Kenwood KA-5700+KT5500 - Lights keep blowing out

[MaxPax86]

Hi everyone!
I'm new here and my expertise regarding audio electronics, as you will see, is very limited. However, I enjoy fixing things myself where I can and I like learning and understanding things about the gear I use, so here we go:

I recently bought a beautiful Kenwood KA-5700 receiver and a KT-5500 Radio. A couple of days later the 8V bulb inside my receiver died - i promptly found and ordered replacements, soldered a new one in place and was happy for a few days until the lights of the radio died as well. Soon after the new receiver bulb died again, leaving me puzzled, slightly frustrated and completely lightless. All lights in two separate devices blowing within such a short time seemed a bit too much of a coincidence so i assumed the problem was with my power supply. I then bought a power strip with an over-voltage protection and replaced all bulbs once more. And now the receiver light died yet again! I thought that maybe another electronic device in my flat could be responsible but that last time at least, I'm sure, I didn't turn on any other device when it happened - can't rule out the fridge i guess since that is controlled by a thermostat.

Anyway, does this problem sound familiar to anyone? I'd really appreciate any help - I'd love to fix this problem myself, especially since Austria, the country i currently live in, is under some massive lock down because of the corona-outbreak so all shops that could help me are closed.

So, thanks in advance to everyone reading this for taking the time

 

[mbz]

Just some basics,
KA-5700 is an integrated amplifier
KT-5500 is a Tuner
KR-xxxx is a receiver ie, a combined integrated amp and Tuner in the one package.

Will try and track down the service manual but yes, something is going on. Do the units (amp, Tuner) have a mains voltage selector
at the rear? Is it correctly set to the applicable mains voltage? Also, when do the bulbs blow? probably at turn ON or some random
time after turn on?

EDIT: The KA-5700 meter lamps are powered by 8VAC 300mA with a 500mA fuse(FM3).
What did you replace the lamps with? Where did you source them. Most people would
upgrade to LED's as lamps with wanted VAC/mA are becoming hard to find

 

[MaxPax86]

Hi, thx for the quick reply and the clarifications!

Yeah, there is a main voltage selector in the back, giving the option between 120V and 220V. It is securely locked in place at 220V.
Interesting, shouldn't it have a 230V position? The europlugs suggest that it was built for the european market.

The bulbs seem to blow pretty randomly - the first times we only noticed after the fact - the last time we were standing in the middle of the room doing nothing electronically relevant when it went with an audible knocking noise.

 

[mbz]

The amp is expecting 220Vac but feeding it 230Vac and over driving all voltages. There may be a problem with the mains
bouncing around maybe 236Vac maybe ??? I'd up the rating on the lamp 10-12Vac if you can find one, alternatively do the
LED conversion, many posts on AK detailing this. I would measure the bias as per the service manual making sure it's within
range, it should be, 20-150mA is some "spec"

 

[MaxPax86]

Right, Wikipedia just told me that residential voltage in most European countries used to be 220±22Vac till 1987 which explains why a pre-1987 Kenwood device with Europlugs would be set up for 220Vac. Now we have 230±23Vac so technically the device could be getting up to 253Vac - so 11Vac over the pre-1987 expected maximum of 242Vac. I’m guessing the voltage I get from my sockets is not constant which would explain that the bulb only burns out occasionally when the voltage bounces around the upper range – is that the theory?

Could this be harmful for other components or is it just the bulbs that are just inherently weak parts and thus go first when things are slightly off? On second thoughts, that’s perhaps a silly question though since that would mean that you practically shouldn’t use any pre-1987 gear which would probably include some guitar amps I’ve used, and I feel I would have heard about such issues.

If I can I’d like to stay away from the LED conversions for reasons of aesthetics and nostalgia, not sure if that’s silly but who cares

As to measuring the bias, this surpasses my current electronics skills. I do have a multimeter, but I also have a strong instinct of survival that tells me that I might want to gather some more expertise before playing with hot exposed circuits.

So anyway, I can safely replace the 8V 300mA bulbs with a 10 or 12V bulb then? I found this 10V 130mA wedge bulb https://www.bulbtown.com/280_Miniature_Bulb_Glass_Wedge_Base_p/b280.htm

I read in another thread here that someone switched out his with a “#192 wedge style automotive bulb” - https://www.jacksonelectricsupply.com/192-Miniature-Bulb-Glass-Wedge-Base-p/192.htm which runs at 13V and 330mA.

Don’t I have to make sure that the bulb is also set up for around (or at least) 300mA or is it all a matter of range - sorry if these are eyebrow raising questions, as I said, I’m just getting into this

Anyway, thanks again!

PS: I found some specs for the amplifier if that helps

 

[MaxPax86]

Oh, i only now read the edit to your previous answer - I replaced them with 8V 2,4W 300mA T10 from ebay - these were the ones i found inside

 

[mbz]

is that the theory?

yep

Could this be harmful for other components

Depends how much the mains voltage swings, 220Vac +/-10% will be ok, long term 250Vac may damage the transformer, I would
expect significant overdesign in this area. In this situation I would monitor the mains voltage during normal listening times. Please
don't do that due to your lack of experiance, it's mains (=lethal) voltages.

As to measuring the bias, this surpasses my current electronics skills

No big deal. If the amp gets abnormally warm/hot then it may be the bias.

So anyway, I can safely replace the 8V 300mA bulbs with a 10 or 12V bulb then? I found this 10V 130mA wedge bulb

Go with either 10 or 12V. 130mA current draw is better than OEM 300mA.

Don’t I have to make sure that the bulb is also set up for around (or at least) 300mA or is it all a matter of range

No, I would look/prefer a lower current draw to take load off the transformer

 

[MaxPax86]

I haven't noticed any abnormal temperatures so i'm just gonna try some 12V 100mA bulbs that i found online.

https://www.reichelt.at/glassockell...MOjhgzZUhBowJUMDUzjVdpQDgdBoC0Z4QAvD_BwE&&r=1

Let's see if that does it - in any case, I am really most grateful for your the time you have taken to answer me!

Have a nice day!

 

[MaxPax86]

Hi,
so i changed the bulbs to said 12V 100mA. As expected, the light is much darker but still nice. However, a few hours later I suddenly started getting weird noises from the speakers. They seem pretty random and we sometimes go hours without hearing them but then they appear to come in packs. I’ve switched back to the 8V to see if they disappear and for a while it seemed like they did but then they started again.

Haha, suddenly my issues aren’t aesthetics anymore – this sounds worrying!

I’ve added a recording of the noise – does this sound like anything anyone recognizes?

 

[MaxPax86]

UPDATE:
Found the culprit for that nasty noise! It seems like my TV was polluting the voltage on that power strip - roockie mistake, i admit!
Maybe it also caused the spikes that blew out my bulbs, that remains to be seen - i kept the original 8V 300mA in, just out of curiosity, to see how long they will last.
In any case i started an online course on audio electronics - nothing like an annoying problem that needs fixing to spark fascination and obsession

 

[Lark]

I have a KT-5500 with all 3 bulbs burnt out. My question - I have a bunch of LED bulbs left from when I was in the pinball hobby, that are rated at 6.3 vac. Bulbs that are in it read "Stanley 8 VO. 3A". Will my LED bulbs from Comet work as a replacement, without hurting anything? The voltage is lower by 1.5, but there is no amperage rating on the LED. I would think they would be ok, as I know LED are usually less of a current draw, but wanted to see what people that have more experience with electronics had to say, before attempting.