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


It is currently Jul Thu 02, 2020 11:06 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 25 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Jackson VTVM
PostPosted: Mar Sun 29, 2020 5:11 pm 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
I have a Jackson 645 VTVM that I am trying to bring back to life. Couple questions for those more knowledgeable than I. First it has a 6X5GT, 6K6GT, and a 7N7 tube which all check out as good. There is a fourth tube spot that I assume is for the 3A-20 Regulator Resistor. Although the schematic just shows a resistor symbol coming off the AC line I am assuming this is actually a ballast tube for the fourth tube socket. Any clues regarding availability of this 3A-20 or is there a way to fabricate a solid state replacement. What does the 3A-20 designation translate to: could it be 3 amps at 20 ohms or is there no reasonable correlation? Finally it calls for a battery model 2730. Does anyone know the value of this battery and possibly a suitable replacement. All I can find on line are small flat 3.7volt 900-1000Mah cell phone battries. Thanks for any help at all.

ettore


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Sun 29, 2020 5:28 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4711
Location: Littleton, MA
According to Jacobi's Ballast Tube Handbook, the 3A20 is rated for 20 to 41 volts, 300 mA.

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


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Sun 29, 2020 5:40 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3524
Location: Seattle WA US
If that battery number was 2370, it would be 4.5volt "C" battery, weighing 3/4 lb and dimensions 3-15/16x1-3/8x2-7/8 in.
Made by Burgess, 50+ years ago.
-Chuck


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Sun 29, 2020 7:00 pm 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
Chuck,

Thanks for the reply would 3 D cells wired in series do the trick if there is room in the case, I assume the 3,7 volt 900Mah Li-Ion battery would not work. Any thoughts, thanks

ettore


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Sun 29, 2020 8:57 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3524
Location: Seattle WA US
I would try the three D cells, but don't use alkaline cells ! An alkaline D cell contains enough potassium hydroxide electrolyte to do real damage when it leaks ! Use traditional flashlight cells, now called "Heavy Duty" by some manufacturers.
-Chuck


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Sun 29, 2020 10:38 pm 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
Thanks Chuck will keep that in mind, just have see how I am going to fit them in the case.

ettore


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Mon 30, 2020 1:18 am 
Member

Joined: Mar Sat 03, 2007 12:53 am
Posts: 804
There are many 3 D cell battery holders available:
https://www.ebay.com/itm/1-2-3-4x-D-Cell-Battery-Holder-Connector-Box-Case-1-5V-DC-Case-with-Wire-Lead/142740560418?hash=item213c001a22:m:m3L3TwHTSxrEmKZf_dQK9lQ


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Mon 30, 2020 2:29 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3524
Location: Seattle WA US
Ettore-
Just a suggestion: if you can, using existing screws and holes, mount the battery holder on the back of the cabinet. That way, it won't show when the meter is sitting on a shelf, but when the battery leaks, it won't damage the meter internals.
-Chuck


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Mon 30, 2020 7:47 pm 
Member
User avatar

Joined: Jan Sun 15, 2012 4:20 pm
Posts: 654
Location: Lawton Oklahoma
Looking at this auction on Ebay I wonder what he has plugged into the socket where the 3A20 goes?
https://www.ebay.com/itm/Vintage-Jackso ... 0290.m3507

Looks like it has been modded for the 3 d cells too.


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 1:39 am 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
Thanks for the ebay post I do not know what is going on with the 3A-20 tube socket. I have done some trouble shooting on mine and wonder if my thought process is sound.

Since I do not have a 3A-20 I tried the following. From the schematic I determined the 3A-20 regulating resistor is wired through pin 7 (inbound AC line) and pin 2 (transformer primary winding to the on/off sw). So I took a piece of wire and jumped a connection between pins 7 & 2, then connected my meter to one of the plates of the 6X5GT rectifier tube and to ground. The rectifier secondary winding to the each plate shows 150VAC on the schematic. With this all set up I then plugged the Jackson 645 into my Sencore Isolation Transformer and starting at 0V I slowly increase the voltage til my meter showed 150VAC. With 150VAC on the 6X5GT plate the Sencore showed 85 input volts, and 43ma.

