Sansui 7000 Receiver with weak FM

[KilgoreCemetery]

I have an otherwise working Sansui 7000 amp, but FM is incredibly weak and AM gets very strong bleed-over from Aux. I'm hoping to correct both problems, but I'm assuming that they have separate causes and I'm far more concerned with the FM.

At first, I thought FM was just completely out. I could hear constant static, but nothing else. I cleaned all of the controls, including the Muting adjustment on the back, and hooked up a dipole antenna. Now I can faintly hear a local radio station which I normally get in Stereo without an antenna on other receivers. There is no noticeable deflection in either of the meters, which leads me to believe that the problem is very early on.

I do have an old tube-type RF signal generator and a scope, as well as the schematic and service manual, but not enough knowledge to track down the problem. Can anyone give me some pointers, or an explanation of how the front end is supposed to work?

 

[PETERDECO]

The most common problem I have found with weak signal strength on receivers is the first RF amplifier stage the antenna is attached, probably due to static charge from lightning. However, a faulty IF stage will produce the same effect.

 

[KilgoreCemetery]

That would make sense. There's nothing charred or visually out of place, but maybe it's more subtle. What would I be looking for? Open coil? Bad FET maybe? Or is it just kind of random?

 

[bertus]

Hello,

Could you post the schematic of the antenna input section with the first transistor/fet ?
(you could print the page to a new PDF).

Bertus

 

[PETERDECO]

It can be anything but if you have limited knowledge, just replace the front end FET. That might solve the problem. Otherwise, you would need your signal generator and scope to test each stage for amplification from antenna down to the 10.7MHZ IF stages. Posting a schematic would be helpful. Your service manual should contain some troubleshooting tips depending on the problem.

 

[KilgoreCemetery]

I'd have to look into availability and possible replacements would be for the FET. It's a 3SK39. Here's the schematic of the front end

 

[bertus]

Hello,

The dual gate mosfet 3SK39 would be the prime suspect for the low sensitivity.
In a cross reference I found that it may be replaced with a BF960.

Bertus

 

[KilgoreCemetery]

Thanks, Bertus! I hate to be a bother, but I'm having some trouble finding anywhere that I can buy either a 3SK39 or a BF960. When I click on the BF960 link, it takes me to the AllAboutCircuits website where, if it's the right part, they're asking $80 and it doesn't even look like they have any in stock.

I might look through some parts receivers and see if I can come up with something.

 

[Bluejets]

$20 in the US on Ebay.....

 

[73's de Edd]

Sir KilgoreCemetery . . . . . .

Now I can't see, in order to know, if that units FM tuner is being all sealed up like sardine can . . . . . BUT . . . . if you can get access . . .first try this evaluation.
Note the shared connective comonality of the drain of the RF amp and the 22pf cap and the matching tap into the RF mixer stage.
You are needing to be able to get access for soldering a connection into any of that common wiring run.
Take away any antenna terminal input RFconnection.
Take hookup wire in hand and strip 1/4 in of insulation from its free end and solder tin it. Then you spool off extra wire and measure for an end length of EXACTLY 28 and 17 / 69th inches*** of wire ( ± 1 foot) and then cut off to that length.
Then solder tack into that mixer RF connection to then evaluate and see if this input point is outperforming or equalling your previous frontal input sensitivity performance ?
If it is, then there is a RF amp shortcoing.



73's de Edd . . . . .

I'm now at that age where my mind still thinks that I'm 29,
my humor suggests I'm 12,
while my body mostly keeps asking if I'm sure I'm not dead yet.





*** Jes' messin' wit 'cher mind . . . . .
.

 

[PETERDECO]

If parts were still available, you could order an F-1323 front end module. And I'm sure it is a sardine can by the shielding in the schematic. You can check the IF for amplification but if it is OK then you have to open the module. I just found this:

https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20201022005643&SearchText=bf960

 

[73's de Edd]

You'se guys . . . . .

Mine eyes show that to be having a pop off / wrap around tuner shielding.
If that is being a TO-72 casing schmitt der 4 gold flashed leads . . . as a dual gate MOSFET . . . . . . I am still havink sum left over from my ham radio days.
I initiallly was expecting it to be a UHF hockey puck casing
If finding yours bad . . . . one available . . .au gratis . . . . contact me via sites PM.

PICKCHOOR






73's de Edd . . . . .

 

[KilgoreCemetery]

Then you spool off extra wire and measure for an end length of EXACTLY 28 and 17 / 69th inches*** of wire ( ± 1 foot) and then cut off to that length.

Thanks, Edd. I needed that on this snowy Monday. Surprisingly, adding a wire in didn't make any difference. I tried a regular wire, then I tried half of a dipole antenna just to be sure.

 

[KilgoreCemetery]

Update: I found a parts unit! One channel blows the speaker fuse, but FM works and it will even pull in stations in Stereo!

So.. I disconnected the wires between boards F-1323 and F1243, then jumpered from the working unit to the non-working one. Long story short, board F-1323 works on both units. On to the next board! Here's the schematic for this board. However, I notice that the IC is different and even has a different number of legs. It seems like some of them are a bit redundant. Nothing else appears different. All the same land marks, at least.

