SX737 FM tuner problems

[pwauk12]

Hi

I have an SX737 which I am recapping, however the FM tuner section simply produces hiss(which then goes away when the muting button is pressed), and no matter where I move the dial there is the same hiss.

So I have replaced the HA1156 and HA1137 and all the electro and film caps on the tuner board, to no effect.

If I touch some components, or when I have the multimeter checking voltage or the scope looking for signal, I get signal sufficient to give me hiss with the muting pressed, and if I touch one of the 10.7Mhz ceramic filters, or the 22k resistor next Q2, I nearly get a station....

I have checked all the voltages, and they nearly look right (and I am happy to post all the details...), however I think I am missing something basic here, so any advice or thoughts are greatly appreciated.

 

[larryderouin]

You are using an external antenna,correct? A cheap $6.00 Radio shack "T" Dipole should be sufficient to capture a signal in the FM section. By you touching the if filters and or the resistor near Q2 and getting more than hiss, tells me you need an antenna. The bar antenna on the 737 is for AM only.

Take another receiver put it next to the 737 and tune it to 90mHz. Now tune the 737 10.7 mHz higher or 100.7mHz (almost to KIOI(101.3)) and see if you get a null. If you DO, then the If section should be working ok.

The muting switch locks out weak signals. If you have a station you like that has a threshold below that of the muting switch, then the muting switch needs to be off.

I wouldn't mess with it any further, and take it to a Radio shop that does alignments. Check with local HAM Radio groups for their recommendations.

Larry

 

[markthefixer]

I know I didn't tell that trick yet, where did you get it?

 

[larryderouin]

Learned that one from when I was a kid, and playing around with AM stuff. AM you tune 455kc off and FM 10.7mc off.

 

[KKnight]

So Ive replaced the HA1156 and HA1137 and all the electro and film caps on the tuner board, to no effect.

Whoa. May have worsened your problem. Do a search here on AK. You never touch your tuner board or section or replace anything unless youre willing to do a complete recalibration and alignment, and have the equipment or the funds to send it to someone who knows what they're doing and the equipment to do so after any cap replacement, etc, per the service manual. There must be 50 threads and posts about that thru the years chastising anyone for touching the tuner and recapping it without the equipment and ability to realign and recalibrate it. Even those few that have the ability, knowledge, and equipment rarely want to do it.

 

[Watthour]

Learned that one from when I was a kid, and playing around with AM stuff. AM you tune 455kc off and FM 10.7mc off.

That can be even more effective if you tie one of the the two 300Ω inputs together. I found that inadvertently as well.

 

[rcs16]

There must be 50 threads and posts about that thru the years chastising anyone for touching the tuner and recapping it without the equipment and ability to realign and recalibrate it. Even those few that have the ability, knowledge, and equipment rarely want to do it.

Let me clarify this.
Changing ecaps in the tuner, takes them out of alignment, when the ecaps have nothing to do with the tuning operation in the first place?
Show me some specific examples of what parts changed, in what assemblies that do this = take them out of alignment?
Mucking around with multi-ganged tuning cap/coils is a whole other matter.
The vary fact that you have take the dial string off or remove the plastic wheel is the problem here.
That can be over come however, since the dial has a lot of tolerance. It is never bang on across the dial as it is, as 1-2 mm means nothing. It is tracking that really matters.

 

[pwauk12]

tuning issues

hi

so the story evolves.

I have pulled the RF board out so I can see the underside, without removing the dial string, and have found some soldering issues, suggesting that there has been some attempt to repair the unit...so I have found a couple of what could be dud solder joints. However when i test the 2SC535, it has a HFE of about 20 rather than the 100ish I would expect. I also notice when I connect the scope or other test gear, i get a much stronger signal suggesting that there may be an an antenna issue as well.

as to cap replacement, my understanding is that swapping out the electro caps is just fine, as they have little to do with any of the tuning/alignment, but rather do the usual things electros do, such as decouple.

I am contemplating replacing some of the circuit ceramics, as one of these may be open which would sort of align with the behaviors.

When I inject a 98Mhz signal, I can get a 98Mhz reading from the tuning cap, which is encouraging, but I still do not feel closer to the solution.

I am now going to try the suggestion of larry as I have just about finished work on a marantz 150 and so can try this test.

Oh and to test gear, I have an ancient Standard FM signal generator which will give me the FM signal, which I am using whilst trying to resolve why the nice Sencore SG165 I bought off ebay does everything...but send out the FM signal (I think I have figured this out).....so like it or not I am becoming familiar with FM receivers, how they work and how to fix them, though if there is anybody around San Francisco who would like to assist/advise I would certainly be suitably grateful

and thanks to everyone for their input

Peter

 

[markthefixer]

Let me clarify this.
Changing ecaps in the tuner, takes them out of alignment, when the ecaps have nothing to do with the tuning operation in the first place?
Show me some specific examples of what parts changed, in what assemblies that do this = take them out of alignment?
Mucking around with multi-ganged tuning cap/coils is a whole other matter.
The vary fact that you have take the dial string off or remove the plastic wheel is the problem here.
That can be over come however, since the dial has a lot of tolerance. It is never bang on across the dial as it is, as 1-2 mm means nothing. It is tracking that really matters.

