Intermittent display on Sherwood Newcastle R-965 Receiver

[BigPete]

Hi all,

I'm working on a Sherwood R-965 at the moment. It came in with relays clicking constantly and also going into protect mode.
I've re-soldered a lot of dry joints and replaced quite a few capacitors and now it works, but I still have one problem I haven't solved and would like some opinions.
The vacuum display is quite intermittent. Sometimes it won't turn on, sometimes it will turn on but the characters, numbers and letters are frozen on, regardless of which buttons are pushed. And sometimes it works flawlessly. If the display doesn't turn on initially, restarting it can get it to work, sometimes not. Restarting it a few times eventually gets it to the point where the display is on and the display works as it should. Resetting it or reloading the firmware does nothing.
When the display isn't working correctly, button pressing still works and settings change.
When the unit is restarted, the restarted display knows which key presses were done when the display was frozen and displays the correct settings.
I have attached the schematic of the main board and front board.
I'm at the point where I may just order in IC301 - LC75721E - The display driver. I have replaced IC201 - MB90F482 - CPU Microprocessor with no change.
Before I replace IC301 I'm hoping someone may be able to go over the schematics or this information and confirm if replacing the driver is the right thing to do.
I've checked the electrolytic capacitors and they test fine. I haven't tested the ceramic capacitors yet though.
I've spent too much time on this already as re-soldering the dry joints on the two amplifier banks was an absolute PITA, so I'm hoping someone can point me in the right direction. Thanks.

[BigPete]

Replacing IC301 - LC75721E didn't work. I'm now going to replace IC205, the Logic Gate. I don't believe there's anything wrong with the voltages on the Front PCB. My theory now is that the CE (chip enable) pin that goes through IC205 may not be switching on IC301, due to IC205 being faulty. As the Data and Clock also go through IC205, it may be the reason the display is freezing when it is on. I may replace IC199 and IC206 as well at the same time. Only a guess though.

[redwire]

A bit confusing MCU power from IC102 (BAO33T) is 3.3+0.7V for a +3V9 rail. So the MCU runs off 3.3V
But the display is +5V6 from IC101 minus a diode drop.
The bus buffers IC205, IC206 run off +5V from IC103.

It's three voltage regulators, what a mess. I would double check the voltages are in spec for those.

You should be seeing 0-5V swing on the RST, CE, DATA, CLK to the VFD display controller IC. Do you have a scope to check that?
I see bus driver 'HC244 IC205 used to buffer the OBD/75721-DATA from MCU pin 30, and OBD/75721-CLK from MCU pin 29 I guess for the high speed and level-translate from 3.3 to 5V.
75721-RST, 75721-CE go direct from the MCU but has 5V pullup resistors for some special pins from D131.
I guess I would expect 5V swing to the VFD driver IC.

[BigPete]

Hi,
Thanks for the reply.
I do have a scope, but it's packed away as we're moving in a few weeks.
RST does have 5 volts.
CE has 0V
CLK sometimes has 100mv otherwise nothing
DATA has 5 volts sometimes.
I'll double check though, I've been testing those resistors on the Front PCB trying to work out a pattern.
The pins on the Main IC 201 are a bit hard to reach.
It may not be the CE pin as the display works regardless of what the voltage on the resistor is.

When I turn it on after a long period of it being off the display won't turn on, but if I leave it for a few minutes then do a power cycle the display turns on. The display will be frozen, then after a few more minutes starts working.

[BigPete]

To add information to my above reply.
The voltages are quite messed up.
RST is 5V regardless of whether the display is working or not.
CE is 0v whether the display is working or not.
CLK has 5v when pure audio is pushed, when pushed again 4.6, then when the display is actually working 100% it's just mv's rising quickly to 100's mv's with key presses.
DATA can be 5V then 0v when pure audio is pressed.
When the display is working 100% it's voltages are like CLK.
Before the display is 100% CLK and DATA can be 2.8v, 5v, back to 600mv then 300mv just based on pressing the pure audio button.
Pushing pure audio can freeze the display between button pushes as well, but the longer the unit is on the less the problems occur untill it works 100%.
After 30 minutes of the unit being on now it works 100%.
I'll check the regulators when I get back from work tonight.

[BigPete]

IC101 = 11V in - 5.6V out
IC102 = 5.6V in - 3.96V out
IC103 = 5V in - 3.3V out

[BigPete]

As the amplifier display starts working as the amplifier heats up I decided to apply heat to the area IC205 is before turning it on. Pre heating it turned the display on first crack. It's not to say it is IC205, but something in that area at least. I've ordered replacement MC74HC244ADW chips and will swap them out and report back.

[petehall347]

could be dry joints or a capacitor . try cooling the ic after heating the area .

[BigPete]

Just a follow up on this.
I replaced IC205 with a new 74HCT244PW chip.
I also replaced IC205 and IC199 as they were the same chip and I bought a pack of 5 so thought what the hell.
I put it back together and the display fired right up.
I left it overnight, turned it on the following morning and the display fired right up again, it's been going fine ever since.
In the mean time I replaced virtually all the capacitors on the DSP board.
Aside from the 74HCT244PW chips that were replaced, and aside from the parts that were unnecessarily replaced, I also replaced a 5W 2x 0.27ohm resistor and numerous capacitors.
I have permanently disconnected the FM module as it pulls down the 12V rail and puts the amplifier into standby mode. I tried to investigate why. I replaced some capacitors that had high ESRs but 2 chips were still pulling the power down. I wasn't going to listen to FM so pulled the pin on that.
The other problem this amp had was a significant amount of dry joints. I don't know how it became so bad but it was.
If anyone gets into this position, save yourself some time and just resolder every joint from the start.

I've currently got it setup with one of the 240V accessory outlets on the back powering a Bluetooth adapter so there's only one button to push and it runs mint.