McIntosh MX 112 FM/AM Stations Suddenly Gone
I'm still thinking Q201 is bad, but before you order one, let's disconnect the wires from pins 5, 6, and 7 and check the voltage at pin 5, both AC and DC. I want to make sure there isn't voltage backfeeding into that point from somewhere.
That C210 can cap is almost certainly bad, and C201 is suspect as well as it's only reading 75 uF.
My Mac cross reference says that Q201, a 132-072, crosses to a 2N4922. They are long obsolete, but I see a few on eBay - hopefully not Chinese counterfeits.
That C210 can cap is almost certainly bad, and C201 is suspect as well as it's only reading 75 uF.
My Mac cross reference says that Q201, a 132-072, crosses to a 2N4922. They are long obsolete, but I see a few on eBay - hopefully not Chinese counterfeits.
richardg0n
Active Member
Regarding Q201 I found NOS Motorola 2N4922 on EBay. Any suggestions for C201 replacement?
Disconnected the wire on 5,6, and 7. The readings are as follows.
Pin 5 - 2.2-16VAC
Pin 5 - 47VDC
richardg0n
Active Member
w3jn,
I installed new power supplies from Hayseed Hamfest, 2N4922(Q201), and C201(see pictures). There is No change from the previous result. Reading 34VDC at pins 5,6, and 7.
I also checked the pin-points on the left pin 1-2, and right pin 15-23 and are to spec. Any ideas?
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This is the simplest regulator circuit ever, and there are really only two things that can go wrong with it presuming supply voltage is good: 1, incorrect base voltage or 2, bad regulator transistor.
Incorrect base voltage would be a failure of D201, R201, or R202. Failure of the latter two would result in no voltage.
It appears you have Q201 installed correctly, however it's unclear from the photo if the base lead is shorted to one of the other leads.
Let's measure the voltages to ground at the base, collector, and emitter of Q201. Something's off....
Incorrect base voltage would be a failure of D201, R201, or R202. Failure of the latter two would result in no voltage.
It appears you have Q201 installed correctly, however it's unclear from the photo if the base lead is shorted to one of the other leads.
Let's measure the voltages to ground at the base, collector, and emitter of Q201. Something's off....
richardg0n
Active Member
OK, the zener diode is bad, or has a bad connection. I remember in a previous post you did have 16V there, but with the bad filter caps that could have been a red herring. That part crosses to a 1N966, which is a 16V, 400mW zener. Any 16V half watt or 1 watt zener should do fine as a replacement.
Is Q201 getting warm at all?
Be advised that once the power supply is working there is undoubtedly more trouble in the tuner section due to the large overvoltage which we'll have to track down.
Is Q201 getting warm at all?
Be advised that once the power supply is working there is undoubtedly more trouble in the tuner section due to the large overvoltage which we'll have to track down.
richardg0n
Active Member
1N966 installed and voltage reading are now 15VDC at pin 5,6, and 7. Tuner is not working.
Excellent! OK, correct power supply voltages are the first step in getting this working. As I mentioned above I fear that the 35V on the tuner section has ruined some transistors. Hopefully a capacitor has shorted due to overvoltage and protected some of the components. There are also two RCA CA3012 ICs and a smattering of MOSFETs on the IF board. THose are not easy to come by.
Nevertheless, let's press on. First question, do you have an RF signal generator? Second, is AM dead as well? Third, do you get typical in-between station noise on either AM or FM?
Let's check voltages on the MPX board and IF board to start. Check base, collector, and emitter voltage on each bipolar transistor on each board. Let's not worry about the MOSFETs just yet. Also check pins 5 and 10 on IC1 and IC2 (on the FM IF board). It'd be easier if you put them in a table here with expected voltage on each pin and voltage you read.
Nevertheless, let's press on. First question, do you have an RF signal generator? Second, is AM dead as well? Third, do you get typical in-between station noise on either AM or FM?
Let's check voltages on the MPX board and IF board to start. Check base, collector, and emitter voltage on each bipolar transistor on each board. Let's not worry about the MOSFETs just yet. Also check pins 5 and 10 on IC1 and IC2 (on the FM IF board). It'd be easier if you put them in a table here with expected voltage on each pin and voltage you read.
richardg0n
Active Member
Thank you for your continued help. Much appreciated!
I can pick up a RF signal generator if needed.
AM is also dead as well.
No in between noise on AM or FM. Dead silent.
Here are the voltage readings for the Transistors and IC's on MPX board and IF board.
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OK, nothing looks hugely out of order with those voltages.
I just noticed that, for some reason, they are routing the tuner audio into the phono preamp stage so that could have gone south.
