My first receiver restoration attempt: G-6700.

[Overundr1]

Voltage readings on pin 01 and pin02 ground referenced and red probe to pin black probe to ground. Do NOT up from 60w bulb and be relatively quick if you can getting the readings.

 

[Overundr1]

Both channels mis behaving would indicate a wiring error thus needing the polarity and voltage readings as a start.

 

[princever]

Looks like things are coming along Steve. I have been following your thread very closely. I personally would set the bias pots at zero ohms for minimum bias myself. The voltage drop at a set current across the bias pot + The STV diode is what sets it up. So the lowest resistance on the pot means no voltage drop across there, so just what the STV diode gives you by default as a minimum. Remember the bias reading is taken across the emitter resistors, so its not the minimum current through the STV diode you are trying to achieve but the current flowing through the emitter resistors with no signal. Making the outputs less "on" means making the drivers less "on" which means making the voltage difference between the base of the negative driver and the base of the positive driver less. Assuming everything else is working.

I also tend to take output transistors out of circuit and make the voltages at the base solder pads something reasonable, like +0.55V and -0.55V (REF ground). you can also roughly adjust the DC offset this way (so the base of the NPN and PNP are roughly equal but opposite) without destroying something expensive or unobtanium.

Sorry if that first part was rambly. I can try to clarify if needed.

Also didn't mean to intrude on the thread.

 

[ghazzer]

Lee: Please clarify

Voltage readings on pin 01 and pin02 ground referenced and red probe to pin black probe to ground. Do NOT up from 60w bulb and be relatively quick if you can getting the readings.

DBT remains in place, with 60 watt bulb. Volume is at zero. Mode selector was at FM Auto, but I just changed that to Phono 2.
I connect the DMMs to pins, components or test points with power off, then turn it on long enough for the readings to stabilize and safe operate relay to engage. Then I turn power off.

Pins 01,02 &03 of the 2980 board are all the same, and tied to the 7.2vac windings of the transformer.

Readings for main caps were taken across the 3.9KΩ(2) resistor tied to their terminals, effectively from ground to the + or - terminal. All other readings were taken with the negative probe clipped to the black wire of the transformer, the center tap of the 33vac windings, used as ground throughout the P/S.

During troubleshooting I took the screw out of the main caps (non-ground terminal) because the DMM readings were climbing indicating a charging cap somewhere. Readings stabilized when I took the main caps out of the circuit. Screws have been replaced.

The only other wiring that was changed is the jumpers between the 2980 bd & the 3058 bds. I will re-check that. Will also look for any wiring that might have been disrupted in the YEAR since I started working on this thing.

The one time that I had it up & running (when I shorted something on the 3058 bd) the safe operate relay was stable, but I did not think to take any voltage readings before I sparked it. This was the first time it has been powered on since then.

Are there any specific voltage readings that would help to diagnose this problem?

 

[Overundr1]

Pin 01 and 02 on the F 3058 board.

 

[ghazzer]

... I personally would set the bias pots at zero ohms for minimum bias myself. The voltage drop at a set current across the bias pot + The STV diode is what sets it up. So the lowest resistance on the pot means no voltage drop across there...

This sounds like I may have the Bias trimmers in backwards. They are now installed such that full CCW puts the entire ~100Ω in the circuit. Perhaps I misread what I was told earlier, but that would not explain the 27 VOLTS I read for the DC offset.

... I also tend to take output transistors out of circuit and make the voltages at the base solder pads something reasonable, like +0.55V and -0.55V (REF ground). you can also roughly adjust the DC offset this way (so the base of the NPN and PNP are roughly equal but opposite) without destroying something expensive or unobtanium.

Temporarily replacing the 0.33Ω(5) with 100Ω(5) emitter resistors should be protecting the output TRs. Overundr1 & Kevzep said that would increase the bias readings by 300X (100/0.33), but the bias readings I got were ~0 mv.

ALSO: I am getting more confused. The SM says Bias readings are the same points on the G-5700 & G-6700 and different on the G-7700, but the schematics are the same for the G-6700 & G-7700 while different for the G-5700. I have been measuring between the juncture of the two emitter resistors (pins 59&60) and the emitter of TR19/20 (pins 89&90). This is effectively measuring across one (1) of the emitter resistors (R602 in the schematic in post #95). Will someone please confirm this for me.

 

[ghazzer]

Pin 01 and 02 on the F 3058 board.

Pin 01 is orange jumper and pin 02 is brown jumper.

BOTH 3058 bds measure positive 85vdc and climbing when safe operate relay clicks in, which is when I remove power.

 

[Overundr1]

Pin 01 is positive and pin 02 should be negative referenced to ground yes?

 

[princever]

I just started looking at this service manual, and it may just be the hardest thing to follow that I have ever seen. On the schematic itself, they list the bias voltage in mA for one model and mV for another, and it seems like whoever drew the schematic was having fun and really should have gotten a job designing mazes instead. Pin 85/86 doesn't even exist on the 6700 schematic, and on the 5700 it is the same thing as point A/B so of course theres no voltage across that. on the schematic TR19 is an NPN and TR20 symbol is a PNP but they are the same thing, just on opposite channels. I can sit here and continue counting errors.

Instead of their backwards labeling and procedures, what I would do is clip my leads across one half of the emitter resistor, and since they say the Bias current should be 10mA and you know its a 0.33 ohm resistor, thats 3.3mV across one, 6.6mV across both halves. Thats with the stock 0.33 ohmers.