FM detector chip (distortion - you can null the tuning), then stereo decoder (loop filters etc) but a rough hand setting of vco center freq will work. What suffers there is separation and sub-carrier rejection adjustments - and you need a quiet channel (R=full, L= no signal) to set separation.

This essentially takes a totl tuner and puts it in motl to botl specs.

There ARE a few caps around the detector IC that should not be messed with if you want to hold alignment .. i.e. distortion.

Believe me, I have TRIED this and the results and dangers are such that I won't attempt to handhold DIY tuner work in those specific areas. And the demands of time for a hand holding recap specific to each tuner board (and then the damn factory variations) and NOT affect alignment make it impossible for me to do.

Of course if you are going to do, or get an alignment done, this all goes out the window. I even looked into how to whip up a simple DIY stereo signal generator, but ran headlong into the screaming need for a distortion analyzer on the other side of the tuner circuits.

 

[markthefixer]

was it working before you started?

what does the center of tuning meter do? What does the signal strength meter do?

I'm wondering about your 3sk45 q1

plus there's q2 2sc535, AND Q3 2SC535

the R22 is the power / bias setup, and should be decoupled and filtered from the LO by caps and inductors.

start boning up on FM theory if you're going to poke in these areas. Wikipedia has good articles.

 

[samc777]

HP 33X Distortion Analyzer

Of course if you are going to do, or get an alignment done, this all goes out the window. I even looked into how to whip up a simple DIY stereo signal generator, but ran headlong into the screaming need for a distortion analyzer on the other side of the tuner circuits.

Mark, any recommendations on a used distortion analyzer? I'm aware of the price and that is not my main concern.

I see a lot of HP 33X models on auction. Good, bad or ???

Dave

 

[pwauk12]

background

Hi, thanks for the imput

The FM tuner section was not working when I got the unit, everything else (inc AM) works fine, just produces hiss which seems to not change at all when I move the tuning dial. Neither tuning meter nor signal strength meter move at all, although when I injected the 98Mhz signal the tuning meter deflected a little.

I have measured all the voltages on the schematic and will happily post these if that will help, but the ones at the FET and the 2SC535's seem to be right.

the earlier "repair" work is around Q2 the 2SC535, which I also suspect and have found a possible substitute (my transistor tester sees the current 535 as a darlington pair, which does not seem right), -KSC1647OTA- though I also have a couple of ones in an old marantz FM front end which I may be able to use (if they work- and that was another "adventure" seeking a ferrite slug)

The 3SK45 seems to test OK, though replacing it would be fine if I can find a suitable replacement.

as to distortion meters I have a HP8903B which I am learning to operate, though a separate distortion meter could be a good thing

and as to hand holding, completely understand, however I appreciate the insights and experience but full realize it is mea culpa.....

thanks again

 

[pwauk12]

more details

so the 3SK45 measures 3.1v and 11.7v matching the schematic, though the "other half" of the unit has 0v on both legs, which is odd as the 5.6k resistor (R2) has 3.1v at its end (and 12.4 coming from the B+ at the other)

Also I tried to hook a freq meter to see if I could find any variance in the meter count in line with the movement of the tuning wheel and when I touched the base of Q2....I got (for the first time) a station at the lower end of the FM spectrum...noisy and distorted but the first FM I have heard form this unit.

as the Freq Meter is mains powered, and when probing with my battery powered fluke on the same spot, I got increased levels of hiss, I am thinking that there is no connection to earth somewhere in the early tuning stage.The Freq meter has an input resistance of 1M, and yes the 1M R1 measures ok...( i was sort of hoping....sigh)

I also tried larrys tuning offset , and got no null...which implied that the IF section is the culprit, though I do wonder if the LO is not working (perhaps as well as the earth issues)-though I get the correct voltages with Q3 and no uplift in noise/signal

there are just not that many components, and given I can get a signal through, this conundrum has got me....

Thanks

 

[pwauk12]

Conumdrum

so the saga continues:

I have reconciled myself to having to do an alignment, when I get the FM side working, however I feel I missing something very basic.

I have an old sig gen which generates a 98Mhz signal, which I have fed to the antennae inputs at the rear, figuring I would do a trace. To test this I connected one channel of a dual channel 100Mhz scope (1M and 20pf inputs) onto VC2 and have a nice clean signal. (I also tested the 3SK45 and it was OK, however I bought another and fitted- no change). Then I connected the second channel to the incoming signal earth (pin 2 on the RF board) and started to probe.

When I connected the probe to the base of Q2 I suddenly got a good strong signal and was able to tune FM stations...and even had stereo briefly......yeah I thought. I then looked at the circuit assuming that the second channel was providing an earth path and decided to check the inductor L1, which when I removed t tested bad on ring tester (i dont have an inductance meter), so I replaced this. The out come was that when I now connected the second probe to the base of Q2, I get an increase in noise, but no signal....

I have checked the other components that lead to this point in the circuit and all test fine (C9,R4,R10,R11,C11) and all seem OK. I have checked all voltages on Q1, Q2 and Q3 and they are all as per the circuit.

So given that the the system could be coaxed into working when the two probes (both grounded at pin 2 on the RF board), I am feeling that the incoming signal is either not getting through or that the earth side is not being held down.

Any and all thoughts and comments are greatly appreciated

I have attached the relevant part of the circuit


Thanks in advance


Peter