So the tuner was working fine, but now the left channel is weak. It doesn't appear to be the output stage. I am possibly thinking the addition/subtraction stages but these things are greek to me. Mono sounds great, just when I pickup Stereo the left channel is very weak.
Here is a schematic if anyone can point me to the correct section to troubleshoot and give me some tips. I'm not a tuner guy but I do vaguely understand the concepts.
Here is a schematic if anyone can point me to the correct section to troubleshoot and give me some tips. I'm not a tuner guy but I do vaguely understand the concepts.
Attachments
Must be somewhere after the FM detector (area of T4) forward, in the multiplex circuit,
or audio output. I'd first try replacing all the low voltage electrolytics in those circuits,
they're due anyway. Don't mess with the alignment.
Last edited:
Yes, there are electrolytics everwhere. I turned a buzzy, insensitive fm radio back into the crisp highly sensitive unit I remembered from Father's den in the 70's just with new electrolytics. No alignment required. The last electrolytic cratered this winter in the cold, the one under the dial cord. Replaced but getting the dial cord back on the 4 pulleys requires too many hands. ****
Buy the 10000 hour service life variety if you can get them. You don't want to do this again. 105 deg c rating IMHO is not as important in the transistor areas. Digikey and Newark have the service life in the selector table if you ask for it, mouser makes you download the product datasheet and read it. Alliedelec carried nothing higher quality than 1000 hour service life caps last time I bought there.
Buy the 10000 hour service life variety if you can get them. You don't want to do this again. 105 deg c rating IMHO is not as important in the transistor areas. Digikey and Newark have the service life in the selector table if you ask for it, mouser makes you download the product datasheet and read it. Alliedelec carried nothing higher quality than 1000 hour service life caps last time I bought there.
I would check the diodes in the stereo demodulator output T6 as well as the subcarrier recovery circuit T5. There may also be one or more errors in the schematic, C45 vs C43, and I believe there is supposed to be another diode in series with R62.
The DGM-2 can be replaced with schottky switching diodes if they are bad.
Despite the similarities to a TDM demultiplexer it's actually a balanced demodulator with matrix. The outputs are - (R-L) and +(R-L) which when added to the R+L signal give you respectively 2L and 2R.
I have not worked on one of these tuners so can't say for sure, but I would look at the audio levels at C43 and C45 with a scope, they should be approximately the same.
There are a number of electrolytics in the stereo demodulator C35, C36, C37, and C42 which at this point should be replaced - that's where I would start.
Do not twiddle any of the inductors, few today would be able to realign the stereo demodulator correctly. Don't replace any of the non-polar caps unless you want to do a full realignment of the decoder.
C43 - C46 should also be high on the list of items to replace and a number of others in the audio amplifier, although you report that appears to work correctly. (And likely is if mono is not affected)
The PCBs in early Scott solid state gear are quite fragile so get good solder wick and carefully wick out the solder. Be quick and not too much heat. A solder sucker will almost certainly result in lifted pads.
These are fairly rare and valuable. My experience ends with the 4310. Good luck!
The DGM-2 can be replaced with schottky switching diodes if they are bad.
Despite the similarities to a TDM demultiplexer it's actually a balanced demodulator with matrix. The outputs are - (R-L) and +(R-L) which when added to the R+L signal give you respectively 2L and 2R.
I have not worked on one of these tuners so can't say for sure, but I would look at the audio levels at C43 and C45 with a scope, they should be approximately the same.
There are a number of electrolytics in the stereo demodulator C35, C36, C37, and C42 which at this point should be replaced - that's where I would start.
Do not twiddle any of the inductors, few today would be able to realign the stereo demodulator correctly. Don't replace any of the non-polar caps unless you want to do a full realignment of the decoder.
C43 - C46 should also be high on the list of items to replace and a number of others in the audio amplifier, although you report that appears to work correctly. (And likely is if mono is not affected)
The PCBs in early Scott solid state gear are quite fragile so get good solder wick and carefully wick out the solder. Be quick and not too much heat. A solder sucker will almost certainly result in lifted pads.
These are fairly rare and valuable. My experience ends with the 4310. Good luck!
Check K1B and K1C relay contacts. I don't know which one is for the L or R. The schematic shows it in mono mode. In stereo, L and R outputs are taken from the diode matrix, the diode in series with R62 is indeed missing on the schematic.
Stick with the basics first and resist replacing other parts without any proof that they are bad.
Good luck!
When I got the unit it wouldn't work at all.
I replaced C2, C3, C17 and many of the power supply caps and the unit worked great for a little while.
Today I plan to check the diodes and caps around the stereo demodulator. I'll also see if the signal is the same at C43, C45 with a scope.
The signals to K1 are not the same in stereo, the contacts are good and pass the same signal going to it.
Thank you all for helping, I usually hate just replacing things without finding the issue. You guys are the best and helped me save a lot of time by pointing out which section to look at next. I'll report back later on what I find. This unit, probably because of the type of multiplex circuit sounds better than other tuners which are probably TDM circuits. My understanding is with TDM you can get lots of nasty odd harmonics if not filtered out properly.
I replaced C2, C3, C17 and many of the power supply caps and the unit worked great for a little while.
