465 Scope Question


Bill
 

Started out with no -8vdc. Replaced the bridge rectifier now I have -6.5vdc. The power transistor Q1566 checks out as OK. Lifter C1569 but that didn't help. The voltages on Q1566 are ok except the Collector voltage is a little over =3vdc when it should be +5vdc. I'm going on the assumption that something downstream is dragging down the -8vdc but is not a dead short.

Sound reasonable?

All the other Low Voltage power supplies are dead on in voltage.
Bill


Mark Vincent
 

Bill,

Check the ac voltage across C1562. It sounds like your filter condenser is bad. If the cap is bad, replace it with 3300-4700mfd 25V UHE type. These are the best I have found for ESR and life. Filter condensers in these and 7603s seen to give problems and replacement is necessary.

Also check VR1522. I have never seen one of the 9V zeners be good. Yours is likely open. I know it may not make a difference in the output voltage. I change these in my units with BZX85B9V1.

Mark


Dave Hills
 

Bill,
C1562 most likely has failed. If it has you should see a large AC ripple voltage across it.
VR1522 is functional. If it were not, ALL the regulated voltages would be off, as they all
are referenced to it.

Dave


Started out with no -8vdc. Replaced the bridge rectifier now I have -6.5vdc.
The power transistor Q1566 checks out as OK. Lifter C1569 but that didn't
help. The voltages on Q1566 are ok except the Collector voltage is a little
over =3vdc when it should be +5vdc. I'm going on the assumption that
something downstream is dragging down the -8vdc but is not a dead short.

Sound reasonable?

All the other Low Voltage power supplies are dead on in voltage.
Bill


Bill
 

I assumed as much since all the other voltages in the low voltage power supply are spot on. While in there I will replace all of them. Was on my list anyway.

This scope is in really nice cosmetic and physical condition so I decided to get it going again even though I don't need it. This old equipment deserves our respect in my opinion. I will also learn quite a bit along the way!!

Thanks for the reply!!

Bill


Bill
 

The input AC voltage at the bridge rectifier CR-1561 is 12.3vac. The AC voltage on the DC output of CR1561 is 4.5vac.

Thank you,
Bill


Dave Hills
 

C1562 is not doing its job, the AC voltage should be much less, I would expect
less than a volt.
You can calculate the DC load current from the measured DC voltage across R1569.

Dave


The input AC voltage at the bridge rectifier CR-1561 is 12.3vac. The AC
voltage on the DC output of CR1561 is 4.5vac.

Thank you,
Bill


Bill
 

After removing C1562 I tested it:
ESR-9 to 17 ohms, VLoss 1.9%
I tested it several times in my M644 component tester to ensure I had an average to improve the results. Not the best way to test an electrolytic capacitor but will suffice in this case.

Now I know why the -8vdc was low and had AC on it.

Bill