I'm wondering if you ever obtain a dip in the plate current when tuning. The fact the Ip is rising when you apply drive to the amplifier indicates the tubes are getting driven but not that well. Lacking a dip in the Ip would indicate either your band switch is not selecting the proper band or none at all, and perhaps the coupling capacitor from the tube plates to the Pi network is bad.
Not achieving 200ma with drive is still a problem and if you can monitor the drive from the exciter to the SB-200, then make sure it is making the amount of wattage you should expect. It could be that the exciter is limiting itself to the 50W level and will not let you go above that. If you have a manual antenna tuner place it on the output of the exciter and the input of the amplifier. With the amplifier keyed and RF being supplied to the tuner, adjust it for minimum SWR. That may help.
Hope this helps
Tried something else this evening. I inserted a wattmeter/swr device between the rig and amp. Sure enough, the match was non existent. As soon as I keyed the exciter, the swr's rose dramatically with the drive increase, until at about 45 watts the rig wouldn't put any more out. No drive, 80-90ma, maximum drive, 120-130ma. For grins I replaced the wattmeter in line with my Dentron GLT1000 tuner. By tuning out reflected power, I was able to actually get the amp's plate current up to 200ma with 50 watts. Unfortunately, that was it! Still no amplification or grid current. If everything wasn't bad enough, the meter light quit on me this session! Can bad tubes upset the factory set tuned input Z? At least I still have good HV (2600V). Is there a doctor in the house? lol
Thx for the comments Gary. As you can see I had just tried what you suggested. The SB-200 has no input tuning slugs available on the back like AL-811's and others. The manual says it was all taken care of by pretuning at the factory! I tried on 80,40 and 20 and the swr on input was way up around 3:1 or more. Tuning it out didn't get me any more input power, but it did get me to 200ma Ip. I never tried looking at the Ip when tuning, as the manual has you with the meter in rel pwr mode and tuning for max power. I would assume when I get an increase in relative power, it's probably dipping the Ip. I'll check that anyway if I can bring myself to try again before the new tubes get here next week. One big hassle with this amp is the huge number of screws you have to remove to get her opened up.
C'mon guys.... Surely all the experts out there must have an idea or two as to why this thing seems to be such an impedance mismatch and won't amplify. I've tried a few things and even fixed a couple things (anode cap, loose solder conn in plate cct, meter switch and meter lamp), but still no joy. Please save the comments about my inexperience killing me or why would I be so crazy to buy something without a "test drive." I need help, not a ruler across the fingers!
I'll be opening this baby up Wednesday to swap out the 572B's with a brand new matched pair and also doing some maintenance to the T/R relay. After that I guess I'll have to find a real expert to get this thing operating. Tnx, 73
The Heathkit SB-200 was sold as a KIT to be assembled by anyone with basic high school shop skills, has some basic tools/electronics test equipment (VTVM, DVM, etc.), and can read English instructions.
... guess I'll have to find a real expert to get this thing operating.
The troubleshooting guide, in that Assembly Manual, are your first steps.
Heathkit SB-200 Assemly Manual
It is not uncommon to find errors in assembly by previous owner or bad solder joints.
Golden screwdriver owners (changing factory slugs w/o proper test equipment) usually create problems that are difficult to troubleshoot.
Tom Snowden's notes
Jim, N4NAB Rebuild
Last edited by W9GB; 06-18-2012 at 10:08 PM.
We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths. -- Walt Disney
Relax C.R. you haven't been forgotten, at least not yet. The peak in power output would usually be at the same point the Ip dips. There are times where it isn't but it will be close. What you need to look at if you cannot get a Ip dip or peak power out anywhere is the band switch. Look in the manual and see if the band switch is in the proper location for the band you have selected. If it is and you still do not get an Ip dip when tuning then there may be something wrong with the Pi network. It either is not getting any RF to be tuned or it is sending the RF to a different path where tuning is impossible until it is corrected. I would concentrate on the output network because as I indicated earlier you are getting drive to the tubes otherwise the plate current wouldn't go up when you applied drive to it.
