Scott 399 repair (or "am I in over my head this time??")

kirk57

kirk57

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So I have a friend who gave me a Scott 399 receiver in exchange for some work I did for him. This had been in his basement for who knows how long, at least 20 years is my guess.

So far:

- I got the correct tubes for all positions
- replaced a few way-off caps and resistors (but probably not all, there are a *lot* of parts on this thing.)
After this, then unit powered up (slowly, using a variac always) and produced hum and some sound using an Iphone as a source, not loud, but kinda sorta worked.

Happy with that result, I replaced selenium rectifier in the bias supply with 4 diodes.
After I did this, I was just hearing hum, now no music at all.

Thinking it was bad PS caps, I restuffed C1 and C2 caps (20 and 30uF, 450 volts for both cans)
Then I got a lot less hum, still no music output, and R172, a 160 ohm 2W power resistor (right after the rectifier tube) started smoking, so I turned it off

The 160 ohm resistor does have a hairline crack in it, but still measures within spec. I've ordered a new one in any case. I also replaced the for .1 uF Cerecaps coupling caps as read they could be an issue. However they also all read within 20% of spec.

There are total of 8 can caps on this thing(!), but I'd read that the others are usually not bad. Wondering it they also need attention

So taking one thing at a time, any guesses as why
- the 160 ohm resistor was smoking
- Not hearing any music through any inputs after replacing selenium rectifier

I'm a bit nervous about working on this thing; these are pretty rare and quite beautiful, and I don't want to destroy it.
 
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I just tested some of the tubes. The rectifier G34 seems to have survived. There were two Scott-branded 7189 in there, and they are OK, but the two almost-new EL84 (I know, not designed for 400v plate voltages) seem to have gone to that great tube in the sky.

Do I have to buy another pair of 7189 just to test this receiver ? I think I can use the Russian 6P14P-EV?

I have a schematic but it's too big to post here.
 
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hfe_hh_scott_399_service_info_en.pdf
I tried to add 399 schematic, But no soap...............

Trouble shoot bias with output tubes removed.

The balance and bias pots are always a problem. (Clean them and return to previous setting)

Test the -55 volt bias voltage at 7189 grids.

Good luck..
 
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The bias will need to be adjusted. You can take out your el84's and adjust the bias first one side and then the other with your 7189's.
Wait for someone with more knowledge to step up, you may have to change a voltage dropping resistor.
Best of luck!
 
FYI:
If you do a youtube search for xraytonyb, you can find a five episode set of videos, where he brings a very sick Scott 399 back to alive and well status.
It takes several hours of viewing, but he goes thru a very thorough restoration explaining every step, and even ties in getting the FM and AM tuners functioning as they should.
I don't have a 399 but it made for an interesting bit of pandemic entertainment.
 
Thanks all. It’s pretty clear that the bias supply needs revisiting

I’ll have a look at that YT video and see if that helps. I should have stated I’m powering this up slowly with a variac, trying to be careful.
 
Pull the rectifier tube and check the bias supply to see what its doing. Guessing thats going to be the issue since it worked until that change. Not looking at a schematic but I'm guessing the bias supply is also DC heater supply to some tubes? If it happens to be feeding the stuff in the tone control section, loss of heater supply to that point would kill the sound. No bias supply will also make the tubes draw a lot of current which could explain a resistor in the supply cooking.


EL34's are plenty capable of 400+v on the plate, unless you meant EL84 ?
 
Power Supply for reference. R172 is the one that started smoking.

index.php

index.php
 
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gadget73 said:
Pull the rectifier tube and check the bias supply to see what its doing. Guessing thats going to be the issue since it worked until that change. Not looking at a schematic but I'm guessing the bias supply is also DC heater supply to some tubes? If it happens to be feeding the stuff in the tone control section, loss of heater supply to that point would kill the sound. No bias supply will also make the tubes draw a lot of current which could explain a resistor in the supply cooking.


EL34's are plenty capable of 400+v on the plate, unless you meant EL84 ?
Click to expand...

Yeah I need to revisit this bias supply for sure. Yes, I meant EL84, edited my post above, thanks for catching that...
 
kirk57 said:
Yeah I need to revisit this bias supply for sure. Yes, I meant EL84, edited my post above, thanks for catching that...
Click to expand...
Make sure you installed the new diodes correctly so that you get negative voltage out, not positive. The resistor following the new diodes will also need to be increased in value and, possibly wattage to compensate for the improved efficiency.

I would plan on replacing all the electrolytics in the bias supply too. Remember that they also need to installed with their positive to ground since they're filtering a negative voltage supply.

No, you don't need to use 7189s, especially since you can adjust the bias. Plate voltage is only part of the story, it's plate dissipation that really matters, which is determined by the combination of voltage and current.

Since you can control bias current, you can keep the dissipation under control. The maximum plate dissipation is 12w for both 7189 and 6BQ5.

And if you're worried about the voltage rating, see my post in this thread:

https://www.audiokarma.org/forums/index.php?threads/scott-299b.942556/#post-14398266
 
Checked the bias supply. I had assembled the 4 diodes exactly in the orientation shown on the schematic, and the resulting voltage is +55 volts wrtg, not -55v.

So I guess I just ignore the schematic and hook it up in such a way that I get -55v (i.e. flip it around)
 
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The Sams doesn't appear to have it drawn wrong. Cathodes (banded end) are positive and that goes to chassis, the non-banded negative end goes to the supply. Other schematics, no idea.
 
banded end is cathode, which will be positive output. They also have direction of current flow wrong there, but most things do. It actually flows negative to positive but for whatever reason its always taught the other way round. Makes it a lot more confusing trying to figure out how vacuum tubes actually function, or at least it did for me at first.
 
Be sure to replace all the elctrolytics in the bias supply since they were exposed to reverse polarity (even if you already replaced them).

It's counter intuitive, but current always flows from positive to negative. This is the opposite of electron flow, which flows from negative to positive. In almost all cases, you ignore electron flow, and only care about current flow. In a tube, current flows from the plate to the cathode.
 
Update: Bias supply capacitors in C5 and C6 were completely trashed. I'm in the process of restuffing all 8 (!) can capacitors as every one has measured out of spec (exceeding +/- 20%) . That should be done today. Also R167 (10k resistor) was burned in half. That's a 1 watt and I had to order it, now on the way.

I hope I didn't do any serious damage (mostly concerned about OPTs) by reversing the bias supply voltage...
 
andy said:
It's counter intuitive, but current always flows from positive to negative. This is the opposite of electron flow, which flows from negative to positive. In a tube, current flows from the plate to the cathode.
Click to expand...

it is counter-intuitive. Current flow is the flow of electrons, so it can't go in the opposite direction the electrons go.

and current flow in a tube is not plate to cathode.
 
Current is the measurement of electron flow.

Quoting Merlin Blencowe from Designing High Fidelity Tube Preamps: "Electric current is not 'made of' anything, not even charge, any more than wind speed is made of wind."

Great book, at least through the 19 pages I've read. :)
 
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