Lafayette LR-1500TA Restoration

[ChaunceyDog]

Hi All. First post, looking for some advice! I've been working on bringing back to life a Lafayette LR1500TA Receiver and it's been going well. However, I'm having some issues with the FM tuner (and maybe the AM as well). As I tune across the band, the signal meter shows good deflection for FM stations, but the audio coming out of the tuner board (even before getting to the tone and amp sections) is low. Interestingly, the audio is loudest just as I tune close to a station. However, the more centered the tuning and the more deflection the signal strength meter shows, the more the audio level of the station goes down until it completely disappears when signal strength is highest. The Lafayette has some unique tuning circuitry (new to me anyway) including a thing called AcriTune. It's supposed to light when stations are perfectly tuned, but its lamp is always lit, whether a station is tuned or not. Just wondering if anyone has any experience with this model who could offer suggestions as to where the issue might be. I've checked all the transistors and diodes in circuit with a meter and they seem ok. Audio is available at the FM discriminator output, but exhibits the same issue of fading away audio as the signal gets stronger, so I'm assuming the issue is somewhere before the discriminator output (pin 14) and after the IF section that feeds the tuning meter. I've attached the schematic and a picture of the tuner board in question. Thanks for any help or guidance you might be able to provide.

 

[chazix]

the issue is somewhere before the discriminator output (pin 14) and after the IF section that feeds the tuning meter

Welcome to AK! I think I can claim to have had an LR-1500something-or-other cross my bench, but I don't recall if it was a TA, and it didn't have this symptom anyway, so take this with a grain of salt.

It sounds to me like the discriminator is simply far out of tune, or not-so-simply busted. Have you tried any alignment steps? If it was on my bench I'd inject 10.7MHz near the input of the chip that drives the discriminator, and rock the frequency around a little to see whether I could get the discriminator output to show + and - DC voltages. And I guess I'd have modulation, too, so I can listen to what happens around 0 VDC.

Might be worth checking the discriminator transformer for continuity, if access is available.

I can't make out the schematic well enough to take a guess at how AcriTune is supposed to work, but it might just be a window comparator that's looking for near 0 VDC from the discriminator. I think it's troubling that it's always on, but I still think the vicinity to start looking for trouble is the discriminator section itself.

Good luck,

chazix

 

[ChaunceyDog]

Thanks, Chazix! I really appreciate your input.I haven't done any alignment work on this receiver yet since many of the steps require an audio output, which unfortunately is one of the big issues with the tuner section.I will take your advice and try to see what's happening around the discriminator circuit area. I've avoided doing much work in the IF sections of receivers so far because I've only been doing this for a year or so and have much to learn, and the tuners I've dealt with so far haven't had many issues in this area. I've mainly dealt with adjustments that relied on RF inputs to the antenna. Most tuner alignment instructions have been good at listing the rf power/voltage that should be input for the various steps, but not for the IF 10.7 mHz sweep tests. Could you give me an idea of how much voltage you would typically apply when injecting it into the various IF stages?

 

[chazix]

Could you give me an idea of how much voltage you would typically apply when injecting it into the various IF stages?

Good question. I'm guessing here, but I'd be surprised if it took more than 10mV at the input of the last IF amplification stage to drive it into limiting. "Start low" is usually a good rule.

If by any chance you have a small coil hanging around, you can connect the RF generator to the coil and just place the coil near the point in the receiver where you want to inject. That's what I usually do with solid-state gear. Very little worry about overload with that approach (though there can be worries in the other direction, i.e., too little injected signal). A bigger advantage to "sniffer coil" injection is that you don't have to worry about what the generator thinks is "ground" vs what the receiver thinks, and what stray or not-so-stray connections there might be between the two.

 

[ChaunceyDog]

Thanks for your advice. I tried to do a 10.7 mHz sweep, but I'm obviously doing something wrong either with my connections or settings on my scope or function generator, because I never seem to be able to get anything that resembles the waveforms always shown in alignment instructions. That's another reason I haven't managed to do any IF strip alignments up until this point. I've been looking for a good tutorial or 'how-to' on the topic and have found a number of them that show the actual results on a scope, but none that explicitly tell me how to set up the scope or generator. Still looking! That said, I took your suggestion about the issue likely being in the discriminator circuit. I tuned to a local station and made a slight adjustment to the coil after the final IF amp section and the audio popped right in. Sound is pretty good in stereo, but when I switch to mono, it sounds a little muffled and distorted, so I'm sure more adjustments need to be made, but it's a major step forward.

