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Sansui QRX-7500A problem - Little to no AM radio stations, weak signal

B

Bladerun

New Member
Hi Everyone,

New member and new owner of a vintage one owner QRX-7500A. Just replaced all the fuse lights with LED's and fixed a few cold solder joints and fuse issues. Everything is working great including all inputs, knobs, and strong FM.

The only thing not working is AM. It has the original ferrite antenna, and I tried a loop and several other antennas attached to the AM slot. I'm only able to occasionally bring in 1 or 2 AM stations. I'm not sure where to start to troubleshoot this and achieve a stronger signal and pull in more stations. I've done quite a bit of solid state repair but never worked on a vintage stereo. This thing sounds absolutely amazing and I probably won't use the AM radio that much, but I want everything 100% when I put it back in that beautiful wood enclosure.

Any help would be greatly appreciated.

Thanks!
 
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Bladerun said:
The only thing not working is AM.
Click to expand...

Well, the AM tuner uses discrete transistors, so one possible approach would be to check DC voltages on, let's say, TR18, 19, and 20 to see if they are significantly different from what the service manual says they should be. (The SM is available from elektrotanya.com and probably other places.) The voltages on just the emitter terminals of those transistors are probably enough to yield a clue about what's amiss.

HTH,

chazix
 
chazix said:
Well, the AM tuner uses discrete transistors, so one possible approach would be to check DC voltages on, let's say, TR18, 19, and 20 to see if they are significantly different from what the service manual says they should be. (The SM is available from elektrotanya.com and probably other places.) The voltages on just the emitter terminals of those transistors are probably enough to yield a clue about what's amiss.

HTH,

chazix
Click to expand...

Thanks so much chazix, I'll measure those voltages and report back!
 
There definitely appears to be a problem as I'm getting 0V at TP1 on the tuner board. TP2 & TP 3 are correct.

That's as far as I got tonight.
 
As the problem lies in the AM section, you should concentrate your efforts on the circuitry containing:-
chazix said:
TR18, 19, and 20 to see if they are significantly different from what the service manual says they should be.
Click to expand...
as suggested by chazix - for example, around TR18 (2SC403C), you will see voltages of "0.85V" and "10.8V" - measure these and the other voltages marked on the schematic in that area - preferably post them up here for us to look at. ;)
 
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Sorry, it was the middle of the night and I forgot to post those! These are the DC values from the emitter leg of the transistors at the following locations:

TR18 - .690
TR19 - .667
TR20 - 3.37

Thanks!
 
And the other voltages too?

Useful to know as much as possible - as for example if one of the coils is 'open' the loss of a whole stage of amplification could explain your problem. ;)

I am hoping @chazix will chime in again, as I can only help with the basics on this one.
 
Bladerun said:
DC values from the emitter
Click to expand...

Hyperion said:
Useful to know as much as possible
Click to expand...

True, dat. But the emitter voltages alone are a pretty strong indication that all three transistors' DC operating points are in the right ballpark. That in turn also suggests that most of the windings of the interstage transformers are intact.

This has me concerned that there might be a problem with the bar antenna, or with the secondary side of transformer/filter T07 - but I'm probably getting ahead of the game.

Speaking of getting ahead of the game, I should have asked first: what indication (if any) is there on the signal strength meter when the receiver does manage to tune in an AM station?

If you do go ahead and measure the base and collector voltages of the TR18-20, listen to the audio while you're doing it, with the volume turned fairly low. If all three of those transistor stages were working correctly, I would expect there would be a strong noise when you connect the meter probe to the base of TR18 in particular. Probing the collector should inject somewhat less noise. (That pattern - more noise when probing base, less when probing collector - would in principle hold for all three transistors, but the magnitude of the noise would be less for each stage in sequence, so the pattern may not be distinguishable.)

Another thing you might try, if you can read schematics well enough to figure out the test points, is to check whether the bar antenna windings are intact. There are three windings, each of which is connected to ground (chassis) at one end. So, the other end of each of them should show a resistance to ground when measured with an ohmmeter. For ohmmeter measurements, do them with the receiver powered off. For example, the AM antenna terminal on the rear panel should read a resistance of a few hundreds of ohms to ground. Terminal 04 on the tuner board should read a resistance that I'm guessing should be lower, perhaps in the tens of ohms. For the third winding - the one that passes through L702 - there isn't any terminal designation, so you're on your own to figure out where to measure that one. It might be connected right to a terminal of the tuning capacitor (the rotating air-vaned gizmo). I don't have a guess at the ballpark of ohms to expect for this one.
 
Hyperion said:
Useful to know as much as possible - as for example if one of the coils is 'open' the loss of a whole stage of amplification could explain your problem. ;)
Click to expand...

