Scott 299 schematic error?
- Thread starter thornev
- Start date
thornev
Super Member
I did my first debugging of a signal imbalance. One channel was significantly lower in volume than the other. I fed a 1kHz signal into both TUNER inputs, attached a dummy 8 ohm load, added a scope, and with the black leads on ground and putting the red leads at various points, I found the component that appears to be causing the volume imbalance. I started on the cathodes of the output tubes (7189)... imbalance there. Moved back to the cathodes of the preamp section (12AX7)... imbalance there. Moved back to the C12 and C112... imbalance there. Moved back to C11 and C111... imbalance there. Moved back to C9 and C109 (RUMBLE FILTER)... no imbalance... AHA. Moved forward to R16 and R116 (470k), input side... no imbalance. Moved to output side of same resistors... imbalance there... BINGO ! Not sure which side is which channel, but I know which resistor needs to be replaced. I think this is how it works. If I replace the resistor and get my volume balance back, I'll be quite pleased with myself. 
By the way, thanks to those clumsy alligator clips inside slippery plastic sheaths, I got myself a nice 300+ volts shock with 2 white scars an inch apart on my index finger to prove it. OW !
UPDATE... Nope, replacing the 470k resistor didn't fix the volume imbalance. Sigh. Is that not the way to diagnose signal issues? I was measuring the Vpp and Amplitude characteristics of the signal on the 2 channels. I was getting 2v and 5v from those parts of the circuit that I thought were showing the channel imbalance. Vpp was around 6v and Amplitude was around 14v. When I thought there was no imbalance I was measuring Vpp around 38v and Amplitude around 37v.
Anyone knows why this troubleshooting didn't work?
By the way, thanks to those clumsy alligator clips inside slippery plastic sheaths, I got myself a nice 300+ volts shock with 2 white scars an inch apart on my index finger to prove it. OW !
UPDATE... Nope, replacing the 470k resistor didn't fix the volume imbalance. Sigh. Is that not the way to diagnose signal issues? I was measuring the Vpp and Amplitude characteristics of the signal on the 2 channels. I was getting 2v and 5v from those parts of the circuit that I thought were showing the channel imbalance. Vpp was around 6v and Amplitude was around 14v. When I thought there was no imbalance I was measuring Vpp around 38v and Amplitude around 37v.
Anyone knows why this troubleshooting didn't work?
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I don't know the answer to that but am enjoying this thread as I've got a 299 (earlier version) that I just started on. This morning, like you, I ordered the Hayseed (5) can cap set for the Scott 299. Then, to start with, I was only planning on changing out the four paper caps and the two ceramic disc caps marked .02 but I'm not sure if that uF or pF, nor do I have any idea what voltage I need. Anyway, thanks for sharing your experiences restoring your Scott 299 and I look forward to learning more.
thornev
Super Member
nels - Unless marked otherwise, I would assume uF. If you tell me the part number from the schematic, I might be able to verify as I have a parts list for the early 299. You can get it too. It's the SAMS schematic for the 299 (I forget where at the moment). As for voltage, that would be listed on the schematic too and, depending on where the cap is, you could increase the voltage given that these units were designed when 115 vac was the standard and today 120 vac is more common. Thorne
PS - I've made much more progress with my 299 v2. I just stopped logging it here as it seemed the interest died down. I've replaced many more parts and am now about to try some vintage tubes. This Scott is my preferred amp as I've got it sounding really good through my AR-2Ax speakers.
PS - I've made much more progress with my 299 v2. I just stopped logging it here as it seemed the interest died down. I've replaced many more parts and am now about to try some vintage tubes. This Scott is my preferred amp as I've got it sounding really good through my AR-2Ax speakers.
kvflyer
Super Member
And I log on every day waiting for updates and pictures! Please, continue to tell us the progress.
thornev
Super Member
OK kv... I've been replacing caps and other parts. Complete list:
- Replaced the two 20/30 450V can capacitors behind the power transformer which meant removing the 68uF 450V add-on lytic which was connected to one of the can caps.
- Replaced 4 of .047uF 400V Pyramids (C14, C114, C20, C120) with .047uF 600V 5% orange drops.
- Replaced R207 15k with 15k 10W 3% Dale wirewound and R208 2k with 2k 5W 5% Xicon wirewound. Left R209 alone as it measures just over 160 ohms.
