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TEK 7603 REPAIR


 

Dear 7603 owners,
I just got serial no. 704170 'parts and repairs' from ebay. Mostly it works, including the graticule bulbs. The regulator board already has an uprated 30K resistor as standard; there is also a fan.
I took apart the rectifier and regulator boards to check before switching on. All TO-3 transistors had betas in the high 30's. All electrolytics are original and with low ESR; when switched on the minus 50V was spot on.
I took apart everything with gay abandon, not noting what wire came from where. Was able to work out rectifier and regulator board connections and placement, but am now left with the signal output board: where is P339 (for a white plastic 4-pin plug), and where do the three left hand co-ax sockets connect to (one green 4-pin plastic plug and one brown 2-pin plug)?
There is a minor fault in that the trace is dim. Once it brightened up when selecting a low sweep speed, but then reverted to a trace that cannot be adjusted to be brighter (and goes out of focus when you try). Something to with the Z axis circuit? But all transistors are OK on this board, mostly already replaced with 2N3906 & 2N3904.
Next possibility the CRT control circuit, getting bias voltage via two 27pf capacitors?
Thanks & Regards,
David Collier


Jim Ford
 

Uh, (DC) bias wouldn't be provided through capacitors, that much I know.  (32+ years practicing EE; one of these days I'll actually get it right! ;))   Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: David Collier <dc888@tpg.com.au> Date: 12/20/20 4:46 PM (GMT-08:00) To: TekScopes@groups.io Subject: [TekScopes] TEK 7603 REPAIR Dear 7603 owners,I just got serial no. 704170 'parts and repairs' from ebay.  Mostly it works, including the graticule bulbs.  The regulator board already has an uprated 30K resistor as standard; there is also a fan.I took apart the rectifier and regulator boards to check before switching on.  All TO-3 transistors had betas in the high 30's.  All electrolytics are original and with low ESR; when switched on the minus 50V was spot on.I took apart everything with gay abandon, not noting what wire came from where.  Was able to work out rectifier and regulator board connections and placement, but am now left with the signal output board: where is P339 (for a white plastic 4-pin plug), and where do the three left hand co-ax sockets connect to (one green 4-pin plastic plug and one brown 2-pin plug)?There is a minor fault in that the trace is dim.  Once it brightened up when selecting a low sweep speed, but then reverted to a trace that cannot be adjusted to be brighter (and goes out of focus when you try).  Something to with the Z axis circuit?  But all transistors are OK on this board, mostly already replaced with 2N3906 & 2N3904.Next possibility the CRT control circuit, getting bias voltage via two 27pf capacitors?Thanks & Regards,David Collier


Tom Lee
 

Many things can cause diminished brightness. Don't overlook simple possibilities, such as misadjusted or noisy trigger thresholds and holdoff (or any other trigger-related controls).  Check easy stuff like that first, before going on to Z-axis circuit-related hypotheses.

--Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/20/2020 16:46, David Collier wrote:
Dear 7603 owners,
I just got serial no. 704170 'parts and repairs' from ebay. Mostly it works, including the graticule bulbs. The regulator board already has an uprated 30K resistor as standard; there is also a fan.
I took apart the rectifier and regulator boards to check before switching on. All TO-3 transistors had betas in the high 30's. All electrolytics are original and with low ESR; when switched on the minus 50V was spot on.
I took apart everything with gay abandon, not noting what wire came from where. Was able to work out rectifier and regulator board connections and placement, but am now left with the signal output board: where is P339 (for a white plastic 4-pin plug), and where do the three left hand co-ax sockets connect to (one green 4-pin plastic plug and one brown 2-pin plug)?
There is a minor fault in that the trace is dim. Once it brightened up when selecting a low sweep speed, but then reverted to a trace that cannot be adjusted to be brighter (and goes out of focus when you try). Something to with the Z axis circuit? But all transistors are OK on this board, mostly already replaced with 2N3906 & 2N3904.
Next possibility the CRT control circuit, getting bias voltage via two 27pf capacitors?
Thanks & Regards,
David Collier




 

Thanks JIm & Thomas,
Firstly it is correct that capacitors do not pass DC; but they do pass alternating current, as in voltage doublers etc.; in this case the measured 45.5KHz from the 800V secondary of T1225 which is then rectified for the correct CRT control grid voltage. Small 27pf capacitors for small voltage(s).
Secondly, it seems the trigger selector board was indeed involved: connecting the signal output board to P339 (on this trigger board) seems to have restored all functions, though I don't understand how.
Now there is plenty of brightness, and with different intensity between normal and delayed sweep.
It only remains to clean a white oxidation from component leads and solder joints, either with CLR; vinegar, or Coke (sugar free of course!)
Regards,
David Collier.


Jean-Paul
 

David:

Suggest to exercise and clean all controls and connectors.

Please use only iso alcohol or correct cleaners for corrosion, PCBs etc. Vinegar/coke/etc NOT recommended.

Sometimes the trimpots need cleaning or even exercising.

Suggest a complete CAL check and recal if needed.

Kind Regards,

Jon


Jim Ford
 

Come to think of it, the term bias usually implies DC, but not always.   I'm thinking of the case of the old cassette tapes and how the magnetic particles were biased with an ultrasonic tone, IIRC to make them respond more linearly to the audio magnetic field, hence generating less harmonic distortion.  Not that I've played a cassette tape in many years, though.    JimSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: David Collier <dc888@tpg.com.au> Date: 12/22/20 2:16 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] TEK 7603 REPAIR Thanks JIm & Thomas,Firstly it is correct that capacitors do not pass DC; but they do pass alternating current, as in voltage doublers etc.; in this case the measured 45.5KHz from the 800V secondary of T1225 which is then rectified for the correct CRT control grid voltage.  Small 27pf capacitors for small voltage(s).Secondly, it seems the trigger selector board was indeed involved: connecting the signal output board to P339 (on this trigger board) seems to have restored all functions, though I don't understand how.Now there is plenty of brightness, and with different intensity between normal and delayed sweep.It only remains to clean a white oxidation from component leads and solder joints, either with CLR; vinegar, or Coke (sugar free of course!)Regards,David Collier.


 

Thanks Jon,
Not to worry! The procedure is 1. take out board; 2. paint affected parts individually with small brush or toothbrush; 3. allow time to change oxide to acetate or phosphate etc.; 4. wash board in (distilled) water; 5. spray with methanol to remove any water, 6. allow to dry. Avoiding potentiometers: but most are sealed anyway.
Regards