Scott 299D plainful re-capped experience

vmhchau

New Member
Hi Akers,

Greeting! I just want to document my experience for a re-cap job of a Scott 299D, hopefully other people will find it useful. Sorry for the long write up due to my poor English.

I just acquired a Scott 299D without the cover recently with a reasonable price. The previous owner said in order to sell it he had sent this amp to a technician for a minimal restoration mainly for safety concern. When I received it, I found out all the electrolytic caps and the notorious selenium bridge rectifier in the power section has been replaced but not the coupling caps in the signal path. They are still the original white caps from the date it was made. When I hooked it up, to my disappointment, I found out the sound in the left channel is just about 50% of the right channel. No matter how I adjust the stereo balance knob and loudness knob, I still can't get it balance.

So, I do a thorough recap for all the stock caps with orange drop caps and cleanup all the knobs, slide switches and tube sockets with contact cleaner. It was fun. After several hours of labor and correctly biasing the four 7591 output tubes, the moment of true came. I turn it on and expect to receive good sound. To my horror, the sound is terrible. The treble is extremely bright and no bass at all. Worst, the imbalance issue still exist.

I start to re-check all my work carefully to make sure no mistakes were made. Then I notice there is a phase switch at the back of the amp. When I switch it to reverse position, the sound is much better, I got bass now but it still cannot solve the imbalance issue. I checked all the voltage and current for all the output tubes and found out they are all within specs. That rule out the output section. The problem must be in the pre-amp line section. This is the beginning of my nightmare trying to debug this problem.

Since I don't have an oscilloscope on hand. Trouble shooting in large part is just an educational guess. I look through the whole AK's previous threads regarding 299D recap issues particular paying attention to those threads regarding channel unbalance issue. Many suggests the loudness port is to be blamed. So I changed that port without success. Someone suggested the PEC in the tone section is another suspect, so I re-build the PEC with discrete components. Still no success. I just reached a point to replace all single resistor and the four 12AX7 in the pre-amp section. Reheat all the solder joints. Still no success. After spending about a week with this beast and running out of any ideas, I am about to give up.

After cooling down myself a little bit, I start to look for a 299D owner's manual to get some idea. I couldn't found a 299D manual on AK database. The closest manual is a 340B receiver manual. Since it got similar features as 299D, so it is a valuable reading. During the reading, I found out in the speaker connection section that it describes a procedure to test the phasing of the speakers. It said when the speakers is suspected to be out of phase, switch the phase switch to the reverse position. I did it already in the very beginning of the recap job. However, the manual gives a follow up instruction to physically reverse one speaker connection leads and SWTICH BACK the phase switch to "NORMAL" position afterward. This step I didn't know before. I immediate follow this step and you know what, perfect balance right on.

My painful experience for this amp maybe laughable but I think this is a common problem with other people first time dealing with antique American amp. The speaker terminal connection nomenclature Scott used is H and O and is very confusing. They didn't specify which is the "+" Red lead, which is the "-" or "Common" Black lead. So, I follow other AKers suggestion to connect my speaker red lead to H, black lead to O and didn't even think about the phasing problem which won't be any problem for modern amp with color identification lead. This teach me a lesson, don’t overlook simple thing and try to complicate the problem.
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One last thing, please pay particular precaution when working with live voltage of those antique tube gear. Working with 400+ volt is a serious matter. I accidental short pin 3 and pin 2 of one of the 7591 tube while I was taking voltage reading and smoke and flame immediate came out of the socket. I immediate switch off the power. It scare the shit out of me. Luckily, nothing harm done to the amp. This beast is very robust. Remember, no matter how good those gear sound, it is not worth dying for.


So, the long story have a happy ending. Now I have a beautiful sounding Scott 299D in my house to compete with my other Fisher X100 and Dynaco ST70. Even better, I got a beautiful after-market wooden cabinet to match with it for very minimal price.


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Thank you for sharing. Those Scotts can sound really good when working right. The switches and pots are a major problem area for any amp that is 50+ years old. I know of a super talented tech who refused to rebuild vintage units. Probably because of the issues you ran across. If you wanted an amp he would make you one. If you needed a preamp he would do that too. cheers.
 
Nice story and a very pretty amp. Thanks for sharing.

I went through a similar story with a Fisher KX-100 that I'm still working on. It's very rewarding to work on them yourself.

Did you ever test the tubes to determine their condition? That's one of the first things I did when working on mine.
 
I love how you rebuilt the PEC, I have noticed those in the receiver I'm currently working on and had no idea how to replace them, if needed. Nice work! :rockon:
 
Good job! I re-did all three of my LK-72's and fortunately did not run into any problems. They were all original so I didn't have to figure out what the other guy did.
 
Sorry for uploading so many repeat photos. This is my first time to upload pictures in AK. I am not too familiar with the system to upload photos.
 
I love how you rebuilt the PEC, I have noticed those in the receiver I'm currently working on and had no idea how to replace them, if needed. Nice work! :rockon:
That part is easy. You just get the circuit diagram of the 299D or LK72B (they are similiar), the PEC circuit is there. You just follow the diagram to build it. Since this PEC only consist of 3 resistors and 3 capacitors (The earliest form of IC cicruit), you even no need to use a mini circuit board. However, it is much tidy if you use a circuit board.
 
Nice story and a very pretty amp. Thanks for sharing.

I went through a similar story with a Fisher KX-100 that I'm still working on. It's very rewarding to work on them yourself.

Did you ever test the tubes to determine their condition? That's one of the first things I did when working on mine.
Since I don't have a tube tester, the only test I can perform by using a multi-meter only is just testing whether the tube is short internally (bad tube) or not.
I can't distinguish which one is a strong tube or weak tube. Since my 299D have individual bias port to adjust biasing current. I don't need to worry too much about matching tubes. For the diver tubes 6A8U, you can use your ears to determine. A good 6A8U should have no hum from the speaker. A bad or weak one can give you a slightly hum. For the 12AX7, they seldom failed since they were not driven too hard. Only around 11 to 11.5 Volt at the heater fialment. If one failed, there will be no sound since the 4 12AX7 tube are connected in series in the pre-amp and phono section.
 
At this stage in the 299Ds life it is going to have problems. Sounds like you found them all and got it sorted out. Once they are working right they do sound great! Congratulations!
 
Hey everyone, just as a side note. Has anyone opened one of those PEC filters? What type of parts are inside?
 
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