| Posted by Brian | October 23, 2018 - 05:11 pm |
| Art, One other thing: Are you by chance in touch with Dave at all? There is a discrepancy in the ST-35 EFB assembly instructions. On PDF page 6, in the EFB schematic, it shows resistors R3 and R4 for the left channel. On PDF page 7, for the overall amplifier schematic, it shows R1 and R2 for the left channel. The output tubes for both the LC and RC are labeled V2 and V3, also. One side should be V5 and V6. I was looking at them closely to determine how voltage at the test point could be causing V6 to run away but not V5, and was trying to determine which resistor I should look at. That's when the noticed the discrepancy. |
| Posted by Brian | October 23, 2018 - 05:04 pm |
| Art, I didn't mean to go radio silent on this thread. I haven't had the time to dedicate to this project daily, so progress has been slow. I did get the capacitors in and installed. Bottom line is the tube in the same problematic socket continues to run away on occasion. The RC is rock solid. I can set the cathode bias through the EFB trim pot and it only fluctuates a few hundredths of a millivolt. The LC is basically unchanged with the capacitor replacements. The cathode voltage, as measured on the EFB LC test point, will sometimes be stable-ish, but always moving around 20-40 mv. Occasionally it'll bounce from 300 mv to 600, and then up to 1.2 v... sometimes it'll stay there and the tube will run away. Sometimes it'll go back to 600 and then back to 300 mv. I swapped V6 with V3 and the problem stayed with the socket, so it's not a tube issue. I need to go back and perform the other test measurements again, and will also check for unlikely leakage between socket pins. Due to work constraints I won't be able to try anything until this weekend. Just wanted to provide you the latest, even if incomplete from a troubleshooting perspective! |
| Posted by Art Grannell | October 17, 2018 - 10:00 pm |
| Brian, The purpose of C4 and C5 in the ST-35 is to pass AC signals without allowing DC to appear on the grids of the output tubes, The left side of each of these capacitors is connected to pins 1 and 3 of the 7247, which operate at 215VDC and 112VDC respectively. The right side is connected to the 6BQ5 control grids which need to operate at 0 VDC in this circuit. All capacitors will allow some DC current to pass through them, but with the type used in this application, it should be so little that it isn't measurable, except under laboratory conditions. This is called "leakage", and will sometimes increase from the effects of time, heat and use, eventually causing the problems that you've seen. All of this is generally independent of the value of the capacitor, which in this case is in microfarads. From your measurements your capacitors are within 15% of the specified .1mfd value, and I would not consider this to be unusual for "vintage" capacitors made in the 1960's. Because of the way that capacitors are constructed, it is more difficult to maintain close tolerances than it is with resistors. Modern materials and manufacturing methods, however have improved both, and I'd guess that your new parts will measure closer to the desired value. The only effect of this slightly lower value would be at very, very low frequencies, and probably could not be heard with any "normal" loudspeakers. |
| Posted by Brian | October 17, 2018 - 12:24 am |
| Art, Once again, thank you so much for sharing your expertise with me! I took your advice and removed C4, C5, and E6 from both channels. I ended up removing all the capacitors from the left channel, actually (the problem side). Not only did all the resistors measure within their respective tolerance (5% or 10%) but C4 and C5 for both channels measured 84 - 86 nf, or 0.085 ish uf. That is below the 0.1 uf specified, but it seems odd that only that one output tubes would "run away" if all four of the capacitors are measuring low, right? Or does that explain why it was somewhat intermittent? In other words, why, with the old tubes, was I able to swap the RC and LC and get it all biased and operating? Were all the output tubes just teetering on having the same issue, based on all of them measuring 15% low? Or perhaps the one problematic C5 was being affected by heat more than the others? Regardless, the take away is the resistors all seem to be fine (and in non-cracked condition), so when the capacitors show up I'll install them and see how it all goes. Thank you again, Art, for all your time and effort helping me troubleshoot! I can't thank you enough! |
| Posted by Art Grannell | October 14, 2018 - 10:04 pm |
| Brian, I hope that the new capacitors will solve the issue for you. Now, while you are waiting for parts, would be a good time to check the PC-13 resistors. With power off, the tubes out, and C4 and C5 removed, it should be a quick and easy job. I'd take a close look at R8 and R9 and replace them if they don't measure close to the original value of 470 Kohms. They could be involved in the issue you have. Although they would not be suspected of contributing to your problem, the more critical ones are R3, R5, R6, and R7 and should check within 5% of their original value. The remaining 10% tolerance resistors are less critical, and almost certainly would not be involved with your issue but are worth a quick check. To get an accurate measurement of R10, you will need to temporarily remove the feedback wire from the 16 ohm output tap of the output transformer. This step will also improve the accuracy of the measurement of R3. You can either lift the lead at eyelet 6 or at the speaker terminal strip. Also, visually inspect each resistor and replace any that show charring or cracking of the resistor body. Good luck! |
| Posted by Brian | October 13, 2018 - 12:36 am |
| Art, Thank for you sharing the photos of your beautiful rebuild! Looks really nice, and I'm sure it sounds even better than it looks. And thank you for helping me troubleshoot my ST-35. I have a sentimental attachment to it, as I have a photo of my dad building it in 1963. I also have the original manuals with his hand-written notes, and the serial numbers on the manuals match the PAS-3, FM-3, and ST-35. My goal is more to rehabilitate the set rather than rebuild entirely... make it last long enough to pass on to one of my children. I agree that the age of the components is worrisome, especially since I measured the pin 2 voltage on the sockets tonight and confirmed your suspicion - when cold, the voltage from pin 2 to ground was actually negative, but after warming up the voltage climbed and perhaps 30 seconds in to the measurement it was at 60mV and continuing to climb, so I unplugged the amp. If it were any other amp, I would do a complete PC13 replacement and try to make my amp look as good as yours! Given my desire to keep whatever I can, I did place an order with Mouser this evening for a complete set of capacitors. I'm hoping the original resistor values are still holding. The cloth-covered wire from the output transformers isn't in great condition. Getting the wire moved from pin 9 to 8 as part of the EFB mod was dicey, and I had to use shrink tubing over some of the crumbling insulation that was closest to the output tube. If there are future problems and I have to swap the sockets, I'll have to plan on repairing the transformers, too - either sending them to a place that rebuilds transformers, or attempting the repair myself, in order to get new leads put on. I probably won't get the parts until later next week, so I'll report back how things go once I get them swapped in. Again, I really appreciate all your help and sharing of expertise on helping me troubleshoot! - Brian |