Diode ID help please

I do have a scope, a Tektronix 2235 I bought to diagnose an old PET 2001 and a more modern C64. I don't think it can do the linked test though.

I've socketed the 4605, I suppose once I've checked everything else I could remove it and measure the supply voltage.

Back after a bit of work. I pulled the 4605 and powered up without it, I'm seeing 26VDC and about 0.045VAC across pins 6 and 4 (supply and ground). About 140VAC between pin 6 and the chassis, and about the same VAC between pin 4 and chassis.

I should put it on the scope but I think the VAC values are probably about right. But 26VDC seems a bit high to me. I'm going to let it discharge a little and go for a ride before checking out the remaining components you mentioned up near the transformer.

1084st bloke said:

Back after a bit of work. I pulled the 4605 and powered up without it, I'm seeing 26VDC and about 0.045VAC across pins 6 and 4 (supply and ground). About 140VAC between pin 6 and the chassis, and about the same VAC between pin 4 and chassis.

I should put it on the scope but I think the VAC values are probably about right. But 26VDC seems a bit high to me. I'm going to let it discharge a little and go for a ride before checking out the remaining components you mentioned up near the transformer.

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As I've already explained in post #20, the chip is powered from the transformer - so you can't do tests with parts removed. In the case of removing the chip you've no supply from the transformer to feed the chip, and it's high because it's startup resistor is fed from the main HT rail, relying on the current the chip takes to keep it low enough.

If you're only getting 25V then either C2 or D1 are breaking down due to over voltage, and you may have killed either or both.

Why are you attempting to measure DC voltages on AC ranges?, and why are you trying to measure primary side voltages referenced from the isolated secondary side?.

Sorry, but there wasn't a single useful test you did there.

I'm learning, part of learning is generally looking around to see what's happening in a circuit. I appreciate this mightn't be helpful to those trying to guide me through though.

The only anomaly I can see is that CP09 (an electrolytic which would be C2 on the diagram you provided) is reading on my ESR meter as 160uF and 0.36ohms (out of circuit), whereas the component's value is 220uF 16V. I understand electrolytics can have large tolerances so I'm unsure if this warrants replacement or not. DP04 (which would be D1 on your circuit) seems to be fine. RP04 (which would be R1 on your circuit) also measures well at 12k for a 12k value.

1084st bloke said:

I'm learning, part of learning is generally looking around to see what's happening in a circuit. I appreciate this mightn't be helpful to those trying to guide me through though.

The only anomaly I can see is that CP09 (an electrolytic which would be C2 on the diagram you provided) is reading on my ESR meter as 160uF and 0.36ohms (out of circuit), whereas the component's value is 220uF 16V. I understand electrolytics can have large tolerances so I'm unsure if this warrants replacement or not. DP04 (which would be D1 on your circuit) seems to be fine. RP04 (which would be R1 on your circuit) also measures well at 12k for a 12k value.

Click to expand...

160uF and 0.36ohms is probably out of tolerance, and while it may not be causing any issues at the moment it will only get worse, until it does cause problems. However, powering it up without the chip in place won't have done it any good, 26V across a 16V capacitor isn't a good idea, and presumably it was only the capacitor breaking down that prevented it going higher.

The normal symptom of those capacitors failing is the PSU pulsing and refusing to start - how it works (using your reference numbers) is that CP09 is charged through RP04, the voltage across this gradually rises until it reaches about 12V, the chip then bursts into life and provides drive for the output transistor. During this time the chip is powered solely by the charge in CP09, which obviously starts to fall. However, as the PSU has now started running CP09 is recharged via DP04, and the chip is then powered by DP04.

As the capacitor fails it can't store as much charge, or provide as much current - so it charges up via RP04, the chip starts running, but CP09 is discharged too much before DP04 can take over, so the chip shuts down, and the whole process starts again - resulting in the PSU pulsing ON and OFF.

ok thanks for that explanation, it's very helpful. It reminds me a little of a Commodore PET I had issues with (I had a lot of help on that due to the complexity of the video output circuits) where I eventually discovered a 555 timer chip wasn't letting the CPU start up, because of a failed capacitor on one leg (the capacitor would charge and the 555 timer would turn the CPU on a second or so after bootup).

I took the value of CP09 before I powered it up without the chip. I've just found a local electronics place in Trafford Park that sells components off the shelf so I think I'll wonder over there and see what they have. With any luck I can replace it today and I think I'll just power up to see if it goes pop again, or starts.

ok I got a bit further. Replaced CP09 and for good measure also replaced RP04 12k resistor next to it. Now powered up, the green LED on the monitor is pulsing. I shut it back down after a couple of seconds, before I could see if the tube was powered up, I didn't hear any static so I doubt it did.

Remove the line output transistor, and monitor the voltage on where it's collector connects (the LOTX), see if that stops the pulsing and see what the voltage is (probably something like 120V?). If that's OK?, then fit a 60W 240V incandescent bulb between chassis and where you just monitored the voltage - as a dummy load - and see what happens then. If you're on 120V mains use a 30W incandescent bulb.

No charge on the CRT. Thanks again for the advice, I'll do that this week. Is the bulb to simulate the normal load the PSU would expect to see?

1084st bloke said:

No charge on the CRT. Thanks again for the advice, I'll do that this week. Is the bulb to simulate the normal load the PSU would expect to see?

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Yes, try it without a load first - and if it doesn't pulse then, add the dummy load.

Tried it without the mosfet, no led activity at all. I can't see right now where in the circuit the LED is though.

1084st bloke said:

Tried it without the mosfet, no led activity at all. I can't see right now where in the circuit the LED is though.

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No one mentioned removing a MOSFET?

You need to remove the line output transistor, as a fault in the LOPT stage could make the PSU pulse..

Apologies, that makes sense, I misread what you wrote. I'll have to find it first, with work etc it may take me a few days.