Restoring a Scott 340-B: Phase 2, power issues

[elbreapoly]

Hi everyone,
I recently ran a thread discussing a newly acquired Scott Stereomaster 340-B that had a nasty homemade bridge rectifier mod on it. We concluded to remove it and this work has been done.

Now I must wander through the other steps in restoring this amplifier, and am in phase 2 and could use some feedback.

First, one can cap looked burned to a crisp from underneath, so I replaced it (photos 1 and 2) with a new CE multi-section (note the cap labels no longer match the schematic, but it is wired in the same way from which the previous cap was). See the first two photos for this work.

But with the replacement installed, and running the unit on low voltage (just enough to see if tubes lit), I was able to see that heater voltage seemed to be dropping from one tube to the next. This was especially apparent in the 4 7591 row, and other tubes such as the 12AX7s failed to light at all with the reduced heater voltage.

Then I saw something terribly wrong. Whoever this goofball is who installed that terrible bridge rectifier mod (in a place where the schematic calls for no rectifier), to say this most kindly, seemed to not be able to read the schematic, or simply didn't know what is going on (lest I think--I may be wrong as only being moderately educated in this area). So in photo 3: A black wire leading from one heater lead from the transformer (those nasty, big dark green wires and nasty soldering work) was directly connected to the chassis ground. My multimeter read zero resistance from the transformer lead to the chassis. So, I unsoldered that black wire and bent it upwards, as you can see in the third photo. I only didn't clip it off just yet in case I'm not understanding something here. But, the schematic indicates that the heater leads from the transformer go straight to each of the 4 7591s, then to two 6U8s before being grounded.

Then, still running at low voltages, the decaying heater voltage across the 7591s was no longer evident. Instead at this point, I measured higher heater voltage on tubes 1 and 3 in the 7591 chain, and lower on 2 and 4. The tubes lit up just barely with the low voltage, but clearly 1 and 3 were getting more power.

So now what should be my next step? In picture 4 you see four .1 MFD caps. Two of the four show leakage (about 15 kilo ohms from continuity on my multimeter, but I only measured while they were still in circuit and have not taken one of the leads out), but the other two do not and read zero continuity. 1 and 3 don't leak; 2,4 do.

SO, my replacement 0.1 MFD 600V caps have come in the mail. I'm ready to replace all four and will, probably one at a time to make sure I don't create some terrible short or whatever.

My questions for the expertise of AK members are then the following: how have I done so far (be brutal if needed); was my assessment correct in that the heater lead should not be directly connected to chassis ground; and finally, do you think these four caps are causing this issue, and if not, what steps should I follow to determine the problem?

Thanks in advance AK members, I couldn't do this without you!

 

[cademan]

I'd have to see this particular schematic but sometimes that is a valid wiring scheme. My Curtis-Mathes AM/FM receiver is wired where one side of the 6.3 volt tap goes to the chassis.

If the 340 is anything like my LK-48, you're not going to see the 12AX7 tubes light up until full voltage is applied as they may be in series and be part of the 48 volt bias supply, and maybe this character thought he was trying to rebuild it/replace it with those four diodes. Four diodes can be wired as a bridge rectifier so I wonder???

 

[dcgillespie]

Elbreapoly -- A couple of points to consider:

1. In Scott designs, it is common to power the output and phase inverter tubes on AC voltage directly from a winding in the power transformer -- and ground one side of the winding to produce minimum hum. Within the actual wiring itself, it does not matter where the actual ground connection is made -- just that it is made. The schematic may show the ground symbol after the full run of wiring on one side of the circuit, but that doesn't mean that is always where it is actually grounded in the unit -- which is not particularly important in audio circuits. What is important, is that one side of this heater circuit IS grounded at some point. With all the questionable work that was done however, make sure the previous owner didn't inadvertently end up trying to ground both sides of the heater circuit, which would then represent a dead short to the heater circuit.

2. Measuring your coupling caps in circuit as you have done is basically meaningless. Both sides of those caps can have connections that work their way to ground, which then effectively shunts the cap giving a false reading -- which can be different across each cap depending on its position in the circuit. When you test components, you must lift at least one lead away from its connections, but you can leave the other end attached. Connect the negative lead of your meter to the lead that is still attached when taking your readings.

3. Heater elements within a tube are very non-linear devices with respect to current draw and visible light emitted. Therefore, at lower than rated operating voltage, the actual light emitted can vary from tube to tube. This can be true within the same tube type, even from the same manufacturer. Therefore, a comparison of heater operation based on light emitted at low voltage being applied can possibly return widely varying results. Quite possibly, if you move the tubes around, the same tubes that seem to not light very well in one socket on low voltage, may still react that way in a socket you feel is receiving proper voltage -- if that is only based on how a different tube reacted in that socket.

Now, if you actually measured a significant heater voltage drop along the wiring that forms the heater circuit, then use your volt meter to determine where the drop begins. The heater wiring is nothing more than a daisy chain of connections running between the tube sockets. Therefore, either a short, or a bad connection are the only possible wiring faults -- but don't completely rule out the possibility of a bad socket (dirty or damaged pins), or a tube being the problem as well. A semi shorted heater in one of the tubes can cause the symptoms you are having as well.

Similarly, the glow in 12AX7 tubes is much harder to see to begin with, and their (likely) operation from DC in this set will affect the point at which their glow will become evident, due to the non-linear drop their current draw will produce across the filter circuit in their DC supply under low voltage conditions.

The bottom line is that if you feel that there are issues in the heater circuits of this unit, but are concerned about applying full power, then disable the high voltage circuit (lift and isolate one lead of the HV winding), and then slowly apply full power by bringing it up to that level through a variac or dim bulb process. Only on full power can you make a proper assessment of the heater circuits and their operation, as only then will normal current flow take place to make a proper assessment of any voltage drop issues.

Within the output tube wiring, it is common to lose about .25 volt or so along the length of the wiring from the transformer, to the last tube in the circuit. Much more than that however should be investigated.

That's a wonderful receiver you have. Good luck with it!

Dave