Thanks Hyperion! Yes, I'm wondering if one of the AM IF or OSC coils is bad, if I'm understanding what you're referring to as a coil. I'm guessing I can check these from the bottom side to see if they're open?
 
chazix said:
True, dat. But the emitter voltages alone are a pretty strong indication that all three transistors' DC operating points are in the right ballpark. That in turn also suggests that most of the windings of the interstage transformers are intact.

This has me concerned that there might be a problem with the bar antenna, or with the secondary side of transformer/filter T07 - but I'm probably getting ahead of the game.

Speaking of getting ahead of the game, I should have asked first: what indication (if any) is there on the signal strength meter when the receiver does manage to tune in an AM station?

If you do go ahead and measure the base and collector voltages of the TR18-20, listen to the audio while you're doing it, with the volume turned fairly low. If all three of those transistor stages were working correctly, I would expect there would be a strong noise when you connect the meter probe to the base of TR18 in particular. Probing the collector should inject somewhat less noise. (That pattern - more noise when probing base, less when probing collector - would in principle hold for all three transistors, but the magnitude of the noise would be less for each stage in sequence, so the pattern may not be distinguishable.)

Another thing you might try, if you can read schematics well enough to figure out the test points, is to check whether the bar antenna windings are intact. There are three windings, each of which is connected to ground (chassis) at one end. So, the other end of each of them should show a resistance to ground when measured with an ohmmeter. For ohmmeter measurements, do them with the receiver powered off. For example, the AM antenna terminal on the rear panel should read a resistance of a few hundreds of ohms to ground. Terminal 04 on the tuner board should read a resistance that I'm guessing should be lower, perhaps in the tens of ohms. For the third winding - the one that passes through L702 - there isn't any terminal designation, so you're on your own to figure out where to measure that one. It might be connected right to a terminal of the tuning capacitor (the rotating air-vaned gizmo). I don't have a guess at the ballpark of ohms to expect for this one.
Click to expand...

Thanks chazix! I did actually hear changes in noise when measuring the transistors, and turned down the volume to eliminate the interference. I'll measure the base and collector also and report back.

I was able to originally pull in an obscure station or two around 1200 on the dial, but can't seem to now. Signal strength is almost non-existent, although it bumps up a little on the far lower end of the dial around 500. I have tried multiple secondary antennas (through the AM slot in back) but can't remember which one pulled in the weak station.

I'll do the other tests you mentioned as soon as I can. The ferrite antenna looks fine, but I can't see inside of it. Thanks for the details of testing that out!
 
Bladerun said:
wondering if one of the AM IF or OSC coils is bad
Click to expand...

If the transistors' DC operating points continue to look OK, it is evidence - not definitive, but reasonably strong - that 2/3 of the coils within the IF transformers are OK. The ones that might still be in the "likely suspects" category are the secondary (rightmost) coils in T07 and T09. I would almost rule out T09's secondary, though, because if that was open I wouldn't expect you to get much audio or signal strength indication at all.

You can indeed check the IF coils to see if they are open, except for T09's secondary, which is internally connected to a couple of (I think) ceramic resonators.

If/when you're able to tune in a station, and if you find that the dial scale pointer is in about the right place for that station, that would be pretty strong evidence that everything is OK in OSC-land. The OSC coils could be checked with an ohmmeter.
 
Here is what I have so far:

TR18

E - .690
B - 1.35
C - 10.91
TR19

E - .667
B - 1.32
C - 11.28
TR20

E - 3.37
B - 4.07
C - 9.78

There was noise when connecting to the base of TR18, and definitely less when measuring the collector. It did decrease with the others.

The antenna measured out fine at all points and right in line with your ohm estimates.

I have not had a chance to pull the tuner board and check the coils. I will do that next when I get a chance.

Thanks again for all your help chazix and Hyperion!
 
Bladerun said:
Here is what I have so far:
Click to expand...

All looks fine to my eye.

I'm pretty close to being out of ideas. It might be as simple as a relatively severe out-of-alignment condition. Given that the section is so deaf at present, though, I think an RF generator would be a must-have to attempt alignment. I don't suppose there is one of those handy? (That would also enable some signal tracing, in the event that alignment doesn't help.)
 
Problem solved!

T06 was the problem child. Replaced it and AM stations come through now, although I need to purchase some sort of additional antenna to supplement the original ferrite one. I noticed the generic AM loop antenna is doing nothing additional.

MVIMG_20190927_072521.jpg

Thanks again chazix and Hyperion!
 
chazix said:
Whoa! Where the heck did you find a replacement?
Click to expand...

I have quite a few parts on hand or can generally locate things quickly, but vintage sansui parts are really tough to find! I actually bought a sansui six am tuner board on eBay a few days ago since it shared quite a few common components with mine.

Hakko desoldering gun makes the swap nice and easy. :beerchug:
 
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