Does the phono section work? You don't need a phono to check, you can use your finger as a signal generator - just touch the center pins of those RCA inputs with your finger and you should get a healthy hum output. When you do so, touch just the center and don't have your other hand or other fingers touching the chassis. Also make sure and discharge any static on your body by touching the chassis first.
Turn the selector switch to AM and touch pin 25 (yellow wire) - you should get hum again
Turn the selector to FM stereo and touch pins 7 and 8 on the MPX board - again you should get hum in the respective channel.
If the above tests pass, turn the selector to FM mono and touch pin 11 on the MPX board (this will test if the MPX board passes a signal through it)
If these tests don't pass, let's start by checking voltages at pins 20, 21, 22, and 23 on the power supply board. These supply power to the phono preamp. If those are all OK then let's check the voltages at each transistor pin in the phono preamp.
I'm hoping we won't need a signal gen, but NBD if we do.
I just noticed that, for some reason, they are routing the tuner audio into the phono preamp stage so that could have gone south.
Does the phono section work? You don't need a phono to check, you can use your finger as a signal generator - just touch the center pins of those RCA inputs with your finger and you should get a healthy hum output. When you do so, touch just the center and don't have your other hand or other fingers touching the chassis. Also make sure and discharge any static on your body by touching the chassis first.
Turn the selector switch to AM and touch pin 25 (yellow wire) - you should get hum again
Turn the selector to FM stereo and touch pins 7 and 8 on the MPX board - again you should get hum in the respective channel.
If the above tests pass, turn the selector to FM mono and touch pin 11 on the MPX board (this will test if the MPX board passes a signal through it)
If these tests don't pass, let's start by checking voltages at pins 20, 21, 22, and 23 on the power supply board. These supply power to the phono preamp. If those are all OK then let's check the voltages at each transistor pin in the phono preamp.
I'm hoping we won't need a signal gen, but NBD if we do.
richardg0n
Active Member
- Phono works when I touch RCS's
- AM pin 25 has no effect when touching pin
- FM pin 7 and 8 does respond when touching
- FM pin 11 no response
- Pin 20 - 10.95 - Expected 10
- Pin 21 - 10.95 - Expected 10
- Pin 22 - 10.07 - Expected 9.5
- Pin 23 - 10.07 - Expected 9.5
OK, we've determined that the MPX board isn't passing a signal. One point, did you have mute on or off? Should be off when you test.
Looks like the phono boards are OK. I'm thinking those zero voltages you got for Q306 and Q312 are errors since the voltages that those points come from test OK in other transistors you did. Q315 and Q316 are a bit off, but it's working, so let's deal with that later if need be.
Let's first check for any audio on TP2, which is the discriminator output. Hook up an amp to the TP2 RCA in the back, turn to FM, and see if you get interstation hiss or any semblance of stations. If there's no audio there then we'll probably need a sig gen or a scope, or both.
Next, let's check all the voltages on the transistors on the MPX board, making sure you have MUTE off and mode in Stereo.
Many thanks for formatting these voltages!! Makes it easy to go thru.
Looks like the phono boards are OK. I'm thinking those zero voltages you got for Q306 and Q312 are errors since the voltages that those points come from test OK in other transistors you did. Q315 and Q316 are a bit off, but it's working, so let's deal with that later if need be.
Let's first check for any audio on TP2, which is the discriminator output. Hook up an amp to the TP2 RCA in the back, turn to FM, and see if you get interstation hiss or any semblance of stations. If there's no audio there then we'll probably need a sig gen or a scope, or both.
Next, let's check all the voltages on the transistors on the MPX board, making sure you have MUTE off and mode in Stereo.
Many thanks for formatting these voltages!! Makes it easy to go thru.
richardg0n
Active Member
Mute was off when testing MPX pins.
TP2 output connected and hearing very low Interstation hiss.
I re-checked MPX voltages verifying selector switch is in Stereo and mute is off. MPX readings are identical to previous readings posted.
Formatting voltage is my pleasure. Anything I can do to make this easier for troubleshooting. Thank you!
Ah yes, my mistake on the MPX board voltages. While they are in the realm of reasonablness I do see some areas of concern. I am betting that those CA3012 ICs are shot since their absolute max voltage is 15V, but let's cross that bridge later. They will be a big PITA to replace, so let's see if we can get the interstation hiss through the MPX board.
