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Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)

 

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 °C) 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
 

Excuse me, but why you would want a functional PAL signal generator?

El 19/02/2025 a las 17:31, Luca via groups.io escribió:
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 °C) 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

--
Este correo electrónico ha sido analizado en busca de virus por el software antivirus de Avast.
www.avast.com
 

To fix PAL standard televisions?

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?

El 19/02/2025 a las 17:31, Luca via groups.io escribió:
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 °C) 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
 

Luca,

R1156 is too hot. Replace with a 5W. Look around the TP1100 for bad parts. The thermistor could be open. Dirty pins/sockets could be the problem. Check the ripple in the power supply. At the age of this unit, recapping may be necessary.

Mark
 

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 °C 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.
 

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)
 

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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