edwardre Posted August 29, 2002 Share Posted August 29, 2002 So, ironing out the kinks in my first dedicated foray into tube ampsville. Modest setup is a stock ST-70 amp with a stock PAS2 preamp connected to my Khorns. The first indication of problem occured the other night when I thought to discover how 192/24 DVD-A would sound through tubes. That wasn't the problem, the problem was that the wife was sleeping so I had to turn it way down. That's when I noticed that the output level between the right and left were not equal. One was clearly louder than the other. Troubleshot this to the preamp being the culprit and reading the string posted recently, I now suspect a tube is to blame so I wanted to switch tubes around to try and isolate the fault. I came home last evening with this mission in mind. Powered up the amp and went off as, based upon what I've read, you warm it up. I've generally been letting it heat about 1/2 hr+. So I'm out in the living room watching Judge Judy or something like that when I hear this god awful noise that I can only liken to the sound of a radio between channels. Only loud enough to have me hopping over to kill power in an effort to save speaker. I repowered long enough to determine that this issue was generating from the ST-70, but not long enough to determine if it was limited to one or both channels, it was that intimidating. It's now unplugged awaiting tonight's attempt to identify and repair. Any ideas on what happened or how I should proceed? Thanks. ------------------ Ed Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted August 30, 2002 Share Posted August 30, 2002 Mdeneen is right here. Also, did you clean ALL your sockets with contact cleaner? Do so, and remove and insert the tubes several times as well (carefully, and always holding the BASE of the tube). Sometimes, a dirty contact will cause strange things. Although your description sounds more like a tube problem or something else letting go. Give it a big cleaning in the tube sockets and after, switch channels to test tubes or isolate problem. I usually try tubes first since it is the easiest to test then moving to deeper problems such as caps, resistors, or poor joints. kh This message has been edited by mobile homeless on 08-30-2002 at 04:07 AM Quote Link to comment Share on other sites More sharing options...
Randy Bey Posted August 30, 2002 Share Posted August 30, 2002 Since you mentioned 'first foray' and 'stock ST 70' can one assume that you have recently powered up and run a 30 year old component that may well have been sitting in a box for the last 10 years? My understanding is that the electrolytic caps are ruined by this long dormancy (or even w/o dormancy from just plain old age). I have no idea how to check a cap for correct operation. Once upon a time I had a meter that could check capacitance, but you had to remove the cap from the circuit (of course) before you could get a good reading. Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted August 30, 2002 Share Posted August 30, 2002 If the caps are in the PS and are going bad, it usually brings about a low level hum that gets worse and worse. kh Quote Link to comment Share on other sites More sharing options...
edwardre Posted August 30, 2002 Author Share Posted August 30, 2002 All, thank you very much for your tips. I will use tham as a starting point this weekend. I didn't get the opportunity to 'dink' with it last night as the missus was adamant that we watch the 5th and 6th episode of Sopranos, 3rd season. Ahhh, the obligations one must endure to keep peace in the nest.... To answer a couple of questions that I left out the first go around.....the condition of the unit is fairly clean. To Randy's question(s), the amp sat dormant in the garage at my father's place in northern WA (atmospherically wet) for the past 18 years until last year when he brought it down on the yearly CA visit. At that time, one of the large tubes (EL34) and a 7199 were cracked. The circuitry exhibited signs of corrosion, but not severely. One or two tube sockets were fairly bad, but responded (I thought) well with contact cleaner, as did the rest of the 'semi-fuzzy white' corrosion. The non-critical stainless steel base showed light rust speckling overall, which was easily lifted out with Wrights polish restoring a gorgeous luster. It was at this point (circuit corrosion removed with contact cleaner) last year that I made grievous error number one. Like I stated, there were a couple of busted tubes. Not knowing beans from beef jerky about the ramifications of my actions, I moved all the good tubes to one channel with the idea that I'd see if the thing even worked at all and thus determine if it was worth it to try and aquire additional tubes. So I hooked up one speaker, hooked up the PAS2, hooked up a Sansui tuner, and fired it up. Evidently, as I've come to find out, that was the wrong thing to do. There were a few minor flashes from tubes before the big flash and meltdown because, as I mentioned in a previous post, I also jammed in a 20 or 30A automotive fuse in lieu of the one that was missing. Though the smoke would tend to indicate differently, the damage appeared to be restricted to 2 tubes (another EL34 and 7199), and an almost total meltdown of the primary side of the xformer....fuse holder exploded, AC