Nikko TRM 40ic

[Busted Audio]

Hi, this is going to be a newbies first Techy thread, I introduced myself here ...

... and there's a (very) brief introduction to my Nikko TRM 40ic there too.

It has an issue that I can't fathom out, probably because I'm still a bit green on diagnosing electical issues.

The 40ic has 7 selectable inputs, 6 on the function dial and the Tape Monitor switch.

With the unit all hooked up correctly using the appropriate input phono's per selection this is what I get...

Aux... Nothing

Tuner... Nothing

Cer Phono... Nothing

Mag Phono... Right channel input being played out of the Left speaker (nothing out of the Right), really low volume that won't turn up or down, Balance, Base and Treble appears to do something but the volume is so low it's difficult to determine what exactly.

Tape 19cm... same as Mag Phono

Tape 9.5cm... same as Mag Phono

Tape Monitor... full control, sounds fine, Left and Right channels as they should be, it produces enough volume at a quarter turn to make my bench speakers distort.

I think I understand that the Tape Monitor bypasses the Pre-amp? so that indicates the issue lies in the Pre-amp?... The 40ic doesn't have a seperate board for that, it's just the one board with everything on it, although I believe I can discern what's power and what's signal by looking at the arrangement of the components.

Doing my searches on AK, I found the thread talking about the NA-90003 module, to which I probably went and did the n00b thing and jumped to the conclusion that this was my problem, so built a discrete component copy to slave in (photo in the intro thread shows this)... didn't change a darn thing... although I did only try it on the one channel before I came running to you guys for help.

There doesn't appear to be much info around on the 40ic, I did find the service manual for the 40B but there is some significant differences between the 2 units. The transistor and capacitor counts differ and the 40B manual states the transistors should be Germanium PNP, this 40ic has Silicon NPN's... (double checked with a Peak Atlas meter too)



... I'm pretty sure that this 40ic has not been messed with before, it all looks original inside and I've had it shut away in my loft for 40 years and although it had noisy controls when it went up there it was still working then. Plus it originally came from another family member and he'd had it from new.

I am going to restore this 40ic and I've already bought all the components by speccing out what was in there and getting the suitable replacements (I also found the thread on here about the bad Transistors that should be shot on sight), what I'd like to do is identify the problem it currently has and fix it before just shotgunning all the capacitors/transistors and what-not hoping that it fixes itself that way.

So I come to you guru's... cap in hand... got any ideas on what I should do next?... you may have to explain to me exactly what to do or how to do rather than just say 'do this or that'.

I do have a reasonably equiped bench in my workshop, so I'm not for wanting of a multimeter or such.

Cheers, Chris

 

[sregor]

Amplifier description sounds similar to amp section of STA-501S receiver. It uses the NA9003 module and has silicon outputs. (FWIW Japanese semiconductors use A or B for PNP transistors, and C or D for NPN transistors, so a 2SC373 transistor will have C373 on it and so forth.)
Before you get too far into the trouble shooting, have you cleaned the switches thoroughly? Here you need to pay attention to the selector switch and the tape monitor switch. I would normally say check supply voltage (pin 8 on the NA9003 module, but if it is wired like the 501, it also supplies voltage to the tone controls, which is after the tape monitor circuits. As you have figured out, all the inputs except the tape monitor go through the NA module. The phono and tape head circuits are high gain, and should sound distorted if you are using a normal level input. So stick to Aux or Tuner for initial trouble shooting.
STA 501A schematic is here: https://audiocircuit.dk/downloads/nikko/Nikko-STA501S-rec-sch.pdf

 

[Busted Audio]

Hi, yes, first thing I did knowing that the unit went up in the loft with the noisy controls was get some deoxit (D5 & F5) in from Amazon on next day delivery. When I run a signal into it on the tape monitor the controls are all as quiet as a church mouse now.

Thanks for the steer towards the STA-501S diagram, I'll take a look and see how well it stacks up to the circuit board and feed back.

Advice on the trouble shooting inputs well noted, I'll stop pumping signal into the high gain circuits.

 

[Busted Audio]

... you need to pay attention to the selector switch and the tape monitor switch.

Just thinking on this... is it credible that a tape monitor switch can be sufficiently clean so as not to produce any noise, but still not be functioning correctly?

It only produces the correct output when set to 'Tape monitor' and I don't get the very limited/corrupted output from the other inputs at all when set to 'Tape monitor', only when set to 'source'. But what if it's still a bad connection in the switch?... what's the way to test that?... is it only by substitution?... or can I 'jump' the source contacts?... i.e. hard-wiring it into source.

