KT-7500 panel lamp substitutions and safe current draw

ifr2lax

Member
This is basic electrical stuff, "E-shop 101", but appreciate input as I have no feel for how far you can push things in terms of loading...follows a cut/paste from the TUNER INFORMATION CENTER on lamps by Bob...
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"""Kenwood KT-7500 and KT-7550
8 volt 300 milliamp X 3 green-tinted bulbs with rubber grommet base and wire leads - two for dial and one for meters.

Our panelist Bob says: "Here's a solution for the front panel lamps in the KT-7500. I just used common bulbs from Radio Shack that worked well. There are three bulbs in series, driven directly off the power transformer secondary AC winding, which is 29 VAC. The stock tuner uses a series 27 Ohm resistor to drop the voltage to 24 VAC, as 8 volt bulbs were used. When using common Radio Shack 14.4 volt bayonet replacement bulbs, you can choose to leave the resistor in place, for a really dimmed look, or jumper across it. I prefer to jumper it - it looks a little better to me, and is still dimmed compared to stock. I used the 14.4 volt/120 milliamp bayonet bulbs, which I hope last a long time due to the fact that they are running at only 9.7 volts. You still need to solder them in place - I don't know of a solution that gets around that.""
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OK at Radio Shack I found three 14.0 volt/270mA WEDGE base lamps...look like they would slip into the Kenwood rubber lamp socket, and the exterior leads could be straightened and soldered to. easier than bayonet type.

In the example from TUNER INFO by Bob, he uses three 14.4 volt/ 120mA bulbs. Is this a correct assumption...?

a. OEM is_______________three 8.0 volt/300mA = 2.4 WATTS

b. Bob's swap__________ three 14.4 volt/120mA = 1.728 WATTS

c. My RadioShack bulbs__ three 14.0 volt/270mA = 3.78

Given the KT-7500 transformer circuit has 29 Vac, reduced to 24 Vac, is this a safe choice to make with out overloading the circuit or overheating the plastic panel..? Of no consequence?

I think I am leaving something out of this...like is the total amps more important to the circuit than the total watts of "heat"...aren't they about the same thing...

Aside from ruining something, wanted to avoid buying the regular flashlight "bayonet?" type bulbs and having to solder leads onto the base...(is that what you guys normally do anyway?)

Thanks, Pete M
 
The bulbs won't actually USE 270 mA, since the voltage will be lower. So actual wattage would be about 240 mA (a guess) times 29V which is about 7 watts, less than the original 8.9W (three bulbs plus resistor). If solder sticks to the wires, they will be fine - you might have to clean them with ]fine sandpaper and use a little extra flux.
 
KT-7500 panel lamps are in...the lights are on...!

Well.

The KT-7500 lights are on, first time in 15 years...(now I just need the c945 transistors E.W. recommended to fix the muting circuit...??...ordering those from Mouser this evening...)

The lamps I picked at Radio Shack:

#194 Wedge-base lamp (RS#272-1126)
14.0 Volt/ 270 mA

May be OK from a total electrical load standpoint, but physically they are a really a little too long.

On the KT-7500 the lamps are supported in a silicone rubber? jacket/plug and the lamps I picked, a wedge-base (an automotive part), are about 5/16ths inch too long compared to the OEM 8 volt/300mA units.

I am using the lamps by inserting the rubber mounting jacket back from its intended "groove" that it is meant to seat in...still a tight fit. not likely to come loose.

The problem was the lamp for the meter psition was ALMOST touching the lucite plastic lens that it sits in...I moved it back by just sliding the rubber sleeve back, and it is probably OK, full1/4 inch clearance and will probably not overheat the lucite lens that illuminates the two meters.

In retrospect, wish I had removed the original lamps and compared them to the replacements...the new ones work, but just barely...they are long...

The leads on the wedge bulbs had to be sanded to get any solder to adhere, I do not know what they are coated with, but it isn't solder friendly.

So, the simple gets complicated...I am sure a lot of you are nodding your heads, "get used to it"...

Thanks Pete
 
You shouldn't have any problems with those. If you wish to redo the effort with something that matches very well, you can contact AK'er dgwojo at his website: http://www.dgwojo.com/. Dave is the go-to guy for vintage lamps, IMO.

Here's what a Kenwood tuner looks like with his wedge bulbs and green condoms:
IMG_0046.jpg


Have you done any of the TIC mods on your 7500?
 
the green tinted lights are cool...but the web site came up dead

Westend,

Thanks for the input...DGWOJO's website didn't come up as current, perhaps there is a new url change...could you look around at your resources...?

The little green "condoms" would be great...I was amazed I could peel the one on the meter bulb off and reattach it to a new bulb.

The two outer bulbs had been replaced 20 years ago and the green jackets were gone...would like to get some new ones...even the correct lenght bulbs...the bulbs that are "too long" can be used, but would rather have the correct pieces, and the little green..."condoms"...new.

For the muting circuit I have ordered new Qb10 and Qb11 replacement 2SC945 transistors from Mouser, (per EW's recommendation) and hope to have the muting circuit working and then move on to the mods...

When I go by the hardware store I am going to pick up some rubber bushings to lift the transformer off the chassis...

Does anyone have the PARTS LIST from the KT-7500 Service Manual...? And readable PC Board copies...?

It would help for compiling a list of caps to replace.

I would gladly pay for a KT-7500 Service Manual if the photocopies of the PC BOARDs were readable...

I paid for a KT-8100 amp manual and the PC board copies are not adjusted for contrast/brightness and pretty useless...the pages are there, but as they just throw the copy of a copy on to the copy machine, manual pages that are "shaded or highlighted" are useless...jut black. I don't want to pay for that.

I am having a lot of fun with this project, and appreciate all the help!

thanks all, Pete
 
When I rebuilt my GAS amp I chose new bulbs from Mouser.com. Their website gives complete specs of all bulbs and I was able to choose a set that a) worked without modification, b) uses less wattage and generated less heat than the original bulbs, and c) dimmed down the meters for a nice, easy going glow during night time but still easily readable in the day. My avatar shows basically what it looks like.
 
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