Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Sun 08, 2019 5:11 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 40 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Nov Mon 18, 2019 9:32 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
That is good to hear! I was just thinking that if I never took that vtvm apart, I would never have seen that bad resistor. Glad I made the effort to go through this one thoroughly. Thanks for the reply bob91343.

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Nov Fri 22, 2019 5:26 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
I have a couple of carbon comp 1/2 watt resistors that are out of spec. Can I replace these with carbon film, 1 watt resistors rated 500 volts? Reading through some other posts mentioned voltages affecting parts choices. I don't have a lot of vtvm repair experience, and want to avoid a problem.

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Nov Fri 22, 2019 6:08 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4235
Location: Littleton, MA
Frank wrote:
I have a couple of carbon comp 1/2 watt resistors that are out of spec. Can I replace these with carbon film, 1 watt resistors rated 500 volts? Reading through some other posts mentioned voltages affecting parts choices. I don't have a lot of vtvm repair experience, and want to avoid a problem.

Yes, you can use the carbon films to replace the 1/2 watt carbon composition resistors. They were rated for 350 volts, and you carbon films are rated higher, so no problem.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Last edited by stevebyan on Nov Fri 22, 2019 7:11 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Nov Fri 22, 2019 6:58 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
Thanks Steve. I'm planning to work on this over the weekend, and feel more comfortable about replacing those carbon comps now.

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Mon 02, 2019 6:43 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
Update on this and a question about the 'zero' function.

As of this weekend, R4, R25, R26, R30, & R34 have been replaced. Capacitor C8 was replaced with a 10 mfd @350 volt capacitor, and the diode was replaced with a 1N4007.

I was able to perform the calibration process. What I am noticing that seems unusual is the zero function. First, it takes a few minutes for this VTVM to stabilize enough to get zero to hold steady, which seems normal. But when I switch between the two lower ranges and the upper ones, Zero has to be reset. I also notice that when in the AC function, the meter takes a few seconds to land back to the zero line. This does not happen with the DC ranges.

I have tried two sets of tubes, but that has not made any difference. I am using a 6189 (12AU7-W) that tests 'balanced', and a 6AL5-W.

I let the meter run for about 12 hours now, and no changes with the zero function. Not sure where to look next, and I sort of doubt that what I've described is normal operation. Any advise is appreciated.

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Mon 02, 2019 7:42 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 787
What determines zero meter reading? If the tubes are balanced that means something else isn't. A resistor may be drifting or a poorly soldered connection may be generating chemical potential. A corroded tube socket.

Switch it on and start poking around to see what causes the meter to move.

If the ac has residual noise that could cause off-zero reading.

It may be painstaking but there is a reason for the odd behavior.

Bob


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Mon 02, 2019 8:34 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
Bob,

I think I took a look at that. I checked every resistor, and replaced a few that were a little high in value. I cleaned the pots and switches, tube sockets look new, circuit board is clean and joints look like they should. One thing I noticed is that there are two capacitors in the parts list that say .1mfd @ 600 volts ceramic, and .05 @ 600 volts ceramic, and instead of ceramic caps in this vtvm, there are two that look like maroon orange drops. They sure look factory installed, and I would guess that they are some type of film capacitor due to their appearance and newer age of this meter (Block letter 'RCA' on the meter and light blue color case vs earlier series).

Nothing looks out of place, and moving things does nothing. Hope I have described the issue clearly, but in case not, basically, after warm up, the zero line up on the R1 and next up range will cause a slight change from zero in relation to all of the other higher ranges. They seem to hold steady. Also, on the AC range, there seems to be a delay after taking a reading before the needle lands on zero. Not sure what range that was, and I did not test all AC ranges. I am judging this by what I think the manual states in that there should be no zero change between ranges nor function. Maybe this is true or maybe it's nit picking. I don't know at this point what should be expected from it.

