Fault with Tek 7704A - UK Vintage Radio Repair and Restoration Discussion Forum

delaitt · 27th Jul 2019, 3:18 pm

Hello

This is a follow-up on this original thread for a fault with a Tektronix 7704A which can be found at https://www.vintage-radio.net/forum/...d.php?t=124922

I've replaced all capacitors on the following boards 2 x regulator boards, Horizontal output including C4373 and C4394 (3uF), tantalum on Z Axis, tantalum on readout board, main interface circuit board and logic board. The voltages are spot on: +5V, +15V, -15V, -50V, +150V. I've not done the 7000 plugins.

The problem is still there after having replaced the capacitors on the boards

I can see the horizontal sweep starting to oscillate when the signal gets distorted on the screen. I was suspecting the horizontal output board but can see its horizontal input signal at J4315 also oscillating.

I've also tried in XY mode as per shown in this video https://youtu.be/7etKWHRisb4 ; the problem is the same with different time base plugin or when using an input plugin on the horizontal input.

In the video I have an input signal plugin connected to the horizontal slot. The signal gets distorted on the screen when changing the position on the input signal of the board connected to the horizontal plugin.

What do you think I should do next? Could this be an issue with the horizontal interface at the back of the main interface board? This one is not easily accessible.

Thanks

Thierry

delaitt · 27th Jul 2019, 5:05 pm

Another video https://youtu.be/aAYO1OpjVF8

delaitt · 27th Jul 2019, 5:43 pm

Scope on the right in the video below shows the output of the horizontal interface. So the problem might be in the final output horizontal board then?

https://youtu.be/fptsBpQsUmE

Thanks

T.

delaitt · 27th Jul 2019, 6:09 pm

Scope on the right in the video now shows the collector of Q4321 & Q4331.

https://youtu.be/hn0Mq3SgIHc

We can clearly see an oscillating signal but not sure of the cause?

Thanks

T.

delaitt · 27th Jul 2019, 9:02 pm

found the issue. R4377 was open circuit instead of 8.45K ohm

Thanks

T.

MrBungle · 27th Jul 2019, 9:34 pm

Excellent work finding it.

RogerEvans · 28th Jul 2019, 7:38 am

Was C4373 one of the ones you changed? If not then now could be a good time to swap it out.

Did R4377 show signs of heat damage or was it just a random failure?

Roger

delaitt · 28th Jul 2019, 9:26 am

Yes C4373 was one of the ones I replaced.

R4377 did not show any sign of heat damage. pic attached.

Thierry

27th Jul 2019, 3:18 pm	#1
delaitt Pentode Join Date: Jul 2005 Location: London, UK. Posts: 193	Fault with Tek 7704A Hello This is a follow-up on this original thread for a fault with a Tektronix 7704A which can be found at https://www.vintage-radio.net/forum/...d.php?t=124922 I've replaced all capacitors on the following boards 2 x regulator boards, Horizontal output including C4373 and C4394 (3uF), tantalum on Z Axis, tantalum on readout board, main interface circuit board and logic board. The voltages are spot on: +5V, +15V, -15V, -50V, +150V. I've not done the 7000 plugins. The problem is still there after having replaced the capacitors on the boards I can see the horizontal sweep starting to oscillate when the signal gets distorted on the screen. I was suspecting the horizontal output board but can see its horizontal input signal at J4315 also oscillating. I've also tried in XY mode as per shown in this video https://youtu.be/7etKWHRisb4 ; the problem is the same with different time base plugin or when using an input plugin on the horizontal input. In the video I have an input signal plugin connected to the horizontal slot. The signal gets distorted on the screen when changing the position on the input signal of the board connected to the horizontal plugin. What do you think I should do next? Could this be an issue with the horizontal interface at the back of the main interface board? This one is not easily accessible. Thanks Thierry __________________ BVWS Member.

27th Jul 2019, 5:05 pm	#2
delaitt Pentode Join Date: Jul 2005 Location: London, UK. Posts: 193	Re: Fault with Tek 7704A Another video https://youtu.be/aAYO1OpjVF8 __________________ BVWS Member.

27th Jul 2019, 5:43 pm	#3
delaitt Pentode Join Date: Jul 2005 Location: London, UK. Posts: 193	Re: Fault with Tek 7704A Scope on the right in the video below shows the output of the horizontal interface. So the problem might be in the final output horizontal board then? https://youtu.be/fptsBpQsUmE Thanks T. __________________ BVWS Member.

27th Jul 2019, 6:09 pm	#4
delaitt Pentode Join Date: Jul 2005 Location: London, UK. Posts: 193	Re: Fault with Tek 7704A Scope on the right in the video now shows the collector of Q4321 & Q4331. https://youtu.be/hn0Mq3SgIHc We can clearly see an oscillating signal but not sure of the cause? Thanks T. __________________ BVWS Member.

27th Jul 2019, 9:02 pm	#5
delaitt Pentode Join Date: Jul 2005 Location: London, UK. Posts: 193	Re: Fault with Tek 7704A found the issue. R4377 was open circuit instead of 8.45K ohm Thanks T. __________________ BVWS Member.

27th Jul 2019, 9:34 pm	#6
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Fault with Tek 7704A Excellent work finding it.

28th Jul 2019, 7:38 am	#7
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: Fault with Tek 7704A Was C4373 one of the ones you changed? If not then now could be a good time to swap it out. Did R4377 show signs of heat damage or was it just a random failure? Roger

28th Jul 2019, 9:26 am	#8
delaitt Pentode Join Date: Jul 2005 Location: London, UK. Posts: 193	Re: Fault with Tek 7704A Yes C4373 was one of the ones I replaced. R4377 did not show any sign of heat damage. pic attached. Thierry Attached Thumbnails __________________ BVWS Member.