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Author Topic: FT-901DM Recieves, but no transmit  (Read 2460 times)
KB9LQA
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Posts: 16




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« on: December 28, 2012, 06:25:00 PM »

I have been able to repair the receive function of the FT-901DM.
But no transmit, i replaced all the caps on the rec. A board, and the rec. C board.
2 of the caps were visibly blown, and ALL the rest were leaking goop.
I have not replaced the 2 big caps on the rec. B board, they LOOK to be OK for now.
Still the tubes don't heat or light. I have tryed a known working set of tubes with the same results.
Is there something that would commonly be damaged when the caps blew? Or could someone
provide a few ideas as to proceed with troubleshooting? And ideas would be appreciated, THANKS, CHRIS....
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KE3WD
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Posts: 5689




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« Reply #1 on: December 28, 2012, 08:15:01 PM »

If the tube filaments do not light, check the Heater switch on the front panel, make sure it is turned on as well as the power switch. 

The Heater switch is primarily there so that the rig could be drawing less current in standby mode, consider that there was once a time when hams ran these hybrid rigs in the mobile to understand why that would be of interest to them.  If your switch contacts have gone south, jumping the switch won't hurt a thing, just use the on off switch normally.  of course, replacing a broken switch is always a good thing, but consider the age of the rig and the fact that you may have to find a used one from someone parting out a rig.  The good news there is that Yaesu used the same switch in several different models from that era. 

If still no light from tube filaments, the heater switch is certainly suspect, they can and do fail, check for filament voltage at the switch terminals, also at the tube pins. 

If the Heater switch checks out good but still no filament voltage, trace it back to the transformer and make sure you have filament voltage coming from the transformer. 


73
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KA4POL
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Posts: 2093




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« Reply #2 on: December 28, 2012, 11:57:12 PM »

The heater switch also should activate the fan. So this should be your hint.
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KB9LQA
Member

Posts: 16




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« Reply #3 on: December 29, 2012, 06:59:17 AM »

Thank you for the replies, yes the fan works when the heater switch is turned on. And there is power to the top of the tubes (915 VOLTS). I will proceed with checking for filament voltage thanks, Chris......
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W9GB
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Posts: 2656




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« Reply #4 on: December 29, 2012, 08:50:36 AM »

Chris --

In addition to the Yaesu FT-901DM Service Manual (200+ pages) which you should have on your repair workbench,
FoxTango (Carol Maher) has extensive photographs, service bulletins, and notes for DIY Repairs on this radio.
http://foxtango.org/FT901/FT-901-902%20Web%20Page.htm

Yaesu FT-901DM SURVIVAL GUIDE
http://foxtango.org/Files/FT-901DMsg.pdf

C-1701 (100 pF, 3 kV) is a common problem on the PB-1715A PA Board as these radios have aged,
it is the coupling capacitor between the Driver tube Anode and the PA grids.
When it becomes leaky or goes bad, PA tube damage is possible.

The rectifier A and C boards are known to "blow" the Elna (Japan) capacitors --
On the Rectifier A board, you need to CHECK IF the diode modification has been performed in the Screen Supply of the PA with S/N radios prior than 08xxxx, otherwise ARCing of PA tubes can occur.
http://foxtango.org/FT901/graphics/FT-901_A_Rectifier_Unit.jpg

http://foxtango.org/FT901/graphics/FT-901_C_Rectifier_PB-1717.jpg
so replacement was a good idea.  

Given the age (> 30 years) of these Yaesu models, the power supply/rectifier boards should be re-built with modern 500 VDC, low ESR, 105 degree capacitors.
Quote
I have not replaced the 2 big caps on the rec. B board, they LOOK to be OK for now.
Still the tubes don't heat or light.
The Rectifier B board supplies the 850 VDC for the PA anode (tubes).
Carol notes that R1405 and R1406 should be checked for proper value (470k), which is part of voltage divider.  IF they differ in value (unbalanced), then one of the 2 capacitors will be handling close to its 500 VDC voltage value !!
« Last Edit: December 29, 2012, 09:20:06 AM by W9GB » Logged
KG6YV
Member

Posts: 515




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« Reply #5 on: December 31, 2012, 11:11:21 AM »

Doesn't the 901 have a screen grid cutoff switch on the back panel?  It is off when using a transverter but must be on for the 6146s to conduct and put out power....

It may be marked either "SG" as on the kenwoods or "Transverter".

Greg
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KB9LQA
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Posts: 16




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« Reply #6 on: January 01, 2013, 04:13:47 PM »

Thanks again, for the replies.
I have printed off the service manual, and I will read up a little before I proceed.
Thanks again, Chris......
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