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Mon 02, 2019 8:39 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4235
Location: Littleton, MA
Frank wrote:
What I am noticing that seems unusual is the zero function. First, it takes a few minutes for this VTVM to stabilize enough to get zero to hold steady, which seems normal. But when I switch between the two lower ranges and the upper ones, Zero has to be reset.

How much does the zero shift? A few minor divisions of the 5.0 DC scale?

The shift is due to grid current in the tube, perhaps due to gas. As you switch to lower ranges, the resistance in the grid circuit of V2A increases, while the resistance in the grid circuit of V2B is constant. This is the source of the imbalance.

If the shift is small, a few minor divisions on the lower two resistance scale, just accept it. Note that the calibration instructions have you reset the zero for the 0.5 V DC scale.

If it is greater, then try aging the tubes in-circuit in the meter for 36 hours or more, per the instructions in the manual. This gives the getter time to remove some of the gas.

Frank wrote:
I also notice that when in the AC function, the meter takes a few seconds to land back to the zero line. This does not happen with the DC ranges.

This is normal, it's charge stored in C1 and C2. The manual for the WV-97A has a note about this. In part, it says "This delay is normal and is caused by circuit constants so chosen that recurrent pulses with low repetition rates may be accurately measured."

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Mon 02, 2019 8:50 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4235
Location: Littleton, MA
Frank wrote:
I am judging this by what I think the manual states in that there should be no zero change between ranges nor function. Maybe this is true or maybe it's nit picking. I don't know at this point what should be expected from it.

Where does it say that? I don't see that in my copy of the manual. In fact, page 12 of my manual says the opposite:
Quote:
NOTE: When the ".5V DC" position is used, remove the probe top from the voltage being measured and re-set the ZERO control to position the meter pointer at the left-hand "0".

My manual also has you re-set the ZERO every time you change the function selector.

The manual on BAMA is the same as my copy.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Mon 02, 2019 9:15 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
Thanks guys for the replies. I'll let that meter run and get some hours on the tubes. Then see how it acts. I know there is a label on the back of the case that states a reset is needed when using the 0.5 volt range. The AC explanation makes sense to me as to the dampening action on that range.
I'm learning a lot from this. Thanks!

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Tue 03, 2019 8:07 am 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 260
It is normal to readjust to zero on the meter when changing between ac and dc ranges.

DM


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Tue 03, 2019 6:57 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 787
A well designed meter will not need constant zeroing. If you put it on dc and switch between + and - there should be a zero adjustment such that the pointer does not move. Then switch to lowest ac range (shorted leads) and adjust for zero with the ac adj. Now zero should be zero no matter what the switch settings.

If you get offsets, there may be a problem with noise or contact potential. My HP 410B doesn't have any problem with zero. In the first adjustment I mention here, the mechanical zero of the meter movement (power off) should coincide with the zeroing with power on.

It sometimes helps to tap the meter gently to overcome minor friction error. (Note my use of the word gently.) In addition, some meters have a sensitivity to static charge on the glass.


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Tue 03, 2019 7:26 pm 
Member

Joined: Nov Thu 07, 2013 3:29 am
Posts: 237
Location: Leesburg, Virginia
bob91343 wrote:
A well designed meter will not need constant zeroing. If you put it on dc and switch between + and - there should be a zero adjustment such that the pointer does not move. Then switch to lowest ac range (shorted leads) and adjust for zero with the ac adj. Now zero should be zero no matter what the switch settings.

If you get offsets, there may be a problem with noise or contact potential. My HP 410B doesn't have any problem with zero. In the first adjustment I mention here, the mechanical zero of the meter movement (power off) should coincide with the zeroing with power on.

It sometimes helps to tap the meter gently to overcome minor friction error. (Note my use of the word gently.) In addition, some meters have a sensitivity to static charge on the glass.


I like my RCA Voltohmyst. And I like my HP-410B. And, they both are if recall spec'd to about the same accuracy. But at least my HP was much easier to make solid than the RCA.