The IC on the good receiver is the LA1111 mentioned in the schematic. The parts unit has a Toshiba TA7027M, which I can't find hardly any info about. My Google-Fu brought me to an Ebay listing for one that claims it replaces an ECG 726, and I was able to find a datasheet for it, but who knows if that's accurate.

 

[KilgoreCemetery]

Well, I found at least one bad part.. CF202, which is a three legged.. capacitor? I'm not at all familiar with this type of ceramic, but it doesn't have anything going on between any of the legs. I tracked it down by checking for signal at the transistors. Collector of TR201 had signal, but the base of TR202 didn't. R209 was well within tolerance. So, I pulled CF201 from both units and they both showed 49pF between two of the pins. One of the CF202's had 49pF, but the other had no connection on any pins.

Stations now come in a little louder, but not by a whole lot. Given how strange it is to find a bad ceramic, I think I'll just put it back and swap the boards entirely

 

[Technomaniac]

The CF device will be a ceramic filter, which probably is tuned to 10.7 MHZ. They DO give trouble. The frequency may be marked on it. It will be easy to find another one. Don't expect to be able to test it, out of the tuner. Replacement is the go.

 

[Technomaniac]

If that schematic is pertinent, you have SIX ceramic filters to get a signal through. I doubt that using a piece of wire (tuned or untuned) would be a good source of signal in these circumstances. What you MAY try, is a cap of say 100 pF held across each pair of ceramic filters from input to output, in turn, one pair of filters at a time, to see if you witness an increase in signal. You can also try the cap across each stage from input to output. Make sure you are tuned to a station and have aerial input. If your signal strength improves because of the cap, you will know that the particular stage isn't amplifying. Measuring the 2 or 3 volts on the emitters could let you know if the I.F. amplifier transistors are functioning. Try that first. If your signal generator has FM output you can carefully set it to 10.7 Mhz and try to get it through the 10.7 MHz IF amplifier, because the stages have ceramic filters it will be very narrow, and you will need to rock the dial of the generator carefully back and forth until you strike the correct position, your signal generator's dial may be out a bit. If it is an AM only generator, switch off its modulation so that you have just the carrier frequency coming out. You will need a small capacitor in series with the generator's output in case the generator has circuitry which might short out any DC in the receiver where you connect it. The cap will block the DC path. You can connect a DC voltmeter across the electro in the ratio detector circuit to indicate signal level. Rocking the dial of the sig gen will show on the meter when you're right on. Then you can use the sig gen to jump from stage to stage to check their individual gains.Start on the last IF stage nearest to the detector. The signal output from the generator needs to be kept as low as possible to prevent the action of the automatic gain control circuitry in the receiver, otherwise you might get confused as to what is going on.
BUT, as someone has already said, the trouble is more likely in the front end. The RF amp transistor/FET. Though, it'll be good experience for you to experiment with the above tests, you might learn something.
Note that a receiver in a metal box should NOT receive anything without an antenna connected. I didn't see the schematic of the front end, and by the IF transistors used, the device looks to be a bit vintage. But if I'm wrong, and it happens to have something other than just an RF transistor and a local oscillator up the front, check that you don't have to set the channel spacing somewhere to be correct for wherever you are living, countrywise.If it has a digital frequency display and you know the frequency you are receiving, you can see if its correct. If you have a dial cord and tuning condenser this doesn't apply.

 

[KilgoreCemetery]

Thanks for the info, Technomaniac! This is indeed a vintage receiver from the early 70's. The problem was definitely on the F-1243 I.F. board where I found the bad ceramic. I probably could have continued tracking down the bad components individually, but given my current knowledge, it was just easier and faster to swap out the boards. It worked great after that and somehow even the AM issue cleared up for some reason. I'm not sure why, but I'm not going to look a gift horse in the mouth.

I do still have the bad board and the parts unit. I might reassemble it and do some experimenting just for learning purposes. Do you know where I could find a 3-legged, through hole, 10.7MHz ceramic filter? It doesn't look like Digikey has them (or I'm looking them up wrong).

 

[Technomaniac]

KC, Just copy and paste 10.7 mhz ceramic filter into eBay and lots come up. But there are different bandwidth versions, maybe other numbers on the filter will tell us that - have a look at the ones on eBay and see if you can find a match. You have so many of them in the board ( they might not all be the same electrically) that maybe one thats a bit wider or a bit narrower in its response mightn't make much difference. If you were going to try to peak up the coils it would need to be the correct filter. What was the bleedover problem on AM ? Are you hearing AM radio on Aux position ? One would think that they would switch the power off on the IF and tuner front end when you're on Auxiliary. But you have to get a ceramic filter fitted before you start working on the set you pinched the board from. Point Z (if you look at the left hand edge of the schematic of the IF board) looks to be the power to the tuner as a whole, and probably comes from the function switch, which is off the page. One would think that would kill the tuner's power supply and should stop any possibility of bleed through in AUX. as the tuner audio wouldn't be getting produced ! Anyway, good work, swapping boards is absolutely acceptable practise in this throwaway era.