Sansui 5500 - low FM output

[anti-doom]

Sansui 5500 has FM issue.

Before: signal meter did not response to any station. Completely dead. NO FM at all. Found out two transistors among TR201 ----> TR206 were shorted.

Replaced all transistors TR201 --> TR206 with new ones (2SC930 equivalent)

Now: FM Signal meter responses to the stations. But FM is very low. Volume is up all the way but barely hear FM sound.

All measured voltages at new transistors are in line with schematic.

Stereo light lit all the time.

AM works as normal

Any though/suggestions.

Much appreciated.

 

[kevzep]

Where did the transistors come from and what are they? What part number?

Whats going on around the detector, is the detector chip working?

 

[anti-doom]

Where did the transistors come from and what are they? What part number?

Whats going on around the detector, is the detector chip working?

Those transistors are labelled as TR201 - TR206 ( 6 total) on the schematic. Don't know if the detector ship is working or not as I don't know what it is.
Thanks.

 

[kevzep]

Those transistors are labelled as TR201 - TR206 ( 6 total) on the schematic. Don't know if the detector ship is working or not as I don't know what it is.
Thanks.

Okay, I'll phrase it a different way.
What are the transistors you purchased, and where did you get them from? I need a manufacturer and part number. They need to be legit, and they need to have the correct gain and bandwidth.
If you don't know what a detector is, then this is probably outside your skill set....The Detector strips the audio out of the 10.7mhz IF which is stripped from the carrier.
You really need an RF generator which can produce a carrier between 88-108mhz and then also do Frequency Modulation with an Audio frequency so you can look at what is going on and where the audio might be getting lost.
Anything outside this is just guess work, but who knows you might get lucky.

The detector chip is IC201.

 

[anti-doom]

Okay, I'll phrase it a different way.
What are the transistors you purchased, and where did you get them from? I need a manufacturer and part number. They need to be legit, and they need to have the correct gain and bandwidth.
If you don't know what a detector is, then this is probably outside your skill set....The Detector strips the audio out of the 10.7mhz IF which is stripped from the carrier.
You really need an RF generator which can produce a carrier between 88-108mhz and then also do Frequency Modulation with an Audio frequency so you can look at what is going on and where the audio might be getting lost.
Anything outside this is just guess work, but who knows you might get lucky.

The detector chip is IC201.

Thanks for all the info and you are right. I don't have any equipment you mention. Original part is 2SC930 and I got exact replacement 2SC930 from ebay. Before, no signal detected, now I got signals but no sound.

 

[llwhtt]

There's an optocoupler, MPL251, in the mute circuit that goes bad in the 5000, unit stays muted when bad. Don't know if there's work around other than bypassing it. Very common problem. I have one sitting here in my pile with that problem but have zero interest in it.

Craig

 

[kevzep]

Thanks for all the info and you are right. I don't have any equipment you mention. Original part is 2SC930 and I got exact replacement 2SC930 from ebay. Before, no signal detected, now I got signals but no sound.

Yeah worst place to buy components from is eBay, they are likely to be fakes, so there's your first problem.
Those transistors have not been manufactured since the early '80s most likely, so to get ones off ebay with the same part number is a sure sign they are not legit.

 

[kevzep]

There's optocoupler, MPL251, in the mute circuit that goes bad in the 5000, unit stays muted when bad. Don't know if there's work around other than bypassing it. Very common problem. I have one sitting here in my pile with that problem but have zero interest in it.

Craig

No optocoupler in the 5500 though, so this is not the issue here, but a good one to consider, and its totally possible that the muting circuit needs to be adjusted.

 

[llwhtt]

Oops, I guess I need new glasses, I saw low FM and immediately thought 5000, never mind.

Craig

 

[kevzep]

The other thing to look at is the LPF on the output, you should bypass that.
I have had a string of these things which have failed, mostly in QRX receivers, but as this gear is getting old, anything is possible...

Look in the alignment instructions and see which is the adjustment for the muting, see if thats the problem...

 

[kevzep]

Oops, I guess I need new glasses, I saw low FM and immediately thought 5000, never mind.

Craig

Good call though, quite often I have seen muting circuits causing this problem too, so good call on that I say!!

 

[anti-doom]

Thanks for the inputs.

The detector chip (IC 201) looks OK to me. Nothing happens around it. Mine has F-6021 board (circled in red) which does not present on the schematic. Look attached images. One question : Muting circuit is located on F-1402 board right? I have an oscilloscope and just bought an RF signal generator. Once I have enough equipment, maybe easier to track down the problem.

BTW, STEREO light is on all the time. Does not matter getting signal or not. Maybe something wrong there.

 

[anti-doom]

Yeah worst place to buy components from is eBay, they are likely to be fakes, so there's your first problem.
Those transistors have not been manufactured since the early '80s most likely, so to get ones off ebay with the same part number is a sure sign they are not legit.

If so, what is the best sub for 2SC930? Thanks.

 

[kevzep]

If so, what is the best sub for 2SC930? Thanks.

I would put a KSC1674, available from mouser. But be careful the pinout will be different so consult the datasheet to make sure you get them in the right way.

 

[kevzep]

Thanks for the inputs.

The detector chip (IC 201) looks OK to me. Nothing happens around it. Mine has F-6021 board (circled in red) which does not present on the schematic. Look attached images. One question : Muting circuit is located on F-1402 board right? I have an oscilloscope and just bought an RF signal generator. Once I have enough equipment, maybe easier to track down the problem.

BTW, STEREO light is on all the time. Does not matter getting signal or not. Maybe something wrong there.

You need to verify its working, but before that, pull those dodgy ebay transistors out and get the right ones in there.

Then you need to verify the Local Oscillator is working which is in the front end pack. That uses a 2SC930 as well.

Once you get the RF oscillator, it will then be easy to quickly go through the tuner and probe about looking at where the signal is getting lost.
Its all guesswork until then.

 

[anti-doom]

You need to verify its working, but before that, pull those dodgy ebay transistors out and get the right ones in there.

Then you need to verify the Local Oscillator is working which is in the front end pack. That uses a 2SC930 as well.

Once you get the RF oscillator, it will then be easy to quickly go through the tuner and probe about looking at where the signal is getting lost.
Its all guesswork until then.

Thanks for all the info you provided. Gotta order now and will update the result. Thanks

 

[anti-doom]

Problem solved. It was the IC201 (part number TA-7027M). Replaced it with new ones and FM radio works fine now. Next is to do the alignment. Thanks all for help.

 

[ghazzer]

Did you replace the fleabay "NOS" 2SC930 TRs with KSC1674 (or equivalent)?

 

[anti-doom]

Did you replace the fleabay "NOS" 2SC930 TRs with KSC1674 (or equivalent)?

Forgot to mention. Yes I did replaced them with KSC1674 from Digi-key. Works fine now.