To prevent wires from melting due to heat from power resistors, used plastic tie to tie wires away from the resistors.
- Replaced cap cans 100-100-10 above C214 and 20-20-25-25. Ruined 47k resistor connected to 20-20-25-25 so have to order one.
- Replaced lytic C214 10uF 25v with 47uF 50v.
- Replaced cap can 100-100-10 below C214. Just need to finish rewiring 1 can and cans are done. Much more challenging than Fishers.
- Replaced C31 and C131 .001uF 2500v white cylindrical paper with .001uF 3kV TDK blue ceramics.
- Replaced 2 Pyramid .047uF 400v with 600v orange drops on on/off/volume switch. Scraped off corrosion on area behind front panel. Lots of irreversible corrosion on slide switches, pots and multi-function switches. Cleaned what I could.
- Inserted 4 of 10 ohm 1/2w 5% metal film cathode resistors between all 7189 pin 3's and pin 3 leads. These resistors are for measuring voltage to set bias.
- Finished attaching leads and 47k resistor at can cap with C202 and C203. All can caps are done !
- Inserted a CL-80 thermistor between fuse and large, spike-protecting (?) disc cap.
- Moved the 2k 5w power resistor farther away from wires.
- Replaced lytic C201 4uF 250v with aluminum-cased 4uF 350v. Kept piggy-backed (parallel) R206 47k on it.
- Added better insulation on and around power resistor R208 2k 5w.
- Replaced C12 and C112, black Pyramids .0012uF 400v, with 1200pF 630v film caps.
- Retensioned 7189 tube sockets.
- Replaced C2 and C102 (.047 uF 400v) with .047uF 600v, and C6 and C106 (.022uF 400v) with .022uF 630v.
- Replaced C9 (.0033 400v) with .0033 630v.
- Replaced C26, C126, C28 and C128 (Sprague 0.1uF 600v) with Sonicap 0.1uF 600v.
- Replaced C118, C119, C18 and C19 (.0068uF 400v American Radionic Ceracaps) with .0068uF 630v.
- Replaced the 5AR4 rectifier tube with a solid state "Yellow Jacket" rectifier (https://www.yellowjacketstc.com/). The voltage initially shoots up to 480 vdc and then settles at 410 vdc, 11 volts more than with the tube. Power resistor gets hotter though.
- Replaced the selenium rectifier with a 3A Rectron silicon rectifier.
- Replaced C25 and C125 (.022uF 600V brown kidney shaped) with .022uF 630V orange drops on AC Balance pots.
- Replaced concrete power resistor R209 (160 ohms 20W) with chassis-attached aluminum-case 160 ohms 25W. Applied NT-H1 thermal grease to resistor bottom.
- Soldered 4 leads to 7189 cathode pins (3) and 1 lead to V5 plate pin (7), all leads threaded thru chassis for external test points so I don't have to keep opening the chassis every time I want to swap tubes and set BIAS.
Thorne
- Replaced the two 20/30 450V can capacitors behind the power transformer which meant removing the 68uF 450V add-on lytic which was connected to one of the can caps.
- Replaced 4 of .047uF 400V Pyramids (C14, C114, C20, C120) with .047uF 600V 5% orange drops.
- Replaced R207 15k with 15k 10W 3% Dale wirewound and R208 2k with 2k 5W 5% Xicon wirewound. Left R209 alone as it measures just over 160 ohms.
To prevent wires from melting due to heat from power resistors, used plastic tie to tie wires away from the resistors.
- Replaced cap cans 100-100-10 above C214 and 20-20-25-25. Ruined 47k resistor connected to 20-20-25-25 so have to order one.
- Replaced lytic C214 10uF 25v with 47uF 50v.
- Replaced cap can 100-100-10 below C214. Just need to finish rewiring 1 can and cans are done. Much more challenging than Fishers.
- Replaced C31 and C131 .001uF 2500v white cylindrical paper with .001uF 3kV TDK blue ceramics.
- Replaced 2 Pyramid .047uF 400v with 600v orange drops on on/off/volume switch. Scraped off corrosion on area behind front panel. Lots of irreversible corrosion on slide switches, pots and multi-function switches. Cleaned what I could.
- Inserted 4 of 10 ohm 1/2w 5% metal film cathode resistors between all 7189 pin 3's and pin 3 leads. These resistors are for measuring voltage to set bias.