 

[sregor]

One thing to check - did someone put shorting plugs in the tape out jacks? Not likely, but would give all the symptoms so far.
Do you have voltage on pin 8 of the module" 501 schematic shows 21 volts.
To jump out the control, you probably should ohm out connections (with power off) signal should go from aux to selector switch, then back to board (goes to input coupling cap before the input of the module - pin 3) . Then there are a second set, coming from the different feedback resistors through the selector switch to the feedback input (pin4) of the module. Diagram fro, 501 schematic.

 

[Busted Audio]

There's no shorting plugs fitted to the tape out jacks... I'll get back up to my workshop later today, it's built in the back corner of the garden, not in the house, so report back in a few hours.

 

[Busted Audio]

Could only spend about an hour up the workshop tonight, but to the best of my ability it does appear that the section of the 501 schematic diagram from the function selections on the left to the output transistors 2sc1030's is exactly what the 40ic has in it, as best as I can tell at least all the transistors and capacitors are a match (I didn't verify all the resistors). The only difference is the 501 has x2 Aux inputs and x1 Phono, where-as the 40ic has x2 differing Phono inputs and only x1 Aux.

checked the voltage at pin 8 on the module and it's showing 19 volts... I also checked at pin 1 which the diagram says is 7.2 volts, I got 18.6 volts... and at pin 5 where it says 1 volt I got nothing.

I checked the Ohms from the Aux input jack to pin 3 and got a reading in mili-ohms, I wrote it down but left the bit of paper up at the workshop , I then checked to pin 4 and got a reading in ohms, but again it's on that bit of paper I left up there

And then for no particular reason I thought I'd just put the meter into continuity and just move along the path from the Aux input jack towards the modules checking I had it all the way along (which I should have had if I had next to no resistance in the previous steps?)... the very first component after the jack is a resistor and I lost continuity across it and from that point on, nothing... nada... nix... as far as continuity was concerned it was deader than a door nail... I don't understand that.

It's at that point I put the probes down, out of time for the day more than any other reason, pic below of the position of that resistor...



... there's 2 little blue circles marked on that picture showing where that resistor is mounted on the underside of that little board that's mounted next to the jacks, you might have to enlage the pic to see the blue wire that comes from that point to the function selector switch sitting in the foreground.

And a cropped 501 diagram showing just what I believe coincides with the 40ic (granting there is a Aux/Phono miss-match)...



Thoughts anyone?...

 

[Busted Audio]

Panic averted... turns out it's what we would call in the aircaft industry as a 'seat to steer interface problem'... I was being an unedjucated nitwit, sometimes you can't get a continuity signal across a resistor because the meters signal doesn't have the oopf to overcome the resistance, so you get no beep.

Managed to grab another few minutes with it tonight, injected a signal into the Aux jacks, set the tape monitor switch to source then jumped the switch contacts for source to see if it made a difference... no change... so I'm going to take that as it's unlikely to be related to a bad contact in the source switch.

Those readings I got at pins 3 and 4 of the module were 3.7 mOhms at 3 and 280 Ohms at 4.

I think that next I'm going to pull the 3.3mf/35v caps between the switch and the modules and check them out of circuit.

 

[sregor]

checked the voltage at pin 8 on the module and it's showing 19 volts... I also checked at pin 1 which the diagram says is 7.2 volts, I got 18.6 volts... and at pin 5 where it says 1 volt I got nothing.

Something isn't right here. Pin 5 is connected to ground, so should be zero, so that's ok. Ran a sim to get voltages - with a 20 volt supply, I got pin1 at 6.7 volts, pins 2 and 3 at .6 volts, pin 4 at 20 mV (.02 volts) pin5 ground, pin 6 .86 volts and pin 7 at 14.1 volts. Only way I could see getting 18 volts at pin 1 would be if you used a different resistance than 120K for the resistor between pins 1 and 8 (power)??
Also, are you using the original NA modules, or your breadboarded version?

 

[Busted Audio]

On both the remaining module in circuit and my breadboard, just been back up there and rechecked all pins on both...

Left Channel which is the original module...
1 = 0.7 volts but bleeding down
2 = 0 volts
3 = 0 volts
4 = 0 volts
5 = 0 volts
6 = 0 volts
7 = 0.15 volt bleeding down
8 = 18.7 volts

Right Channel which is my breadboard...
1 = 18.5 volts
2 = 0 volts
3 = 0.3 volts bleeding down
4 = 0 volts
5 = 0 volts
6 = 0 volts
7 = 18.5 volts
8 = 18.5 volts

those that were bleeding down was very gradual but didn't look like they were going to stop

 

[Busted Audio]

This is how I re-drew the circuit from the Nikko service bullitin so that my brain could undertand it, the Nikko supplied drawing have the pins arranged on both sides of the circuit and I needed to reconfigure it to the pin alignment on the actual module for me to understand how to build it (so pins in my drawing are 1 to 8 left to right), I was pretty sure I'd got it right but happy for anyone to correct me if I've goofed.

