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Chuck,

I must be misunderstanding something? In your first post you wrote that all voltages produced by the LV transformer gradually drop after some minutes.

The 503 HV transformer is quite different than that of several others in the 500 series. T620 here is loaded by (almost) all other consumers and not just by the HV circuit itself. Moreover feedback is from -100 V, not from the HV voltage. I didn't think of this earlier, perhaps the term HV transformer is a bit strange. I now wonder what would happen when the CRT heater winding shorts to ground. Probably diode V692 would not survive this.

Albert

Chuck,

I had the same fault as in Joel his 515A in one of my 502As.
It seems you checked filament voltages delivered by T601 and concluded that these were good. But the CRT winding fault is not a short in the winding itself. The problem is that the winding leaks to ground (or whatever) and heavily loads the CRT cathode circuit. When easy to reach a check would be to disconnect both CRT heater leads from the transformer.

Albert

Better check the data sheet. On the 74S00 NAND, pins 1 & 2 are input, pin 3 output, on the 74S02 NOR, pins 2 & 3 are input, pin 1 output

I've pretty much verified that the LV supply (T601) is good. When the
T602 voltages drop the +500V output from T601 doubler
stays constant, as do the filament voltages.

ChuckA

On 5/2/2024 6:13 PM, Mike Dinolfo wrote:

The voltage breakdown of the CRT heater winding, which can indeed be
resolved by installation of a new 6.3 VAC filament transformer, is a not
uncommon failure mode for many 500-series scopes.? But it applies to the
60 hz power transformer (T601 in the 503), rather than the ~25 khz
transformer (T620 in the 503).? The original poster (ChuckA) indicated
the failure was in T620- ????? Several years ago, I myself fixed a 504
(similar power supply) which had the same power supply, and the same
symptoms that ChuckA described (i.e, erratic voltages derived from
T620).

I prepared a writeup of the process that I went through to identify the
problem area (T601 vs. T620) and posted the results to the Tekscopes
site, but I don't recall where I put it on the server.

Mike D? N4MWP

The voltage breakdown of the CRT heater winding, which can indeed be
resolved by installation of a new 6.3 VAC filament transformer, is a not
uncommon failure mode for many 500-series scopes.? But it applies to the
60 hz power transformer (T601 in the 503), rather than the ~25 khz
transformer (T620 in the 503).? The original poster (ChuckA) indicated
the failure was in T620- ????? Several years ago, I myself fixed a 504
(similar power supply) which had the same power supply, and the same
symptoms that ChuckA described (i.e, erratic voltages derived from T620).

I prepared a writeup of the process that I went through to identify the
problem area (T601 vs. T620) and posted the results to the Tekscopes
site, but I don't recall where I put it on the server.

Mike D? N4MWP

I have a 495 spec analyzer with a phase lock failure. Theres a post on this cite in spanish that was a fix. Is there a translator on this cite? I havent explored other apps yet.

Better check the data sheet. On the 74S00 NAND, pins 1 & 2 are input, pin 3 output, on the 74S02 NOR, pins 2 & 3 are input, pin 1 output




John KK6IL@...

On 5/2/2024 2:43 AM, Tom Lee wrote:

Hi Glenn,

As to U241, the internal gates are just used as inverters by tying the inputs together, so NAND or NOR makes no difference here. Each is a drop-in replacement for the other in this case; no PCB layout changes required. I can't think of a good electrical reason to prefer one part to the other in mid-production -- they differ negligibly in prop delay, for example. Perhaps it was just what they could get at the time, or get at a good price, perhaps.

-- Tom

Hi Glenn,

As to U241, the internal gates are just used as inverters by tying the inputs together, so NAND or NOR makes no difference here. Each is a drop-in replacement for the other in this case; no PCB layout changes required. I can't think of a good electrical reason to prefer one part to the other in mid-production -- they differ negligibly in prop delay, for example. Perhaps it was just what they could get at the time, or get at a good price, perhaps.

-- Tom

My memory isn’t good on this, but 30 years ago I had to wire in a separate filament transformer for the CRT. I don’t recall the symptoms that led me to this fix. I used a junk box filament transformer which as I recall has the high voltage on it.

On May 2, 2024, at 2:37 PM, ChuckA via groups.io <chuck@...> wrote:

I picked up a Tek 503 scope recently and decided to get it running. Found only one bad filter cap. But it appears the
HV transformer T620 (pn# 120-199) is bad, all the voltages out of it gradually drop after running for a couple of minutes.
The +500v supply voltage is solid, I changed the 6DQ6 oscillator and the 6BL8 control tube but no change.

My memory isn’t good on this, but 30 years ago I had to wire in a separate filament transformer for the CRT. I don’t recall the symptoms that led me to this fix. I used a junk box filament transformer which as I recall has the high voltage on it.

On May 2, 2024, at 2:37 PM, ChuckA via groups.io <chuck@...> wrote:

I picked up a Tek 503 scope recently and decided to get it running. Found only one bad filter cap. But it appears the
HV transformer T620 (pn# 120-199) is bad, all the voltages out of it gradually drop after running for a couple of minutes.
The +500v supply voltage is solid, I changed the 6DQ6 oscillator and the 6BL8 control tube but no change.

On May 2, 2024, at 2:37 PM, ChuckA via groups.io <chuck@...> wrote:

I picked up a Tek 503 scope recently and decided to get it running. Found only one bad filter cap. But it appears the
HV transformer T620 (pn# 120-199) is bad, all the voltages out of it gradually drop after running for a couple of minutes.
The +500v supply voltage is solid, I changed the 6DQ6 oscillator and the 6BL8 control tube but no change.