- Finished attaching leads and 47k resistor at can cap with C202 and C203. All can caps are done !
- Inserted a CL-80 thermistor between fuse and large, spike-protecting (?) disc cap.
- Moved the 2k 5w power resistor farther away from wires.
- Replaced lytic C201 4uF 250v with aluminum-cased 4uF 350v. Kept piggy-backed (parallel) R206 47k on it.
- Added better insulation on and around power resistor R208 2k 5w.
- Replaced C12 and C112, black Pyramids .0012uF 400v, with 1200pF 630v film caps.
- Retensioned 7189 tube sockets.
- Replaced C2 and C102 (.047 uF 400v) with .047uF 600v, and C6 and C106 (.022uF 400v) with .022uF 630v.
- Replaced C9 (.0033 400v) with .0033 630v.
- Replaced C26, C126, C28 and C128 (Sprague 0.1uF 600v) with Sonicap 0.1uF 600v.
- Replaced C118, C119, C18 and C19 (.0068uF 400v American Radionic Ceracaps) with .0068uF 630v.
- Replaced the 5AR4 rectifier tube with a solid state "Yellow Jacket" rectifier (https://www.yellowjacketstc.com/). The voltage initially shoots up to 480 vdc and then settles at 410 vdc, 11 volts more than with the tube. Power resistor gets hotter though.
- Replaced the selenium rectifier with a 3A Rectron silicon rectifier.
- Replaced C25 and C125 (.022uF 600V brown kidney shaped) with .022uF 630V orange drops on AC Balance pots.
- Replaced concrete power resistor R209 (160 ohms 20W) with chassis-attached aluminum-case 160 ohms 25W. Applied NT-H1 thermal grease to resistor bottom.
- Soldered 4 leads to 7189 cathode pins (3) and 1 lead to V5 plate pin (7), all leads threaded thru chassis for external test points so I don't have to keep opening the chassis every time I want to swap tubes and set BIAS.
Thorne
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kvflyer
Super Member
Thorne,
Thanks for the follow up! Now, pictures when you get a chance? I know, a lot to ask for. But any Scott amplifier is always worthy of documentation. Good job, I need to get them back on my bench. Oh, BTW, I have a 299C on the bench right now. 100% original except for one phase inverter/driver tube. Even all of the capacitors. Surprisingly, it works very well. But... there is a rushing noise and it beckons a restoration. What an amplifier!!
I should start a thread?
Thanks for the follow up! Now, pictures when you get a chance? I know, a lot to ask for. But any Scott amplifier is always worthy of documentation. Good job, I need to get them back on my bench. Oh, BTW, I have a 299C on the bench right now. 100% original except for one phase inverter/driver tube. Even all of the capacitors. Surprisingly, it works very well. But... there is a rushing noise and it beckons a restoration. What an amplifier!!
I should start a thread?
thornev
Super Member
299-C was what I originally wanted because it sounded like, from what I read, that was the model where HH Scott got it all right sound-wise. Start a thread if you're up to it. I will be partly interested although it's not relevant to my work. I think I'm done buying vintage gear until I somehow unload at least some of the 4 pieces I have now. No more room and want to tighten the financial belt buckle as they say.
Pictures... I have an underside picture where I notated all the part numbers. I've attached it here. It's not the latest picture so still shows parts I've replaced. Hopefully you can blow it up to see the part labels.
Pictures... I have an underside picture where I notated all the part numbers. I've attached it here. It's not the latest picture so still shows parts I've replaced. Hopefully you can blow it up to see the part labels.
Attachments
Thanks Thorne, I need to get the SAMS schematic, next on my list. I do have cap replacements on order for all the Pyramid caps (except those on the switches/pots), the two paper caps (4uF & 10uF) and the two white caps. For the two .02uF disc caps I opted to go with a pair of vintage, nos, Chicago Vitamin Q .02uF 600Vdc PIO caps.
You've certainly done a lot to yours so far, thanks for the updates and pics. I see Rectron 3A rectifiers with values of 100V and 600V, which should I use? Thanks again.
After several months of accumulating parts I set to work on my Scott 299, first version (7189 tubes running back to front. I replaced all 5 can caps on top of the chassis with a set of replacement can caps from Hayseed Hamfest. Then I replaced all the capacitors underneath with the exception of the ones on the tone controls and the 2K 4W power resistor that got a bit toasty. I think the resistor got toasted when the one output tube gave up the ghost during our preliminary testing and created a short which made that baby get, burn the skin right off your finger, hot! We didn't know the tube went bad at first, we saw a leaking can cap that also was exceedingly hot and attributed the loss of sound to that. We found out later when we were testing when that same resistor got really hot at around 80-85 volts, so we went looking.