Today I plan to check the diodes and caps around the stereo demodulator. I'll also see if the signal is the same at C43, C45 with a scope.
The signals to K1 are not the same in stereo, the contacts are good and pass the same signal going to it.
Thank you all for helping, I usually hate just replacing things without finding the issue. You guys are the best and helped me save a lot of time by pointing out which section to look at next. I'll report back later on what I find. This unit, probably because of the type of multiplex circuit sounds better than other tuners which are probably TDM circuits. My understanding is with TDM you can get lots of nasty odd harmonics if not filtered out properly.
The signals to K1 in stereo won`t be the same, but the level should be fairly close for most FM stereo programs. If 1 channel is significantly weaker, it should be apparent at this test location.
Temporarily disconnect R54 and R55 to isolate R52 & R53 separation controls from affecting stereo separation. There is still some stereo separation even with them taken out of the circuit. We don't know the state of R52 and R53 so by taking them out of the circuit temporarily we can better isolate the problem. Just don't forget to reconnect R54 and R55 later.
If you have a 2-channel scope, connect one channel to the 38kHz sub-carrier present between R47 and R50 and use it as your trigger. Use the remaining scope channel to look at the diodes and resistors on T6's output. Doing this will show you the MPX switching action and confirm if any of the parts are good or bad.
I have no service manual for this thing but based on my experience with tuners that used similar scheme, Q25 is the 38kHz amplifier and the 2 diodes that precedes it is a full-wave rectifier which is the 19kHz frequency doubler. Q15 & Q16 is the 19kHz amp. T6 is adjusted for maximum audio level/stereo separation/minimum THD, T5 for maximum 38kHz at the diode cathode junction, L8 is adjusted for 19kHz level/phase/separation and L7 is part of the SCA trap adjusted for minimum 67kHz. The MPX topology is a bridge-type time division decoder, it's a TDM.
As you can see, each adjustments affects more than 1 parameter so without any test equipment, you are bound to be make things worse by adjusting any of them. I'm not suggesting that you adjust any of these but more for understanding the circuit and know what to expect when probing these locations.
Can you post a pic of the underside where it shows the rest of the parts? I'm just curious as to the type and quality of components used.
Temporarily disconnect R54 and R55 to isolate R52 & R53 separation controls from affecting stereo separation. There is still some stereo separation even with them taken out of the circuit. We don't know the state of R52 and R53 so by taking them out of the circuit temporarily we can better isolate the problem. Just don't forget to reconnect R54 and R55 later.
If you have a 2-channel scope, connect one channel to the 38kHz sub-carrier present between R47 and R50 and use it as your trigger. Use the remaining scope channel to look at the diodes and resistors on T6's output. Doing this will show you the MPX switching action and confirm if any of the parts are good or bad.
I have no service manual for this thing but based on my experience with tuners that used similar scheme, Q25 is the 38kHz amplifier and the 2 diodes that precedes it is a full-wave rectifier which is the 19kHz frequency doubler. Q15 & Q16 is the 19kHz amp. T6 is adjusted for maximum audio level/stereo separation/minimum THD, T5 for maximum 38kHz at the diode cathode junction, L8 is adjusted for 19kHz level/phase/separation and L7 is part of the SCA trap adjusted for minimum 67kHz. The MPX topology is a bridge-type time division decoder, it's a TDM.
As you can see, each adjustments affects more than 1 parameter so without any test equipment, you are bound to be make things worse by adjusting any of them. I'm not suggesting that you adjust any of these but more for understanding the circuit and know what to expect when probing these locations.
Can you post a pic of the underside where it shows the rest of the parts? I'm just curious as to the type and quality of components used.
Kinda, I replaced C45 and C46 and still nothing when connected. But when I lift C45 off the junction of R61 and R62 it comes back. Solder it back on and nothing. Weird.
Maybe something in the separation controls?
Forget about the separation control, I asked you to take it out of the circuit for a reason. To simplify things and rule it out.
Make sure that C45 is not reversed-biased. Circuit board silkcreen can be wrong if you rely on them too much. Measure the DC voltage on the individual C45 pins reference to ground.
Turn off the unit and measure R64, is it close to 100k or appear to be shorted? Disconnect C45 and C46 when measuring R64 and leave K1 relay.
The K1B relay contact can develop tin whiskers and short the connection. It is quite common if the relay's flexible metal that holds the contact are tin-plated.
Okay I will take separation control out of the circuit for my next testing. Sorry I skipped over that part you suggested, makes sense.
The amp doesn't have any circuit boards at all. It's all point to point hand wired. C45 is as in the schematic, negative to the 10k resistors and the + to the relay. I'll measure the DC voltage in circuit in reference to ground.
R64 measure 99.8k ohms. I'll Disconnect C45 and C46 and recheck.
I'll also recheck the relay for any sort of shorts, possibly suck all the solder off it and cleanly reflow.
Thank you for all your help I really do appreciate it. The tuner was a very good sounding unit, one of the best I've heard so I really want to get it working again.
Last edited:
Well I appreciate the knowledge and experience you and others share so thank you.
I am in the process of trying to learn as much as possible about radio transmission and detection, it's great fun to work on this sort of gear but at the same time can be complex and time consuming to me.
Thanks again Kevin and everyone
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.