HH Scott Solid State

[jlovda]

Another question... Does anyone know what the gain of the uA703 IC is. It just has an inverting and non-inverting input and output. Can it even be called an op amp? According to the way it is hooked up in the IF (see above) there are no resistors determining a closed loop gain so the circuits are run at their intrinsic open loop gain. Can the gain deteriorate over time?

 

[Bob@FM]

Another question... Does anyone know what the gain of the uA703 IC is. It just has an inverting and non-inverting input and output. Can it even be called an op amp? According to the way it is hooked up in the IF (see above) there are no resistors determining a closed loop gain so the circuits are run at their intrinsic open loop gain. Can the gain deteriorate over time?

View attachment 1975881

That schematic looks very similar to the internal circuit of a later IF chip, the Sanyo LA1222. It was used in the IF gain section of many 70's tuners. The LA1222 has 2 gain sections, though, but internals of the first LA1222 gain section is very close. It certainly contains a long tailed pair fed by constant current source on the 2 emitters.

Can they go bad? Sure, typically what happens is the plastic package "leaks" through the lead openings, and the semiconductor section gets deteriorated. It is not common, but I have replaced LA1222s that were bad, seen once in a blue moon. I would compare the gain of all those parts to determine if they are bad. Look at the input and output levels (10.7 MHz) with a low level RF signal. A bad one will have no gain at all. A high impedance FET probe will help. These are cheap on Ebay (<$20), sold as almost finished "kits"
Bob

 

[Ylli]

Guess on my part.... The uA703 has a typical transadmittance of 35 mmhos.
Transadmittance is the change in output current referenced to the change in input voltage, so:

35e-3 = i(out)/v(in)
i(out) = 35e-3 * v(in)

So now you need to know the output impedance.
v(out) = i(out)*Z(out)

Combing them, we get v(out) = 35e-3 * v(in) * Z(out)

I'm confident someone will correct me on this, but I though I would prime the pump.

 

[jlovda]

Well, I replaced D306 and D307 with 1N34a diodes. No change in the signal strength meter deflection better or worse. I notice the deflection of the center tuning meter is quite small all across the band except for three very close station. They are also the only three stations that bring the signal strength off zero. I give up. I really think I live at a location with low signal strength and poor reception. Many years ago I lived in the NW side of Chicago about 15 miles from the Hancock Building and Sears Tower. I could see them from my roof. What wonderful reception.

 

[rmp]

My 312C with ICs badly needed renovation. No new diodes were needed. Among other tasks, my tech performed "meter zero adjustment" and "meter sensitivity potentiometer contact cleaning".

 

[jlovda]

The meter adjustment wasn't helpful for me. In the signal strength position the pot would just move the needle off of zero but not affect the "sensitivity." It would just shift the zero position up. I cannot figure out what the multipath board is doing. I do not know if the gain of the single transistor is relevant in the Signal Strength position. I need more gain from the IF. I connected my Fluke meter to the Vertical scope input. It would jump to 5-6 volts for the three really strong stations but was only at 25-30 millivolts for most other stations.

 

[backmd]

The meter adjustment wasn't helpful for me. In the signal strength position the pot would just move the needle off of zero but not affect the "sensitivity." It would just shift the zero position up. I cannot figure out what the multipath board is doing. I do not know if the gain of the single transistor is relevant in the Signal Strength position. I need more gain from the IF. I connected my Fluke meter to the Vertical scope input. It would jump to 5-6 volts for the three really strong stations but was only at 25-30 millivolts for most other stations.

I had a similar problem with the Scott LT112B tuner, could not for the life of me get the signal meter or multipath meter to work on the bench. So I tried it in the installation area near the coveted Sanus rack. I did not take off the tuning cage and just adjusted the trimmer cap with the tuning cage on and a flashlight for guidance. Lo and behold the signal meter and multipath meter started to respond properly. I did the same adjustment as specified in the service manual at like 106 Mhz but again using the tuning cage and making all adjustments with the cage on, rather than the cage off. I do not think the manual is specific as to the tuning cage, but one cannot get the alignment correct with the cage off!

 

[backmd]

I had a similar problem with the Scott LT112B tuner, could not for the life of me get the signal meter or multipath meter to work consistently on the bench. Sometimes it would work and other times not, very inconsistently. So I tried it in the installation area near the coveted Sanus rack. I did not take off the tuning cage and just adjusted the trimmer cap with the tuning cage on and a flashlight for guidance. Lo and behold the signal meter and multipath meter started to respond properly. I did the same adjustment as specified in the service manual at like 106 Mhz but again using the tuning cage and making all adjustments with the cage on, rather than the cage off. I do not think the manual is specific as to the tuning cage, but one cannot get the alignment correct with the cage off!

