Scott 299 schematic error?

[thornev]

Sanity check... I replaced electrolytic C214 that was 10uF 25v with a 47uF 50v. The additional capacitance is OK, yes? I figured it can't hurt to have a little headroom. It's not like it will cause the amp to function differently, right? C214 lives in the filter cap section just after the selenium rectifier (soon to be a silicon rectifier).

 

[larryderouin]

I don't know about Sanity Check, but C214 should be OK with a 47uf/50V lytic.

 

[thornev]

You don't know about sanity checks? Are you teasing me? Just in case... I ask for a sanity check to make sure I'm not doing something insane. I'm known for doing things that normal people wouldn't think of doing. That's my adventurous side. Somehow I didn't get the filtering that most people seem to have both in action and speech. Thank you for paying attention to me today. It was a trying day electronically for me.

 

[thornev]

299 STATUS UPDATE...

I replaced all 5 cap cans in the last 2 days. What a task ! There are so many more leads attached to Scott cap cans than in Fishers and so many of the leads are buried under other leads and other components. I ruined a 47k resistor in the process (leads too short for heat-absorbing gator clips) so had to order a few. I also replaced C31 and C131 (.001uF 2500v !), and lytic C214. Earlier this week I replaced 4 of .047uF 400v Pyramid caps with 600v orange drop caps. I replaced R207 15k with a 10w 3% wirewound version, and R208 2k with a 5w 5% wirewound version. I left R209 alone as it seems to have preserved its resistance and I couldn't find a suitable 160 ohm replacement. The 3 power resistors' heat has really beat on several leads so I tied leads together to try to put a little more distance between them and the resistors. Those leads have become brittle and I should probably replace them at some point.

On deck is 10 ohm cathode resistors, rectifier replacement including figuring out what ohmage dropping resistor to use to drop down the increased voltage from the new silicon rectifier. From all I've read, it's trial and error trying to match the voltage out from the selenium rectifier. I found that I have an IN5408 in my stash which I can use. I just have to figure out how to mount it.

I'm using all my powers to keep from powering up this baby I'm so excited. It's a labor of love and it's going to be so sweet when I hear those first sounds of music.

 

[thornev]

I replaced 2 Pyramid .047uF 400v caps with 600v orange drops on the on/off/volume switch.

While back behind the faceplate I noticed there's a lot of corrosion on the sliding switch terminals, multi-function switch terminals and pot terminals. I cleaned what I could and used some isopropyl alcohol where I could to soak up the leftover corrosion powder. (it'll be dried up by the time I power it up so I'm not worried about flammable)

What do most people do about corrosion like this? It starts out like a dried up, liquid brown substance that I scrape off. I read that DeOxit can create its own problems in the long run. Is that the sentiment on AK? If not, I could spray some of that on the parts, but because the parts are so tightly packed, I won't be able to get into some areas to wipe off any excess.

 

[thornev]

Back from my vacation in NoHo (MA) and Boston. Listening on my Fisher KX-200 to vinyl I picked up at Planet Records near Harvard.

As for my 299, I added 10 ohm cathode resistors to 7189's so I can measure voltage to monitor bias setting and a CL-80 thermistor in line to the fuse. Found that I ordered .047uF and .068uF caps that are 200v but should be at least 400v. Dummy. So I ordered some 630v. Waiting for some other caps with which I can replace some Pyramids and Ceracaps.

 

[kvflyer]

Good to hear that the Scott is (almost) back on the bench. So many choices when buying parts, it's easy for "stuff" to happen. Note also that when you are ordering parts at DigiKey, if you need 6 of an item, it may be cheaper to actually buy 10 because of the price break. Usually the search engine points that out to you.

Good luck, keep us informed please.

 

[thornev]

For no other reason than I like the web site user interface, I've been using Mouser, not Digikey, as my 1st choice for parts. Replacing that 160 ohm 20w resistor has been quite the challenging journey. Every part I've ordered has been too large or won't fit for some other reason. I decided to take you, Don, up on your suggestion with attaching to chassis with thermal grease the aluminum-cased resistor. Grease should be here tomorrow. Just got delivery on some 1200pF 630v 10%, .022uF 630v 10% and 4uF 350v parts so that's the order of business on this 4th day of vacation.

And thank goodness the temperature will finally melt some of this ice all around us.

