SX-3800 display

Matt Mincer

Matt Mincer

Active Member
Hey All,

I have an SX-3800 that works great except the frequency display is jumping around like crazy. The thing tunes fine, it's just the numbers that jump around. It happens on AM and FM. Any clues?

Thanks
Matt
 
ah yes... I paid stupid money to get this issue figured out on my unit. When you pop the cover off the receiver and look on the board directly above the tuner knob there is a little IC that fails and causes the tuning frequency to get all wonky. I can't recall what the IC is BUT that was the part on my restored SX-3800 that was causing the tuning display to show numbers that were all messed up. Basically what the IC did was translate whatever analog tuning frequency you were at to make it display as a LED segment on the fluroscan display. The part that was pulled was made by Mitsubishi IC # M54451P 9ZE0. I believe it is called a BDC to decimal converter IC.

It would take me a whole whopping 20 minutes to desolder this and put a new one in.
 
ah yes! thanks rcs16 for clarifying that. Back in 2010-2017 I couldn't figure out (and was afraid of frying things) after I did my recap and the display for the tuner went all silly on me so I hired a shop here locally to troubleshoot and repair and I got hosed on that along with restringing my tuner... never again will it go to that shop in town. Price gouging at it's best :(
 
Here's the problem with that, I have an SX-3900 on the bench as well. I took the entire display board out of that one (It was a pain but don't ask) and the display did the exact same thing. I also noticed that the frequency to my o-scope doesn't change when I turn the tuner.

Matt
 
If it's acting wonky in both AM and FM it could be the Pre-scaler IC M54451P. The one costly IC mentioned above.
Pins 1 and 7 are from the AM and FM FET's acting as buffers. Probably good as both going bad are a long shot. Plus if bad they may load down the oscillators and cause poor tuning.
Pins 2 and 8 are the input power. 5.5V's?
Pin 10 is the output? Scope?
Check C29, 470/10 Ecap for leakage?

Data Sheet, page 3-53

m5441p.PNG

m5441p-2.PNG
 
Last edited:
zebulon1 said:
If it's acting wonky in both AM and FM it could be the Pre-scaler IC M54451P. The one costly IC mentioned above.
Pins 1 and 7 are from the AM and FM FET's acting as buffers. Probably good as both going bad are a long shot. Plus if bad they may load down the oscillators and cause poor tuning.
Pins 2 and 8 are the input power. 5.5V's?
Pin 10 is the output? Scope?
Check C29, 470/10 Ecap for leakage?

Data Sheet, page 3-53

View attachment 1286114

View attachment 1286121
Click to expand...

Here are some pix of my o-scope re M54451P. Pin 2 reads 6 VDC and pin 8 reads 7.5 VDC. I also noticed that none of the sweeps change, AM or FM, when I change stations. The thing tunse fine though.

https://flic.kr/s/aHsmsN35Jg

Thanks
Matt
 
I don't have one to check but I think the output should be in the form of a square wave and change as you tune the dial.
 
Pin 10 is the TTL level o/p, 0-2.4V min, if you hook a frequency counter up, it should measure the divided LO, usually /100 for FM. The LO freq is the tuned freq + the 10.7MHz IF offset.
if it is stable then it might be the PD5009 part, xtal reference, other comp?
method of elimination.
 
Is the counter the only way to see the output?
Thanks for the lessons.
 
what o/p? LO? I guess so, that and ext instrumentation as I describe above.
You could apply an ext TTL level signal into PD5009, to test it out, but its i/p has to be isolated from the M54451P pin 10 o/p.
 
It turns out it was a power supply problem. I don't recall why I checked this in the 1st place but the 5.5V was up over 8V and had about 1/2V ripple. I thought it was a problem with Q210 or one of the caps in that circuit but they all tested okay. The solder on the transistor looked a little funky so I went ahead and heated it. When I got everything back together, the display worked. I guess I was lucky this time!!

Pre-repair, I had a working SX-3900 on the bench (basically same display circuit) I put the scope on pins 1, 7 & 10 of both M54451P. The link has pix of the o-scope display of both. Hopefully, it'll help someone in the future.

Thanks
Matt

https://flic.kr/s/aHskFip8H6
 
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