Products > Test Equipment

Vintage HP 3721A Correlator : Repair, Restoration and Enhancement

<< < (6/13) > >>

dazz1:

--- Quote from: tautech on October 05, 2024, 04:44:13 am ---Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....

--- End quote ---

The resistors are all 1% metal film.  Even the logic pull up resistors are 1%.  The only engineering in this instrument is over engineering.  It is almost like the Scottish design team were trying to show the US HP office how good the Scottish office were, rather than building a commercially viable test instrument. 

The big HV resistors look ceramic types.

Today I washed the HV board with IPA to clean away any contaminants that might lead to arcing.  That made no difference.

I am leaning towards the fault being the wires/connections between the crt deflection plates and the Y deflection amplifier.  Bad connections would be consistent with the step changes in Y gain I am seeing.  Maybe a floating deflection electrode in the CRT would explain the interaction with intensity.  Right now it is the only thing that fits the facts. 

I ran the HP 3721A for about 6 hours today to attempt flush out the fault.  The 3721A gets HOT HOT HOT.  I measured 51 deg C on the metal near the power transistors.  Even with the ugly fan, the rear half of the 3721 was uncomfortably warm.  I am going to fit TO-3 heat sinks to try and dump the heat into the air and not into the chassis.  I am also going to replace the super ugly external fan with an internal 80 x 15 12V fan and make a nice bezel with a period correct grill to cover the hole through the side of the enclosure. 

I tried to video the HP 3721A fault seen on the crt, but I got the exposure completely wrong.

tautech:

--- Quote from: dazz1 on October 05, 2024, 06:49:07 am ---
--- Quote from: tautech on October 05, 2024, 04:44:13 am ---Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....

--- End quote ---

The resistors are all banded 1% metal film.  Even the logic pull up resistors are 1%.  The only engineering in this instrument is over engineering.  It is almost like the Scottish design team were trying to show the US HP office how good the Scottish office were, rather than building a commercially viable test instrument. 

The big HV resistors look ceramic types.

--- End quote ---
FTFY

Still, have you measured them ?
IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.

dazz1:

--- Quote from: tautech on October 05, 2024, 06:59:04 am ---
--- Quote from: dazz1 on October 05, 2024, 06:49:07 am ---
The resistors are all 1% metal film.  Even the logic pull up resistors are 1%.  The only engineering in this instrument is over engineering.  It is almost like the Scottish design team were trying to show the US HP office how good the Scottish office were, rather than building a commercially viable test instrument. 

The big HV resistors look ceramic types.

--- End quote ---
Still, have you measured them ?
IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.

--- End quote ---

OK I will take a closer look.  I have been able to avoid faults on CRTs until now so still on the bottom left edge of the learning curve.

factory:

--- Quote from: tautech on October 05, 2024, 06:59:04 am ---
--- Quote from: dazz1 on October 05, 2024, 06:49:07 am ---
--- Quote from: tautech on October 05, 2024, 04:44:13 am ---Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....

--- End quote ---

The resistors are all banded 1% metal film.  Even the logic pull up resistors are 1%.  The only engineering in this instrument is over engineering.  It is almost like the Scottish design team were trying to show the US HP office how good the Scottish office were, rather than building a commercially viable test instrument. 

The big HV resistors look ceramic types.

--- End quote ---
FTFY

Still, have you measured them ?
IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.

--- End quote ---

Knowing the age of this (late 1970s), there will be very few carbon comp resistors, by then they had swapped out these parts for film types, except for the few needed in special cases, such as high frequency areas.
The precision film resistors do go bad, this is a problem sometimes found with the 140/1 & 180 series scopes, they tend to go high in value in PSU regulator circuits and other areas, sometimes this is obvious due to discolouration, but not always. The focus/intensity pots can go noisy, or open circuit too. Sometimes the small plastic transistors can fail in strange intermittent ways too, had this with the CRT HT regulation circuitry in the last 180A I worked on.

Can you please add a picture of the board in question, as it's hidden by the cover in the previous pictures.

David

dazz1:
Hi
Here are photos of the HV section for the HP 3721A as requested.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod