Yet Another Color Classic with a doomed analog board

[ironborn65]

Hi folks,
before posting here seeking for help I read and checked the material I found here and elsewhere.

So; I got a CC that does not boot up.
The logic board is very nice and clear, but as usual I recapped it with tantalum caps.
The analog board seems just fine with no evident leaking but I recapped all the caps in the high and low power section as in the attached pic.
I removed and tested ok the optocouplers, they went back in with a socket.
I replaced both the TDA4605-2, I don't know how to test them. I tried the -3 as well. (I added a socket)
I replaced both the IRFBC40, they test OK though.
The fuse is ok.

Both boards inserted, grounded, keyboard, mouse, 3V battery, RAM, VRAM.
I power it up and press the start-button, no chime, no spinning HD, no fan.
No +V at the HD molex.

If I bench test the analog board with only the AV plug I do not measure any voltage in the fan contact nor in the connector as shown below.
My frustration level is very high, I'm considering collecting brooms, they always work.

I can measure 220V AC in the input of the bridge rectifier and 330 DC in the output, so it seems just fine at that point.
At the end of the transformer I got 0 V.
I do not have an oscilloscope but I wound't know how to use it in any case.

Anything I'm missing or doing wrong?

 

[ironborn65]

typo, "AV plug" was intended "AC plug"

 

[bibilit]

Is the Egret chip nice and clean ?

 

[ironborn65]

Hi mate, yes it is. There is no gunk on the board or along the EGRET pins. The board is one of those that have been used in a clean environment.
While bench testing the AB I was expecting a +5V to power the fan, as I read here in the forum someone saying it should spin even without the logic board.
The +5V is usually the always on standby circuit, at least with soft power in LCs, and there is not a relay just after the AC input. So the PSU should work without the LB, the soft power should "just" power the monitor and the LB.

 

[AwkwardPotato]

With the logic board removed, what voltages do you get on pins 3 and 6 of each TDA4605?

 

[ironborn65]

+5.6V on pin 3
+7.5 on pin 6

+4.6V on pin 3
+10.44 on pin 6

Does it tell something to you?

 

[AwkwardPotato]

Are the +5.6/7.5V from chip IP7? If so, I get the same 7.5V on my working analog board (I have 2V instead of 5.6V on pin 3, but that's due to the lower line voltage here in the US).

Could be that there's a problem with the feedback circuit on yours, or there's a short on one of the power supply outputs. First thing I'd do is check the solder joints on the transformers and diodes, specifically the one labelled "157-0144-A" and the big diode on a heatsink next to it. Then make sure that diode isn't shorted.

Does your analog board make any noises with the logic board/hard drive removed? On mine there's a faint buzzing noise with the monitor section of the board running, and the computer power supply section makes a ticking sound (expected when there's no board/drives connected).

 

[Cam]

I have two of these boards that don't work and since analog electronics is not my strongest skill I'd like to ask what might be a stupid question.

Is it safe to power up the AB without having a load on it?

 

[AwkwardPotato]

As far as I'm aware, it's safe to power up the analog board without any of the computer-side parts (LB, drives) loading it. You'll hear the protection circuit ticking without a load. That said, you should definitely keep the CRT anode cap and the yoke connected; if it comes to a point where you need to test the analog board out of the chassis, desolder and remove the horizontal output transistor first.

 

[Cam]

Thanks for the information.
I can't believe I have 2 boards exhibiting the same behaviour, one I recapped (which I need to review) and one from a machine I bought for parts.

This is definitely a discussion I'll be keeping an eye on for tips about this AB.

 

[ironborn65]

Are the +5.6/7.5V from chip IP7? If so, I get the same 7.5V on my working analog board (I have 2V instead of 5.6V on pin 3, but that's due to the lower line voltage here in the US).

Could be that there's a problem with the feedback circuit on yours, or there's a short on one of the power supply outputs. First thing I'd do is check the solder joints on the transformers and diodes, specifically the one labelled "157-0144-A" and the big diode on a heatsink next to it. Then make sure that diode isn't shorted.

Does your analog board make any noises with the logic board/hard drive removed? On mine there's a faint buzzing noise with the monitor section of the board running, and the computer power supply section makes a ticking sound (expected when there's no board/drives connected).

yes, +5.6/7.5V are from the IP7.
The solder joints are ok, in any case I reflowed them. The two diodes are ok.
The board does not make any sound at all, no buzz, no ticking sound.
I measured the optocoupler feedback input and I have +108V

 

[gobabushka]

I’ve been having a similar problem with mine. Every time I press the soft power button, I get a light blink and a click. Years later, I still haven’t figured it out. Also it makes for the most awesome looking paperweight.

 

[ironborn65]

I was wondering, what about injecting DC +5V +/-12V ?
I was asking this for a classic some times ago https://68kmla.org/bb/index.php?thr...n-external-psu-into-a-classic-ps-board.37479/
There was a guy who actually did it

Classic / Classic II ATX power conversion.

Hello, I am new here. Figured I'd jump in with something outrageous. Preamble: I've acquired a non working Classic and a Classic II with early (EAB) and late (LAB) analogue boards respectively. I have recapped both ABs and LBs. The LAB would blow the fuse immediately, eventually 'solved' by...

68kmla.org

Can this be done for a CC?

 

[AwkwardPotato]

In principle I think you should be able to do that with the CC too. Don't be angry with me if it blows up though

That is, unless the root problem isn't with the computer power supply on the analog board. I'm not sure whether a fault on the monitor-side B+ supply/deflection circuitry will cause the entire computer to shut down. Later today I'll get my analog board out to investigate.

 

[ironborn65]

While doing the reverse engineering of the AB, I decided to buy a refurbished working AB.
I got it for a reasonable price and .... IT ALL WORKS.
So the LB is just right.
A happy news is that the HD is WORKING as well!
System 7.5 is installed, 12MB
The CRT is not very crispy, I'll investigate how to improve the focus

It's slow as hell but ok, I will try an SCSI2SD/BlueSCSI

thanks

 

[techknight]

While doing the reverse engineering of the AB, I decided to buy a refurbished working AB.
I got it for a reasonable price and .... IT ALL WORKS.
So the LB is just right.
A happy news is that the HD is WORKING as well!
System 7.5 is installed, 12MB
The CRT is not very crispy, I'll investigate how to improve the focus

It's slow as hell but ok, I will try an SCSI2SD/BlueSCSI

thanks

Not crispy becuase you replaced the analog board. You gotta recalibrate it. Focus, G2, and H-STAT for convergence.

 

[Mu0n]

posting to compare notes later, I also have issues with my CCII AB.

 

[ironborn65]

Not crispy becuase you replaced the analog board. You gotta recalibrate it. Focus, G2, and H-STAT for convergence.

I did it, yes, it was quick and easy
Lovely Mac, now with a RJ45 network card.
Still slow with 10MB RAM

 

[ironborn65]

Pls ignore