HH Scott 340 Receiver Rebuild

The busy work. The insides of switches can look like this selector. While some say the corrosion is conductive, I find not cleaning them can result in scratchy sound.

I'm using DeoxIT with makeup Q tips as they are more tightly wound than regular cotton swabs. Saturating the wafers with DeoxIT can cause issues without completely removing corrosion so I soak the Q tip with DeoxIT and lightly rub the metal without bending the contacts. I fold paper wipes and saturate to clean between contacts. I've read some guys use a dollar bill.

Still have a little more to do. The forward section of selector is much harder to get at.

Thanks Ivan!

xschop wrote: »

Are you renewing the wiring as you go or is the insulation still good after all these years?

So far, I have not encountered any burnt or brittle wiring so I'm not replacing wiring. It's like 22-gauge solid tinned wire. I've had to snip ends off some desoldered wiring and remove a little insulation and it's still pliable, no insulation cracking off. I'm able to loop wire around terminals and squeeze tight with needle nose before soldering.

I watched a video of a Scott 399 restoration, and that unit had some shorting that damaged some wiring.

Greetings Tony! Maybe it's a test of friendship...

More parts arrived from Mouser. Today I worked on the output tube sockets. I replaced all the 1/2 watt carbon resistors with 1% metal film as some were beyond 20% out of spec. Four 270K from DC balance pots to terminal strips, four 1K ohm terminal strip to pin 5 of 7591 and four 330K ohm terminal strip to pin 6 which are grounded.

The Scott procedure to adjust DC balance in the 340 'A' cause it's not a B:

Connect a 16 ohm resistor across speaker terminals of one channel.
Remove the 6U8 phase inverter for channel being adjusted.
Connect a VTVM or scope set to line across the resistor.
DC balance is achieved with a 120 cycle response from the scope or a minimum reading on the VTVM.

I wanted to make biasing the 7591 output tubes easier. I removed the ground wires from pin 6 cathode ground. One end of 10 ohm resistor to pin 6, the ground wires and 330 ohm bleed resistor to other end. I added tip jacks connected across the 10 ohm resistor so we can easily measure mA of each output tube from top of chassis. Along with chassis grounds, there are other wires grounded to pin 6 and all sockets are daisy chained together.

Mouser provided one incorrect nut for a tip jack. I had nothing in my stash that would fit. My parts donor Eico 232 came to the rescue with the nut from the ground probe. Due to chassis design and a small metal cover at top left corner, I had to shift locations of the tip jacks for the left channel.

Carl came to the rescue with two old 7591 tubes. I have two Scott branded Westinghouse that tested good, one bad Scott, and an oddball (rubbed clean) that measures way higher. Before we buy a matched quad I want to fire, not literally, this thing up. The replacements should measure closer and be easier to bias.

Thanks! Tired of slinging solder so.....

Set screw in plastic hub of FM station indicator dial, what could go wrong? I say frequency indicator as the 340 has a center knob to turn dial that has way more finesse in dialing in stations. Dial near cracked in half.

I made a couple loops of 22-gauge tinned hookup wire. I twisted them snug around hub which is tapered to rear. I dabbed some super glue on dial cracks and around wire to hopefully soften plastic on hub. Them I twisted the wire tight, nipped off the excess nubs. Hopefully it holds.

Thanks for the kind words!

More rambling from the bench.

All electrolytic have been replaced, voltage value set, still works, so it’s film caps time. I love the look and feel of the old ceramic Ceracaps, but they are sixty-three years old. Lots of brand choices and what’s best opinions. How much do you want to spend? Maybe the best comment I read, anything you put in there will be better than the old ones.

I used various brands. Sometimes they don’t offer that specific value in the series you select, or the voltage spec for that value is lower than required. Sometimes they don’t offer axial so you use a flat radial style. Voltage values were mostly 400v with three at 160, 200v, 20% tolerance. I used 10% tolerance.

Cornell Dubilier / Illinois 473MWR series film caps. Reasonably priced. Good rep.
Vishay BFC2647 series for .0018uf and .0027uf.
Vishay Roederstein MKP were chosen for a 1.0uf and a .22uf and .033uf were piggy backed for two .25uf values. Some folks would just use a .22uf and that would be OK too.

I measure all new caps to ensure they are the correct values. Most new manufacture film caps do not identify outside foil end. (The Vishay Roederstein MKP did). “The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground.”

The band on end of the old Ceracaps indicates foil end. On old electrolytics, it indicates negative as I had one like that. In one of his very informative project threads, Ken showed us how to ID outside foil end of capacitor using an oscilloscope while holding the capacitor cap between your fingers.

Some say it doesn’t matter, yet you see folks making mild to wild foil side finders with LED’s, buzzers and even an Arduino controlled one. I took the time to ID foil end of all caps and marked them with a Sharpie. My big scope is on shelf above bench which requires me to stand. I wanted to sit so I used this cute mini scope. The side with lowest reading to negative is foil side.

I measured all the caps removed. Most tested within 10% of value, some near 20%. One .25uf was bad. No reading at all, open circuit. It was across pin 3 and pin 7 of a 6U8 pentode (grid 3 pentode grid 3 / internal shield.) I guess this is why we recap.

I read some folks suggested replacing a few caps and powering up to test before proceeding. At first, I thought what a waste of time. The other side of that is if you replace all the capacitors and have a problem, did I create it and where is it?

So after replacing a few caps in a section or area, I’d review my work. Point to point is very busy. Did I create a solder bridge? Bump something with the iron or needle nose so it’s touching something it shouldn’t? Those terminal strips. They ground where they are attached. The terminals between attaching points are not grounded. Sometimes they run leads through the attaching point on wafer which is very close to chassis. I always check no solder ran down from the strip where it’s mounted to wafer to chassis. Then I power up and test operation.

It still works, sounds great! Next up, check out the phono section....

Hold cap between fingers, connect scope leads. Reverse leads. Lowest reading to neg probe indicates foil side.








One of the tight areas. I removed the .047 and slid the .0027 out. Worse are the ones at front panel bass/treble controls. You can see the 1.0uf Vishay with line that indicates foil side.



You cannot rely on printing being the same on all caps. Foil side can be on either end.

I purchased an inverse RIAA filter from Hagerman to test the response of the RIAA filter in the Scott 340 with my old, tired 2235. I have a two-channel signal generator but only one channel supports sweep, so I must input frequency manually for both channels.

Connections are signal generator CH 1 & 2>inverse RIAA R & L in>out to phono R & L in>tape monitor R & L out to scope. I went through frequencies from 100Hz to 20kHz, all looks good. No use posting them all. Scope set to both channels and chop, so you see wave form of both channels traced sequentially.

If you are inputting consistent amplitude in, you should see the same amplitude in output. I've seen videos where they remove the inverse RIAA and the pattern shrinks, so I'm good. Ken was right. No worries about replacing the PEC RIAA/NAB tape EQ filter if it tests OK.





For those who like a square wave...