 

[jlovda]

I had a similar problem with the Scott LT112B tuner, could not for the life of me get the signal meter or multipath meter to work on the bench. So I tried it in the installation area near the coveted Sanus rack. I did not take off the tuning cage and just adjusted the trimmer cap with the tuning cage on and a flashlight for guidance. Lo and behold the signal meter and multipath meter started to respond properly. I did the same adjustment as specified in the service manual at like 106 Mhz but again using the tuning cage and making all adjustments with the cage on, rather than the cage off. I do not think the manual is specific as to the tuning cage, but one cannot get the alignment correct with the cage off!

What is the "trimmer cap" you are referring to? I cannot figure out how to put numbers on a photo but here is a picture of the front end box. If the holes are numbered 1 - 6 left to right, where is the trimmer cap? Is the trimmer at the very front end adjusting the RF gain or part of the LO adjustment?

Thanks

 

[backmd]

I will go back in there tomorrow and let you know which hole the adjustment is made. It is not the trimmer for the local oscillator though, and likely I remember correctly.

 

[backmd]

I will go back in there tomorrow and let you know which hole the adjustment is made. It is not the trimmer for the local oscillator though, and likely I remember correctly.

number 4 which is labelled FM RF. It is not number 2 which is FM oscillator. It is amazing how much difference there is during an RF alignment with the tuner cage installed.

 

[backmd]

You have to shine a flashlight through the hole and thread a screwdriver through the hole with the proper orientation. Kind of like surgery.

 

[jlovda]

number 4 which is labelled FM RF. It is not number 2 which is FM oscillator. It is amazing how much difference there is during an RF alignment with the tuner cage installed.

Thanks so much. I will try it out tomorrow.

 

[jlovda]

number 4 which is labelled FM RF. It is not number 2 which is FM oscillator. It is amazing how much difference there is during an RF alignment with the tuner cage installed.

Well I apologize for being lazy. I used a photo of the front end box off Google above. I just took off the cover of my unit and it has a sticker with descriptions of the holes. This front end must have been used in other units; probably higher end FM and FM/AM receivers. I wonder if the AM capacitor plates are there?

It looks like the circuitry "moves" from right to left inside the box. From the schematic C203 (hole 6) is the antenna trimmer and C210 (hole 4) is the RF trimmer. Did you try adjusting the antenna at all?

Thanks again.

 

[backmd]

I did the Scott EZ align 3 times with the cover off. Then it was not working well, so I decided to do the alignment with the tuner cage cover on, and just used the RF trimmer capacitor adjustment while listening and also watching the meter when the meter switch was turned all the way clockwise, which on the Scott LT112B is labeled align. After such adjustment 85 stations, I am happy with that. Hopefully your adjustments will be successful also.

 

[jlovda]

I have a copy of the LT-112B instructions. Awhile back I tried to figure out where things around the IF and detector changed in the align position since the last version of the kit is virtually identical to version 1 of the 312-D. Time to go back.

I just looked in side the box. If you shine a bright flashlight on an angle into the RF Osc hole you can see the RF trimmer screw quite clearly. That is a very thin tiny slot! I may have to order a proper alignment screwdriver if I don't have anything that narrow.

 

[backmd]

The service manual, at least the one I have is super deep. The third time I read it I understood it. Clearly it is written by knowledgeable engineers rather than English majors with their doctorate of jurisdiction.

I have a copy of the LT-112B instructions. Awhile back I tried to figure out where things around the IF and detector changed in the align position since the last version of the kit is virtually identical to version 1 of the 312-D. Time to go back.

I just looked in side the box. If you shine a bright flashlight on an angle into the RF Osc hole you can see the RF trimmer screw quite clearly. That is a very thin tiny slot! I may have to order a proper alignment screwdriver if I don't have anything that narrow.

I do have the proper alignment tools, but this particular trimmer capacitor responded quite nicely to a standard craftsman philips and standard miniature screwdriver set. like this one
Craftsman 2 Pc Micro Screwdriver Set 9 41543

 

[backmd]

Also on my tuner, I found a bad 300 ohm to 75 ohm transformer that was external to the tuner. It was working on the bench but not on the rack secondary to this transformer issue. Found it by swapping coax, then swapping transformers.

 

[rmp]

We have a rare period of three 4312 tuners on the bay.

 

[jlovda]

We have a rare period of three 4312 tuners on the bay.

I was looking at some of the chassis pictures. Boy, do they fall into an age when the transistors tried to look like miniature tubes. I wonder if they used wire-wrap inside? LOL