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Nikko TRM-1200 Amplifier (1970) Repair Questions

JamesWest...

JamesWest...

New Member
Hello all,

I have what seems to be a bit of a rarity; A Nikko TRM-1200 preamp-amplifier (approximately 1970). Which I am having a couple of issues with.

First, the VU meters seem really insensitive. They move a little if I turn on the loudness switch. Any thoughts? As a note: I did replace the electrolytic on the vu circuit with a Wima film.

Second, the two Nikko 18005 Integrated Circuits (which are part of the preamp) are dead (only emitting a "lovely" noise that could only be described as the last groove on a record that is covered in sand). Anyway, I can't find replacements. I have the schematic for the ICs (see attached image), so in theory I could make a replacement, but I don't know which transistors would be suitable.

Notes:
I gave it a full electrolytic recap (with lower uf electrolytic caps being replaced with film).

I am aware of the 6 Noisy/faulty 2sc458lg transistors. I plan to use KSC1845 transistors. From what I've read, those are the best replacements. Are these indeed the correct replacements?
By the way some of the 2sc458lg transistors are orange and some of them are black. Is there some difference (other than the discrepancy in color)?


Any help is appreciated. Pardon me if I am oblivious of something blatantly obvious, I am still rather new to working on electronics.

18005 IC schematic (as taken from the repair manual).
{I think that the "HB18005" (on the schematic) and "18005" IC (on the part in the unit) are the same.}
upload_2023-4-29_20-51-13.png
 
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So working backwards, yes, the KSC1845 is a good replacement for the 458. See the following thread for the listing of 'bad' transistors. I would highly recommended to eradicate the system of these as well as any other 'see it, replace it' transistors (along with your recap) before you go any further. Re-evaluation the the behavior of the unit once complete.

As far as the VU problem, do the meters appear to barely have the needles move? Does it look like they are almost stuck in place? How far up do they actually go? I am asking this question because many times I encounter a situation where VU meters seem not work when in fact they are, it's the meter itself that is the problem. Two reasons for this. Either the meter coils are gummed up, or and this may sound strange but, the meter needles are being pinned down by a decal (that has the numbers/graduations on it), came unglued and slipped down inside the meter. You can test meter health by taking a watch battery and directly wiring it to the meter to see if it moves freely. I guess you could do it also with AAA battery, but be careful not to let it stay connected for long....those coils do not need much juice to power them so the needle should theoretically fly across the panel.
 
Alright, I thought that The KSC1845 was a suitable replacement just wanted to make sure.

I did the battery test on the VU meters earlier and they both (while at least one of them at any rate) seemed to operate properly. Another thing that is happening with the VUs that I neglected to mention is if you switch off the speakers, they kind of bob back and forth (even with no input) this seemed to be mitigated to an extent by the cap I replaced on the VU circuit.
 
Bulletin probably uses obsolete parts too. KSC1845 should work - a few people have had issues with it as the input transistor (marantz phono stages) - KSC1815 would be another option. Somebody rebuilt an NA-90003 phono module for an STA-701 receiver with KSC1845 and it worked fine My 2 cents
 
Thanks for the help, I'll get both for the preamp circuit if the ksc1845 prove troublesome I'll swap in the ksc1815.

Any more ideas on the VU meter issue. Being noncritical to function, I don't care to much if they work or not, but it would be nice to have a fully functional unit.
 
The meter circuit is pretty simple. There is an adjustment control (though no information on how to adjust it in the manual)
Does it look like anyone changed the diodes. The 1N60 is a germanium diode, so if someone puts in a silicon, it won't work very well. You can also use a 0.2 volt Schottky type diode.
As an aside - I have had some luck scavenging germanium diodes from old receiver or tuner boards.
 
Ok, it doesn't look like the diodes have been swapped. I'll order some Schottky diodes to replace them, see if that fixes the issue. Thanks for all the help. :)
 
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