 

[thornev]

Does anyone know how to remove the metal front plate - not the faceplate - on which the controls are mounted so one can access the parts attached to the amp controls (e.g. bass and treble)? Frankly it doesn't look possible at all without removing each control. The components attached to that wall have leads attached to components that are attached to the chassis making separating all the controls at once from the rest of the chassis impossible. I must conclude that in order to access parts attached to a control, that control must be individually removed from the metal wall.

STATUS update: Replaced electrolytic C201 4uF 250v with an aluminum lytic 4uF 350v keeping the piggy-backed R206 47k attached in parallel. Also better insulated power resistor R208 (2k 5w) because its leads are just too close to other metal, especially the metal enclosure in which the power transformer sits. All 5 cap cans are done (replaced with Hayseeds). Also added a CL-80 thermistor between the fuse terminal and the mystery (spike saver?) ceramic capacitor.

I think I'm pretty much done with under-chassis restoration, just a couple of smaller Pyramids (C12 and C112) to replace. I'd like to replace some faceplate parts as the Ceracaps look like they are bulging at one end. But as I said above, access to the terminals of the controls is very challenging. It's probably a good idea to polwer the amp up with a variac after all the work I've done to see if this thing works at all. Some of the components attached to the controls look very "tired".

PS - Just replaced C12 and C112 .0012uF 400v black Pyramids with 1200pF 630v film caps.

 

[kvflyer]

Does anyone know how to remove the metal front plate - not the faceplate - on which the controls are mounted so one can access the parts attached to the amp controls (e.g. bass and treble)? ...

Are there not three self tapping 1/4" head screws on each side of that panel? I thought you could loosen and remover them to "swing" the panel away.

 

[thornev]

There are too many SHORT lead attachments between components attached to controls and to chassis to "pull away". Maybe I can bend down slightly to expose parts, nut that's about it.

 

[thornev]

This post moved from another forum...

Larry... I've got the correct schematic - 299 version 2 from Scott, not the 299 version 1 from SAMS. The picture you attached in post 26 matches the schematic I've been using.

Yes, the lead from pin3 V104 attaches to an outer tab on the cap can. That can is NOT attached to the chassis. None of the cans are. They are all on wafers.

Yes the 7189 pin 3's are daisy chained. And I did put a 10 ohmer between V104 pin3 and the can. You're saying to remove it?

V4 pin3 does daisy chain to V5 pin3.

The outer tab on the can (to which the V104 pin3 is attached) is also attached to a gray wire that attaches to another outer tab on another cap can and NOTHING ELSE is attached to that other can's outer tab.

I do have the 10 ohm resistors attached as you suggest. That is I removed the lead, soldered the 10 ohm resistor to each pin 3, soldered the OTHER side of the resistor to the lead that was attached to pin3. I did that for all four 7189 sockets INCLUDING the lead from V104 pin3 whose lead attaches to the outer tab of the cap can (which I think you are saying I should remove, yes?).

The only issue I see with your illustrations and text and my wiring is the grounding. As I said, the lead to the outer tab of the cap can doesn't attach to any grounds anywhere. HOWEVER, V104 pin 3 has a wire attached to it that emanates from V104 pin 5 which has a wire attached to it that emanates from the power transformer. And that pin 5 has other wires attached to other components.

The schematic I'm using is attached. Or go here to page 24:
https://web.archive.org/web/2013112.../Pioneering-Wireless/eMagazines/VTV/VTV12.pdf

PS - The 2 cans referenced above are "can negative" if that helps. By the way, when I was notified of your post, I had the variac on at 50v, connected to the amp, monitoring voltage. No smoke or weird noises and the tubes weren't lighting up yet.

 

[thornev]

Got the variac at 62v. The 3 lights on the front light up which is good. All voltages that I checked (about 6 locations) are looking good except for 2 locations...

1) V3 pin 1 is supposed to be 75v according to the schematic. I'm getting 174v and that's at half power ! Maybe a resistor preceding that pin is falling down on the job. I'll have to hunt.

2) And at power resistor R207, I'm supposed to get 300v. I'm getting zero.

3) I think the bridge rectifier might be a problem. The "+" side, which I think is the B+, is generating nothing. The "-" side is supposed to generate -50vdc and is measuring -20vdc at half power which seems OK.

At 72v on the variac, the tubes start to glow. The bad news is that the bias current is completely uneven among all four 7189 tubes. I'm getting .36v, .33v, .26v and .16v. I don't understand this measurement because all 4 tubes are daisy-chained on their pin 3 but with a 10 ohm resistor attached to each pin 3. I think the 10 ohm resistors might be the cause of the differing voltages because the farther away from the power transformer the 7189 is, the higher the reading.

