Panasonic VP-7722A Repair attempt - Page 1
Forum section: electronics / repair

author: psykok
author_rank: Frequent Contributor
timestamp: August 05, 2020, 09:29:55 pm
content: Hi,Today I got the faulty Panasonic VP 7722A I purchased last week.When tuning on the instrument it get stuck in some booting initialisation process, mostly all the led of the front panel are on and no key is responding.By opening the case I found some good trace of corrosion, looks like this instrument has not been store in a dry place.

author: psykok
author_rank: Frequent Contributor
timestamp: August 18, 2020, 10:48:35 am
content: So I have replaced all the caps but nothing changed, the instrument is still not starting normally.Any suggestions what to do next?

author: Haenk
author_rank: Super Contributor
timestamp: August 18, 2020, 11:58:42 am
content: corrosion might not even be visible - e.g. in the trimmer pots and switches; maybe even the buttons on the panelcertainly you need to check all the connections and clean themcorroded metal parts might be making contact to PCB tracesI'd give the PCBs a good cleaning (IPA) and check for those rusty parts.Check supply voltages.*Then* you don't know, if it even worked when it was stored away a decade ago, a defect might have been the reason for being decommisioned. That would raise the bar quite a bit, as you will need the service manual...

author: psykok
author_rank: Frequent Contributor
timestamp: August 18, 2020, 12:15:30 pm
content: I cleaned all the PCBs with IPA and looked for rusty parts.Only two relays were really in bad condition, I will replace them with new ones (still waiting for the part)Power supply rails are all ok according to the service manual.I may have to check all the switches to check for bad ones, visually I have not seen any corroded one.

author: thlee
author_rank: Contributor
timestamp: August 18, 2020, 12:41:58 pm
content: If all supply voltages are okay, you may check the CPU board.

author: thlee
author_rank: Contributor
timestamp: August 18, 2020, 12:50:06 pm
content: You may visual inspection of the Ni-cd battery on the CPU board.

author: psykok
author_rank: Frequent Contributor
timestamp: August 18, 2020, 01:00:07 pm
content: I visually checked the CPU board and haven't notice something.The backup battery has been changed, the old one was completely out.I also checked all the eeprom by reading them multiple time and comparing the output.This procedure was successful but that dosen't means that the content is ok.

author: fzabkar
author_rank: Super Contributor
timestamp: August 21, 2020, 02:46:21 am
content: One would expect that the EEPROM would be checksummed. Can you upload the dump?

author: psykok
author_rank: Frequent Contributor
timestamp: September 21, 2020, 07:40:25 am
content: After searching and trying different stuff, it appears more and more obvious that this repair could not be successful without having a second working unit.By chance I found another working VP 7722A on ebay for a reasonable price because the unit was out of specsAfter a session of card swapping, I was able to identify the faulty boards :- A14 Master : was preventing the device from booting- A13 Interface : some buttons and led were not responding- A11 A/D Converter : not all the measurement were performed correctly

author: psykok
author_rank: Frequent Contributor
timestamp: October 05, 2020, 02:10:07 pm
content: I finally managed to get the faulty unit working, I almost broke the working unit in the processEverything is working, I just need to go through the adjustment process and to check if everything is ok according to the specifications.At the end I had to replace couple of 74hc and the special DAC unit from Mitsubishi MA6197.By chance I was able to source this chip from utsource.com from a decent price.From the 2 units I bought only one was working correctly but I got refunded for the faulty one.Unfortunately due to a short during the repair process, I damaged the DAC from the working instrument.I have ordered again 2 DACs and now waiting for delivery

author: Golds
author_rank: Contributor
timestamp: October 11, 2020, 04:05:36 pm
content: I can't provide technical assistance, but if you need parts of the equipment, it may be possible to help you.

author: psykok
author_rank: Frequent Contributor
timestamp: October 14, 2020, 07:24:56 am
content: Quote from: Golds on October 11, 2020, 04:05:36 pmThanks for the proposal.If the DAC I ordered are working correctly, I should not need anything else apart from a calibration

author: USMC_Spike
author_rank: Regular Contributor
timestamp: November 28, 2022, 12:55:57 am
content: Hi, I see you got your opened and working on it.Now you've got two. I assume you got your repaired one working?Question, I've have one also and wondering what is a good method to cleana few of the front panels push button switch/LEDs? I have a few that stickand takes about 1/2 dozen or more times pressing and releasing to getit to work.I've not opened mine up to look yet to see if there is easy access to the front panel switches.That said, I was thinking of the following:1. Flood the area with ISO, and then compressed air?2. DeOxit D 100 - D5, then compressed air.3. DeOxit shot of the F5 - Fader lube.Any thoughts or other ideas?

author: 2016kat211
author_rank: Newbie
timestamp: July 20, 2023, 08:35:23 am
content: Hello Allandthanks Psykok for sharing great repair works on VP-7722A,I got a working VP-7722A, after clean up and seems no need for recap,Tried to controlled through Labview, I might lost my mind .. here,I put the meter on a fordable plastic chair, it's small than meter,and I power on, then the meter behaviors are different than before.no sparks no strange sounding,power off, and remove from chair, sitting on the floor , then re-power on,strange items are,1. the frequency and THD is hard to LOCK and stable, keeps blinking, especially THD, sometime it lost in somewhere, blank THD displaytried Manual setting then return to AUTO, most time it reads THD correctly, but still blinking.(the reading is same as before, no changed)AUTO range related circuit defective?2. S/N ratio measurement, without SLOW response selected, it jumping around 30-40db, and relay ticking loudly,once "Slow" response activated, it reads correctly,3. Left/right channel has same situation, show no difference.(not caused by L-R pre-amp boards, also implied not 1st/2nd notch filters boards?)Tried to figured out where to start,the PDF service manual I downloaded , lost one important map, Figure 3-9 digital block diagram,Could anyone advise where to check?and/or share the Figure 3-9, digital block diagram?thank you very much!CS