Tek 561S 5A1S amplifier help - Page 1
Forum section: electronics / repair

author: VIPQualityPost
author_rank: Newbie
timestamp: July 22, 2019, 04:35:18 pm
content: Hey everyone-I have recently started to try and repair a 561s that's been sitting around for a while. I've isolated the error to the vertical amplifier plug in unit (power supply, calibrator, and timebase all seem fine). Basically, there is no amplification happening in the unit. The trace for both channels shows up, but neither of them will have any deflection when a signal is applied (calibrator or signal generator), they are almost off screen (can be brought up using focussing) and the positioning knob has a miniscule effect. There are two circuits that compose the amplifier, the input amp and then output amp. I can't tell which one is bad, so I've been testing both. From checking the upper and lower deflection plates on the output amp, the voltages on the deflector plates aren't in spec with the references on the schematics (with the proper conditions setup on the front panel). Working back from here, I tried to check V364 and V374 and it seems the anode voltage is low, somewhere like 30V instead of 48V. I'm not sure what could cause this, since the transistors that drive those tubes, Q364 and Q374, seem to be operating fine. I'm not sure how to check the voltages of the gates on the pentode (I'm new to tube electronics), or what those values should be, as they're not marked as test points on the schematic. I'm also not sure how I could check the integrity of the tubes, if those were to be the issue. They look in good shape and there are no loose parts inside, and the getter is still silvery, but maybe something else can break that I'm not aware of. When I take either of these pairs out (V/Q 364/374), then the trace returns to the middle of the screen, but irregularly stutters/pulses. I feel like this is indicative of a leaky cap or leaky transistor, but I'd like to iron out the first problem with the trace amplifier not working properly.The manual is here, with schematics located at the bottom of the manual:http://w140.com/tek-561s.pdfHere is the area I am suspecting has issues:I appreciate the help!

author: orbanp
author_rank: Regular Contributor
timestamp: July 25, 2019, 10:46:30 am
content: Hi VIP,Welcome to the forum!Read up on tubes, the 48V is on the cathode, not on the anode! If you are familiar with BJT and FET transistors, it will not be that hard and it will definitely help.Do check all the voltages on the preceding stages as well, as indicated on the schematics. This is a DC-coupled amplifier, so the origin of that "faulty" 30V reading could result from one of the earlier stages. Q364-374 transistors must work in their linear region, the base voltage should be lower that the voltage on the collector.The tubes you would be able to check with a tube tester, if you have one.As this is a fully differential amplifier, you could check the tube voltages on both sides (top and bottom), and if the matching tubes have the same voltages on them then chances are that the tubes are fine.You can also work out the grid voltage on V364-374, the voltages are given on the transistors and diodes (D364-374) and with the resistor network you can calculate it (probably it is easier just to plug it into a circuit simulator).But most probably the fault is somewhere else.None the less, this sounds like fun!Good luck, Peter

author: VIPQualityPost
author_rank: Newbie
timestamp: July 31, 2019, 03:51:05 am
content: Hi Peter! Thanks for the reply.I have checked some more documentation on tubes. They definitely seemed a bit foreign at first, but you are right, they are very similar to BJT/FET.I borrowed a curve tracer from a university and it seems that the two transistors that I thought were the issue (Q364, Q374) are indeed fine. The tube voltages also seem fine (V364/374), I'm not too worried about those. I am having some difficulties in parsing the schematic- there are cases where there are paired amplifiers, like Q376,374. The numbering makes sense to have them as a pair. In the schematic, I note that there are some places where it will reeference a part A and part B, see the output amplifier V353A and V353B for example. I have the same revision scope as in the manual, so I'm not sure what the discrepancy is, but on my plugin, physically there is only a V353, no A or B marking, and there certainly is not two of them. Is there something I am missing about this that is standard in schematic notation? I'm mainly confused because the circuit would indicate that both are needed to operate, not like they are stand-ins or something for each other. This has caused me some trouble in tracing out the differential amplifiers. I can upload a picture if you find any of that confusing, it is certainly driving me in circles.Re: checking grid voltages for the tubes, do you recommend any particular circuit simulator? I am not familiar with any of them, but I see some mentions of SPICE on different forums.Matei

author: VIPQualityPost
author_rank: Newbie
timestamp: July 31, 2019, 03:58:27 am
content: Ah, just kidding, I just looked at the datasheet for V353A/B, which is 6DJ8. It is twin triode, A and B are the two triodes within the tube. I will get back to some testing and let you know if there is some more help I need to find the problemsMatei

author: VIPQualityPost
author_rank: Newbie
timestamp: December 01, 2019, 05:50:54 am
content: For anyone searching in the future, I figured it out.Mainly, I noticed that the problem was the trace drifting off screen during some warm up period, and for some reason I didn't really connect that with a tube warming up, no other component should take that long that you can see it reaching operational state. Once I narrowed it down to that, it was a lot easier. Tracing the problem back farther shows that the signal path for output amp is not receiving correct power, so it has to be input amp. I found that V123 was faulty and not working, verified by borrowing a tube tester. This tube helps to set DC offset so it makes sense why this would drive the trace off during warmup. I swapped V123 with V223 and it was working well enough that I could use it until the replacement tube showed up.Matei

author: med6753
author_rank: Super Contributor
timestamp: December 02, 2019, 06:23:58 pm
content: Good deal on fixing it. Keep in mind that the 561S is a unique animal. You can only use a 3A1S Vertical plug-in on the left side and preferred 3B1S Time Base in the right side...although an 2 or 3 series (non sampling) plug-ins can be used.The 561S was a special order for IBM and has higher bandwidth than a standard 561A or B but less vertical sensitivity.