UPDATE: I got the variac up to 96v and I have stereo ! However, the balance between the 2 channels is way off and I cannot figure out how to use the AC and DC balance controls, and the BIAS control on the back of the amp. Measuring the 10 ohm resistors on pin 3 of each tube is constantly changing and way different from each other as mentioned above.

UPDATE 2: The channel imbalance was because the variac voltage was only at 96. I turned it up to 104 and the balance is coming back ! I got the variac up to 110v and the sound is fabulous !

 

[thornev]

And then there's this... There's a 330 ohm resistor that's not even on the schematic. It's attached to V4 pin4 and V3 pin5 and it measures at 1.7 ohms. I'd say that's just a bit out of tolerance ! A second resistor with the same specs is attached to V104 pin 4 and V103 pin 5, same bad measurement which leads me to believe it's OK - what are the odds of both resistors failing the same amount?

What's interesting is that the 299 version 1 amp schematic has two 330 ohm 10w resistors, but on the version 2 amp, they are combined into a single 160 ohm 20w resistor. I wonder if someone who worked on this amp previously installed those 2 resistors using the wrong schematic. Not likely because the two 330's on the schematic are not even close to where they are in my amp.

UPDATE... Those 2 resistors are on the schematic as R217 and R218, 3.3 ohms 1w. I missed them because they are on the schematic as attached to pin 5 of the two 6BL8 tubes. I guess I misread the color bands (orange, orange, gold not brown).

 

[kvflyer]

There are too many SHORT lead attachments between components attached to controls and to chassis to "pull away". Maybe I can bend down slightly to expose parts, nut that's about it.

I can't disagree with you there! If I recall, yes, you can take the retaining nuts off of the controls but the slide switches are riveted in so they will have to stay. And, working on a control or selector switch whilst it is not attached is a recipe for disaster because of all of the movement during the operation. It will surely break a few leads.

I do admit to changing all of the Pyramid capacitors on controls etc. in my 299. But it took a long time. Many have said that a leaky capacitor on the tone control or selector switch may not cause a lot of problem since there is essentially very little voltage on them. But... I went ahead and toughed it out. It is especially difficult because Scott make very good mechanical connections before they soldered the connection. Patience and them more patience. No hurry...

 

[thornev]

Thanks to great assistance offline from Larry, I found I had miswired the 10 ohm resistors (daisy-chained the pin 3 leads at the socket pins when they should be daisy-chained at the non-socket side of the 10 ohm resistors). I corrected that, balanced the DC BALANCE and cranked the BIAS on the back of the unit and now my 299 v2 is singing although the 7189 output tubes are not singing with the same voracity:
plate = 391 vdc
V104 = .213 vdc
V105 = .209 (391 x .0209 = 8.2w)
V4 = .242 (391 x .0242 = 9.5w)
V5 = .209

Might be time for an IBAM or maybe an IBBA this time. The bridge rectifier is holding up while I await a silicon rectifier. It's going to be a long and arduous journey updating parts behind the faceplate. Parts are on order. I love the brown on gold faceplate.

So far I've played music through the B&W DM601's. Excited to try this baby through my AR-2AX' which will require the KX-200 to make way.

 

[thornev]

Geez, this thing gets really hot ! The left side is 148F ! In part that's because I have it on its right side so all the heat is traveling upward onto the left side, but geez. The rectifier tube I measured as hot as 239F. I didn't know that Scott's could double as stoves ! Power resistor R207 I measured at 127F.

 

[thornev]

In case anyone with an HH Scott 299 version wants a labeled chassis, see the attached.OK, uploaded files get shrunk. Start a forum conversation with me if interested. Thorne

PS - It seems like this baby can't push enough sound into my AR-2Ax speakers. I read that might happen. I'll try again after I've completed all restoration. They sound great with my B&W DM601 bookshelf speakers (6" woofers).

 

[larryderouin]

I'll wager if you check the temps on your KX-200, they'll be similar. 148*F isn't close to meltdown. When you get to 180*F start worrying HARD! 239*f on the Rectifier is about Normal.

AR-2ax's have a reputation for sounding pretty good, and also for being hard to drive (very inefficient). I gave up on trying to power them with tubes.

 

[thornev]

The AR-2Ax sound fabulous with my KX-200 and 500-C. But the Scott 299 just doesn't have the power. However, I notice that one channel is significantly louder than the other so there might be something else that is causing the poor sound.