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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.

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.

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'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

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

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

Before you drill mounting holes, experiment with orientation in case the field gets into the CRT or other circuitry.

Dave Wise
________________________________
From: [email protected] <[email protected]> on behalf of ChuckA <chuck@...>
Sent: Friday, May 3, 2024 9:30 PM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] 503 HV Transformer


To finish this thread the problem ended up being leakage from the CRT
filament winding in the LV transformer.
Seems I didn't "hear" Alberts description of the problem until I got an
email from Ferrous Steinka and the light went on.

I removed the CRT leads from the LV transformer and all the voltages
from the HV transformer came on and stayed on.
I connected a spare transformer I had to the CRT leads and now have an
excellent display on the CRT.

I was able to find a NOS 6.3V transformer with 3KV isolation for $5
that should fit on the bottom of the scope cabinet.
And another TEK scope joins the "herd"

Thanks to everyone who shared their ideas on fixing the problem

ChuckA

--
See Early TV at:

www.myvintagetv.com<>

"Other circuitry" is a risk too. The field can induce voltages in nearby wiring, so you want to stay away from the amplifier front-ends. I had trouble with this when I was modifying a 500-series Type W plugin to use 6DJ8's instead of 8416's.

Dave Wise
________________________________
From: [email protected] <[email protected]> on behalf of ChuckA <chuck@...>
Sent: Saturday, May 4, 2024 8:28 AM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] 503 HV Transformer

Yep,

Gonna mount it on the opposite side of the CRT, full metal plate between
them.

Have a oversize temp transformer there now with no issues.

ChuckA

On 5/4/2024 11:22 AM, Dave Wise wrote:

Before you drill mounting holes, experiment with orientation in case the field gets into the CRT or other circuitry.

Dave Wise
________________________________
From: [email protected] <[email protected]> on behalf of ChuckA <chuck@...>
Sent: Friday, May 3, 2024 9:30 PM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] 503 HV Transformer


To finish this thread the problem ended up being leakage from the CRT
filament winding in the LV transformer.
Seems I didn't "hear" Alberts description of the problem until I got an
email from Ferrous Steinka and the light went on.

I removed the CRT leads from the LV transformer and all the voltages
from the HV transformer came on and stayed on.
I connected a spare transformer I had to the CRT leads and now have an
excellent display on the CRT.

I was able to find a NOS 6.3V transformer with 3KV isolation for $5
that should fit on the bottom of the scope cabinet.
And another TEK scope joins the "herd"

Thanks to everyone who shared their ideas on fixing the problem

ChuckA

--
See Early TV at:

www.myvintagetv.com<>

--
See Early TV at:

www.myvintagetv.com<>