Scott 299D repair question

W

wgrr

Super Member
I am starting to repair and rebuild a Scott 299D that I pulled out of my attic several months ago. I am starting with the hardest repair first and need some advice from you kind folks that have seen the insides of an early model 299D.

My problem is a very bad previous repair that replaced the four section 100mfd/75vdc x 4 capacitor with a couple of electrolytic capacitor hot glued to the side of the power transformer. So what I have is a hole and no capacitor after I removed the old repair.

The filter cap is not my problem. I had a gentleman in Wichita build me a can com pos 100mfd/75vdcx4 capacitor that should fill the hole just fine. The wiring of this capacitor is fairly simple. My problem is that when they nuked out the old capacitor for the repair I think they also took out the 50/75 cap that is filtering (?) the bias power. I see it in the schematic but it is not in the amp bias power circuit. The 10K ohm resistor to the bias has several wires that have been cut off the terminal the resistor is soldered to. A wire that is soldered to the ground of the terminal for the power to and from the (soon to be solid state) rectifier looks like it has been cut off and used as a ground for all the wires in the bad cap repair job for the 12AX7 heaters. Could this be one of the wires from the missing 50/75 cap? Anyway, I am going to try to post a picture of the whole mess.

Thanks

wgrr

link to schematic: http://www.hhscott.com/pdf/fs/299D_1.JPG
 

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The 50UF 75V caps negative lead was soldered to the 10K resistor and also a wire from this same junction went over to the two bias voltage pots. The positive lead of this cap was soldered to one of the outer can com positive twist tabs of the filter can.
 
NOSValves said:
The 50UF 75V caps negative lead was soldered to the 10K resistor and also a wire from this same junction went over to the two bias voltage pots. The positive lead of this cap was soldered to one of the outer can com positive twist tabs of the filter can.
Click to expand...

Craig,

I know that both you and I have several Scott models. Maybe we should take some close up pictures of the underside of these guys, 299, 299B, 299C, 299D, LK-72, LK-72B etc. and post them in a picture gallery. Might help a lot of people?

Wadda ya think? I will be willing to do it but can't for about two weeks.

I have several of each model and some 222 and LK-48 versions...
 
After you get the bias/DC filament supply capacitor sorted out, I might suggest that you replace the white ceramic American Radionic Ceracap .1µF capacitors going to (through the resistor) pin 6 of the 7591 output tubes. They are usually leaky and will upset the bias of those expensive output tubes. Replacing all of those paper caps is a good idea but at least replace those four as soon as possible.

Just a suggestion...
 
I will be replacing all the ceramic (ceracaps) and the paper caps with orange drops. the two orange 4/250 caps in the picture will be replaced with Sprague Atoms. The old selenium rectifier was replaced with diodes many years ago. This repair is also shoddy. I will replace it with a Radio Shack 4 amp 400 Volt bridge rectifier and will replace the 10k resistor with a 33 ohm 5 watt resistor. All the can caps will be replaced. Would a 47/500 cap work to replace the missing 50/75 cap.

This leads me to another question. I had to get three section 30mfd 525vdc to replace the old two section caps. That means I have two 30/525 caps that will not be used. Any Ideas what I could use these extra caps for in this amp.

Once again, thanks for the help. I have built several amps but this is my first vintage amp rebuild.
 
wgrr said:
I will be replacing all the ceramic (ceracaps) and the paper caps with orange drops. the two orange 4/250 caps in the picture will be replaced with Sprague Atoms. The old selenium rectifier was replaced with diodes many years ago.
Click to expand...

There's an early model and a late model of the 299D. I think the late model 299D had diodes, not a selenium rectifier.

wgrr said:
This leads me to another question. I had to get three section 30mfd 525vdc to replace the old two section caps. That means I have two 30/525 caps that will not be used. Any Ideas what I could use these extra caps for in this amp.
Click to expand...

If you have the late version 299D, maybe you could increase the filtration where it would do the most good. Use 60µF (strapped) for the first power supply cap instead of the original 40µF. This heads over to the OPT center taps.

Just an idea.

. . Falcon
 
Before you replace that 10KΩ resistor, please read this below. 33Ω vs 10KΩ is a big difference. I don't think that the 10KΩ resistor is the one that you need to check.

I know that the 299D is a little different than the other amps that Scott produced. Most of them had an 18Ω resistor after the selenium bridge. I think that the 299D early and late were different.

