Tandberg TR2080 troubleshooting

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samgraysound

Well-known member
Joined
Jun 9, 2014
Messages
284
Location
Olympia, WA
Hi All,

I'm working on a Tandberg TR2080. I rebuilt the the R power amp which was extremely damaged. Now it passes signal but the quiescent  current adjustment does nothing.

Can you help me troubleshoot it? I have run out of ideas.

Attached is a schematic
 

Attachments

  • hfe_tandberg_tr_2075_service-1-14-14.pdf
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scott2000 said:
Could the trimmer be bad??

Dang can't see anything.....

all replacement parts the same as original????

Trimmer was fried and has been replaced. I used some equivalent transistors when I could not get the originals.

Sam
 
Pretty simple amp topology... check voltages around the idle current trim transistor, and check output stage current (voltage drop across emitter resistors).

Lots of pretty easy things to check... look for diode drops between base-emitter for power transistors (carefully).

JR
 
JohnRoberts said:
Pretty simple amp topology... check voltages around the idle current trim transistor, and check output stage current (voltage drop across emitter resistors).

Lots of pretty easy things to check... look for diode drops between base-emitter for power transistors (carefully).

JR

Q913 (current trim transistor)
Collector 1.084V
Emitter -0.823V
Base -0.358

B-E Diode drops of 0.06V  on both power transistors

20mv across emitter resistor R959

This is all with trimmer R931 all the way counterclockwise.
 
samgraysound said:
Q913 (current trim transistor)
Collector 1.084V
Emitter -0.823V
Base -0.358
Those voltages look possible... for a NPN bias transistor... the (fuzzy) schematic looks like a PNP, but the circuit and voltages look consistent with an NPN there. 
B-E Diode drops of 0.06V  on both power transistors
0.06V is too low for a power transistor in any kind of forward bias.
20mv across emitter resistor R959
I can't read numbers but 20mV seems in the ballpark.
This is all with trimmer R931 all the way counterclockwise.
Your measurements seem inconsistent.

If you have 1.8V across the bias string, it is unlikely to have 0.06 Vbe in power transistors. BUT you have darlington connection with driver transistor Vbe in series too... look at driver Vbe also.

JR
 
JohnRoberts said:
Those voltages look possible... for a NPN bias transistor... the (fuzzy) schematic looks like a PNP, but the circuit and voltages look consistent with an NPN there. 

It is a NPN. Attached is a clearer schematic. I guess when I split off this page originally it degraded the image quality.

JohnRoberts said:
0.06V is too low for a power transistor in any kind of forward bias. I can't read numbers but 20mV seems in the ballpark. Your measurements seem inconsistent.
Sorry, I think I misunderstood what kind of test you were asking for. I did the diode drop test on my multimeter, with the amp off and transistors in circuit. Were you were asking for the voltage drop B-E with amp on?

Q927 Vbe = 0.248V
Q929 Vbe = 0.562V

JohnRoberts said:
I can't read numbers but 20mV seems in the ballpark.
The service manual wants you to set the voltage drop across R959 to 40mV. On the working channel I can adjust it well past 1v. But on this channel it maxes out at 20mV

JohnRoberts said:
If you have 1.8V across the bias string, it is unlikely to have 0.06 Vbe in power transistors. BUT you have darlington connection with driver transistor Vbe in series too... look at driver Vbe also.
Q925 Vbe = 0.532V
edit Q923 Vbe = 0.549V
 

Attachments

  • hfe_tandberg_tr_2075_service-1-14-14.pdf
    267.1 KB · Views: 4
925 and 929 at roughly 0.5V looks good.

927 at more like 0.250 not so good

what is 923 voltage, should be similar to 925?? 

I can't even find a 926? but I stopped looking when it wasn't in area I am interest in.

JR

 
JohnRoberts said:
925 and 929 at roughly 0.5V looks good.

927 at more like 0.250 not so good

what is 923 voltage, should be similar to 925?? 

I can't even find a 926? but I stopped looking when it wasn't in area I am interest in.

JR

Typo. I meant Q923 not Q926.
 
JohnRoberts said:
OK 927 voltage is still low...

but what is voltage collector-emitter on the bias trim transistor on the working channel?

JR

Collector = 1.23V
Emitter = -1.16V
Vce = 2.395V

the working channel power transistors Vbe are 0.629v and 0.594.

This is all with the trim set to 40mv across the emitter resistor.
 
samgraysound said:
Collector = 1.23V
Emitter = -1.16V
Vce = 2.395V
OK... 2.4V in the good channel and 1.8V in the not good channel... that's the rub

The bias trim transistor is what we call a Vbe multiplier. The way it works is pretty simple. The base -emitter voltage is set by semiconductor physics at roughly 0.5V (give or take).  The total resistance across base-emitter defines a current (using ohms law). This current must be supplied by the resistance from base to collector. So the base -emitter current, times the base -collector resistance + the base voltage = the emitter-collector voltage. A little awkward description but this trimpot varies the voltage across the output bases...

These trimpots can go bad... At least they did the design correctly (open trim pot wiper = minimum drive voltage).

CHECK THE TRIMPOT

JR

the working channel power transistors Vbe are 0.629v and 0.594.

This is all with the trim set to 40mv across the emitter resistor.
 
JohnRoberts said:
OK... 2.4V in the good channel and 1.8V in the not good channel... that's the rub

The bias trim transistor is what we call a Vbe multiplier. The way it works is pretty simple. The base -emitter voltage is set by semiconductor physics at roughly 0.5V (give or take).  The total resistance across base-emitter defines a current (using ohms law). This current must be supplied by the resistance from base to collector. So the base -emitter current, times the base -collector resistance + the base voltage = the emitter-collector voltage. A little awkward description but this trimpot varies the voltage across the output bases...

These trimpots can go bad... At least they did the design correctly (open trim pot wiper = minimum drive voltage).

CHECK THE TRIMPOT

JR

Also perhaps some relevant information:

The range of adjustment for Q913 on not working channel:
Collector = 1.06v to 1.09v
Emitter = -.775v to -.824v

On the working channel:
Collector= 0.408V to 1.4V
Emitter= -0.365 to -1.33v
 
samgraysound said:
Also perhaps some relevant information:

The range of adjustment for Q913 on not working channel:
Collector = 1.06v to 1.09v
Emitter = -.775v to -.824v

On the working channel:
Collector= 0.408V to 1.4V
Emitter= -0.365 to -1.33v
With power off, measure Base-emitter and base collector with VOM...

Trimpot is apparently AFU (or bad solder connection).

JR
 
JohnRoberts said:
With power off, measure Base-emitter and base collector with VOM...

Trimpot is apparently AFU (or bad solder connection).

JR

Base-Emitter = 150ohm
Base-Collector = 470ohm

Trimpot is brand new 100ohm. Old one was blown up. Along with resistors R929, and R933. Q913 was also replaced with a BC548C. Just checked that on my transistor tester again and double-checked the pinout.

Trimpot tests at 0ohm when all the way counter-clockwise. 107ohm when clockwise. R929 reads 470ohm, R933 reads 150ohm.
 

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