After replacing the above passive components, I cleaned all the tube sockets with Noalox anti-oxidant. I'm amazed at how good this amp sounds and I'm not quite done. I intend to retention the tube sockets There's an issue that doesn't seem to affect the sound in stereo but when I switch to just channel B, I get nothing. Perhaps it's the switch itself?
I still need to replace the selenium rectifier with a diode bridge but my lack of overall experience working on vintage gear has me wondering if the square, metal "thing" with two orange colored cloth leads from the power transformer on the two lugs on one side and a single white wire and a single black wire, both small guage wires, on the two lugs opposite the other two lugs. Printed on the chromed metal in black it is the following: "B60 C200" and under that it reads: "Kc0,6e 22/2". The lug that the black wire is on is marked "+" and the one the white wire is on is marked "-" with both lugs that have the orange wires on are marked with a wavy line, sort of an elongated, backwards "S", but I looked on the internet and didn't find this symbol.
After replacing the above passive components, I cleaned all the tube sockets with Noalox anti-oxidant. I'm amazed at how good this amp sounds and I'm not quite done. I intend to retention the tube sockets There's an issue that doesn't seem to affect the sound in stereo but when I switch to just channel B, I get nothing. Perhaps it's the switch itself?
I still need to replace the selenium rectifier with a diode bridge but my lack of overall experience working on vintage gear has me wondering if the square, metal "thing" with two orange colored cloth leads from the power transformer on the two lugs on one side and a single white wire and a single black wire, both small guage wires, on the two lugs opposite the other two lugs. Printed on the chromed metal in black it is the following: "B60 C200" and under that it reads: "Kc0,6e 22/2". The lug that the black wire is on is marked "+" and the one the white wire is on is marked "-" with both lugs that have the orange wires on are marked with a wavy line, sort of an elongated, backwards "S", but I looked on the internet and didn't find this symbol.
Okay, so now I know, that is the selenium rectifier and I found it on the schematic, it looks like it's -50V but I see no amperage value but since you replaced your selenium rectifier with a 3A Rectron silicon rectifier, I'm guessing my 50v-3a bridge rectifier is just the ticket. Please feel free to chime in and let me know, whether I'm right or wrong, I'd appreciate it. Thank you.
Nelsress & Thorne;
THE "~" indicates A.C. VOLTS. On your keyboard it should be to the left of the 1 key. There should be two of the ~ on the bridge. Clip off the wires at the connector and attach them to the new Silicon bridge at the ~ connections (on the table looking bridge they are catty-corner from each other. Then clip off the "+" wire and attach it to the new diode at it's connector. Then the "-" wire is last. Mount the bridge on the bulkhead and you're done.
Bump up the voltage on the rectumfrier to at least 100V. You'll need to figure on surge voltage when you turn on the unit and that will be above 50V. 3A is fine for a 7591/7868 unit for bias purposes.
THE "~" indicates A.C. VOLTS. On your keyboard it should be to the left of the 1 key. There should be two of the ~ on the bridge. Clip off the wires at the connector and attach them to the new Silicon bridge at the ~ connections (on the table looking bridge they are catty-corner from each other. Then clip off the "+" wire and attach it to the new diode at it's connector. Then the "-" wire is last. Mount the bridge on the bulkhead and you're done.
Bump up the voltage on the rectumfrier to at least 100V. You'll need to figure on surge voltage when you turn on the unit and that will be above 50V. 3A is fine for a 7591/7868 unit for bias purposes.
Thanks Larry. I've got the first iteration of the first version so I'm running 7189 tubes, not 7591 tubes but the advice to double the voltage of the bridge rectifier is much appreciated! So, I need to get a different bridge rectifier, no biggie, they're cheap, I'm off to go find one on ebay!!
You know Thorne, I've always wondered why no Scott forum too with all the gear they put out and being one of the leaders in the early days of tubes. There's a Yamaha forum, not that there's anything wrong with that, I just wouldn't have thought that they're more popular than Scott or contributed more to audio but, I don't know, I'm often wrong, could this just be one more time?
