2275 - FM tuner static only

Jailtime

Jailtime

Standin' on a corner
Took in an ailing 2275 recently, the FM tuner is nothing but static across the whole dial, no meter deflection. AM tuner works fine, but doesn't drive the signal strength meter. I've started tracking things down in hopes of fixing it. First thing, I only have 10.2V from the power supply where I should have 13.4. The board is mostly rebuilt, I figure I should take a look at the diodes here. With that in mind, here's the info from the tuner boards.

The front end seems to be OK, I was able to sniff out the 10.7MHz from the front end fed to J201 of the IF board using my scope. Not sure how strong this signal should be, but it's there. H201, 202, 203, and 209 on the IF board are all reading 0 on all legs. H204 thru 208 and H210 all seem to have reasonably close voltages to the schematic, considering the weak power supply feed.

It's obvious to me why the tuner doesn't work, H201 - 203 killed the IF, but why are they dead when other transistors in the IF stage are fine?
 
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If c208 or c209 are shorted (look below h203 on schematic), it would kill the voltage on those transistors, but not on the others, due to the 47 ohm resistor in the line. It would probably drop the 13.4 volts line down also. Just a thought. Good luck
 
I've had coils / windings go open in RF transformers once or twice maybe, screwing up the stage they were in. Get that p/s working or find out if it's being overloaded as the latter may give you a good hint.
 
Power supply makes 13.4V without a load. So I pulled the 13.4V feeds to the IF board (J206 and J211), no more pull down to 10.4V. So the IF board is pulling down the power supply.
 
The most likely cause is there is a short after R224. Take a resistance reading from both sides of R224 to ground.
 
previd01 said:
The most likely cause is there is a short after R224. Take a resistance reading from both sides of R224 to ground.
Click to expand...

Looking at the component side of the board, measured 43 ohms right leg of R224, 0.3 ohms on the left leg.
 
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We're making progress now. C208 was the culprit. When I did a baseline test on this receiver, the FM performance was poor, I could pick up only the strongest station in the area. So I went on recapping the boards, and didn't really suspect much when the FM went static. I had a tiny solder bridge across C208, fixed now. Power supply voltages are good, and H201 - 203 are alive now.

The FM reception is still lacking, I can get maybe 3 of the strongest stations in the area, so we're back to my baseline when I first tested it. Signal strength meter seems to be dead on FM, it works great on AM though. The FM tuning needle is well right of center when a station is tuned in.
 
Well that's a step. Don't have any suggestions, but I'm staying tuned (bad pun) to see what happens next.
 
Kyle,
I assume you have cleaned and lubed the tuning capacitor.
If you have a known good front end from another receiver, you can substitute that for your weak FE as a test. Sometimes a nearby lightning strike will take out the first transistor in the front end of the tuner.

Tom
 
^^^ Yes indeedy, I learned this trick from Tom. I have a spare front end on hand with a long RG-174 coax on it to attach to the sick unit's IF board input. A very handy tool/method. I just power it from a bench supply.

I've seen a bad front end transistor in a 2500 front end. Now that was fun to get at!!
 
I cleaned the FM tuning cap in the 2275, it made a marginal improvement. I can pick up a few more stations, but they're still weak, no signal strength meter, and definitely no stereo.

So I took a known good 2220B, and connected its front end to the 2275 IF board. Results were great, 2275's signal strength meter works, and it picks up everything in the area in stereo. So we know the problem lies in the 2275 front end.

Earlier in testing, I had 10.7MHz signal from the front end to the IF board. It was pretty weak, not sure how strong it should be.
 
If you have a calibrated RF source for testing, you can check the conversion gain of the tuner, I generally look for around 30dB of voltage gain from the antenna input (at around 100MHz) to the input (10.7MHz) of the IF strip.
Since you are getting 10.7MHz at the IF input and can tune stations with the 'weak' tuner, we can be reasonably sure that the local oscillator and mixer portions are working. I would replace the RF amp transistor first, I suspect that will fix the weak reception problem.
Fortunately, the RF amp transistor is a 2SK19 junction FET and is easily replaced. Some tuners use a dual gate MOS FET (3SKxx) with no built in ESD protection and must be handled carefully, especially before installation.
They used to come with a small wire wrapped around the leads which you would leave in place until the part was soldered in circuit, then you could remove the wire.

Tom
 
Try aligning the front end for max sensitivity. Follow the instructions in the service manual but use fm stations if you don't have an rf generator.
 
T4ry aligning it first before replacing parts. Sometimes it's not the transistors, it's bad caps.
 
I looked around at some voltages in the front end this morning, H102 looks pretty good. H101 has good 12V on leg D, 37mV on leg S where there should be .6V, and 37mV on leg G where there should be 0. Interestingly, the 2275 receives in stereo when I probe the 12V leg of H101. Still no signal strength meter action though.

I don't have the equipment to do an FM alignment, just a scope.
 
Well,
from your measurements, H101 doesn't appear to be passing current. Looks like H101 is bad. Make sure you ground yourself with a wrist strap when you replace it, and wrap a thin wire around the new transistor leads to short them together, then remove the wire once the transistor is soldered in. This will protect it from ESD damage, You will need to align the front end when you are done, but as i said above, you don't need special equipment for this part of the alignment.
All you do is follow the instructions in the service manual about aligning the sensitivity. When it says, tune to 88MHz, you tune to A known station on the low end of the dial, and adjust the coils in the tuner for the highest reading on the signal strength meter. To do the high end, tune to a station at the top end of the dial and adjust the trim caps in the tuner for the highest reading on the signal strength meter.
 
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