[GreenKeys] Dovetron Repair

Sun Feb 18 09:21:12 EST 2024

Thanks Jeff, Yes a lots needs to be sorted. Not just for Dovetron, but 
other TUs, too. I have a HAL ST-6000 almost ready after finding an open 
1Meg bias resistor preventing any horizontal "CRT" loop display. This 
unit also needs some additional parts now ordered from Mouser for the 
auto-start circuit.

The Dovetron MPC-1000T is mostly working but it will not be ready for 
prime time soon. I moved the factory shifts to more useful 2125/2295 and 
2125/2975 and afsk audio does work with the polar input driven off TxD 
from RS232. The KOS-1000 board still lacks the schematic. For the 
display, to get the center LEDs and the lost signal LED to work, I 
removed an "intentional looking" hard melt solder bridge across the 
hidden back side of the front displays PCB socket. I can only speculate 
how it got there as it was inaccessible without removing the display 
from the front panel. Possibly the display was added later by the owner 
and never factory checked before the parts kit was shipped?  Another 
remaining issue is the cause of a dim loop LED. None of my schematics 
show the LED circuit without also showing the DC loop supply.

But, to get it on the air, I might add back the loop supply. All the PCB 
positions are empty and could be populated with the diodes and caps 
needed. The transformer does have the "HV" winding soldered to the 
board. Should be easy. Optionally I could use an external loop with an 
SSR and optical couplers.

For the time being, I put the repairs on the side, and am now focused on 
getting on the air. Likely with the ST-6000. Maybe in a couple weeks. 
Some of the locals are thinking about running a RTTY trial on 6M. Should 
be fun with a real machine!

73, John W1AN

On 13-Feb-24 05:16, Jeffrey Golas wrote:
> On mine the adjustments are on the back, and theyre only for sending. I left the factory 170 and 850 shift, but I changed the 425 option to be 170 shift centered at 2000 for nets/2m use. I might retune 850 to be centered at 2k as well.
>
> With the giant knobs up front receving is "whatever works just spin the knobs".
>
> What trips me on mine, is mine has the cw ID board. Theres certain situations that trigger it and while its sending, it wont decode/encode. You really need to read some of the manual to understand what these things are doing, cuz you almost need h&r block to sort it out lol.
>
> Have fun!
>
> Jeff KC3GJX
>
>
>> On Feb 13, 2024, at 12:05 AM, John Spigel <w1an.dxusa at gmail.com> wrote:
>>
>> Hi guys, thanks for all the suggestions. I hadn't thought about actually using a polar relay on the +/-outputs to place in the loop. As of Sunday I have the afsk audio working after replacing a bad XR2206. It now has the audio and I am thinking about changing from the factory settings to the more useful 2125 with 170 and 850 shifts and the apply a +/- input to see if afsk actually is working. My unit has the four audio shift adjustment trimmers mounted on a tiny circuit board hanging on the front panel wide/narrow switch. Much does not follow the standard circuit diagram! I'm still trying to get all the LEDs working. I have no signal loss LED and the loop light flashes very dimly and not at the right threshold setting. Some discrete LEDs on the SSD don't light as well. Possible bad 741s somewhere. I have already replaced a couple.
>>
>> I have a few polar relays to try. And yes, there are some pretty clever device testers out there. Some can even identify what you plug in! Just got to figure out which one I need!
>>
>> 73, John W1AN
>>
>>> On 13-Feb-24 03:33, B. Smith wrote:
>>> A handy device is an op amp tester.  The one pictured came as a kit. It has a zero insertion force socket which is kind of neat.
>>> Note that some op amp brands out perform others and I think that is mentioned in the Dovetron manual.
>>> See attached.
>>>
>>> Polar relays can be tested and adjusted.
>>> http://k4che.com/Polar%20Relay%20Test/Polar%20Relay%20Testing.htm
>>>
>>> K4CHE
>>>
>>>> On 2/12/2024 8:25 PM, Tom Hunter wrote:
>>>> John Spigel,
>>>>
>>>> I have recently repaired two 1000Ts and am fighting with an MPC 1000. All Doverons are basically similiar.
>>>> The mark and Space tuning, the filters, etc are all the same.
>>>>
>>>> It helps to have a decent oscilloscope and a handful of 741 opamps for substitution.
>>>>
>>>> I believe you said you have a 1000T. If you want to key a loop with it you must use a polar relay (255A - tricky to adjust) or if you can find one, a 314A which needs no adjustment. I connect my relay to the +/- 12 volt output and use it  to key an external 60-mil loop. The 'scope will show you if you the polar outputs are working before you connect the relay.after that the relay chatter will prove they are working.
>>>>
>>>> My problem with my MPC 1000 is that the signal loss light flickers on space. When signal loss is activated it ties the unit up in mark hold. Mine works fine on the bench but acts up in the rack. I am stumped!
>>>>
>>>>
>>>> Tom   N3CRK
>>>>
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