Now for the input you would need to make a tuned input for each band the exciter is going to operate on with the amplifier. The tuned circuit on the input matches the impedance of the amplifier tubes to the impedance of the exciter. If your rig was an older style tube unit you would not need to do this but the modern rigs are setup to be 50 ohm units and they don't like it when the SWR gets to 2:1 and over. As you noticed you did get that high or higher of an SWR. What Heathkit meant when they said the amplifier was already tuned at the factory is that the tube units of that time could drive it with no problem. Nowhere does Heathkit state the input of the amplifier will be a perfect 50 ohms.
BTW in case you hadn't noticed it, troubleshooting anything via the internet is slow and frustrating. Where an Elmer would be looking over your shoulder and giving you step by step instructions you'd be done by now. That's just not the way it is at present. We try hard to help but sometimes the postings just get put on the back burner until surprise, someone posted something. I'm told there are ways around that, something about subscribing to your postings or something like that, but as of yet I haven't done that. If I had I would have noticed your replies long ago and responded. Guess I got to get going on that.
Hope this helps
Last edited by KO6WB; 06-19-2012 at 01:42 AM.
Tnx for the links OM. I'm determined to get this thing going. I've d/l'd all the refurbish, repair, rebuild articles that you provided and a couple others. After the tube swap out and checking, cleaning the T/R relay, and maybe trying a "golden screwdriver" on one of the band input slugs (if movable) to improve impedance match and still not getting the unit to work, I will go back to square one and follow the repair articles. There is no evidence of any mods (other than the RF input terminal change out from RCA to PL258), so following the original manual will be easier.
In the meantime, I've bought another older (FL2100B) linear and am negotiating on another newer unit (AL811). ONE OF THE THREE SHOULD WORK!! I can at least get back on the air and fix the other(s) at my leisure. Then I'll sell at least one, if not two and keep the best one (most likely the Ameritron). I have the ARB-704 buffer unit which I'll use on all of them.
Once again, thank you, and the original authors, for the data.
Thanks for the comments OM. I'm a derelict from the 70's, with 12BY7's driving 6146's driving 572B's and 100+VDC switching relays! I haven't owned a linear since 1980 and I didn't have a solid state exciter until buying the 751-A in 1998. The problems mixing the two eras was something unexpected. Ip dips, ALC, T/R relays etc., etc. has been almost a forgotten art after 20+ years of pumping 100 "no tune" watts into a mostly resonant antennas (10M monobander, R5 vertical and homemade 40M dipole). Even with my occasional bouts of "CRS" I think I can re-learn
C.R., I have a AL-811 that has been fine. The only problem I had with it was the SWR going into the 811 was high on 80/75 but I managed to work around it until I got up enough steam to work on it. Hooked up my SWR meter and another coax jumper and proceded to tune the coil slug for best SWR. Had the tuning tool at the ready. I keyed my rig and the SWR read really good. Hmmmm....head scratching moment. Put everything back the way it was and high SWR. I then removed the 2 foot coax jumper and replaced it with a 6 foot jumper and bingo, perfect SWR.
The Al-811 still works but I have replaced it with an ALS-600. Same power just no more tuning. My antennas are only rated at that power level otherwise I would have gone for an AL-80B.
As for learning new stuff, I had to work on my little TS-50S awhile back. Just an ALC glitch that needed attention. I got in there and probed around. Have you seen this stuff????? SMT is really tiny. I remember resistors and capacitors, tuning coils, IF transformers, real transformers with choke filtering power supplies, oil filled capacitors, tubes, sockets, stuff you could actually see, understood and could work on. Now it's all specialized IC's and micro-miniature, multi-layered PCBs and displays that rivial the TV sets we had when we were kids. Wow, is this a great time to be an amateur or what?
CRS, what's that, who are you, why am I here? Must be medication time....sigh.
Hang in there C.R.