HH Scott Solid State
- Thread starter rmp
- Start date
Can anyone with a 312D-1 help me. I have one that I recapped and sent out to be professionally aligned. It works quite well. The problem is that, while the center tuning meter works fine, the signal strength meter only moves for the strongest stations. It either sits at one or jumps to 9-10. Never in the middle (4-7) It this normal? There is a pot on the multipath circuit board that I fiddled with but it didn't make any difference.
I should note that the reason I bought it was that I bought one new in 1968 and kept it in my college dorm for 18 months. It replaced a KLH Eighteen and was superseded by a Mcintosh MR73 (bought new in 1970). It's been a long time but from memory, the Scott was not really better than the KLH and The Mac was definitely no better or worse than the Scott. I did get a black glass front panel though! To be honest I am still not really a fan of the 312-D dial scale. It's impossible to read. The 312-C looks much nicer.
I should note that the reason I bought it was that I bought one new in 1968 and kept it in my college dorm for 18 months. It replaced a KLH Eighteen and was superseded by a Mcintosh MR73 (bought new in 1970). It's been a long time but from memory, the Scott was not really better than the KLH and The Mac was definitely no better or worse than the Scott. I did get a black glass front panel though! To be honest I am still not really a fan of the 312-D dial scale. It's impossible to read. The 312-C looks much nicer.
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Another comment. I found a website of someone who resurrected a Scott HHS-20 "Scottsman" one-piece compact audio system. I copied the photo. The IF strip also has the four uA703 design with regular transformers. Probably just had a one FET front end like the other cheaper Scott tuners.
KentTeffeteller
Gimpus Stereophilus!
My example suffers from the same problem as yours does.
You inspired me to fiddle with the tuner to see if I can get the meter to work better.
The center tuning comes off the output of the detector and the signal strength appears to come off the output of the third IF IC. Does anyone know if diodes D306 and 307 (DGM-2) are normal small signal Si diodes or are they Ge or varactor types? The two diodes look like a voltage doubler. Maybe the gain of my IF strip is low but I don't know it because the AGC is pushing it up.
KentTeffeteller
Gimpus Stereophilus!
That is an avenue worth me pursuing. The Service bulletin (the one 312D specific service manual) and the related HH Scott tuner section are on the net if you need them, and I am reasonably sure I have PDF scans of them on my computer. Do bear in mind I have the first version of the 312D.
By "first version" I assume you mean the one with IF cans rather than crystal filters. That's what I have. I have a copy of the service bulletin which has schematics but no parts lists or alignment procedures. The IF circuit above was snipped from the bulletin. If you have a copy or know the link of the complete manual I would appreciate getting it. The vertical scope output should be related to the signal strength as well.
Bob@FM
Well-Known Member
"Does anyone know if diodes D306 and 307 (DGM-2) are normal small signal Si diodes or are they Ge or varactor types?"
I am betting they are low drop germanium types. That circuit looks very similar to a RF detector probe, that you use to convert 10.7 MHz to DC for use in IF sweeps. You want the voltage drop on those diodes to be as low as possible, and also they should have low current and low capacitance specs - diodes made for RF signal detection. Definitely not varactor type. Measure the drop with the diode function on a multimeter - they should both have a 0.5V - 0.6V drop, or lower. Same deal for the ones in the detector, DGM-1, except those should also match. And for sure replace the 22uF C313 if not already done.
Old germanium types have been known to increase in voltage drop over time, in some cases. So best to always check them. The supply of NOS ones is usually good, try ham radio parts site. You can also use a special HP diode made for mixers, which is a more reliable modern type, but has slightly higher drop. I think the HP ones are special Schottky types.
Bob
I am betting they are low drop germanium types. That circuit looks very similar to a RF detector probe, that you use to convert 10.7 MHz to DC for use in IF sweeps. You want the voltage drop on those diodes to be as low as possible, and also they should have low current and low capacitance specs - diodes made for RF signal detection. Definitely not varactor type. Measure the drop with the diode function on a multimeter - they should both have a 0.5V - 0.6V drop, or lower. Same deal for the ones in the detector, DGM-1, except those should also match. And for sure replace the 22uF C313 if not already done.
Old germanium types have been known to increase in voltage drop over time, in some cases. So best to always check them. The supply of NOS ones is usually good, try ham radio parts site. You can also use a special HP diode made for mixers, which is a more reliable modern type, but has slightly higher drop. I think the HP ones are special Schottky types.
Bob
Bob@FM
Well-Known Member
Thanks for showing that curve. My take is that no companies made germanium diodes anymore.
But I may be wrong. I have purchased obvious fakes on Ebay, with 0.7 V drop, so the source is very important.
This guy for instance, apparently has the real deal
https://www.mikeselectronicparts.com/product/1n34a/
Usually they are marked, but you may need to remove them and use a magnifying glass to read the marks.
I was off in the voltage drop for germanium diodes, at least some sites like the one above say they have a typical 0.3V drop. It really depends on the specified current for the spec., as your chart shows.
If your multimeter uses a different current level for the diode test, what you see will be different results from the typical "spec." My 2 meters use different diode test currents, so I see different results depending on the meter used.
