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Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)
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Hi all
Here /g/TekScopes/topic/107425848 H?kan kindly provided the service manual for this signal generator. I had time to have a look inside and found a shorted cap on the -15V supply (C1200 of A13 board), now the power supply seems to behave a bit better (the 3.6 V rail is still off, at 3.3~ ish Volts) According to the manual the oven temp normal light should come on but this never happens (this circuit doesn't seem to use the 3.6V rail so I ignored it for now) The manual says that TP1100 should be around 4.1-4.2 Volts for the "oven temp normal" light to come on but that never happens.. it hovers around 3.5 V and varies a bit. I noticed that R1156 gets really hot (>100 ¡ãC) but I don't know if this is a fault or not. I took the oven apart to check the thermal fuse but it seems ok... so I'm not really sure where to go from here. I updated the album: /g/TekScopes/album?id=296482 Can I plug the oven in without enclosure for testing? Disclaimer: I obviously have no idea what I'm doing... there doesn't appear to be any notable occurences of this SG on the net so I will try to find faulty or out of spec parts and hope any of you have a good idea where to start Thanks Luca |
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Excuse me, but why you would want a functional PAL signal generator?
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El 19/02/2025 a las 17:31, Luca via groups.io escribi¨®:
Hi all --
Este correo electr¨®nico ha sido analizado en busca de virus por el software antivirus de Avast. www.avast.com |
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To fix PAL standard televisions?
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Yes, people are into preserving antique televisions and equipment. Now if 145 PAL does not refer to test equipment for the non-NTSC standard televisions them I am out. Frank DuVal On 2/19/2025 11:44 AM, EB4APL via groups.io wrote:
Excuse me, but why you would want a functional PAL signal generator? |
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Hi all
Thanks for your inputs... yes it's mostly for preservation reasons... how many people or institutions are left with a functional Tektronix 145? I initially picked it up for spares but it's so well made it would be sad to trash it if repair is possible. If I can't get the crystal oven to work or other BER situation I can still gut it. I do have old PAL televisions to repair/restore so it could also theoretically serve a purpose. I'm not sure replacing the resistor will help with anything.. it reads within spec, is ¡ãC 130~ too hot for a larger resistor? I checked all the socketed transistors and they're OK. The thermistor is not open, though I'm not sure about its value (I should probably remove it out of circuit to check). I will have a look into the power supply section and try to get it within spec before I continue. After 30 mins of waiting, the TP1100 voltage stopped at 3.95 V (the manual says it should come on before 20 mins - the recommended warmup time). Maybe it does need the PSU section dialed in. |
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Hi all
I have dialed in the voltages, which brought back most of the logic that generates the patterns, but there is still something wrong with the heater circuit. It generates a picture now, though it's missing two colours and has artifacts in it: /g/TekScopes/photo/296482/3889912 Unfortunately Mark is right, the unit needs recapping because ripple is all over the place and out of spec on all rails (10 V rail has 2V of ripple - yikes) |
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The capacitors are fine... I did change the ones on the PSU board, but left the large cans be.
However, there was an open diode in the bridge rectifier circuit which was the cause of the ripple on the 10V rail I also made a mistake and do have all the colors, it was a setting on the monitor. Now there is still ripple on the 3.6V line which appears to be caused by the 3.6V regulation going into current limiting. It looks like something on the A3 field timing board, though nothing appars to get hot. The A3 board is quite complex.... /g/TekScopes/photo/296482/3889932 |
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Hi
I'm still looking for why my 3.6V supply has issues. The current draw of the boards doesn't seem excessive (though I don't know what's their normal power consumption - Bar Timing board draws almost half an Amp). Q850 gets scorching hot. /g/TekScopes/photo/296482/3910379 Any ideas? I do get a picture, though sometimes I loose the colors and the crosshatch pattern's vertical lines aren't perfect |
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Luca,
Q55, as Ed B. said, is suspect. Use an hfe meter to test yours. I have seen the originals too low, beta <10. The beta would be nominal 110 at the current range the load wants by the chart for a 2N3055. Make sure to get a replacement from a reliable source. There are a lot of counterfeits out there - especially on ePay. The old phrase, you get what you pay for, is true. You can check the other series pass transistors to confirm their beta. Check Q850, 2N2192A, and get a spare or several in case. Getting that hot could have changed the parameters even if checks good. the 3.6V supply is limited at 600-650mA by R841. At this point or higher, the supply will shut down by turning on Q840. It is possible the ripple is high on that supply. A 594-MAL205655103E3, Mouser p/n, could be used as a replacement. See if the decoupling, 100mfd, on the output of the 3.6V supply is good. You may have already replaced this. Mark |
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Considering the circuit, there is more than enough voltage in the base circuit to use a darlington transistor in place of the 2N2192A.? The circuit should perform normally with a slightly elevated?voltage at the base of Q850.? There are a number of high current NPN TO-92 darlington transistors on Mouser.? You may have a few of the older variations in your junk drawer,?just make sure the IC will handle about 100mA.? It would be worth a try to see if the issue persists. - Ferrous
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Hi
Thanks for all your suggestions. Q55's hfE value is around 580 on those china testers (in circuit). Check Q850, 2N2192A, and get a spare or several in case. Getting that hot could have changed the parameters even if checks good. the 3.6V supply is limited at 600-650mA by R841. At this point or higher, the supply will shut down by turning on Q840.Mark, how did you determine the current rating? I'm not an EE just a hobbyist but would like to learn about this because now at least I can try to measure if the load of all modules connected at once exceeds this value. I did replace the capacitor (the old was slightly tired but not broken), and without a load the power supply ripple is within specifications. I should have a spare for the 2N2192A *somewhere* but if I can't find it I could try it with the suggested part |
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Luca,
The current is determined by the voltage drop across R841, in the 3.6V case. Ohm's law V/R (ohms)=I. This is the limit (600-650mA), not what the load draws. Measure the voltage across these resistors in the current limit parts to determine the load current. If a supply overloads, usually tantalums are the first thing to check as these will go bad (short). Test Q55's hfe out of circuit. While it is out of circuit, test the transistor for leakage/open. It would be best to get a new one and any other spares you want. Be sure to put on new compound when you replace the transistor(s), MX-4 for example. Test ripple while under load. Mark |
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Something's not right here. If Q55 has plenty of gain, then there's no reason for Q850 to run hot. Are you absolutely sure Q55 checks out OK? I wouldn't necessarily trust the measurement without knowing more about the tester and setup.
I assume you've been using a scope to measure ripple, so you would have noticed if there's any high frequency oscillation that can mess things up. If Q55 is definitely good, then I'd focus on Q850 now and figure out where all the currents are going and how big to account for it getting so hot. Ed |
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Sorry that I haven't been following this thread too closely, so forgive me if this has already been discussed, but you may need to retest that transistor's beta at collector currents closer to where the transistor is supposed to operate. Beta is not a constant, so a measurement made under some random condition doesn't tell you a whole lot (other than "this transistor sometimes can transist"). Beta is very much a function of current, and unhealthy devices may check ok in some range but not in others. If you are persistently unable to figure out what's going on, you may wish to rig up something to determine what the beta actually is. If taking the transistor out is too much trouble, you'll need to figure out from other measurements what the actual beta is.
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Good luck! Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 On 4/28/2025 3:02 PM, Luca via groups.io wrote:
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Ok, for checking Q55 at its real load parameters I have to find a way. I could take it out but as far as transistor testers go, all I have is the cheap multi component tester.
Regarding the ripple, without a load it's correct. /g/TekScopes/photo/296482/3910866 The voltage drop across R841 is about 0.17 V While (apparently) overloaded, it's according to the attached picture /g/TekScopes/photo/296482/3910865 The voltage drop across R841 is about 0.4 V I did measure R841 and it is out of spec. It's a 0.2 Ohms at 10% tolerance (according to what's written on it) and it currently reads 0.275 Ohms with my highest precision meter. I'm going to check the rest of the resistors too just in case, and try to find out if any drifted resistors are messing up the power supply. |
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On Tue, Apr 29, 2025 at 9:10?AM Luca via groups.io <la_whg=
[email protected]> wrote: While (apparently) overloaded, it's according to the attached picturesays nothing about what's happening here. |
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Ok, I'm going to bet my $0.02 that C42 is simply dead.
Have you measured the unregulated supply? I'd bet that the unregulated voltage is dropping below 3.6V + the saturation voltage of Q55. At that point Q850 is going to try and hold up the supply by feeding the 10V supply through the BE junction of Q55 for all it's worth. This is no good for either transistor :). Good luck, Siggi On Tue, Apr 29, 2025 at 9:51?AM Sigur?ur ?sgeirsson via groups.io <siggi= [email protected]> wrote: On Tue, Apr 29, 2025 at 9:10?AM Luca via groups.io <la_whg= |
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I think Siggi has called it right if the DCV and ripple on the raw feed to the 3.6V regulator has not already been confirmed OK or fixed. I thought though that all the filter caps had been replaced earlier. If not, this could explain it all. If the C42 has been replaced yet still shows excessive ripple, there may be an open rectifier ahead of it, which would make it half-wave with much bigger ripple at line frequency rather than 2f.
Ed |
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On Tue, Apr 29, 2025 at 12:08?PM ed breya via groups.io <eb=
[email protected]> wrote: I think Siggi has called it right if the DCV and ripple on the raw feed toThat's a good point. Looking at the oscillogram ( /g/TekScopes/photo/296482/3910865), it looks to me that the period is 10ms, which is double 50Hz? The only way I can see the signal flat-topping like that is if there's no (or very little) current drawn from the unregulated rail. Though, as we don't know the DC level, maybe it's not flat-topping at 0V. All this being said, this current limiting arrangement seems a little hokey to me as it's limiting the output of the regulator, while measuring the combined currents from the rectifier and the unregulated output? |
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