Resurrecting an FT-757GX Mk I - No Tx or Rx

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Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby KE0DNZ » Mon Mar 13, 2017 8:42 pm

Greetings!
I just got hold of a Yaesu FT-757GX Mk I (built May 1984); I bought it 'as is' for $25 at a ham club swap (just got my General, so I need an HF unit).

I haven’t opened it up yet to do any testing, but here’s what a preliminary check found:

The unit doesn't TX or RX, although things seem to be operable otherwise. The controls seem to work, and switching between modes (RX) gives different variants of background noise. LSB is a bright noise, USB is a bit darker, the CW modes even darker, AM is brighter, and FM has very little noise.

The VFO knob turns fine, and the display operates correctly, so it appears that the CPU and battery are good.

I checked the +8v and +13v jacks on the back, and they were at proper voltage.

Squelch kicks in at about 11 o’clock, and other controls seem to be in good shape.

From what I've read about the FT-757GX - here on FTI, mainly - the areas that seem to be problems (and, fwiu, automatically changed by Yaesu when units were sent in for repair) are:

1) Diodes - There are something like 14 diodes on the RF board that have a tendency to blow; a post here on Fox Tango recommended changing the stock diodes with 1N4148 or BAV21, which seems to solve the problem.

2) Transistor - The 2C496Y on the display board tends to burn out, cutting off the +8v to the radio. Since the +8v on the output jack is good, though, would that not be part of the problem, or does that +8v come off the IC on the main chassis?

3) Crystals - Replacement of the crystals, along with better quality trimmer caps, was recommended.

The bottom line is that with the exception of the Tx & Rx, everything is good. Granted, that’s like saying “This car is in great shape, except that the engine doesn’t run.”, but I’m thinking in terms of it being just the Tx/Rx fix, instead of having a lovely boat anchor, and having to start from scratch.

What’s the best place to start focusing on once I open it up?

Thanks and 73's!

-Bob / KE0DNZ

ADDED: For the various replacement components, are there brands that tend to work better, i.e., is the Fairchild BAV21 better than, say, the Vishnay BAV21? Same spec, of course, but consistency, etc., can vary from manufacturer to manufacturer...

Also, for the crystal trimmer caps (30pf/10pf), are there recommendations for higher quality/reliability parts than OEM?

Is everything but the crystals (diodes/2C496Y transistor/trimmer caps) available from places like Mouser/DigiKey?

And are the crystals only available from Yaesu, or are they available elsewhere?
KE0DNZ
 
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby w6nfu » Tue Mar 14, 2017 8:29 pm

Bob,
I logged in, then sent a long reply only to hit "SEND" and it told me to log in again. Everything lost. I think I can help.
Dave - w6nfu
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby KE0DNZ » Wed Mar 15, 2017 6:41 am

Dave,
Oh, I HATE when that happens!!! :(

I've gotten into the habit of either: A) Writing out the answer in a text editor, and THEN pasting it in, or; B) Copying it and saving a copy before I hit send. Believe me, I speak from experience... :?

In any case, any help you can give me is VERY much appreciated!

Thanks and 73's!

-Bob / KE0DNZ
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby w6nfu » Wed Mar 15, 2017 12:48 pm

Bob,
Email me thru qrz.com so we can work thru your 757. Afterwards, if successful, you can post results here for the benefit of others.
Dave - w6nfu
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby KE0DNZ » Wed Mar 15, 2017 3:10 pm

Dave,
Thanks! Message sent!

-Bob / KE0DNZ
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby N2OIO » Mon Jul 24, 2017 5:31 pm

Bob,

I have the same radio and the EXACT same problem. Did you ever find a fix?

Mine came from a flea-market. I thought I got a steal for $45 only to find out no RX or TX..
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby KE0DNZ » Tue Jul 25, 2017 11:46 am

N2OIO wrote:Bob,

I have the same radio and the EXACT same problem. Did you ever find a fix?

Mine came from a flea-market. I thought I got a steal for $45 only to find out no RX or TX..


I wound up sending my radio to Tim Moes at Midwest Technical Services in Watertown, SD - he's the BEST tech you'll find - and unfortunately, it wound up that one of the ICs that controls the 2 PLL circuits was bad, and Yaesu doesn't have the part available (of course, right?).

Sadly, you'll probably have to find a good local board to replace it...

Unless, of course, somebody knows of a source for that particular IC!

-Bob / KE0DNZ

P.S. Many thanks to Dave (W6NFU) for all his help!!!
KE0DNZ
 
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Re: Resurrecting an FT-757GX Mk I - No Tx or Rx

Postby N2OIO » Tue Jul 25, 2017 6:38 pm

KE0DNZ wrote:
N2OIO wrote:Bob,

I have the same radio and the EXACT same problem. Did you ever find a fix?

Mine came from a flea-market. I thought I got a steal for $45 only to find out no RX or TX..


Sadly, you'll probably have to find a good local board to replace it...


Thank you, I was able to get receive working, so I may have a bit more hope. TX is still dead though. It was previously in a storage locker for an undetermined amount of years. Based on the other equipment it was with, nothing has seen the light of day since the 80's. The inside looks like new, but someone definitely poked around in there. Outside shows minor use, just needs some cleaning.

I ordered some diodes, and will probe some voltages as per Dave's advice as soon as I can. I will fiddle with it for a while and if I can't get it working I will have to make the decision to pull the plug or send it for repair.. Not sure if its going to be worth the cost of repair.
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