Nikko TRM-1200 Pop and distortion

[JGamez]

First and foremost...hello to all! First post and all...

I've decided I needed to make my life more complicated and full of disappointments so I've started buying vintage amps. My first foray into this world is a very clean Nikko TRM-1200.

Everything mostly works and I've gotten rid of some minor issues by cleaning the pots but one major issue remains. When turning on the amp, there is a loud POP on the right output and the vu meter spikes monetarily. During playback, audio is distorted on that channel but only on that channel.

I should also note that when using the Nikko as a pre-amp, everything is perfect...no distortion.

I'm a novice so I could use some guidance...where do I start? Has anyone had this particular issue with this amp? Thanks for your time!

 

[JGamez]

So I just read how to check DC offset...

Left: 0.0 vdc
Right: 10.50 vdc

Oh boy.

 

[BilboBaggins]

Yep! Hopefully it is just an adjustment, but my thought is something else is causing it, like a bad cap.

I once had that TRM-1200, and I think it is a gorgeous unit that has good power for its ratings. The only fault with mine was it didn't have all its original knobs. Really well made too!

 

[sregor]

Probably bad output transistor or diode. The output stage is fairly simple, but to get zero offset both the top and bottom parts have to match - 10V DC means one of them is out of sorts. If you're lucky, it may just be a bad resistor.
Any DC offset problems are only in the output stage (nothing on the other side of the drive transformer affects offset)
There are no adjustments for bias and offset. There is a service manual available at hifiengine.com . Free, but have to register.

 

[c.coyle]

So I just read how to check DC offset...

Left: 0.0 vdc
Right: 10.50 vdc

Oh boy.

So, you have 10.5 volts of DC offset on one channel. And the speaker relay is clicking at power up? It's not staying in protection?

 

[JGamez]

So, you have 10.5 volts of DC offset on one channel. And the speaker relay is clicking at power up? It's not staying in protection?

No. The relay is working as intended...once that switches on though, the spike on the right channel occurs.

Probably bad output transistor or diode. The output stage is fairly simple, but to get zero offset both the top and bottom parts have to match - 10V DC means one of them is out of sorts. If you're lucky, it may just be a bad resistor.
Any DC offset problems are only in the output stage (nothing on the other side of the drive transformer affects offset)
There are no adjustments for bias and offset. There is a service manual available at hifiengine.com . Free, but have to register.

OK. I have the service manual but there is a learning curve I have to overcome...if the problem is in "the output stage", should I look for the problem on the " audio amplifier " PCB/schematic?

Thanks to all for your help!

 

[c.coyle]

No. The relay is working as intended...once that switches on though, the spike on the right channel occurs. . . .

Then at that point the relay should disconnect the speakers. The relay is just one part of a circuit that is designed to constantly - not just at start up - look for DC at the speakers, and to quickly disconnect them when that DC gets dangerously high. If the protection circuit doesn't do that, then I would say it is not working correctly.

EDIT: Unless the spike is so brief that the circuit can't react quickly enough.

 

[sregor]

The relay is just a turn on delay muting. There is no DC protect function in it. The output stage I referred to is the section after the driver transformer. It consists of the secondary of the transformer, A string of resistors and diodes and output transistors. There is no board picture in manual for the output stage. Look for 3 Watt (relatively large) 600 ohm resistors, 12 ohm resistors, 0.47 2W (also large) components probably soldered to lugs.

 

[c.coyle]

The relay is just a turn on delay muting. There is no DC protect function in it. The output stage I referred to is the section after the driver transformer. It consists of the secondary of the transformer, A string of resistors and diodes and output transistors. There is no board picture in manual for the output stage. Look for 3 Watt (relatively large) 600 ohm resistors, 12 ohm resistors, 0.47 2W (also large) components probably soldered to lugs.

Aha. Transformer coupled? Shouldn't have assumed. How then would DC get to the speakers?

 

[redk9258]

Are you sure you are reading volts and not millivolts?

 

[JGamez]

Are you sure you are reading volts and not millivolts?

Volts...I couldn't get a reading otherwise as I was off the scale. Of course I could be using my multimeter incorrectly...

The relay is just a turn on delay muting. There is no DC protect function in it. The output stage I referred to is the section after the driver transformer. It consists of the secondary of the transformer, A string of resistors and diodes and output transistors. There is no board picture in manual for the output stage. Look for 3 Watt (relatively large) 600 ohm resistors, 12 ohm resistors, 0.47 2W (also large) components probably soldered to lugs.

I think I found what you are referring to...

Some of them are indeed out of spec so I'll order them and change them all. The grouping on the right are the worst of them so its likely the right output channel that's given me problems.

I also came across something odd...the transistor mounted to the heat sinks are different... The ones on the left channel look new really but the ones on the right (Hitachi) look original. Should I change those out as well?



Again, thank you!

 

[sregor]

Aha. Transformer coupled? Shouldn't have assumed. How then would DC get to the speakers?

The transformer is driver, not output. The secondary goes to the base of output transistors with the other side of the secondary to a DC bias voltage. Zero offset depends on both transistors on the output stage having the same current.

 

[sregor]

Volts...I couldn't get a reading otherwise as I was off the scale. Of course I could be using my multimeter incorrectly...

I think I found what you are referring to...
View attachment 956167
Some of them are indeed out of spec so I'll order them and change them all. The grouping on the right are the worst of them so its likely the right output channel that's given me problems.

I also came across something odd...the transistor mounted to the heat sinks are different... The ones on the left channel look new really but the ones on the right (Hitachi) look original. Should I change those out as well?
View attachment 956168
View attachment 956170

Again, thank you!

I would check them first, then change them if one tests bad or they test significantly different. There is a thread somewhere on testing transistors. You might want to pull them and refresh the thermal paste (clean them, then just a minimum coating is recommended. If you want to, you could change them anyway.

 

[c.coyle]

The transformer is driver, not output. The secondary goes to the base of output transistors with the other side of the secondary to a DC bias voltage. Zero offset depends on both transistors on the output stage having the same current.

Now I understand. Cool.

 

[JGamez]

Fixed it!

I read up on how to test transistors and while testing them, I discovered that a wired connection near the bottom of the output stage was loose. After a bit of logical reasoning, I figured out where it was supposed to go.

Now when it starts there is just a light pop on both speakers and there is no distortion!

I checked DC offset after repair...
Left: 7mA
Right: 4.5 mA

Ive still got a little work to do though...I broke one of the potentiometers and the contacts on the paddles need cleaning but overall, it works! Thank you all for your help!