Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Pass Labs This forum is dedicated to Pass Labs discussion.

Nakamichi PA-7II won't power on, limiter resistor blown
Nakamichi PA-7II won't power on, limiter resistor blown
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st May 2020, 12:14 AM   #211
Chamberman is offline Chamberman  United States
diyAudio Member
 
Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
You're probably correct on the bias transistor probably being out of the circuit before. Especially now that your bias voltage readings have dropped significantly.

I'm not sure why the bias voltages aren't responding. The DBT dropping the rail voltage may be messing with it, but I would expect it should still be able to bias up. What is the rail voltage right now?

If the biasing transistor was out of circuit and you were only getting +/- 1.1V before reconnecting it then that was your max bias voltage. It will not get better than that.

If the dc rails are still significantly low then you may want to turn the bias setting to its minimum level and take it off of the DBT and bring it up connected directly to the mains to see how the bias voltage responds with the higher DC rail volts. If the DC rail voltage is only a little bit low right now when on the DBT, like say 5 - 10V low, then this will not correct it either. We'll have to look elsewhere.
__________________
KC
  Reply With Quote
Old 21st May 2020, 01:43 AM   #212
Chamberman is offline Chamberman  United States
diyAudio Member
 
Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
After thinking about it for a bit I have another thing for you to check. The fact that TP12 is moving when you're adjusting the bias pot is not a good thing. It makes me wonder if something is wrong with another one of the Q116 - 119 drivers.

Pull one end of R126, R127 & R128. I know Q119 still removed so you don't need to pull its resistor.

Now use your diode check on your DMM to test the Base to Emitter voltage drop on Q116, 117 & 118. Check it both ways. If they all test normal then that's fine. But I expect the B to E drop should be in the 550 - 650mV range for all of them.
__________________
KC
  Reply With Quote
Old 21st May 2020, 02:22 AM   #213
tenelson is offline tenelson  United States
diyAudio Member
 
Join Date: Mar 2015
Location: Santa Fe, NM
The DC rails are 77.7v and -77.7. So, higher than what the schematic says by 3 volts or so, so I'm assuming the DBT is not drooping them.

I wound up having to completely remove R126, R127, and R128. I figured I would measure them while they were out, and they all appear intact at about 1.4 ohms.

Q116 B to E: 0.492v
Q116 E to B: OL

Q117 B to E: OL
Q117 E to B: 0.519v

Q118 B to E: 0.492v
Q118 E to B: OL

So, a little lower than what you were thinking they should be. What do you think?
  Reply With Quote
Old 21st May 2020, 03:06 AM   #214
Chamberman is offline Chamberman  United States
diyAudio Member
 
Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
Because they're all showing low Vbe then they're probably fine. Didn't you say earlier that you thought you still had a couple of new output transistors? If so you should check them to see what the new one measures for the same test.

The 1.4 ohms of those resistors is taking into account your test lead resistance so that too is normal.

When you put the 1 ohm resistors back into the circuit leave out R128 which is the resistor for Q118 which is opposite from Q119 which is removed. This will leave that transistor out of circuit. So you'll only be using Q116/117 for the front end drivers for now. I'm having you do this so as to avoid any imbalance that might result from having two driver transistors in the circuit on one polarity and one on the opposite polarity. It shouldn't matter just for preliminary testing, but just to be safe leave that resistor out for now.

Do you have a 1k ohm resistor? If so install it into the R163 location which is currently a 750ohm resistor in the bias network. This increased resistance should allow for a somewhat higher bias voltage adjustment range. Turn the bias pot to its minimum position and retry powering the amp up with the DBT in circuit.
__________________
KC
  Reply With Quote
Old 22nd May 2020, 03:42 PM   #215
tenelson is offline tenelson  United States
diyAudio Member
 
Join Date: Mar 2015
Location: Santa Fe, NM
Ok, I swapped out R163 for a 1.1k resistor (I'm out of 1ks). Then I reinstalled R126 and R127.

The bias is still stuck at 1.9mV. TP12 varies from 0.275v to 0.394v.

Q119 base: -0.740v to -0.592v
Q116 base: 0.618v to 0.775v

My lone new A1492 measures:

BJT-PNP
hFE=112
Ie=6.3mA
Vbe=568mV

The Q119 I pulled out measures:

BJT-PNP
hFE=106
Ie=6.3mA
Vbe=557mV
  Reply With Quote
Old 22nd May 2020, 11:39 PM   #216
Chamberman is offline Chamberman  United States
diyAudio Member
 
Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
Sorry for the delayed response. I've been off of work this week and catching up on outside projects. Replacing rotting trim pieces and wood on the house today. Why builders ever decided to use masonite as the exterior trim on homes is beyond me. I guess profit is the utmost importance not a long life. I'm guessing that you do not have that problem in Santa Fe.

