I bought on eBay) may > have lower gain on high frequencies. > > I have posted the schematic of the linear on the photo album section ( > http://groups.yahoo.com/group/emrfd/photos/album/535254555/pic/597076725/view?picmode=&mode=list&order=ordinal&start=1&count=20&dir=asc) > > The yellow markings on the schematic are the feedback paths in question. > > I tried to increase the resistor values in the driver (BLF145 transistor) > to reduce feedback. This helped a bit but with too low value the BLF145 > transistor started to oscillate. I also see that there is a capacitor in > this feedback chain that. This I have not changed since at 28MHz it presents > a very low reactance. > > In the pre driver circuit, the Yaesu engineers have used a 2SC2166 with a > feedback circuit consisting of a series CLR circuit. I wonder if this > circuit may have something to do with the problems I see. > > Question: what would you do to make this amp work well on 10 and 12 meters? > The drive level is fine. All DC voltages also look ok. The bias currents are > according to spec. > > Any hints? > > 73 > LB3HC > > > -- Please avoid sending me Word or PowerPoint attachments. See <http://www.gnu.org/philosophy/no-word-attachments.html> [Non-text portions of this message have been removed]
4769	2010-06-10 05:17:20	lb3hc	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
Hi thanks for answering. I may have formulated myself unclearly. The output on 10/12m is only 20-30W. The output
4770	2010-06-10 05:19:24	lb3hc	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
Hei Thomas. I will try to measure the power after the pre-driver. Need to make a probe to my spectrum analyzer that doesn't load the circuit too much. Will report back with the results. LB3HC
4771	2010-06-10 05:51:06	Tim	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
If the problem is confined to one band or band range ... suspect the bandswitching magic. I don't know what's exactly in a FT1000 but other rigs of that vintage have relay-switched low-pass filters after the PA, and some may have relay-switched filters before the PA, and some even tweak bias levels etc. for the different bands. It would not surprise me at all if the 10M and 12M band shared the same filters. If for some reason the 15M low-pass was being used
4772	2010-06-10 05:56:17	lb3hc	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
Hi, the filter is OK. Have checked it separately out of circuit but thru the relays. Terminated in 50 ohm it measures approx 50 + j3 at 28 MHz. The rig has been owned by hams. The gain control can be set per band, but the input level to the PA is the same on all bands. The gain controls are set to max. LB3HC
4774	2010-06-10 06:24:37	lb3hc	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
Hi all. Have measured RF voltage levels with my high bw. Tektronix scope. The input to the pre driver is even higher on 10 metrs and 12 meters (voltage wise) it looks like. Therefore I suspect the pre driver circuit (2SC2166). The output from the pre driver circuit is lower on 10 and 12 than the input ... Looks like negative gain or unity gain on 10 and 12 meters. When I do the same measurement on 20 meter, the pre driver shows gain. Then the questi
4775	2010-06-10 10:34:43	ajparent1	Re: Linear amplifier. Low output on 12m and 10m. Need advice.

4776	2010-06-10 10:38:48	ajparent1	Re: Linear amplifier. Low output on 12m and 10m. Need advice.
There have been reports of low gain and poor quality transistors (2166 included)