 

[Busted Audio]

This is the diagram from the service bullitin... which is also a slightly different configuration as that drawn out on the 501 circuit diagram.

 

[Busted Audio]

... and just double checked the colour code on the resistor, yes it was a 120K I put in there.

 

[sregor]

View attachment 3776068

This is how I re-drew the circuit from the Nikko service bullitin so that my brain could undertand it, the Nikko supplied drawing have the pins arranged on both sides of the circuit and I needed to reconfigure it to the pin alignment on the actual module for me to understand how to build it (so pins in my drawing are 1 to 8 left to right), I was pretty sure I'd got it right but happy for anyone to correct me if I've goofed.

Try these changes

 

[Busted Audio]

I believe things are moving in the right direction.

I modified my breadboarded surrogate module as per the drawing above, it now looks like this...



Voltages now look like this...

1 = 7.47 volts
2 = 2.303 volts
3 = 0.3 volts bleeding down
4 = 0 volts
5 = 0 volts
6 = 2.45 volts
7 = 2.535 volts
8 = 18.65 volts

additionally, putting a signal into the Aux jacks, with the appropriate source selections made, I can just make out the music I was pumping in at the Left speaker... but I mean just barely... I may have mixed up which channel still has the original module in it and which is my breadboarded copy.

Volume control still has no effect.

Balance control seemed to work, as in it took the signal away from the left when I turned it to the right.

 

[sregor]

I believe things are moving in the right direction.

I modified my breadboarded surrogate module as per the drawing above, it now looks like this...

View attachment 3776792

Voltages now look like this...

1 = 7.47 volts
2 = 2.303 volts
3 = 0.3 volts bleeding down
4 = 0 volts
5 = 0 volts
6 = 2.45 volts
7 = 2.535 volts
8 = 18.65 volts

additionally, putting a signal into the Aux jacks, with the appropriate source selections made, I can just make out the music I was pumping in at the Left speaker... but I mean just barely... I may have mixed up which channel still has the original module in it and which is my breadboarded copy.

Volume control still has no effect.

Balance control seemed to work, as in it took the signal away from the left when I turned it to the right.

First thing that jumps out is that pin 2 voltage is 2.3, yet pin 3 voltage is .3 and falling There is only a 1k resistor between them, and would guess the .3 volts is from leakage. My guess is that there is a bad connection between pin 2 and the base of the first transistor. So all the current going through the 47k resistor is going into the second transistor, and results in higher voltage at pin 6 and lower voltage at pin 7.

 

[Busted Audio]

My next visit to the workshop I'll remake the solder joints on the back of the board, but just checking... you mean at pin 2 it's 2.3?... not pin 1?... I'll go over all joints anyway.

I'll report back when I've done this and metered it again.

 

[sregor]

Yes, it's pin 2.

 

[Busted Audio]

So... this is kinda embarrasing... it wasn't a bad connection between pin 2 and the transistor base... there was no connection between pin 2 and the transistor base!... as in, I had completely failed to put it in... even after it was pointed out as a change I needed to do above, I still didn't recognise that until I went to ring out the connection.

Fixed that...



... voltages are now...

1 = 6.23 volts
2 = 0.601 volts
3 = 0.601 volts
4 = 0.026 volts
5 = 0 volts
6 = 0.81 volts
7 = 13 volts
8 = 18.6 volts

... and the right hand channel now works perfectly!... and sounds glorious!... thank you for the help with this Steve.

Now I need to work out how to package this surrogate circuit into a size that'll fit in the original space, or find somewhere to suspend another board within the casing.

There was a lad on one of the Dutch Audio Forums that had done this...



... and with both surrogate 'modules' in position...



... but I can't visualise the circuit when it's presented that way, so I don't think I could copy it.

I thought about maybe going with a surface mount PCB by drawing up a Gerber File to send to PCBWAY, as I'm sure I can get the size down to the same as the original. But I've never used that kind of service before and would also need to learn how to even create the Gerber.

Using the larger breadboards and going with a larger format is hampered by the selector switch and the long rod that connects it to its control knob, it passes directly over the top of where these modules are located on the main board. I think there is probably room to mount them over the top of the rod, but then I need to be careful about how the connecting wires get around the rod without chafing.

For now, I'm going to remove the left channel module from the board and put some temporary fly-leads in, like I did for the right, and move the breadboard over to check that the left channel is going to be fixed by this also. That'll give me some thinking time on what to do for the permanent fix.

 

[sregor]

Glad you're on the road to success.