But, I think there is a bigger problem here that needs to be addressed first. 27V at the output isn't a bias setting or measuring issue. There is something else wrong. I wish I could be more helpful but I haven't worked on the 5700/6700/7700 before and my 4700 is very different and much simpler. I think Overundr1 has more experience it seems.

 

[Overundr1]

Yup, this series has high aggravation factor, rebuilt more than I care to think about.

 

[Overundr1]

What we are trying to first verify is that the rail voltage is correct in both polarity and value to the two output transistors before backtracking through the circuit. Both channels have the same issue thus whatever is incorrect is wrong on both.

 

[Overundr1]

Remember this reciever was a basket case when Steve started, that many of the wires were dis connected, the two circuits are mirror imaged etc etc.

 

[ghazzer]

I just started looking at this service manual, and it may just be the hardest thing to follow that I have ever seen....

OK. See if you can follow this:

Coming off the main caps, brown is positive and orange is negative.
Positive (brown) goes to the L/R channels of the 2980 bd at pins 51&52.
Negative (orange) goes to the L/R channels of the 2980 bd at pins 65&66.
Positive (brown) is jumpered to pin 1 of the left channel 3058 bd and pin 2 of the right channel bd.
Negative (orange) is jumpered to pin 2 of the left channel 3058 bd and pin 1 of the right channel bd.
This does not agree with the schematic, but was probably done to match the jumpers up with the wiring on the 2980 bd. The L&R channels are (mostly) a mirror image of each other and the NPN & PNP output TRs are at reversed ends of the L&R 3058 bds, to keep them aligned with their connecting components on the 2980 bd.
Positive (brown) goes to the collector of both NPN output TRs (P-PNP).
Negative (orange) goes to the collector of both PNP output TRs (N-PNP).

Either all this is correct or I have spent WAAAAY too many hours staring at the schematic and the boards.

 

[ghazzer]

What we are trying to first verify is that the rail voltage is correct in both polarity and value to the two output transistors before backtracking through the circuit. Both channels have the same issue thus whatever is incorrect is wrong on both.

Per the previous post, I believe that the collectors of all 4 output TRs are connected to the correct main caps.

I also checked the points where I am measuring the bias voltage and they are across one of the emitter resistors.

 

[ghazzer]

The voltage at the speaker terminals starts climbing to 27.5V at power on. When the Safe Operate relay clicks on, it clicks off right away and the voltage drops to ~17V and climbs back to 27.5V. The relay clicks on, then off, and the cycle repeats.

 

[Overundr1]

Verify zener voltages at zd604 +18.8v and zd606 -18.3v.

Verify C628 100uf 25v is oriented with + towards R16
Verify C629 100uf 25v is oriented with - towards R18

I need the actual resistor values of R10 through R18 to work out the values to measure in the divider circuit

-Lee

 

[ghazzer]

I may have popped something – DC offset still climbs to +27.5VDC, but the SO relay no longer clicks on, then off.

Verify zener voltages at zd604 +18.8v and zd606 -18.3v.

ZD 604 reads +17.7VDC & 1.2mv
ZD 605 reads -17.7VDC & 2.0mv

Cannot locate ZD 606 on the board. The side of R614 that S/B connected to ZD606 reads +12.75VDC. The other side of R614 has been lifted from the designated hole in the PCB, but still tied to hole #49. S/B +44VDC but reads +33.5VDC

Verify C628 100uf 25v is oriented with + towards R16
Verify C629 100uf 25v is oriented with - towards R18

I think (hope) you got these resistors reversed.
C628+ is tied to R17&18, then R616, then B+
C629- is tied to R15&16, then R617, then B-

I need the actual resistor values of R10 through R18 to work out the values to measure in the divider circuit

R09&10 = 2.7KΩ
R11&12 = 470Ω
R13&14 = 33Ω
R15&16 = 3.3KΩ
R17&18 = 2.7KΩ

I double checked the color bands on the resistors, and ohm'd them in the circuit. I make it a habit to check component values with a meter before installation, but it is not like I have never made a mistake.

 

[Overundr1]

Re-visited the entire thread. Lets back up a bit and all get on the same page.

VR01 and VR02 need to be initially set to @50ohms across the wiper and checked with your dmm. If at 0ohm (or 100ohm) you will get a high offset reading.

Not too worried about bias yet HOWEVER lets make sure that we are measuring correctly. Reference your post #89. Attach mini grabber leads to the OUTBOARD ends of the two emitters X------.-----X to measure the bias being careful not to ground the board.

Something to think over. If the relay is not engaging how are you getting 27vdc at the speaker terminals?

Wiring to the outputs looks correct.

-Lee

 

[ghazzer]

I have verified that VR01 & VR02 are centered.

The almost identical readings of 27.5V for DC Offset is confusing. I took a lot of resistance readings between the left & right signals, starting at the inside point where the wires are tacked to the speaker connectors. After about an hour I realized that the probes of my oldest DMM were still inserted into the speaker connectors on the outside. Even tho the DMM was off, some of the readings inside were now different.

It was time to get another eggnog and get ready for Rudolph.

 

[kevzep]

Something to think over. If the relay is not engaging how are you getting 27vdc at the speaker terminals?

Wiring to the outputs looks correct.

-Lee

Thats exactly what I was thinking.........

Are you measuring pre-relay?

I'm letting Lee take lead on this one, and will only jump in from time to time so as not to confuse the train of diagnoses....

If you say this voltage slowly climbs to 27VDC, this would suggest there is no signal earth on the front end of the driver circuit, the signal going into the JFET's.

Food for thought....