Q101 takes the baseband signal from the discriminator and is an emitter follower which feeds Q102 through the 67 KHz SCA trap. Q102 is a Darlington emitter follower and one part of the signal goes to the 19 KHz oscillator (Q104) and the other goes to the 38 KHz balanced demodulator Q103 is the muting transistor so let's ignore it for the time being. Q105 drives the balanced demodulator wiith the 19 KHz signal from Q104. Q106 turns on the stereo light, when a 19 KHz pilot is detected. So Q104, Q103, Q105, and Q106 have no role in an FM mono signal with mute off; only Q101 and Q102 do. Since we have a negative voltage on the base of Q103 and Q105, the only way that can exist is if Q104 is good.
C107 is also suspect, since it's a 15V component that's seen 35V. I am also concerned about the collector voltage on Q101 - there's only a small RF choke in series with the power supply, but theres a sneaky path to power via the muting control. Check L101 for resistance. It should be in the range of 100 ohms. And, with power on, lets' check voltage across L101.
If you have one of those cheapo component testers, I'd test Q101 and Q102. If you have a DVOM with a diode test function, use that. And as long as you're in there test D104-D107. Q101 is a BC168C or BC238C. Q102 is a 2N5305. Not sure how easy those are to get or sub. If need be I can further research.
Q101 takes the baseband signal from the discriminator and is an emitter follower which feeds Q102 through the 67 KHz SCA trap. Q102 is a Darlington emitter follower and one part of the signal goes to the 19 KHz oscillator (Q104) and the other goes to the 38 KHz balanced demodulator Q103 is the muting transistor so let's ignore it for the time being. Q105 drives the balanced demodulator wiith the 19 KHz signal from Q104. Q106 turns on the stereo light, when a 19 KHz pilot is detected. So Q104, Q103, Q105, and Q106 have no role in an FM mono signal with mute off; only Q101 and Q102 do. Since we have a negative voltage on the base of Q103 and Q105, the only way that can exist is if Q104 is good.
C107 is also suspect, since it's a 15V component that's seen 35V. I am also concerned about the collector voltage on Q101 - there's only a small RF choke in series with the power supply, but theres a sneaky path to power via the muting control. Check L101 for resistance. It should be in the range of 100 ohms. And, with power on, lets' check voltage across L101.
If you have one of those cheapo component testers, I'd test Q101 and Q102. If you have a DVOM with a diode test function, use that. And as long as you're in there test D104-D107. Q101 is a BC168C or BC238C. Q102 is a 2N5305. Not sure how easy those are to get or sub. If need be I can further research.
richardg0n
Active Member
L101 15vdc and 22 ohms in circuit resistance(do I test resistance in circuit?). I tested out of circuit and it was open.
Pulled Q101 and Q102. Tested good with Peak Atlas DCA55.
Diode test out of circuit with Fluke 87V
D104 .403
D105 .362
D106 .365
D107 .365
OK, the muting adjust pot is in parallel with L101, but there's a 270 ohm resistor in series with it so if l101 was bad, and the muting adjust was all the way over at one end, it would read 270 ohms. Perhaps removing L101 damaged it. The choke is wound on a phenolic form and the enameled wires are likely wrapped around the leads very close to where they emerge from the form. If it's indeed bad, try heating directly at the point where the leads enter the form, and add a touch of solder. However could also be that overcurrent destroyed L101. Here are some replacements if need be https://www.ebay.com/itm/225238544788?hash=item3471437194
Now that we've established Q101, Q102, and the diodes are good, we can try shooting an audio signal into TP2 to see if audio makes it thru the MPX board. Should have done this before, I guess. See if there's any DC voltage at TP2. There shouldn't be, but it's best to check. If there isn't any DC voltage, feed an audio source (CD player, another tuner, audio from your phone, etc) into TP2. Make sure MONO is selected. If we get audio when it's in FM mode then the MPX board is passing audio. That doesn't mean quite yet it'll work in stereo, but we can then proceed to the IF board.
Now might be a good time to try and round up a sig gen. For the IF we'll need one capable of 10.7 MHz and FM modulation. Lets also check the voltages on the rest of the pins of IC1 and IC2 on the IF board.
Another thing, might as well check to see if the local oscillator is working. Tune the MX to 88 MHz, then take a portable FM radio, put it as close to the FM RF section as possible, and tune the portable to 98.7 MHz (88+10.7). Hopefully you'll heard a dead carrier on the portable radio (ie a signal that quiets the radio, but has no modulation), and if you tune the MX a little bit the carrier should also shift around.
Now that we've established Q101, Q102, and the diodes are good, we can try shooting an audio signal into TP2 to see if audio makes it thru the MPX board. Should have done this before, I guess. See if there's any DC voltage at TP2. There shouldn't be, but it's best to check. If there isn't any DC voltage, feed an audio source (CD player, another tuner, audio from your phone, etc) into TP2. Make sure MONO is selected. If we get audio when it's in FM mode then the MPX board is passing audio. That doesn't mean quite yet it'll work in stereo, but we can then proceed to the IF board.
