Yaesu: FT-8900 blown out 6 meter RX (low sensitivity)

[mancow]

I have an FT-8900 in my Jeep and I'm about 90% sure I blew out the 6 meter portion when I was tuning the screwdriver for FT-857 for 6 meters. Apparently the antennas are too close. I was driving to a 6 meter repeater site and was talking on the repeter with a friend. I hit the tune on the FT-857 and soon realized I lost the repeater on the 8900. I wasn't able to pick it back up until I was basically at the site. I haven't yet pulled it and put it on the test set but it seems all other bands are fine except 6 meters.

My question is what is the most likely culprit causing the low sensitivity? I found a manual but haven't had time to dig in. I'm looking for any advice from anyone who has fixed a similar problem.

 

[KB4MSZ]

I have one of these rigs. If the other 3 bands are still normal then it doesn't seem likely that the problem is the front end of the receiver. I now it's quite a coincidence but I would suspect something went wrong with the antenna attached to the 8900.

 

[mancow]

Hmm interesting thought. The antenna is a comet uhv-4 I will sweep it and see.

 

[mmckenna]

I thought the amount of power used when auto tuning one of those antennas was pretty low. Am I missing something?

 

[mancow]

It is the atas 100. Not sure how much power it tunes under but it seems like high power.

 

[AA4TX]

Looking at the schematic, the path for 50 MHz is as follows:

Low Pass Filter to RL1003;
Low Pass Filter to RL1002;
D1144 to Low Pass Filter;
Then RF Amp Q1006;

D1144 and Q1006 are the semiconductors at the beginning of the path.

 

[AA4TX]

D1144 is shared with 29MHz.

 

[mancow]

Thanks, I'm looking at the PDF of it now. The next challenge is to find the replacement parts. I guess if I do I will try shotgunning it and replacing D1144 and Q1006.

It looks like D1144 is a HSC277 and Q1006 is a 3SK296ZQ.

It looks like the 3SK296ZQ isn't that popular. There are only a few on ebay and all are from overseas.

 

[mancow]

I found a spares kit on ebay for the FT8900. It's from the UK but looks like the way to go with all the parts.

Yaesu FT-8800R, FT-8900, FT-8900E, FT-8900R Semiconductor spares Kit | eBay

FT-8800, FT-8900 series semiconductor pares kit. RT1N441U Digital Transistor (30 used in the FT-8900). 2SC4617 Transistor (18 used in the FT-8900). HSC277 Diode (26 used in the FT-8900). 2SA1602 Transistor (1 used in the FT-8900).

www.ebay.com

 

[AA4TX]

I would use a signal generator to inject an appropriately modulated signal and then trace/measure the signal before assuming the components were defective.

Is 10m showing the same symptoms?
If not, D1144 is not the culprit.

 

[mancow]

No, all other bands appear to be fine both RX and TX wise.

 

[KB4MSZ]

Here is what I found stated by a Atas 100 user:

The ATAS-100: Up n Down!

Tuned to 7.255 MHz Tuned to 53.030 MHz

Tune to the frequency, press TUNE momentarily, the radio and antenna will do the rest!

The radio transmits a low output carrier while it sends dc pulses to the ATAS-100. Once the SWR falls into spec, the radio switches back to receive mode.

If this information is correct, I can't see it being an issue with the 8900's receive.

 

[mancow]

I wouldn't think so either but I know that if I tune it to 40 meters it will at times kick on the airbag light so obviously a fair amount of RF is being put out. I wish I had an RF power meter that passed DC to put in line and see what it's doing. Maybe I could use my field strength meter or an SDR with the gain turned way down and key up on CW while the antenna is tuned then detune and again initiate a tune session for the same frequency and see if the observed output near the same level as the previously tuned position as it moves near resonance.

 

[N4GIX]

I thought the amount of power used when auto tuning one of those antennas was pretty low. Am I missing something?

IIRC, my FT-857D uses ~5 watts while autotuning.

 

[mmckenna]

IIRC, my FT-857D uses ~5 watts while autotuning.

Yeah, that's what I thought. Seems like it would be difficult to damage a radio with 5 watts into an inefficient antenna unless they were touching.

 

[Chronic]

kinda sounds like a stuck relay

 

[mancow]

kinda sounds like a stuck relay

Hmm interesting thought.

 

[AA4TX]

kinda sounds like a stuck relay

That would affect 10m as well.

 

[clovisb31]

I have an FT-8900 in my Jeep and I'm about 90% sure I blew out the 6 meter portion when I was tuning the screwdriver for FT-857 for 6 meters. Apparently the antennas are too close. I was driving to a 6 meter repeater site and was talking on the repeter with a friend. I hit the tune on the FT-857 and soon realized I lost the repeater on the 8900. I wasn't able to pick it back up until I was basically at the site. I haven't yet pulled it and put it on the test set but it seems all other bands are fine except 6 meters.

My question is what is the most likely culprit causing the low sensitivity? I found a manual but haven't had time to dig in. I'm looking for any advice from anyone who has fixed a similar problem.

Mine did the exact same thing a year ago. I was in my shop tuning/testing another 6 meter radio. My truck , with my FT8900. was parked a few feet away in the shop. At some point , I don't remember how long it took me to discover it , but I realized the 6 meter RX was deaf on the FT8900.
The service monitor confirmed that ONLY 6 meters was affected, as all the other bands had sensitivity within specs. Looked at the schematic and decided to send it to Yaesu for repair since it was still under warranty. Figured it was a blown RF amplifier transistor in the 6 meter path.

Sure enough, upon return about 3 weeks later that's exactly what the problem was. It's Q1011 in the 6 meter RF path that was the culprit.

I figured it was a fluke and that I just got a weak radio. But I did recall that I had transmitted 6 meter energy on the order of 50 watts inside my shop. I just decided that I wouldn't do that anymore near my FT8900, just to be on the safe side. Now it's looking as If the 6 meter front end on these FT8900 may have a bit of a design flaw in the 6 meter RX path.

Interesting that I'm not the only one.

73,
Shane KE6JZ

 

[mancow]

I tried replacing Q1011 from a scrapped VX7R but still no improvement. Then again, that 7R had been hit with high RF so maybe that one was bad too. I'll try to pull another off it and try again.