wires melted to the chassis, power switch just a glob of goo. Upon finding out that you don't just jam tubes and hookup one side of a biased circuit, I then ordered replacement tubes in pairs from Triode Electronics...I believe they were Sloveks, to replace what I now realize were fairly decent tubes, Mullards. I received the new tubes, replaced the fuse holder and power switch as well as the AC wiring. I also installed the correct fuse. With fingers crossed and standing to the left shielding eyes, I hit the power switch. I thought I heard the 'pull' of current through the transformers briefly, then nothing. No lights, no sound, no flashes, nothing. Upon checking, the fuse was good. At this point, dejected, I shelved the ST-70. Fast forward to a week or two ago. Based upon what I perceived as 'renewed' interest in tubes on this very BB, I decided to get in on the action and pulled the ST-70 out for another go around. This time, with the base seperated and the unit propped up like a car hood, I started to try and identify why nothing happened when the power switch was flipped. Taking voltage readings with my Fluke 77, it was a fairly quick path to identification.....the power switch I installed was faulty. So I replaced that and it fired up. For the next week or so, I 'burned in' the unit on my test bench in the garage using a fairly efficient pair of Bozak/JBL 12" 3-ways. The unit performed flawlessly for a week, with the source being the older Sansui tuner through the PAS2. During this week I spent a considerable amount of time recleaning contacts. After being satisfied that the unit was indeed operational, I then took it indoors and hooked it up to the Khorns. This was cumbersome as everytime I wanted to listen, I had to unhook the Khorn leads from the Denon based HT and rehookup to the ST-70. I listened to this for 2 nights, about an hour per night before I discovered the PAS2 problem, then this problem. That's where I am now, hoping to devote a few hours to both the PAS2 and the ST-70 this 3 day weekend. They're now both back in the garage. I do have a LCR meter if things get to the 'cap-checking' stage, and I believe my Fluke meter will pretty much identify the lion's share of my needs. I have the schematic as well as have found what appears to be a very detailed troubleshooting guide on the Triode website, though I had hoped to be able to 'shortcut' based upon experiences that you all have gone through. Final couple of questions: I note with keen interest that Triode offers replacement tube sockets. Have any of you gone this route and what are your opinions on the best 'bang for the buck' replacements? WRT 'cheap tubes'...ie - Sloveks, Svetlanas, Teslas, Yugos....which are your recommendations? I believe that the bias is towards Svetlanas? I have access to lots of the 'orange drop' and Sprague caps. If I ever get both the ST-70 and the PAS2 up and running, would it be a good course of action to replace all the caps with the polys? Or are there some that need to remain whatever their 'stock-type' are? Of course, there is much written on the web about upgrades to this classic amp, but reading through the myriad of verbage to glean 'quick-fix' tips can be daunting. Therefore, I really appreciate all your insight. What about inputs? With a stock preamp, should I be using the 'spare' input jacks for DVD (CD) playback? Thanks again for your assistance. I'll write when/if I find out anything. ------------------ Ed Quote Link to comment Share on other sites More sharing options...
edwardre Posted September 3, 2002 Author Share Posted September 3, 2002 Yup!! That's me.....the auto-fuse one which should be indicative of my electronics prowess To further that point, earlier I reported that I thought the ST-70 was on the fritz based upon 'static' type noise. This conclusion was based upon the fact that upon hearing the noise, I powered off the PAS2 to no effect. When I turned off the ST70, the noise went away. As it turns out, I had the PAS on the workbench to try and determine the problem with the one channel not outputting to the level of the other. Has the PAS plugged unto my test amp - a Bryston - and there was the static. There was a faulty AC recepticle in the back of the PAS, which I replaced as well as the RCA phonojacks. I then did as suggested here....touched up all the solder joints that looked suspect. The problem seems to have gone away for the moment. So now I'm back to the first problem....one channel not outputting to the same level as the other. Swapping tubes yielded nothing other than to tell me it's not a tube issue. Upon listening, it sounds more like the 'fidelity' has gone out of one channel, almost like a cheap AM radio sound now. Krackley and lacking bass/treble. I have 'ohmed' out components in an attempt to identify glaring anomolies, but there were none. Any ideas now? I was going to replace all the caps with poly's anyway, should I just do that and if the problem still exists afterwards, try and follow the signal back from the output connector? Is the preamp output signal large enough to 'see'? What would I need? An o-scope? Or is the signal measurable with a good digital DMM? Thanks ------------------ Ed Quote Link to comment Share on other sites More sharing options...
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