They are old analog devices. 3% or 4% (I forget) of FULL SCALE when new. If a bit of drifty is of concern my current favorite would be a Brymen 869S. Just sayin. ;-)

_________________
JI
W0MPM


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Tue 03, 2019 7:42 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4235
Location: Littleton, MA
bob91343 wrote:
A well designed meter will not need constant zeroing. If you put it on dc and switch between + and - there should be a zero adjustment such that the pointer does not move. Then switch to lowest ac range (shorted leads) and adjust for zero with the ac adj. Now zero should be zero no matter what the switch settings.

An HP 410B is a much more sophisticated VTVM than Frank's RCA WV-98C. As I've pointed out, RCA's manual says to expect to re-zero on the lowest DC scale, and whenever switching functions. Frank may be able to reduce the offset by selecting for a tube with lower contact potential and lower grid current, though aging per the manual is required when replacing a tube.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Tue 03, 2019 8:07 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 787
If that's true, then it seems to me that it's more trouble to constantly re-zero a poorly designed meter than to just buy a good one. Of course, the RCA has nostalgia value but beyond that it appears it may be too much aggravation. HPs are all over the place for decent prices. And the high load resistance of 122 megohms can be nice, along with the ac response into the VHF region and above.

Mine has become my ham radio rf voltmeter for tuning and measuring output power. And while it sits on the shack desk it can also be used for its other ranges now and then. For workbench measurements I have better devices.

While there is certainly charm in using 'vintage' gear, for serious work I prefer more modern gear. But this is a hobby so one is entitled to enjoy it any way he sees fit.


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Tue 03, 2019 8:14 pm 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 260
bob91343 wrote:
While there is certainly charm in using 'vintage' gear, for serious work I prefer more modern gear. But this is a hobby so one is entitled to enjoy it any way he sees fit.

The RCA VTVM is more than sufficient for antique radio repair.

DM


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Wed 04, 2019 12:33 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 787
True, but for many years I didn't even use a VTVM. My VOM was all I needed to do all the repairs, and in many cases, not even that.

Using a VTVM upgrades the setup without major benefit for most purposes. Upgrading it further is a lot of fun but totally unnecessary.

That's the basis for my comment regarding this being a hobby. Sometimes I measure the line voltage with my HP 3456A set to 6 digit resolution. And sometimes I get distracted and start making line voltage measurements with four or five other meters because it's fun to compare results. Then I think about why they read different values (they always do) and how to get them to correlate better. Is it harmonic content? Asymmetry of waveform? Noise? Or just calibration?

No need to be defensive. I am not decrying the use of one or another meter as long as the user is aware of the frailties of each. I am reminded of a young guy I knew who had a power supply he thought might be 350 Volts at 60 mA. So he put his VOM on it and proceeded to measure the voltage, sure enough not far from 350 but when he tried to measure the current his meter stopped working. He called me and I gave him the bad news, that he had destroyed his meter by connecting it across the power supply while set to current range.


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Fri 06, 2019 4:27 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
I'm getting around 81 volts at the 10 mfd electrolytic; not 88. Can't see a reason for this. Are these all supposed to be 88 volts or was there a range of acceptability for different runs?

_________________
Frank
KD0RUC


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Fri 06, 2019 4:50 pm 
Member

Joined: Jul Wed 11, 2018 7:14 pm
Posts: 49
Location: Calgary, Canada
Plus or minus 10% in the voltage is nothing to be concerned about. If you have calibrated the meter then the meter will work just fine.


Top
 Profile  
 
 Post subject: Re: RCA WV 98C VTVM Battery holder.
PostPosted: Dec Fri 06, 2019 5:16 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1802
Location: Eagan, Minnesota, USA
Sounds good. Over the weekend, I'll recheck the calibration and button this one up.
Thanks!

_________________
Frank
KD0RUC


Top
 Profile  
 
Post New Topic Post Reply  [ 40 posts ]  Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: Peter Bertini and 13 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB