Panasonic VP-7722A Repair attempt

[psykok]

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.

[psykok]

So I have replaced all the caps but nothing changed, the instrument is still not starting normally.


Any suggestions what to do next?

[Haenk]

corrosion might not even be visible - e.g. in the trimmer pots and switches; maybe even the buttons on the panel
certainly you need to check all the connections and clean them
corroded metal parts might be making contact to PCB traces
I'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...

[psykok]

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.

[thlee]

If all supply voltages are okay, you may check the CPU board.

[thlee]

You may visual inspection of the Ni-cd battery on the CPU board.

[psykok]

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.

[fzabkar]

One would expect that the EEPROM would be checksummed. Can you upload the dump?

[psykok]

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 specs



After 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

[psykok]

I finally managed to get the faulty unit working, I almost broke the working unit in the process


Everything 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

[Golds]

I can't provide technical assistance, but if you need parts of the equipment, it may be possible to help you.

[psykok]

I can't provide technical assistance, but if you need parts of the equipment, it may be possible to help you.

Thanks for the proposal.
 If the DAC I ordered are working correctly, I should not need anything else apart from a calibration
 

[USMC_Spike]

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 clean
a few of the front panels push button switch/LEDs?  I have a few that stick
and takes about 1/2 dozen or more times pressing and releasing to get
it 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?

[2016kat211]

Hello All
and
thanks 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 display
    tried 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