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Hello, fixing the TEK portables is not so difficult, many hams, DIYers, and techs suceeded

suggest you get the service manual for,your serial number, see flowchart on power,supply and CRT,CKTs.

The PSU s of most portable tek are not exotic, 120/240...rect>>360 VDC>>Switcher>>DC busses>>final linear regulators.

Photo of the trace will benefit debugging, but suspect blanking in CRT circuit

Search Both TEKSCOPES2 and Tekscopes on groups.io and also,search eevblog.

Bon chance

Jon

Thanks for your reply. I got into it farther and found +7.65 on the +8.6 volt line. It will adjust lower, but not higher. They've done some awfully fancy stuff in the power supply and I don't have it figured out yet. It looks like it's an AC to regulated DC multivibrator to AC to DC again system. I'll try replacing the Zener diode in the voltage adjust and replacing some of the high-value electrolytics, but there's a suspicious-looking FET in its own cartridge. I wonder if you know anything about that. There are no detailed voltage pick-offs around the PS. Frustrating for those of us without BSEEs!

Could be CRT blanking issue.? Do you have total control over the trace intensity?

If not, defocus the trace to reduce the likelihood of screen burn while you fault find.
If you do not have a manual, this seems OK.? Operator/Service manual.

Including schematics.

No cost, just one of those picture block things to negotiate, to prove you're human!.

Regards.

Dave G8KBV
(Ex Tek UK.)

--
Created on and sent from a Unix like PC running and using free and open source software:

Dug out my 2215A after 2 year storage to fix a tape recorder and the horizontal sweep begins with a bright point of light, or a vertical line when a vertical signal is present. Both traces the same, vaveforms display correctly but I think the sweep is short. Ramp signals present on L&R sweep pins on CRT, but not having another scope, I can't tell much about the quality.
Based on the principle that 90% of anything is having seen it before, does anyone have a good suggestion before I start tearing into an unfamiliar circuit and doing the monk-work?

Hi,
On my 7704A which I bought decades ago non working:
When plugged, on the regulator board A32:
TP+7 +7,28V
TP+54 +54,4V
TP+17 +17,00V
TP-17 -16,98V
TP-54 -54,16V

Outputs of the regulators on P32:
-15V -1.2
+5V +0.1
+15 +0.4
+50 +52.6
-50 -53.6
+5 lights +4.5
I found on the -50V an opened resistor R134 which explains the opened loop and -53.6 out.
Explains also the +50 over referenced on -50.
Then I found short circuits. For +15V I tracked to C83 on A20 (main interface) and for -15V to C98 on A43 (H amp)
Next move, I still have the short on +5 and I'll run the regs unconnected to A20 but on some resistors.
And I'll have to replace the faulty caps and even the non faulty I suppose.
As usual, comments, critics etc. welcome,
Renaud F5ZR

Thanks to all.
I think this thread became too long and unclear.
I open another one more specific: 7704A 15V tantalums
Have a look there please!

Renaud are you SURE the issues are shorted rants, eg are the +/- 15 busses clean, harmonica connectors and cables perf, etc ?

the thread is so long and meandering, it will not get the veteran TEK mavens attention.

Suggest you start a new post in all 3 forums, EEVBLOG, TEK and TEK2, with a more speific title eg

Tek 7704 (?) backplane PCB A20 access

include manual schéma page image and a photo of exactly what is the board, caps in the scope.

Bon courage

Jon

Hi Jon,
Yes, I have the pdf but I still wonder how to reach those tantalums on A20.
I won't dare dismantle that A20,22,23,24,25 pile without? the know-how of someone.
Help anybody!
Good day Jon,
Renaud

Rebonjour Cher Renaud

On groups.io you cannot attached a photo!

All replies and posts,are text.

Instead go to the group's photos section, creating the new album, make a recognized name like 7704 repair, then upload images or PDF, etc.

Finally référence that album name in your posts.

Easier to do than to describe

Jon

"There's that which is not much:"
I thought I added a photo which did not show.

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There's that which is not much:

All those are bolted and connected together of course taking about 50 mm in depth. The tants are on near the center of the A20 on the front side here.

I'm really not enthousiastic about dismantling that package you know! I'll quietly wait for more advice from those? who know.