The earlier 299D appears to use the conventional winding on the power transformer for the bias supply with a Siemens selenium bridge, and I believe it is mounted to on the right side of the chassis near the volume control. The first resistor after the bridge is a 33Ω 1 watt resistor. I have not worked on one of these for a while.

The later 299D, SN: 249780 and above used a what appears to be a different power transformer; one that has a center tapped winding for the bias supply. Since it is center tapped, they use just two diodes with in a full wave configuration.

I would leave the 10KΩ resistor alone for now and after repairs, check the bias voltage/current at the 7591 tubes. The 299D is quite different around this bias circuit.

Check out the HH Scott website and download the different schematics so that you can refer to them. I will provide a link below that will take you to the various products and you can check the schematics from there.

http://www.hhscott.com/database/vhhs_00011.html
 
The amp is the early model. I posted a link to the schematic in my original post. I got to looking at the various schematics for the 299x series of amps. The only one that provides power to the bias on the first tap off the negative side of the rectifier is the early 299D. The first resistor off the first tap is a 10K carbon comp. This goes to the dc bias pots. The second tap off goes to a 33ohm 1 watt wire wound resistor then to the 100/75 X 4 can filter cap through three 18 ohm resistors and then provides power to the AX7 heaters.

Will I have to adjust these resistors (10k, 33 ohm) when I install the more efficient solid state bridge rectifier.

Thanks

Bill
 


I put also a Silicone Rectifier in. Want' joking around with Selenium Stuff! This must go.
After I have to put a lot more then here suggested, as new Resistor Value In.
Its over 240 Ohms. Now we have -50VDC. My Replacement Cap are 3 x 22uF/160V Types.

Had put in also a new matched Quad Outputpowertubes from EH. Type is 7591A.
They was not matched. Only Two matched Pairs... w.t.f.? I wrote the Seller, he send me a new Set over.
All have the same Date Stamp 0804, but one is definetively different Lot. Smaller Getter, like a Soup Dish. Not only a Ring.

Had a Lot of Measurements done, until a Friend tells me, that can be the unmatched Quad.
Yes Sir! Bias was ok, but had over 2.45VDC Different Voltages on the Kathodes.
Now I have a new "quad matched-silly-Fun-EH-Set" in. 3 are same physical Look, but 1 its another Story.
The Halo Getter is pretty different. My Cam is inLoading Accus. No Pics herefore. Not Yet.

All 4 seemd to be very good now. Kathode Voltage at -18.45 and -18.54VDC.
Bias here is at (3,0Ohm Resistors= exact 210mVDC. Both Channels good.

Before, I try several Pairs for the Phase Converters,
labeled as 6U8 or 6GH8A, I saw the Blue Glow in Each Pair was different strong.
IFirst the RCA's 6GH8A, left Channel wizhout any Blue Glowing.
Then Syslvania, all in nice Blue, but second from left had less.
I put the Original Telefunken 1x6GH8 and 1x6U8 RCA back, beautifull same Glowing, on Each 7591A.

Now its equal. Looks amazing at Darkness in the Room.

HH Scott 255D: higher Serial Number 249779 are the "newer", with Diodes, not Selenium RF.
 
Thanks for the info and pics. I believe I can get this mess sorted out now. I will post pics when done.

Thanks

Bill
 
A Scott 299D Owner is here always Welcome !

;-)

Yesterday, I have tasted my new Dremel for Cap-Restuffing-Job.
First I make Pics of all 3 Multi Can-Caps.
Then, I draw all on Paper with correct Wire and Resistor - Colors.
After I desolder the First Cap, then un-install the Old Can
Here we need some Protections for Eyes and Enviroment.
The Dremel had a little Router-Disk for Cutting installed.
I cut the old Can right overhad the wider Low-End, near the Contacts.
It smells like burned, or ery new Electronics - not good and was black in the Upper Area. Heat!
Then I drill 3 Wholes. 2 right beside the Two Solder-Lugs. One for the Minus
Pole,in the Middle.

Pics will follow ...
 
kvflyer said:
Craig,

I know that both you and I have several Scott models. Maybe we should take some close up pictures of the underside of these guys, 299, 299B, 299C, 299D, LK-72, LK-72B etc. and post them in a picture gallery. Might help a lot of people?

Wadda ya think? I will be willing to do it but can't for about two weeks.

I have several of each model and some 222 and LK-48 versions...
Click to expand...


I uploaded a bunch of good quality photos and lot of the experts gave me advise on my 299D rebuild, I think it will help:


First Mishap with the HH Scott 299D
 
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