It looks like I had you go the wrong way with R163, so if you have something a bit lower resistance than 750 ohms you could try that.

But first,
Check and verify that R124/125 are still measuring 3 ohms. After that see what your voltage drop is across R124/125 with the bias pot at its max position.

Just looking at the manual and Nak is showing +/- 0.8V for the base readings of Q116/117 so you should be there of very close. Back when you had > +/- 1V when the bias transistor was disconnected you should've been dropping some voltage across R124/125. Its possible one or both of those were damaged. They are fusible, so if they get over driven then they will open up.
__________________
KC
  Reply With Quote
Old 23rd May 2020, 12:35 AM   #217
tenelson is offline tenelson  United States
diyAudio Member
 
Join Date: Mar 2015
Location: Santa Fe, NM
Seriously no worries about getting some stuff done around the house. It's funny, out place has harieplank siding, which is similar to masonite but is hopefully much more durable due to its structure. We'll see. I spent the day (my day off) getting a bunch of landscaping gravel and sand. Very not fun. Ah well.

Both R124 and R125 are 3.3ohms. Do you think it's possible the bias transistor was damaged when it was partially disconnected?

I swapped out R163 for a 560ohm resistor.

The bias is still stuck at 2mV. TP12 varies from 0.303v to 0.695v.

Q119 base: -1.02v to -0.646v
Q116 base: 0.672v to 1.058v
  Reply With Quote
Old 23rd May 2020, 01:14 AM   #218
Chamberman is offline Chamberman  United States
diyAudio Member
 
Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
Funny our builder used hardiplank on certain areas of the house and then mixed in masonite in others. Needless to say the hardiplank is holding up fine.

What's your DC voltage drop directly across R124/125 with bias at min/max?

I think the bias transistor is okay. It's job is just to essentially turn on more and lower resistance between the pos & neg drive as the temp of the heatsink drives its hFE higher. This will lower the DC bias voltage as heat increases which keeps temp runaway from occurring. The positive temp coeff thermistor they added into that circuit assists the bias transistor. Many amps do not have the thermistor.
__________________
KC
  Reply With Quote
Old 23rd May 2020, 05:11 PM   #219
tenelson is offline tenelson  United States
diyAudio Member
 
Join Date: Mar 2015
Location: Santa Fe, NM
bias pot fully right:

R124 rail side: 79.0v
R124 transistor side: 79.0v
R125 rail side: -79.1v
R125 transistor side: -79.1v, but takes a long time to charge up to that voltage, it starts around -50v and slowly ramps up to the rail voltage.

bias pot full left:

R124 rail side: 78.7v
R124 transistor side: 78.7v
R125 rail side: -78.7v
R125 transistor side: -78.5v, no delay in charging up to this voltage
  Reply With Quote
Old 23rd May 2020, 06:47 PM   #220
Chamberman is offline Chamberman  United States
diyAudio Member
 
Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
Quote:
Originally Posted by tenelson View Post
bias pot fully right:


R125 rail side: -79.1v
R125 transistor side: -79.1v, but takes a long time to charge up to that voltage, it starts around -50v and slowly ramps up to the rail voltage.
Thats very odd. So the rail side comes up to -79.1 right away? However the transistor side of the 3 ohm resistor takes time to come up?

So it sounds like Q117 is conducting for a short period of time when you power the amp up. Or the base of one of the output neg polarity transistors is momentarily conducting or breaking down.
__________________
KC
  Reply With Quote

Reply


Nakamichi PA-7II won't power on, limiter resistor blownHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Nakamichi PA-7 Power Transformer sourceoneaudio Swap Meet 6 25th March 2018 04:54 PM
Nakamichi PA-7II: Thermofuse in the potted transformer electrafixion71 Pass Labs 31 3rd January 2015 02:31 AM
Limiter Resistor of MOSFET Power Amp ervinl Solid State 11 23rd July 2010 06:55 AM
Nakamichi PA-7II manuals - HELP Peteb Pass Labs 4 4th October 2007 07:09 PM
Nakamichi OMS-7II help proteo01 Digital Source 0 17th September 2007 03:12 AM


New To Site? Need Help?

All times are GMT. The time now is 05:52 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki