HH Scott 399 w/ Type 335 FM Stereo Multiplexer

SCompRacer

More rambling from the bench. I had no AM/FM and the capacitors for the AM/FM circuits had no B+. The 6X4 rectifier tube was bad. Now I had tested it, and it was good, but I think it may have blew on the first aborted variac/dim bulb power up prior to replacing power supply capacitors. (It shows two shorts now in the tube terser). Looked at schematic and see that the 6X4 sends 190v and 145v B+ to selector switches. How about that.

If you select phono, both AM and FM circuits are shut off (magic tuning tubes go dark), only tube heaters are powered. FM only, AM shuts off. Simulcast, both on.



Checked my work, looked good. Close look at selector, a resistor is rubbing the shaft. Resistor clears flat on shaft but rubs the rounded part. Terminal on selector bent to one side. On opposite end, terminals very close and may have contacted one of the metal retainers that holds this thing together when resistor moved by shaft. I added a short section of lead and ensured no contact with leads on other end. (This receiver was shipped from Georgia double box but just wrapped in bubble wrap, so it got jostled around).




I contacted Aarons Tube Emporium in Rockford (@ALL212) and he had two 6X4's. Sold! Great service with overnight delivery! The FM multiplexer uses a 6X4 as well. Stuck the tube in, powered up and now I have functioning FM but no AM.




I use my chop stick to check for loose grounds around tuner tubes (read about loose ground issues on the Scotts at audiokarma). Touch some wires and briefly have AM. Upon close inspection, one lead on the AM RF-IF antenna coil is being drawn (but not quartered) to rear. Real ugly solder joint. Maybe the foreman got on them about using too much wire that day...there was no slack, taught to rear panel. Lengthened wire (that's clear shrink, not bare) and straightened terminal on coil. All is well, we have AM now. Was worried terminal had pulled apart inside of the coil.






When I was looking at selector closely, I spy one terminal not making contact. For this old man, this is lighted magnifier work. I don't see any tracks to indicate it was at one time so I do not think I did that. We'll see if everything works before pushing it with a toothpick to make it contact.

Next up test it with FM multiplexer then order all film caps.

schwarcw

Wow! Progress! Nobody listens to AM anyway LOL!

schwarcw

pitdogg2 wrote: »

Hey Carl! How you doing pal?

Greetings Ivan! I am doing well! I hope you are doing good. I owe you a text and/or a phone call!

SCompRacer

schwarcw wrote: »

Wow! Progress! Nobody listens to AM anyway LOL!

What if simulcast returns? Maybe he wants to listen to news and music at same time?

It's freaky with one magic eye out. Like it's winking at you.

schwarcw

[/quote]

It's freaky with one magic eye out. Like it's winking at you. [/quote]

I have one of those in my Mac MR-71 and my Scott LT-110. My Marantz 150 tuner has the scope!

SCompRacer

Film caps arrived. I decided to do the 335 FM multiplexer first. I had already restuffed the 40uf/250v x 4 can with 47uf/350v x 4 Nichicon UCY. Vishay BFC2467 polyester were chosen for the four .0027uf and two .1uf 400v film caps. BFC2467 doesn't go up to 400v in 1uf value so I chose an MKP for that. A 25uf 25v Sprague Atom replaced the Callins of same value.

Some were buried. The Callins 25uf 25v was really out of spec measuring at 80uf. Next up the 399 and it has some challenging ones.

SCompRacer

I got covid late Dec and didn't start back up on the 399 until late Feb. It's getting close to a road trip to PA, pick up Carl and head to NY for delivery.

High voltages were an issue running at 125VAC line. I decided to build a bucking transformer with selector that allows line voltage, 3.7% and 7% reduction of line voltage. It's fused with on/off switch otherwise primary on transformer is hot all the time.

Meters show in and out voltages. It has toggle switch to shut display off. That solved the voltage issue as 7189 have a 400VDC max plate voltage. 7189A are rated higher, but you have to look at tube heater voltages as well.



DC after first step down was around 410VDC at 125VAC line.



The 399 is a compact design so not much room around tube sockets to add 10-ohm cathode resistor and tip jacks to measure mA. I have done the math. I measured ohms of each output tube transformer windings. For example, 135 ohms x .020 mA = 2.7 volt drop from where I measured voltage after first step down to plate of tube. The windings are shorter on the inner winding so that one measure 125-ohms. 125 ohms x .020 mA = 2.5 volt drop.

The math gets you close but not exact.

Or you can use these 9 pin mA adapters and plug them directly into a DVM. Tube sockets are recessed so diameter too large. Had to get some tube saver sockets. They measured exact diameter of hole so had to sand them a bit. Otherwise, it's a press fit.




I had replaced the old Selenium rectifier with a BR34 rectifier bridge. I installed the same value resistor after it, a 33-ohm but used a higher wattage chassis mount. The cement resistor would heat the carbon resistors above it, which are negative bias circuit resistors. Heating carbon resistors changes their value. The chassis mount solves that issue.

I suspected I would have to adjust resistor values after installing the modern bridge rectifier. I had to add a 15-ohm in series with the 33-ohm to knock the negative bias voltage down a bit. Otherwise, I hit the bias pot stop before getting correct mA values. I have ordered a larger value chassis mount.




I drilled the mounting holes between resistors on opposite for the chassis mount screws. It won't be fun but it's doable. That is why it is mounted a bit crooked.



Before. Lot's of wires on that little bugger. The three 18-ohm tested well within spec. The 10k on top was way out and replaced.




Tubes are new so I'll have to re-check bias after I get some hours on them. A matched quad isn't 100% exact. The DC balance pot allows setting equal mA for each tube. Running at 22mA right now. Maybe I should of paid extra for the burn in option...

Right channel.



Left channel.



It's actually sitting on right side to allow access, so I gave pic a quarter turn. Owner requested Blue LED's.

msg

Rich, did you retire and get busier?

SCompRacer

I got in over my head. Overwhelmed. Aaron laughs at me.

Speaking of Aaron, he was here the other day with two of his three sons picking something up. I got the these for all three so there would be no confusion as to who was the......



@ALL212