Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Alinco DX-77 Transceiver  (Read 1641 times)
N5ZQB
Member

Posts: 7




Ignore
« on: February 02, 2010, 08:54:32 AM »

Hello: I have Alinco DX-77. It receives on all HF bands, however, it will only transmit on 80 and 160 meters and on the rest of the bands, there is no transmit. Need suggestions where to look? Thank you
Logged
AD4U
Member

Posts: 2173




Ignore
« Reply #1 on: February 02, 2010, 09:21:23 AM »

I am not familiar with this rig, but if it transmits normally on 160 and 80 (full power and proper modulation etc) I would think the finals are OK.

I would look for some problem on 40 - 10 meters in the band switching or in the switching of bandpass filters.  Some rigs do this mechanically with the bandswitch and some use a series of relays.

Dick  AD4U
Logged
AD4U
Member

Posts: 2173




Ignore
« Reply #2 on: February 02, 2010, 09:23:24 AM »

PS:  While it has been in your possession:

Did it work and then quit

or

Did you get it broken
Logged
N5ZQB
Member

Posts: 7




Ignore
« Reply #3 on: February 02, 2010, 09:39:17 AM »

Thanks Dick: Bought as is! Will reply to your first suggestions later. Meanwhile, if you could please look at filter board in the schematic (which is available free at google), may reveal some clue(s) to this problem as I have done some homework after looking at the filter board. Will be in touch! Thank you!
Logged
N5ZQB
Member

Posts: 7




Ignore
« Reply #4 on: February 02, 2010, 11:07:39 AM »

Thanks Dick: I looked at the filter board. There are three transistors (each have two transistors in them). The first Q504 controls 1.8M and 3.5M which activates relay to engage a particular band, is working fine. However, I suspect the remainder two Q506 which activates 7M and 10/14M relays and Q507 for 18/21M and 24/28M are not working/activating relays. All of the capacitors and resistors check good prior to these transistors. I think these transistors act as RF sensed switch and carry small voltage. The wiring of the transistors go back to Main board. All 3 transistor No is FMA4A-T148. Dick do you think I am in the right area? Thanks again.
Logged
AA4PB
Member

Posts: 12890




Ignore
« Reply #5 on: February 02, 2010, 11:26:45 AM »

I'd be really supprised if they are RF sensed. More likely they get a DC control voltage from the processor in the main board, I think. You should be able to measure the DC voltage across the relay coils to see if it is trying to activate the relays. It's possible that one or more of the relays are bad or stuck in the closed position.
Logged
AD4U
Member

Posts: 2173




Ignore
« Reply #6 on: February 02, 2010, 12:21:10 PM »

Since you located something common to 40-10 meters, I think you are on the right track.  Now all you have to do is isolate the problem.

I don't think you can check a "transistor within a transistor" (Darlington pair) with a VOM.  However you should be able to check the switching voltage coming to the base of the transistor combinations in question.  You should also be able to use your VOM to see if the transistor actually switches voltage to the relay coils.

I agree with AA4PB that either the 40-10 meter switching transistor(s) are bad or they are not getting the signal to "switch" from the bandswitch or the CPU (if one is used in that rig).

You are getting close to finding and solving your problem.

Dick  AD4U
Logged
N5ZQB
Member

Posts: 7




Ignore
« Reply #7 on: February 02, 2010, 12:38:35 PM »

AA4PB: Thanks for the valuable input. I shall check the voltage at the relays for the band(s) in question. Thank you. 73 N5ZQB
Logged
N5ZQB
Member

Posts: 7




Ignore
« Reply #8 on: February 02, 2010, 01:00:55 PM »

Hello Dick: Many thanks for the quick response. Now, I shall try to measure voltage at the relays and at the base of these transistors for the respective bands in question. I do appreciate your valuable help. 73, N5ZQB
Logged
G7MRV
Member

Posts: 480


WWW

Ignore
« Reply #9 on: February 02, 2010, 11:16:08 PM »

You may be in the right area electronically by suspecting the filter board, but may be on the wrong board physically!

The relays that switch the filters are controlled by parallel lines from the main board. For so many bands not to work suggests its not the relays or their driver transistors, but something common to them all for the switching. Working back from the relays, the first common point is IC23 on the main board - a serial to parallel driver. It could be that this device has failed (possible to fail partially). This should be simple to check, by testing the parallel output pins to see if their states change with band changes (be aware though that there are twin diodes on the control lines, so it might not be one line-one relay)

Ive only had a quick look at the schematics, so havent checked the entire tx/rx signal paths. You say it receives on all bands, but only tx on 160 and 80m? Its worth checking the sensitivity on each band, and checking the rx signal path to see if that routes via the same filters as tx (there is another filter bank that is diode switched and most likely therefore its the rx filters)

POst up your findings. I'll take a more indepth look at the manual when in not tired after a night shift!

Martin G7MRV
(owner Alinco - DX-70TH, DR-130, DJ-F1E)
Logged

N5ZQB
Member

Posts: 7




Ignore
« Reply #10 on: February 03, 2010, 09:22:22 AM »

Hello Mr. Martin: Thanks lot for the very valuable input. I shall be posting my findings over the weekend.
Logged
N5ZQB
Member

Posts: 7




Ignore
« Reply #11 on: February 03, 2010, 10:03:06 AM »

Hello Mr. Martin: Thanks lot for the very valuable input. I shall be posting my findings over the weekend.
Logged
G7MRV
Member

Posts: 480


WWW

Ignore
« Reply #12 on: February 03, 2010, 01:01:23 PM »

I have the schematic in front of me now, and have had time to familiarise myself with it.

Relay 502 is the tx/rx relay. Its worth checking this is switching on tx on all bands, but its unlikely to be the problem given the symptoms.

Rx uses diode switched band pass filters on the PLL board. Since your rx is fine on all bands, this can be ruled out.

Assuming that by 'no tx' you mean no output but the radio appears to be transmitting (ie tx light on), we can tentatively rule out a firmware /cpu fault. That leaves the VSWR sense circuit and the tx low pass filter board. I think we can rule out the swr sense as if that was faulty 160 and 80 would probably fail as well.

Each LPF uses a pair of relays to switch, each relay pair driven by one half of the driver transistors (SMT PNP pairs, not darlingtons) these switch a 14v line to the relays. Q504 handles 160 and 80m, but the rest are shared by the other two transistors, so its unlikely that one device has failed or you would have more bands available. These transistors get their controls from IC23(?) on the main board, as i mentioned before. The only other thing they share is the 14v0 line. Its possible that this line goes to the 160/80m driver first then on to the others, so may be broken after the first transistor. Checking the emitters of the transistors will reveal if this line is present.

So, from what i can see on the schematic, there are two obvious CPF's (common points of failure) - the 14v0 line, and IC23. Theres also CN505, the plug carrying the control lines. Make sure thats seated properly.

I hope this helps you out a bit. If you dont have a service manual it can be downloaded from alinco's website (where i got it from!)

Note - i dont have a DX-77, and all the above is from reading the schematics. Use my hints in comparison with your testing results.
Logged

Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!