Now might be a good time to try and round up a sig gen. For the IF we'll need one capable of 10.7 MHz and FM modulation. Lets also check the voltages on the rest of the pins of IC1 and IC2 on the IF board.
Another thing, might as well check to see if the local oscillator is working. Tune the MX to 88 MHz, then take a portable FM radio, put it as close to the FM RF section as possible, and tune the portable to 98.7 MHz (88+10.7). Hopefully you'll heard a dead carrier on the portable radio (ie a signal that quiets the radio, but has no modulation), and if you tune the MX a little bit the carrier should also shift around.
richardg0n
Active Member
A
L101 looks in good physical condition(see attached pics). Tried adding solder but could not heat it enough for solder to stick, decided not to risk damaging it and re-installed L101. I also ordered the replacement part from Ebay link.
Checked for DC voltage at TP2 and there was none. Switched to MONO and FM. Fed 1k audio from audio generator to TP1 and connected Main output to amp. Test was successful, 1k tone was heard and passing though MPX board.
Regarding possible signal generator gear, I found an HP 8656A locally for $200 and bidding on a Sencore SG165 on eBay. I would like to add test equipment for FM alignment. By the way, I also have a Heathkit IT-12 Signal Tracer for troubleshooting.
I tried using a portable FM radio tuned to around 98.7 and the MX to 88MHz but was not able to produce a dead carrier on the portable radio by shifting them around.
Here are the voltage readings for IC1 and IC2
L101 looks in good physical condition(see attached pics). Tried adding solder but could not heat it enough for solder to stick, decided not to risk damaging it and re-installed L101. I also ordered the replacement part from Ebay link.
Checked for DC voltage at TP2 and there was none. Switched to MONO and FM. Fed 1k audio from audio generator to TP1 and connected Main output to amp. Test was successful, 1k tone was heard and passing though MPX board.
Regarding possible signal generator gear, I found an HP 8656A locally for $200 and bidding on a Sencore SG165 on eBay. I would like to add test equipment for FM alignment. By the way, I also have a Heathkit IT-12 Signal Tracer for troubleshooting.
I tried using a portable FM radio tuned to around 98.7 and the MX to 88MHz but was not able to produce a dead carrier on the portable radio by shifting them around.
Here are the voltage readings for IC1 and IC2
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First, it looks like you can repair that choke, see the circled area in the pic below, it appears that wire has broken. You can pull off a half turn of wire from the choke so you can re-solder it, or bridge it with a short piece to fix it.
Second, man I wish I knew you had a signal tracer - woulda made some of this easier!
Third, I think the Sencore (if working) would be best since it does stereo. The HP would be excellent for the front end and IF, but not much help to align the MPX. The HP 8656A is a beast of a sig gen, with absolutely top-notch quality. But there are downsides to it, namely the attenuator can get funky, and complexity of repairing it if it craps out. I'd offer the guy a hundred and if so, that'll get you close and you'll have something that will serve you well for years.
Fourth, bummer that the LO isn't working, apparently.
Last, the ICs seem to be drawing a bit more current than expected. You might need to round up a couple of CA3012s. Be advised (as I mentioned) these are a giant PITA to replace, because of the ten pins in a circle. Getting all the pins aligned into the holes is no fun. To remove them, I'd just clip all the leads (but note beforehand where the tab on the IC case is siutated), then pull the leads one by one. If you have a good vacuum desolder station that'll make short work of it though.
Second, man I wish I knew you had a signal tracer - woulda made some of this easier!
Third, I think the Sencore (if working) would be best since it does stereo. The HP would be excellent for the front end and IF, but not much help to align the MPX. The HP 8656A is a beast of a sig gen, with absolutely top-notch quality. But there are downsides to it, namely the attenuator can get funky, and complexity of repairing it if it craps out. I'd offer the guy a hundred and if so, that'll get you close and you'll have something that will serve you well for years.
Fourth, bummer that the LO isn't working, apparently.
Last, the ICs seem to be drawing a bit more current than expected. You might need to round up a couple of CA3012s. Be advised (as I mentioned) these are a giant PITA to replace, because of the ten pins in a circle. Getting all the pins aligned into the holes is no fun. To remove them, I'd just clip all the leads (but note beforehand where the tab on the IC case is siutated), then pull the leads one by one. If you have a good vacuum desolder station that'll make short work of it though.