HELP: Nikko TRM-230 Power Indicator Bulb / LED Replacement

[Gaurang]

Hello everyone,

This is my first post and was seeking some help from you experienced members. I have a Nikko TRM-230 which I use with Bose bookshelf speakers and the sound is just phenomenal for an amp / speakers of this age. Beautiful warm and heavy sound which I have been using for the last 15 years with no problems.

I recently added an Amazon Echo Dot for my internet radio in my library. Unfortunately I had bad weather in the area and my power was cut briefly. When the power came back up I noticed that the amp worked fine, but that the power light was no longer functional (See pic):

Nikko TRM 230 in library

• Gaurang
• Feb 11, 2017

Nikko TRM 230 in library

I opened up the amp and found a bulb which I removed, and then checked for DC current on the bulb leads. There was none. I was told that these older amps have an AC current going through. I even touched the leads together and noticed small sparks - so there is definitely some current passing through.

I don't want to take this to a shop (Where they sometimes do more damage than good!), but does anyone know whether I can use a bulb / led to attach to the leads and whether the current is actually AC or DC and what voltage and type of bulb I would need. Looks like a simple job and would prefer to do it in house.
Thank you all in advance.

 

[jdurbin1]

Connect a multimeter to the leads and see what kind of AC voltage is present. That, and a picture of the old lamp should give us enough info to get pretty close on a replacement lamp recommendation. Keep in mind the unweighted voltage will probably be a bit higher than it is with a good lamp connected, and also that some guessing as to the replacement lamp milliamp rating is probably involved - that is, unless another Nikko owner or expert happens to know the spec for the OEM lamp.

John

 

[Gaurang]

Will post a photo of the lamp / bulb when I get to it. I have a cheap multimeter that does not give me the AC voltage but the fact that sparks are flying upon touching the leads, gives me some hope that current is still flowing through (Either AC or DC!).

Tried to get info from the below thread - but it did not help much:
http://audiokarma.org/forums/index.php?threads/replaced-my-nikkos-lamps-with-leds-pic.186105/

 

[jdurbin1]

A multimeter that doesn't read AC? Never seen that... have you got it on the input with the squiggly line?

Yes, sparks are a good sign but I wouldn't test that again or often... risk of damaging whatever is supplying voltage to that lamp which could be a secondary of the power supply transformer, or a transistor or other semi-conductor.

John

 

[Gaurang]

Thanks again John - Sorry I wasn't clear. I have a basic multimeter that only reads high AC voltage (110V +). This may be a low voltage AC current, but that's just my hunch. Waiting for some stand up Nikko owners to come through. Will post the bulb photo soon!

 

[Gaurang]

Just got the photos sorted now - because I have a terrible phone camera and borrowed an iPhone 7 to take these shots. One of the leads has some miniscule writing in green. I will try and read that with a loupe later. In the meantime I'll be looking for a decent multimeter which can read voltages below 110v.

Any thoughts people, on the voltage (AC / DC) and whether I can use a bulb / LED - including the voltage?

 

[sregor]

Both trm210 and trm500 schematics (available at hifiengine.com - free but must register) show an 8V 40 mA bulb connected to AC from transformer (not line). One side should run strait back to transformer. An LED would work fine, with a diode and a limiting resistor. A cap could be used to smooth the rectified signal if it flickers too much.

 

[jdurbin1]

edit: doh! sregor beat me to it... but check below for a good source:

Looks like a typical "grain of wheat" lamp, more technically a T-1 3/4 lamp with harness. That's assuming the glass diameter is in the 5mm range - if it's smaller, "grain of rice" or T-1lamp which is a tad over 3mm in diameter.

Unlikely the writing on the leads will identify it but worth checking I guess.

I looked at a couple of other Nikko TRM models (210 and 500) and both show an AC-driven power lamp of 8V/40mA in the schematics. I'd be surprised if yours isn't the same, the 210 looks a lot like the 230 in terms of layout, style etc.

There are some for sale on German eBay, Vintage Electronics has some that are close... but I'd check with AK member dwgojo as I am pretty sure he carries this type. If not, they're available in bare wire type fairly readily and it's not hard to tack those onto the existing harnesses, but I think you'll be able to find them with the harness already attached.

John

 

[Gaurang]

Thank you guys so much (sregor and jdurbin1). Will post pics (Nice Ones) when I'm back and ready with the replacement. Will look for the 8V 40mA bulbs. Just a few quick questions (I always sucked in my electrical engineering classes - ECE 101!):

1. How do you know the lamp is AC driven, I could not see that in the schematic?
2. sregor - Isn't a LED a diode by default? Is there a need for a diode or a limiting resistor? Not too sure what a cap is either. (Sorry for these basic questions!)

Hope to have this fixed and will show off as soon as it's complete. Living in DUbai is not to easy to source parts!

 

[Gaurang]

Ok - got your concept with this photo -

R= V / I

So with a 4V 20 mA LED, I'm guessing I need this (Assuming power source is 8V 40ma):

Forward current across the circuit is 4V (8V Supply - 4V Required);
V=4, I = 20mA (0.02A) so R = 4 / 0.02 = 200 ohm resistor, right?

Can you recommend a cap to use - because I never did get that far! Thanks again.

 

[jdurbin1]

Not quite sure why someone would go to that much work (not to mention adding three extra in-line/non-board mounted components!) to a vintage piece when the original part is so easily sourced... ymmv, but me? I'd just drop Dave (dwgojo) a line and get one of his ready to install lamps. But, if the objective is to solve it with something you have on hand or you are itching for an electronics project, no worries.

John

 

[Gaurang]

Going to drop Dave a line too! Thanks!

In case I go in for the project - can someone recommend a capacitor? Need it to light not much longer than the amp is on, to smooth the flicker (Incase I go with this solution!).

 

[sregor]

Ok - got your concept with this photo -

R= V / I

So with a 4V 20 mA LED, I'm guessing I need this (Assuming power source is 8V 40ma):

Forward current across the circuit is 4V (8V Supply - 4V Required);
V=4, I = 20mA (0.02A) so R = 4 / 0.02 = 200 ohm resistor, right?

Can you recommend a cap to use - because I never did get that far! Thanks again.

Almost. Your reasoning is correct, but the rub is that the peak voltage for 8v is around 12 volts, so including drop across the diode, you will end up with just over 11 volts DC. The cap doesn't have to be very big at all (100uF should be ok), resistor should be larger. If you have a 1k pot, you can use that to get a desired brightness, then measure the resistance and put in nearest resistance. Also, my own prejudice, but I prefer lower currents on LED's (they last longer) Diode is both protection and rectification. (some people use 2 LED's back to back also)