 

[chazix]

10.7 mHz sweep

Lots of art to this, and I'm not the guy who can really set you on the path, but there's one clue that might be germane: If you do not have a "detector probe" (and you would probably know if you did), then you won't be able do the sweep procedures that call for a sort of inverted "U" curve. You may be able to do the procedures involving what's usually called an "S" curve, and those happen to be the ones that involve the discriminator. ("S" curve response can actually be used to align the whole IF strip, IMHO, but alignment procedures are not usually written for that technique.)

I'm guessing that another thing that isn't clear for you is how to synchronize the 'scope trace with the generator sweep, which is crucial for both kinds of curves. If you say what kind of 'scope and what kind of generator you have, then with a lot of luck, someone familiar with them may be able to comment. (Oh, and if you happen to have a Sencore SG-165, get the manual for it and read the generic alignment instructions, which are quite good.)

Nice going with the seat o' the pants adjustment, in any case!

 

[ChaunceyDog]

The function generator is a Rigol DG1022, 25 mHz unit and the scope is a Tektronix 2235 100mHz. The Rigol can do sweeps with or without modulation. The Tektronix has x/y capability plus external trigger. As you said, I don't seem to be able to get everything synced together. I'd love to get a Sencore-165, but they seem to be pretty scarce around here. Cost from outside Canada with shipping, customs and exchange rate is usually prohibitive.

 

[chazix]

Rigol DG1022

The Rigol manual doesn't seem to specify what comes from the Sync Out when the generator is in sweep mode. If the Sync Out produces a pulse that corresponds to the sweep rate, you could connect that to the 'scope's trigger input. I'd set the generator to something like linear sweep, 10.7MHz center, 400KHz width, no modulation, and just a tad more than 20 millisecond sweep time - say, 20.1mS. The scope's timebase would be set for 2mS/div (I'm assuming that would give a 20mS total trace time) and external triggering. The scope's input would be set for DC coupling, maybe somewhere around 0.5V/div for starters, and connected to the Lafayette's discriminator output.

If you can then figure out some way to couple the generator's output to some point in the IF section, you should see something like a sideways "S" on the scope. Again, my preference for coupling would be to use a "sniffer coil" connected to the generator output, and place the sniffer near an IF transformer. If you have to make a direct connection between generator and receiver, it should generally be AC-coupled with something like a small-value ceramic disc capacitor - 0.001uF should be plenty. You will likely need to adjust the scope's vertical sensitivity to get a convenient curve. Use the lowest generator output that produces a convenient curve.

The "S" curve should go through 0V right in the middle of the sweep, have symmetrical peaks on either side of the middle, and make a very straight line between the peaks. One of the discriminator coils will mostly affect the 0 crossing point, and the other will mostly affect the linearity.

I'm kind of concerned I'm leading you down a garden path. I'd suggest not getting too, too serious, if you decide to try this at all - but if it works out, it works out.

Couple of side notes:

It's normal for mono to have less treble than stereo. Most of the stereo separation - the difference between L and R - is in the treble, and those differences are prone to cancel out when they are combined to make a mono signal.

The Sencore SG-165 manual is great, the generator itself... less great. (Good enough for receivers of this vintage, though, IMHO.)

 

[ChaunceyDog]

Thanks, again. I'll try out the settings you suggested tonight and see what happens. I've also downloaded the Sencore manual. Looks like a lot of good general information on the topic.

 

[ChaunceyDog]

Still no luck getting anything resembling the curves in the alignment instructions, but I'll keep plugging away! I'll take some time over the weekend to read through the information in the Sencore SG-165 manual to learn more about alignments in general. I'm finding a lot of new (to me) circuitry in this receiver, so it's a learning opportunity. I've got several schematic and service instruction documents for the unit (all labelled as LR-1500TA), but they seem to have different labels for some of the components and different alignment instructions, so that's providing an interesting conundrum.Thanks again for your help in getting the discriminator working properly!

 

[amptramp]

Still no luck getting anything resembling the curves in the alignment instructions, but I'll keep plugging away! I'll take some time over the weekend to read through the information in the Sencore SG-165 manual to learn more about alignments in general. I'm finding a lot of new (to me) circuitry in this receiver, so it's a learning opportunity. I've got several schematic and service instruction documents for the unit (all labelled as LR-1500TA), but they seem to have different labels for some of the components and different alignment instructions, so that's providing an interesting conundrum. Thanks again for your help in getting the discriminator working properly!

This sounds like the leadership training courses where people are required to do basic orienteering - but with maps that are not correct. I have a Sencore SG-165 and have reviewed it here for some of the problems I have encountered:

https://www.antiqueradios.com/forums/viewtopic.php?f=8&t=129016&start=60

For those of you rebuilding an SG-165, check the voltage at the source of TR-316. If it is low, you may be OK, but mine was at 6 volts (well within the MPF-102 spec) and that puts 18 volts across 400 ohms (counting R368). When I got mine, the output was clipped in the negative direction for the 400Hz output. The simple reason was R365 was a 1/2-watt resistor carrying 22.222 mA and dissipating 0.8666 watts. The 1/2 watt resistor had gone up to 2195 ohms. I put in a 1 watt resistor and it looked OK but was running too hot to touch, so I then added a 2 watt 390 ohm resistor and turned down the voltage balance on the power supply so I would get 12.1 volts on the positive output but 11.4 volts on the negative output. There is no reason to balance the power supply voltages - nothing depends on it.

I replaced the filter capacitors (input and output) in the power supply because the 15 volt rating on the 12 volt outputs means the reliability will be low, especially if the unit is turned on and the output voltage rises before the regulator has a chance to stabilize it. If I was going to be totally anal about it, I would change the rectifiers to fast recovery types as ordinary rectifiers tend to ring at 30 MHz and harmonics thereof and the third harmonic would be in-band for FM. A snubber capacitor may do the same thing, just less effectively.

I had to replace TR302 and TR316 because they were blown. I also added a 1.5 meg resistor to ground from the base of TR301 because I am not comfortable running transistors with a 15 volt Vceo with the base open when the resistor will bring it closer to the 30 volt Vcbo. I also replaced TR314 because it appeared to be shorted going by the diode voltage test on my DMM, but that was a mistake - it was actually seeing R358 and the removed transistor tested OK.

I can't complain about the SG-165 I got for $10 with these defects plus one channel of the audio meter was butchered - leads cut and not reconnected for some reason, but I never expect to use that function anyway. It was a piece of test equipment designed to a price to enable the average TV repair and stereo store to have AM - FM - FM MPX repair capability and it succeeded in that mission. I used PN3563 transistors for the oscillators, which are plastic versions of the SE-3002 transistors on the schematic and the 2N3563 transistors that were actually installed. The Vceo goes from the 12 volts of the SE-3002 to 15 volts on the PN3563 / 2N3563, critical in this circuit.

I should add that working on this beast is like building a ship in a bottle - the mechanical design seems to be biased in the direction of making it as difficult as possible, so when you get in there, fix everything. You won't want to do it again.

 

[bobtolen]

Hi All. First post, looking for some advice! I've been working on bringing back to life a Lafayette LR1500TA Receiver and it's been going well. However, I'm having some issues with the FM tuner (and maybe the AM as well). As I tune across the band, the signal meter shows good deflection for FM stations, but the audio coming out of the tuner board (even before getting to the tone and amp sections) is low. Interestingly, the audio is loudest just as I tune close to a station. However, the more centered the tuning and the more deflection the signal strength meter shows, the more the audio level of the station goes down until it completely disappears when signal strength is highest. The Lafayette has some unique tuning circuitry (new to me anyway) including a thing called AcriTune. It's supposed to light when stations are perfectly tuned, but its lamp is always lit, whether a station is tuned or not. Just wondering if anyone has any experience with this model who could offer suggestions as to where the issue might be. I've checked all the transistors and diodes in circuit with a meter and they seem ok. Audio is available at the FM discriminator output, but exhibits the same issue of fading away audio as the signal gets stronger, so I'm assuming the issue is somewhere before the discriminator output (pin 14) and after the IF section that feeds the tuning meter. I've attached the schematic and a picture of the tuner board in question. Thanks for any help or guidance you might be able to provide.


Test the audio at TP 14 disciminator out. Jump it to the tape play input and select monitor. A good signal tracer will work also. Make sure you not losing the audio through the MPX decoder. I am familiar with the 1500TA. The tuners were pretty good.

 

[S Phillips]

Old thread but need some advice. I just picked a LR-1500TA, like new on the outside. Before plugging in I pulled the cover for a visual and found it very clean on the inside as well. The only thing that got my attention was the gap on the power transformer cover and the black material showing in the gap as shown in the attached pics. Not sure it that’s normal or not, any input greatly appreciated.

Steve

 

[bobtolen]

I know this is an old thread. That's the way they look. That amount of corrosion is normal being it is 70 years old. I have seen them a lot worse and still they worked fine. I have detailed a couple of these including painting the power transformer. Some people are willing to pay for it. They want

 

[bobtolen]

This is what I did with one. I put a Chinese amp with speaker protection. It needed modifications to the power supply to get the +- 35VDC from the 70 VDC.