Winters
Active Member
As I mentioned in my recent thread about choosing the best location for the thermistor protection, I had decided it's high time to dig deeper in my 500B I purchased seven years ago, do a proper recap, add the IBAM and do several minor modifications, for instance modify the 'Tape Head' input to act a second 'Phono' input. I have never had a problem with the unit other than one 7591 tube that gave up the ghost several years ago. I did some basic maintenance on my 500B when I purchased it - silicon rectifier, .1uF coupling caps, screen resistors, cathode to ground resistors etc. It always sounded rather fine, but lately it seemed a bit under the dog - lost some power, some very faint squealing appeared etc.
I decided to tuck the new replacement caps underneath and I purchased Nichicon UPW low ESR electrolytics which are supposedly good for the job.
I started with some minor work - replaced the "death cap", started with the 'tape head' input conversion etc. If the latter works out fine, I'll make a "how to" post.
So I started to do some planning on the recap and noticed that there is something wrong in the power supply area...
First thing that caught my attention is that the R125 (22K) resistor looks overheated.
And then I discovered that someone had been there before me and replaced R131 (1.2K 1W) with a 12 ohm resistor!
Also, R133 seems to be missing!
In this photo you see the R131 resistor which looks way newer than the other components and is a 12 ohm resistor (1.2k on the circuit diagram). It connects C84A to C78C. I have deliberately cut one end off to measure it with an ohmmeter.
And where is R133? It's supposed to go between C88 and C84 whereas there are only two things connected to the positive side of C88: CR3 and an orange wire that goes to the B point on the schematic (395V)
I think I'm going to bring it back to original (as shown on the schematic). But why would someone introduce these changes? Is this indicative of some problem?
I decided to tuck the new replacement caps underneath and I purchased Nichicon UPW low ESR electrolytics which are supposedly good for the job.
I started with some minor work - replaced the "death cap", started with the 'tape head' input conversion etc. If the latter works out fine, I'll make a "how to" post.
So I started to do some planning on the recap and noticed that there is something wrong in the power supply area...
First thing that caught my attention is that the R125 (22K) resistor looks overheated.
And then I discovered that someone had been there before me and replaced R131 (1.2K 1W) with a 12 ohm resistor!
Also, R133 seems to be missing!
In this photo you see the R131 resistor which looks way newer than the other components and is a 12 ohm resistor (1.2k on the circuit diagram). It connects C84A to C78C. I have deliberately cut one end off to measure it with an ohmmeter.
And where is R133? It's supposed to go between C88 and C84 whereas there are only two things connected to the positive side of C88: CR3 and an orange wire that goes to the B point on the schematic (395V)
I think I'm going to bring it back to original (as shown on the schematic). But why would someone introduce these changes? Is this indicative of some problem?
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