Tune a Moto U71LHT-1400B

[old_bill]

All,

I am trying to convert an old Motrac U71LHT (low band, 2rx 4tx, 110 w, extender) for use on 6M (52.525). Crystals are due in a couple of days. Originally used around 48 Mhz so I think the band split/segment is pretty close. Rx worked perfectly; tx unknown.

I used to have the service manual, but it has disappeared, and being a tube final, I know there is a specific tuning sequence/ power setting so as to not wreck the PA. About 8 tuning slugs, a couple of capacitors, and of course the standard metering socket.

Would anyone have the pull out pages from the manual, or tell me where to start, tuning what slug to what peak in what order and so forth. I can easily measure the peak, RF power and deviation.

Any/all ideas welcome.

O_B

[Will]

Kinda like riding a bicycle but I have to think about it because the radio is not under my hand.

Do you have the meter?

[old_bill]

Thanks for the quick reply. No I don't have a test set. I was hoping to decipher the metering socket pin out and just put a VTVM between the right pin and ground for the peak. Actually the test set meter position switch corresponds to the 12 pin socket so that is simple enough. So it would be the slug, the metering switch setting, and peak or null.

Here's what I have for the Tx:

T101- T107. T107 is on the PA deck
L101-L102. L102 is on the PA deck
PA Plate adjustment
Ant. Trimmer Adjustment
Ant. Coupling Adjustment
F1/F2 warp (pretty self explanatory)
Hi-Lo power. Presume we first tune on LOW then move to HI

That ring a bell? I can snap an image if it would help.


Thanks again for your reply.

[old_bill]

Doesn't make much sense in replying to myself but to anyone who took the time to read, and might be wondering where all this is going, after some effort I now have the receiver working. A couple of slugs had to be replaced with aluminum substitutes (actually better than copper/brass) and the receive sensitivity on 52.525 MHz is at least passable down to -108 dBm. Certainly not motrac design specs but a huge 10MHz leap from where it was last working.

The transmitter however has me baffled. I checked, and the channel element is active, putting out RF, and working right on frequency, but from there, it just sort of stops. No output from the exciter. I have a variety of T's and a couple of L's and a whole metering socket to work with but no matter what slug I turn, nothing in the metering socket changes any, and the RF stops at the crystal. It would be immensely helpful if anyone who has a schematic could tell me if the first multiplier is a doubler or tripler, so I could try to go to that frequency and try to trace something. Of course, it may be that what I am feeding it is just too far out to do anything, but I was able to coax the receiver into working.

Thank you for reading and for any good ideas you care to share.

[WB6NVH]

What you write about the receiver is not what I have experienced. A 42-50 MHz "L" receiver will tune to 52.525 after changing three capacitors in the multiplier section which follows the channel element input. Presumably this is where you changed slugs instead, which I suppose would work as well. The front end requires no modification.

The receiver front ends are in two models, Extender and Standard. The Extender model has a delay line and a bunch of small coils in addition to the three large ones. The circuits in the front end are over-coupled and tricky to align, it is usually necessary to pull the cover and inject a signal closer to the front end's output when beginning alignment and then replace the cover and touch everything up. An "L" receiver should produce 20db quieting on 52.525 with about 0.3 to 0.35 microvolts. The Extender should not be tuned to a frequency anywhere near the actual operating channel.

-108 dbm is horrible sensitivity. Meter # 2 should be checked for idle current. It should idle at about 10ua or better. If it doesn't have idle current, this usually means the Permakay filter is defective, and the sensitivity will be about 1-2 microvolts or worse depending upon which section of the filter has failed.

The transmitter will tune to 52.525 without modification, but trying to do so without a manual or a test set is just likely to lead to frustration or damage of the equipment. The manuals are common and show up on eBay all the time and places like W7FG (I think) have copies for sale.

I have seen test sets with the Motrac adapter cables for less than $ 20 on eBay and that's really the way to go. You can try using a VOM with a 50ua scale, but be aware that there is high B+ on some of the pins on the transmitter socket.

[old_bill]

Thanks for the good ideas. I will try for a manual first.

[WB6DGN]

I'm pretty sure I've got that manual, maybe even a couple of them Give me a couple of days to look for it (them). I can scan the alignment page in several pieces that you can print and piece back together. I've got a Motran with the "L" receiver and an "L" transmitter so the above post is helpful to me as well. Any advice for the "L" transmitter anybody??? Send me your email address and, if I have the manual, I'll reply with the scans. My email is tallinson2 <<at>> yahoo <<dot>> com.
Tom DGN

[WB6DGN]

I've got most of the information you need, tuning charts, parts location, pictures, etc. but the schematics for the transmitter are not with the book. Don't know whether they are still around of if they've been lost. I have all of the information for the receiver as that's the same one that's in my Motran and I've got the full manual for it. If this is useful to you, drop me an email and I'll get you a scan of the pages you need.
Tom DGN

[old_bill]

Tom,

Thanks so much for your most generous offer. I will correspond directly to you at the email address you reference, in just a sec. Since the last post, through some measures I sure didn't find in any instruction book, we were able to get the exciter to come alive and now can get a good 75 watts out of it. But the last exciter slug, T108, is removed completely with no peak, and detuning with brass or aluminum slugs won't peak it. So there is some work to do there and it is most likely I am not doing it right. Also, the best I can get so far out of the receiver is about -108uV, which is around 1-2 uv and while I am happy to have that, referencing the earlier post-er we should be able to do a lot better. So while the immediate problem is passed, and it looks like it is working, for sure we should be able to get more out of it. Actually the receiver is the main focus now, as 75 watts is a good 2/3 of its maximum, and those tubes are likely 40 years old. Rather not drive them so hard.

Thank you all again for your most appreciated assistance.

[WB6NVH]

Instructions for converting the 42-50 MHz Motran (a fully solid state radio) to 6 Meters are on my website:

http://www.wb6nvh.com

The "L" receiver would be the same one. These instructions presume you have the manual. I don't recall converting a 100W transmitter to 52.525 but the 50-80 Watt versions with tube PA's always tuned up without any mods for me.

[old_bill]

Thanks for the info and I'll go have a look right now. Unfortunately, the service manual is the one thing I don't have, but it looks like the other poster may be able to give help.