Given this information the schematic shows input voltage of 100-125VAC to the Regulator Resistor (3A-20). Using the upper value (125) would leave 40 volts that need to be dropped, 125 minus my Sencore input voltage of 85 volts to get 150VAC on the rectifier plate. That 40 volts divided by 43ma gives me 93 ohms, and 40 volts times 43ma gives me a needed resistor of 94 ohms at 17.2 watts minimum.

Since the tube socket is an octal would it work to simply solder this resistor into pins 7 and 2 of of an empty tube socket and plug it in in lieu of the 3A-20 Ballast Regulating Tube.

Any thoughts, feedback or advice would be greatly appreciated.

ettore


Last edited by ettore on Mar Tue 31, 2020 12:09 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 12:06 pm 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
Everyone plz note in my lengthy post above it should be .43amps or 43ma NOT .43ma. I believe I have corrected the post. Sorry for the mix up.

ettore


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 6:40 pm 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 714
Rather than measuring the B+ which is not very accurate, measure the input to the transformer, it requires 80 volts on the primary. If your line voltage is 120 you will need to drop 40 volts. You will have to measure the current draw of the primary when it is at 80 volts to determine the resitance and wattage needed. The 3A20 actually regulates the the voltage to the primary of the transformer keeping it a 80 volts as long as the line voltage is 100-125 volts.

DM


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 6:55 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 4019
Location: Florida
The filament voltage is a better reference then the HV. Adjust the ac in until it's 6.3 for whatever nominal ac line voltage is shown on the schematic.

RRM


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 7:12 pm 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
Thanks RRM, I did check filament voltage in my above post and it indicated 5.6VAC. Did not try bringing the filament voltage up to 6.3 and see how hi the plate voltage would show and what input voltage from my Sencore Isolation Transformer would result. Do you think filament voltage is a better reference for my resistor need computations?

ettore


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 7:14 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 4019
Location: Florida
Yes. The tubes will be happier.

How about posting the schematic?

RRM


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 8:39 pm 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 714
Tube filaments are 5.7 volts not 6.3. Adjusting your input to get 6.3 will throw everything off. Also, filament windings are not too precise.

DM


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 8:48 pm 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
RRM,

Here is a copy of the Jackson 645 Schematic and parts list, hope the come out okay.

ettore


Attachments:
IMG_3548.jpg
IMG_3548.jpg [ 1.34 MiB | Viewed 587 times ]
IMG_3549.jpg
IMG_3549.jpg [ 1.5 MiB | Viewed 587 times ]
Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Mar Tue 31, 2020 11:41 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 4019
Location: Florida
From the schematic it looks like they are running the filaments about 10% low. That was most likely to increase tube life. In any case I'd use the filament voltage as the reference to set the ac line input, keeping in mind that without the ballast tube there is no regulation.

What's the spec for the ac line in?

RRM


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Apr Wed 01, 2020 12:38 am 
Member

Joined: Nov Sat 24, 2012 5:05 am
Posts: 141
Location: Forest Lake, MN 55025
RRM

Not sure what you mean by spec for the AC line. The only numbers I know of are on the schematic, it says 100-125V for the input to the regulator resistor (3A-20 Ballast Tube). So you are suggesting I increase my Sencore input voltage til I read 6.3VAC on the filament pin, then use that indicated input voltage to do my math for the correct size dropping resistor and not worry if my 6X5GT plate voltage exceeds 150VAC. Thanks.

ettore


Top
 Profile  
 
 Post subject: Re: Jackson VTVM
PostPosted: Apr Wed 01, 2020 12:50 am 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 714
I suggest you re read my above post about the transformer input voltage and forget about the filament voltage..

DM


Top
 Profile  
 
Post New Topic Post Reply  [ 25 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 9 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB