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Hi Mark & Morris:

Update - I isolated pin 3 of the crt by unsoldering the wire at the junction of R830/C829 & C830 and measured the voltage on the wire. It was around -1070V even though it was not connected to anything. Maybe this is expected, I don't know. Unfortunately, I don't know of anyone with a crt checker.

Also checked for continuity between pin 3 and the cathode at the junction of R857 and C858 - open circuit. Then checked for continuity between pin 3 and the wiper of focus pot R846 - also open circuit. It doesn't seem to be an internal short in the crt, but it's still strange that the grid at pin 3 is at -1070V when not connected to anything.

I think this is now beyond my capabilities, so I may have to consider parting it out :(

Gordon

On 5/10/25 00:36, Tom Lee via groups.io wrote:

sometimes the attenuators will burn out first to protect the fuse. ?

maybe that's why these parts were kept in an envelope in a scope-mobile drawer with a 7904A on it!
The 7A19 must have been nearby at one time too...

Hi John,

The 7A29 has no input fuses (thankfully), but the 7A19 does (along with two spares). The 7A19's fuses and attenuator modules share the same form factor. so that they can be inserted into the signal path with the same minimal perturbation of microstrip line impedance. The attenuators don't have fuses (intentionally) integrated with them, but sometimes the attenuators will burn out first to protect the fuse. :(

There are three attenuator modules in the 7A19, with 10x, 5x, and 2.5x attenuation factors (gain factors are then 0.1, 0.2 and 0.4, which probably explains the "0.2" seen on the pic of the modules you linked to in your OP).

-- Cheers
Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070

On 5/9/25 12:19, Adam R. Maxwell via groups.io wrote:

John, those look like 7A19 fuse or attenuator elements. See photos on Tekwiki, or I can take one apart when I get home from work.

Looks similar, except some different circuitry on the flat side. The box it was in was labeled as fuses.
Is the fus *IN* the attenuator? Are there five or ten kinds?

I can confirm that the original TM502A fuse drawer fits the replacement power entry module (Schurter p/n 4303.1093)

Jason,

I wonder if the caps inside are going bad even though it appears to work. The oscillation could also be decoupling is too low or not at all. If the original electrolytics are still there, replacing would be best. Condor Audio has a kit. The owner owner is on this group. What Siggi said sounds good. That could be your problem. Dirty contacts will cause weird problems.

Mark

On Thu, May 8, 2025 at 8:59?PM Jason B via groups.io <sydbowen=
[email protected]> wrote:

I've tried DC balance. I was mistaken on the channel 2 waveform on GND.
It's on AC. I have it set right now at 2 mVAC and 2 nS on channel 2 with no
probes connected and readout on frequency is 850 MHz with a perfect sine
wave at 3.55 mV. Lol, I think I acquired a basket case maybe?

Interesting.
Below 50mV/DIV the preamp amplifies the incoming signal, so it's perhaps
within reason that it's amplifying something at the BNC. I guess it might
also be oscillating.

Have you measured the DC resistance of your inputs? It should be as near to
1MOhm as makes no difference in DC coupling and infinite in AC coupling.

What happens:
- In 50Ohm coupling?
- In DC coupling?
- When you connect a probe?
- When you connect coaxes of different lengths?
- At other attenuation settings?

The CH1/CH2 preamp hybrids should be interchangeable (though the
calibration is preamp specific, so keep track of which is which), so you
might see whether this oddness follows the preamp.
It's also possible that the hybrid contacts are oxidized, leading to
weirdness, so before you swap them, try just re-seating them and/or
cleaning their contacts.

On Fri, May 9, 2025 at 09:48 AM, John Griessen wrote:

John, those look like 7A19 fuse or attenuator elements. See photos on Tekwiki, or I can take one apart when I get home from work.



thanks,
Adam

I have a few of NFM's Peak to Peak (067-0625-00 clone) PCBs if anyone would like one for free. I only needed one for myself to play around with.



msg me privately and we'll sort it out.

Radu, you are always exceedingly polite!
I’m surprised you had difficulty working on a Wavetek generator. I repaired my model 166 function generator about 8 years ago, and I don’t remember it being particularly difficult or inconvenient.

Unfortunately it’s back to its old trick of the square and pulse outputs flatlining after a few minutes of operation. Last time, it turned out to be a flaky connector between the power supply board and the main board. I bent the offending pin so that it made a positive connection. I will check this weekend what’s ailing it now.

Good luck and have a great weekend everyone!

Jim Ford
Laguna Hills, California, USA

On Fri, May 9, 2025 at 06:29 AM, Roy Thistle wrote:

It could be.
But welding machines get hot. And so, that drives out VOCs that wouldn't
normally be driven out in such concentrations, at room temperature.

Roy Thistle

The smell in the welders was evident on brand new units and replacement PCB's
Over time it went away.

Other brands from Europe (Kemppi, ESAB, etc), Australia (Cigweld) and USA (Lincoln) didn't have the same strong smell.


Jared

I have the same experience with some Waveteks - a sweep/signal generator in
my case - and working on it has not been fun. Furthermore, I think the
design was fundamentally less than stellar (to be exceedingly polite), but
even as much as trying to repair it was underwhelming and
sometimes frustrating.

I've been avoiding similar units since, though Wavetek has some very good
metrology units that may change my mind if I try to fix them.

On Thu, May 8, 2025 at 8:46?PM naikm via groups.io <naikm=
[email protected]> wrote:

Today, was trying to fix a Wiltron 610D sweep generator with no output.
Opening it up revealed the very difficult path to just accessing any of
their boards. No slack in connecting wires, no way to access components,
etc. Maybe, I'm used to the cool parking lot layout of Tektronix. Anyone
have any experience on Wiltron? I won't dare try to fix another one again.

Today, was trying to fix a Wiltron 610D sweep generator with no output. Opening it up revealed the very difficult path to just accessing any of their boards. No slack in connecting wires, no way to access components, etc. Maybe, I'm used to the cool parking lot layout of Tektronix. Anyone have any experience on Wiltron? I won't dare try to fix another one again.

Yes, my address is good in QRZ. I'll certainly pass it on if someone needs it before I do (and I hope I never do but...).

Thanks!
Barry - N4BUQ

How about I send it your way, with the promise that you'll pass it along if someone needs it before you do? Is your address good in QRZ? If so, I'll pop it in the mail.

73,

--- Zach

I've tried DC balance. I was mistaken on the channel 2 waveform on GND. It's on AC. I have it set right now at 2 mVAC and 2 nS on channel 2 with no probes connected and readout on frequency is 850 MHz with a perfect sine wave at 3.55 mV. Lol, I think I acquired a basket case maybe?

Gordon,

If you have a crt checker, you may be able to test the crt for leakage and 'blow' the short by the rejuvenate part. If not, you need to find another crt. I would like to find a P11 for my 545B/547 to replace the P31.

Mark

Thanks Morris.

I must admit the thought of an internal crt fault crossed my mind. I will re-check it to be sure I wasn't seeing things.

I will also check the crt pins and plug as you suggest.

Gordon

I'm sorry to have break some bad news but it looks like you may have a grid-cathode short, or at least leakage, in the CRT. Either way there should not be HV on the grid if you have isolated it by lifting R830. If you're lucky the short will be in the wiring rather than in the tube. You can try unplugging the CRT socket (with the power off!!) and measuring the resistance between CRT pins 3 (grid) and 2 (cathode). It should be an open circuit. If it is, do the same for the socket. That should also be an open circuit if R830 has ben lifted and C829 and 830 are good.

Lifting the grid supply is generally not a good idea because if there is any DC leakage path to ground from the grid, the full HV will be impressed between grid and cathode which would not be good for the CRT, to put it mildly.

If there's a grid-cathode short in the CRT then it's toast.

Morris