HP 3335A help pls - Page 1
Forum section: electronics / repair

author: e_pigreco
author_rank: Newbie
timestamp: May 19, 2024, 12:36:19 pm
content: Hi All,I have a HP3335A that recently has started to give strange erratic problems (including missing display and unlocked msg) . All voltages are correct in the test points on the board, except the 10mhz reference board. PSUs works perfectly. I have +15v -15v -5.29v and +5v spot on. And the unregulated as expect.Seems not cap depending (board has been already recapped recently) , it seems more cable dependent.For example sometimes the display output disappears (1.000.000hz) and if I tap with the screwdriver on the 10mhz connector on the main board, it comes back.(well, until ytd, today not anymore even tapping)If I leave the unit off, and restart after one day, again the display seems disappeared. Tracing back voltages, I do not find any of the +20V and +15V expected on the A17 board (the 10mhz temperature Stabilized oscillator board). This seems to explain the unlocked msg. Notice that when I was able to establish back the display, the voltages were still missing on the Temp. Ref. Board. So I would say the unlocked msg might be related to the A17/A10 board, but the display problem is bit of erratic. The fact that sometimes comes back make me think is not ROM memory dependant.I will try to add an external reference now and see if I can at least check the behaviour with an ext reference.The A10 board controller board that should provide the 20V+ unregulated to the 10mhz reference has only few IC (I think 2) but it's bit difficult to workHas anyone got similar problems ?Thanks for any help in advance.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 19, 2024, 01:44:39 pm
content: Disconnect or remove the Oven board and see if your display and the voltages that go to the oven board come back.

author: e_pigreco
author_rank: Newbie
timestamp: May 19, 2024, 05:19:46 pm
content: I tried but nothing.Tnx for answering

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 19, 2024, 11:24:13 pm
content: If you do not have voltages to the Osc board, even after disconnecting heater and oscillator, I would track down where the missing volts come from, If you are missing a voltage, it might be a supply to other things like the display.Also you said work was done on the caps, double and triple check that. You also said the problem was intermittent at one point, again look at the recent work.I do not know about this unit but some HP units had over amp circuits that would shut some things down if there was a short. A common cause of this was leaky caps on some boards. Not necessarily on the PS unit.Anyway It is a place to start.

author: e_pigreco
author_rank: Newbie
timestamp: May 20, 2024, 09:26:44 pm
content: Hi,I sorted out the Display board problem now.The problem was simply matter of removing each rom and CPU and scratch the legs to remove any old patina and clean with isopropyl also the pin holes.So now I have all display and keyboard working as it should.Voltages are all correct on all test points, except the 20V and 15+ points on the reference oven board.I start to get output on some ranges... like 14-19 etc... but with wrong frequency...mhz, 28-39 get 44.4Mhz like in a video I have seen on the youtube. But in that video they don't explain how the same problem was sorted.Anyway I am now quite happy to have at least a display and kb working.I wrote to the youtube guy, not sure if he will ever anwser, hope so.

author: DeepLink
author_rank: Regular Contributor
timestamp: May 22, 2024, 07:33:16 am
content: The +15V on the reference board are derived locally from the +20V, and not related to the +15V in the power supplyOn the 3335A the reference oscillator is powered when mains is attached, regardless of the power switchMeasure the +20V on the reference board, both DC value and AC value (look for ripple)Do this measurement with the 3335A in standby (turned off) and operating (turned on)Guess you already measured this, but didn't mention the result

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 23, 2024, 07:09:11 pm
content: Did you try an external ref now that the unit is responding?