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Tek 7904 HEPS operation with dummy loads -- Need advice
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Hi folks,
I decided to create a new thread with with question so other (as inexperienced as I am) may benefit from the answers. While trying to find why my 7904 was showing a blurry trace, I heard a tick and now there is no trace anymore, just a glow. Checking the Voltages at the Z axis test point I found that +15V and +5V were down. So I decided to remove the power supply to isolate the problem; is the Regulator boad broken or something outside is drawing too much current? In order to test the HEPS out of the scope I built a dummy load as suggested here (). In this article, it seems that there is just one flat cable to be connected to the regulator board at P1482 ( voltage output to mainframe) However, there is a voltage feedback from the mainframe that arrives at P1483 (sense voltage return). Can I test the HEPS with just the loads connected? I vaguely remember seing another dummy load where voltage sense signals were derived connecting each voltage rail point at the load back to the corresponding voltage sense input using a second flat cable. Is it required? Thank you. |
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On Thu, May 2, 2024 at 12:51 PM, Rogerio O wrote:
....== On 7904 LV regulator there are resistors connecting outputs to their sense inputs in case motherboard doesn't provide sense return. For example R1429 is such a resistor for +50V supply. For debugging sense connection is not needed.? Did you check for capacitor shorts at sheet <9> rectified outputs and capacitors on sheet <1>? Ozan |
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Thank you for your reply Ozan.
I have measure the resistance to ground on the power rails at Z-axis boards and they were similar to the ones described on the famous article from Servicescope. From that measurement I am deducing that there are no "active shorts" t o grounds, although there might be some shorted caps where the series protection has opened. I am also confused about the need to connect the light common to the dummy load. It is P1482 pin 10. It seems to be connected to ground, but it goes to main interface and I got lost. Regards, Roger |
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One thing I should mention is the 7904 on its best day has
a very blurry trace. It is not as sharp as, say, a 547, or 545B. It is not as sharp as a 2465, either. The reason it is inherently blurry is the extreme bandwidth of the solid state amplifiers, and the high magnification of the secondary mesh lens that was required to keep the CRT's length and the transistorized vertical amplifiers reasonable. So, be sure that the fuzzy trace isn't normal before you try to fix it. Secondly, there are a lot of internal reflections that happen inside of the CRT that make for halos, and moving spots that you might never have seen on other scopes. On the power supplies, the 7904 is fairly unique in that tektronix put ALL of the voltages in one transformer, including the anode voltage. So, be mindful. If you run the inverter wide open, it is likely that you will arc over the HV winding of this very hard to repair transformer. -Chuck Harris It is not as s On Thu, 02 May 2024 12:51:01 -0700 "Rogerio O" <rodd414@...> wrote: Hi folks, |
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On Thu, May 2, 2024 at 03:45 PM, Rogerio O wrote:
== Hi Roger, Could you clarify your question about lights common? +5VL is one of the supplies monitored by the switcher, it needs some load. You can tap lights connections at P1417 (violet colored connector) which will be empty once you remove PSU outside the unit. It is next to P1415 (green) connector.? Ozan |
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Thnak you all for the replies.
I am lost about where to connect the load to the +5Vlamp rail. As far as I can make sense of the schematics the pin 10 of P1482 is connected to ground. According to Ozan's reply (and the schematics...) the output to the mainframe of +5Vlamp is on P1417 pins 3 and 4. So, I will need to make another flat cable to connect the dummy load to P1417 and the +5Vlamp resistor load should be connected between P1482 pin 10 and P1417 pins 3 and 4. Is this correct? Thank you |
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It is fairly typical of the beast, but any mesh/quadrapole
lens fast scope is fuzzier than, say a 547, because the beam granularity is magnified over that of a classic CRT. The thing that most don't expect is the internal reflections. -Chuck Harris On Thu, 02 May 2024 22:50:41 -0700 "Froggie the Gremlin" <jonpaul@...> wrote: Chuck: Never had an issue on my 7904 for sharpness vs 7104, 2465B |
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On Fri, May 3, 2024 at 04:07 AM, Rogerio O wrote:
Thnak you all for the replies.==== Your suggestion would work, as an alternative pins 2&6 of P1417 should be same ground as pin 10 of P1482 (you could confirm with a DVM). Load could be between pins 2&6 and pin 3&4. You need 0.3A or greater, if you pick a resistor for e.g. 500mA connecting only one pin of the supply/ground is fine. Ozan |
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I tested my dummy load after disconnecting the HV section and the HEPS went to ticking mode.
However, it worked when connected to the scope. I think I have double checked all the connections, but I may be wrong... Does anyone used the resistors values that Dennis Tillman suggested for the dummy load? I wonder if the current drawn in one (or more) rail(s) is causing the protection circuit to turn on. Thanks |
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> I tested my dummy load after disconnecting the HV section and the HEPS went to ticking mode.
> However, it worked when connected to the scope. > I think I have double checked all the connections, but I may be wrong... > Does anyone used the resistors values that Dennis Tillman suggested for the dummy load? When I made my dummy load for the 7904 I used the following load resistors: +130? 2K2 20W +50? 240R 20W +15? 82R? 5W +5? 10R? 5W -15? 100R 5W -50? 240R 20W I also connected indicator LEDs to all the outputs for a bit of extra current through the following resistors: +130? 5K6 5W +50 and -50? ?2K 5W +15 and -15? 750R 1W +5? 220R 1W I? installed a couple of LEDs in the dummy load through 470R resistors to the appropriate pins of P1417 to check the light supplies. I know there resistors in the HEPS between the sense inputs and the outputs but I installed a P1483 plug which is the sensing one and connected it to the corresponding outputs from P1482 "just in case". It works well and helped me fix the HEPS. FWIW the problems I've encountered have all been faulty LV rectifier diodes and caps which cured the ticking, and faulty focus &? DC restorer diodes and 22M resistors in the HV section which cured focus and display problems.? It's such a pain to work on that HEPS that once it's apart I preferred to change them all rather than have to go back in. Good luck! Morris |
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On Sat, May 4, 2024 at 09:34 AM, Rogerio O wrote:
I tested my dummy load after disconnecting the HV section and the HEPS went to ticking mode.=== Hi Roger, I looked at the schematic of Dennis Tillman load. It is for a different model and missing the load on 130V line. Minimum load currents you need on 7904 (from H?kan): -50 V: 0.08 A -15 V: 0.8 A +5 V: 0.4 A +15 V: 1 A +50 V: 0.2 A + 130 V: 0.036 A +5 Lights: 0.3 A Also see Morris's reply. Ozan |
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On Sun, May 5, 2024 at 08:31 AM, Rogerio O wrote:
Thank you all for the reply.==== +130V is the main voltage switcher regulates, all other pre-LV regulator voltages (+7V, +54V, etc.) depend on +130V being correct. ?It is sensed at the top of R1292 and adjusting R1293 to set +130V is one of the calibration steps. No need to touch it if +130V is correct, adjustment pot is in an inconvenient place when the PSU is connected. +130V comes out at a test point on LV board.? Ozan ? |
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On Sun, May 5, 2024 at 11:10 AM, Rogerio O wrote:
Thank you for your reply Ozan,======== Hi Roger, What I meant is if the +130V measures within target, while loaded in the mainframe, then no need to touch it. First PSU needs a minimum total load so that switcher section works in its intended operating frequency range. Then if +130V is not loaded switcher will be able to build 130V without much work and other windings that are loaded will be at lower voltage. If it is too easy to build 130V, PSU may again fall out of its intended operating point. When +130V is loaded similar to other windings once +130V is correct other voltages (prior to LV regulators) will be closer to their target value.? Ozan |
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Thank you for your reply Ozan,
I found one old Tektronix Reference document about the min and max loads that I have uploaded to the files section (/g/TekScopes2/files/Tek%207000%20scopes%20power%20supplies%20specs.pdf). I will proceed with the troubleshooting and close this topic. Thank to all. |
Froggie the Gremlin
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