Cheers,

Renaud

Bonjour Jon,

Yees! Salt water is good to keep flesh. That's why I'm in good shape probably... For electronics that's obviously another matter.

Your reference is what my wife calls "ton catalogue". Ha ha! It's in my hands half of the time those days for I ride constantly with it between the house and the lab (60 meters only).

Do we have the same for 7904 Jon?

Bonne journée à toi,

Renaud

Renaud bonjour

Odd that so many PS seem shorted. Unlikely that several tant fail in one unit.
A20 acess should be in the service manual under mechanical diagrams.



After decades of storages near salt water, upon turn on, caused electrolytic plating on conductive salts on the the boards.

Try to wash the suspect boards with a flood of 91..99% isopropyl alcohol....

just a long shot..

Amicalement

Jon

I realize I didn't properly answered your question Eric.
None of the regulated supplies were correct because of the wrong -50 reference as you said:
1 -15 -1.2
2 +5 +0.1
3 Gnd 0
4 +15 +0.4
5 +50 +52.6
6 -50 -53.6
7 Lights com 0
8? 0
9 +5 +4.5 (lights)
10 0?

And I measured that +15 & -15 are shortened on A20.
Now that I suppose I fixed the -50 I want first to fix those shorts before I fire it.
Thanks,
Renaud

Hi Eric,
You are right the -50V serves as ref to the others. That's why I looked at it first.
I found an opened loop on it because of the R134 opened. I believe it is fixed though not tested.
Now, as I found plus and minus 15 shortened I won't plug it anymore before I find the tantalums on A20.
Regards,
Renaud

Another one:
There's also 4 cables from the PSU bloc to A20 or around. I didn't find them on the diagrams.

Just a short add-on to my Q2 above:
If I disconnect P32 H&G to A20 to have the regulators working alone I should probably also disconnect the 20 kHz from A31 to HT to A41?

Hi everybody and thanks for having me here.
I also post on TekScopes but don't see why I could not here as well.

Just me for the context:
Retired EE, analog and RF mostly, built my first scope when 18, long ago!
Measuring equipment: Fluke multimeter and PM3382 scope.

I own a 7704A and a 7904 both in need of serious attention.
Two 7A26, 7A19, 7B53AN & 7B87.
All this not working since decades and never used by myself.

7704A, no plug-ins:
When plugged, a 20mm spot. Intensity not working. Reduced by turning CW R 4135.
As I didn't want obviously to burn the CRT, all my following measurements done one by one unplugging each time.

We are in 230V mains here and C3016 get 156V, C3017 153V. When I cut the mains the 309V slows down very slowly.
I consider that fine. Will give the inverter more attention later probably.
Apart from A32 which on top collected decades of dust, A30 and A31 as well as mains caps etc. look very clean, visual control could not find anything obviously bad. Nothing leaking or burned.

ON A32:
TP+7 +7,28V
TP+54 +54,4V
TP+17 +17,00V
TP-17 -16,98V
TP-54 -54,16V
As I have +50, -50 -even very out of tolerance- and +5 lights, I understand that the inverter is working, ok maybe incorrectly.
I know that the main regulator is -50 from the 9V zener reference. Then all the others take that -50 as a reference.
So inverter fine. -50 regulator wrong.

A32:
Diode and cap on Q130A good as were res and all others around. Q115, 137 141 good. Q139 (power) untested but my money's on it beeing ok.
But R134 (divider from -50V sens) opened.
It's a 32.4k I changed for two series res totaling 32,37K. Of course it explains why the -50 went to -54 with a loop open.

Q1:
Just a thought then: R32140 (470 ohms), between -50 and -50 sens is no use when A32 is connected to A20 because short circuited by the bridge on A20 between P20G -50 and P20H -50 sens. I would guess that R140 is used to have the loop working when P32G&H are disconnected from A20.
In short may I try A32 disconnected from A20?

Q2:
Now I didn't dare fire it (litterally maybe). Rather I disconnected P32 H&G (to A20) and found shorts from A20 only Q2:for +15V and -15V.
If not mistaken, from A20 they only reach A33 and the plug-ins. The shorts stayed as I disconnected A33 and I have no plug-ins so I will have to look on A20.
I will need help there because C2081/C2085 are not the easiest to reach.

Any help gratefully appreciated,
Renaud F5ZR