Hi,
I have a Sony STR-7035 that I purchased for parts to restore, and that I wanted to use to practice what I have been learning online and with books about FM alignment. The alignment procedures for this particular receiver in the service manual seemed simple enough, and I used the opportunity (we all need a 'reasonable' excuse) to purchase a Panasonic VP-8193 signal generator, which does also multiplex.
First, I had very poor reception, which resulted to be a gain issue in the first IF amplifier due to an open base to emitter junction in Q201. This solved the poor reception: I was able to align the IF stage as for service manual, zeroing AM and maxing the FM signal across C217 at the FM detector. I was also able to obtain full deflection of the signal strength meter with a 60dBuV unmodulated signal, which is what the specs call for.
Now, the issue is that there is something wrong with the stereo section of the receiver: I do not seem to get stereo, all the adjustments are unresponsive (apart from RT501), Q303 (38kHz amplifier) appears to always be off, regardless of the strenght of the incoming signal, and the collector of Q302 shows always a wrong voltage of 7.7V DC. Here is where I am stuck.
I checked all the transistors, resistors and capacitors in this part of the circuit, and they all work and are in spec. I replaced all the electrolytic caps just in case. If I inject a 19kHz signal from an audio generator directly to the base of Q301, I get an amplified signal at the collector; I also get a very very noisy 38kHz signal at the cathode of D301 and D302. Regardless of the amplitude of the injected signal (up to a few volts), I can only get a maximum DC voltage of about 0.63~0.65V at the base of Q303, from a 200-300mV with no signal and about 20-30mV with the stereo switch in the mono position.
If I use the pilot signal from the RF sig. gen. injected to the antenna terminals on a 93MHz carrier, I can't see any decent 19kHz signal downstream of the detector, but I see a very weak very noisy signal (80mV pk-to-pk), almost impossible to trigger on the scope. I have now run out of ideas. I even tried looking at the frequency response of the front-end and IF stages, and there is a lot of filtering going on: a carrier FM modulated with a 400kHz signal (I think I set it at 60dBuV) gives me a 400mV peak-to-peak at the detector, but 200mV with a 4kHz modulation, and it carries on going down as I increase the frequency, to nothing visible at 19kHz.
Does anyone have any idea on the possible issue, or what else to check and try?
Attached are the section of the circuit, the 38kHz signal at the cathodes of D301-D302 when I inject a 19kHz signal at the base of Q301, and the same when I use the pilot tone modulating a 93MHz carrier injected to the antenna.
I have a Sony STR-7035 that I purchased for parts to restore, and that I wanted to use to practice what I have been learning online and with books about FM alignment. The alignment procedures for this particular receiver in the service manual seemed simple enough, and I used the opportunity (we all need a 'reasonable' excuse) to purchase a Panasonic VP-8193 signal generator, which does also multiplex.
First, I had very poor reception, which resulted to be a gain issue in the first IF amplifier due to an open base to emitter junction in Q201. This solved the poor reception: I was able to align the IF stage as for service manual, zeroing AM and maxing the FM signal across C217 at the FM detector. I was also able to obtain full deflection of the signal strength meter with a 60dBuV unmodulated signal, which is what the specs call for.
Now, the issue is that there is something wrong with the stereo section of the receiver: I do not seem to get stereo, all the adjustments are unresponsive (apart from RT501), Q303 (38kHz amplifier) appears to always be off, regardless of the strenght of the incoming signal, and the collector of Q302 shows always a wrong voltage of 7.7V DC. Here is where I am stuck.
I checked all the transistors, resistors and capacitors in this part of the circuit, and they all work and are in spec. I replaced all the electrolytic caps just in case. If I inject a 19kHz signal from an audio generator directly to the base of Q301, I get an amplified signal at the collector; I also get a very very noisy 38kHz signal at the cathode of D301 and D302. Regardless of the amplitude of the injected signal (up to a few volts), I can only get a maximum DC voltage of about 0.63~0.65V at the base of Q303, from a 200-300mV with no signal and about 20-30mV with the stereo switch in the mono position.
If I use the pilot signal from the RF sig. gen. injected to the antenna terminals on a 93MHz carrier, I can't see any decent 19kHz signal downstream of the detector, but I see a very weak very noisy signal (80mV pk-to-pk), almost impossible to trigger on the scope. I have now run out of ideas. I even tried looking at the frequency response of the front-end and IF stages, and there is a lot of filtering going on: a carrier FM modulated with a 400kHz signal (I think I set it at 60dBuV) gives me a 400mV peak-to-peak at the detector, but 200mV with a 4kHz modulation, and it carries on going down as I increase the frequency, to nothing visible at 19kHz.
Does anyone have any idea on the possible issue, or what else to check and try?
Attached are the section of the circuit, the 38kHz signal at the cathodes of D301-D302 when I inject a 19kHz signal at the base of Q301, and the same when I use the pilot tone modulating a 93MHz carrier injected to the antenna.
Attachments
yes, the switch appears to works fine. Before starting looking at the tuner section I had to deal with some amp issues as well, and I recapped and cleaned everything, which was a total pain becasue unlike other Sonys this model was not designed with serviceability in mind at all.
A slight aside: when I worked for Pye TV in the late '70s we pulled apart some of the latest Japanese TVs and wondered how you would ever service them. Little did we know at the time but they never broke-down!
This one is so badly layed out that a couple of caps are under the frame of one of the switches, meaning the switch has to come out. To remove the switch the entire board needs to be disassembled and let free hanging from the wires, but the wires are so short that it's almost impossible to do that without also unscrewing the tuner board which hangs slightly over it. Everything is so tight that I ended up having to refit the dial string cause it came off despite all the tape I used to secure it in place. What's even more surprising is that there is plenty of unused space in that case.
oops, that makes me realise that I wrongly posted the thread in the analogue source forum, rather than in the solid state one... could it be moved there?
As for your question, yes. I do have good reception of a number of FM radio stations
As for your question, yes. I do have good reception of a number of FM radio stations