Six Weeks of Troubleshooting...I think I found it!

[Chris K]

Before Thanksgiving, I got stalled trying to align the sound and video IF traps on a 1949 RCA KCS 28B chassis in an RCA 9-TC-240 cabinet TV. After making some progress (With a tremendous amount of help from Penthode, Tom C and Bob A) I began to have issues with the HP 8600 Sweep Generator and the HP 8601 Digital Marker combination. Without describing the long ride to standstill, and after several "Ah Ha!" moments that I thought solved the issue, I all but gave up after the last failure. The furthest I got was narrowing the issue down to the 8600 marker. Sometimes the issue would disappear with banging or circuit board flexing only to show up an hour later or a couple of days later. I figured it was a ghost in the machine I'd never find.

Six weeks of actual troubleshooting using a multimeter and a schematic instead of a club led me to the 5V circuit on the regulator board. It was low at 4.5V even with the adjustment pot on the board set for maximum voltage. The voltage is regulated by a 10 gold pin HP can IC, part #1820-0196.
It's proprietary...nobody but HP made it. When I froze it, voltage went to +5V DC. The machine worked perfectly until the voltage returned to 4.5V and the problems reappeared. I then froze it to restore function and instead of waiting for it to warm up, I immediately adjusted the voltage down using the regulating pot and as soon as I got to 4.5V, the marker wouldn't function correctly. I repeated this several times and was able to reproduce the phenomena every time. The regulator is a 500 ohm pot and it measures perfect. Below is the offending board and the SOB of a "chip in a can" part.







After fruitless searching for weeks, I finally came across the chip NOS on eBay for $10. So it should be here in a few days , I'll instal and test the unit. If it's now reliable, it's back to alignment on the RCA.

While thankful for all of the contributors in the other thread (and there were mare than the 3 I mentioned), I want to give a huge thanks to Penthode who rode with me through this half year process giving advise and imparting his experience and wisdom while taking a heartfelt interest in seeing me succeed. Besides the thread we exchanged emails as well. I hope he'll be pleased I have not given up!

Additionally, I bought an HP 8643A Signal Generator. It's a beast. Other than a dead memory battery, it was tested and certified and appears to work well. I can sweep with it but if I'm going to look at a scope XY trace, I need to figure out where to tap into the generator to access the horizontal sweep output. The scope gets the Y input from the test point in the TV, the sweep signal goes into the input lug of the particular tube but the scope needs an X input. It's probably on the inaccessible back of this station wagon on my bench!

Thanks everyone!

[bandersen]

Regarding HP8643 - beast is an understatement! You'll find the x-axis output on the back of the device

[Chris K]

Originally Posted by bandersen

Regarding HP8643 - beast is an understatement! You'll find the x-axis output on the back of the device

Of course it is! Thanks Bob.

[Tube TV]

Nice work Chris getting that debugged. You sure don't run into those early IC's to often these days. When you get it running it will be a hell of a nice sweep generator.

I have a sync board in my GBC small studio video cameras that has Faichild epoxy dome IC's that are sort of like these. There's about 12 of them and thankfully they're all good.

[bandersen]

The HP8643A is awesome too with 3 programmable markers.

[Chris K]

Originally Posted by bandersen

The HP8643A is awesome too with 3 programmable markers.

I got the X axis hooked up and its works great but on Auto Sweep I can't get it to do more than one sweep per second. I can see the cursor trace and it looks like it's supposed to but... Can it sweep faster than that? I don't think it's useable like this.

[Chris K]

Originally Posted by Tube TV

Nice work Chris getting that debugged. You sure don't run into those early IC's to often these days. When you get it running it will be a hell of a nice sweep generator.

I have a sync board in my GBC small studio video cameras that has Faichild epoxy dome IC's that are sort of like these. There's about 12 of them and thankfully they're all good.

Thanks. I'm a very inexperienced tube radio and TV guy. Solid state isn't something I'm comfortable working on. Upside is there's no 350V B+ to getcha!!!

[Penthode]

Originally Posted by Chris K

Thanks. I'm a very inexperienced tube radio and TV guy. Solid state isn't something I'm comfortable working on. Upside is there's no 350V B+ to getcha!!!

The HP8643A is a terrific piece of equipment. But it is a bit of an overkill for this application. I hope it works for you.

[Chris K]

Originally Posted by Penthode

The HP8643A is a terric piece of equipment. But it is a bit of an overkill for this application. I hope it works for you.

Hey there you are!!! Welcome back to my adventures! Yes, overkill for sure and once the 8600 is up and running, I'll be using it and the 8601 but I wanted to see what this could do sweeping and I'd like to try a bunch of other stuff with it too. I have vintage signal generators but accuracy and reliability of the analog stuff has led me down a lot of unnecessary rabbit holes. This piece of equipment should be all I'll ever need.

[bandersen]

HP 8643A supports 20 ms to 10 sec sweep times. Press sweep time to set it

[Chris K]

Originally Posted by bandersen

HP 8643A supports 20 ms to 10 sec sweep times. Press sweep time to set it

Yes I see that in the sweep range limits but when I try to enter it I get an error message "Sweep time too small". I have a 1MHz sweep with 21.25MHz as the center frequency. Something else is wrong. It's set to the default phase continuous sweep.

[bandersen]

What value are you entering? I think it's in seconds so 0.1 should give you 100mS sweep.

[Chris K]

I’m concerned it can’t do it. If you look in the table it says if your a range where 21.25MHz sits, minimum is 0.5 seconds. I tried to enter 0.1 seconds and got the same too small message. When you see a MHz frequency displayed on your HP of this type, how many digits to the right of the decimal does it display? My display has about 8 zeros.

[bandersen]

Use phase continuous mode. It is enabled by activating special function 112

[Chris K]

Yes I've been going to that. It tells me my frequency span is too large for phase continuous. I'm scanning 500KHz to either side of the center frequency. What the hell? Something basic in my setup can't be right but the trace on the scope looked fine. The sweep just wasn't fast enough for me to see the scan as a line.