My first receiver restoration attempt: G-6700.

[OverLoad]

Are you're taking voltage measurements with the DBT in use? :yes: Although the DBT is mainly used for current limiting, the DBT will reduce the main voltage as well (especially a 60W). It's coming out of protection so I would move up to a 100W bulb and confirm that the bias adjustment are responding. If you're OK here then reduce the bias to MINIMUM. Remove the DBT and go for full mains power up. NOW you can make your amplifier adjustments.

 

[DCinDC]

^^ that!


oh and congrats!

You're an old hand now!

 

[ghazzer]

You are faster than I am! I went thru the whole house, and the garage, and there is not a 100W bulb to be found. I have a bag of bulbs from traffic lights and there were some 135W bulbs there.

A 75 W bulb raised the main caps up to ±42.5 and virtually no change in the other readings.

The 135W bulb raised the main caps to ±51.3, the R15/16 and R17/18 readings are up to ±18.2, and the emitter resistors to -1.91 (left) and +2.26VDC.

Now I need to go back and see if I can adjust the bias current.

Is it correct to have the junction between the two emitter resistors at opposite polarities between the left/right channels?
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[DCinDC]

Y
Is it correct to have the junction between the two emitter resistors at opposite polarities between the left/right channels?
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no

set the bias to minimum and then adjust it without the dbt.

 

[Overundr1]

Turn the leads around

 

[Overundr1]

Those readings don't make sense? 2v squared/.33 ohm equals 12 watts which would smoke those emitters. Using mini grabbers attach one to each end of one of the emitter resistors and report back with reading. Don't ground out those leads or bad things will happen.

 

[ghazzer]

Those readings don't make sense? ... Don't ground out those leads or bad things will happen.

The reading of 2.2vdc was taken with the negative probe on chassis ground.
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Sorry I didn't get anything accomplished last nite. I decided to goof off and watch the bowl game between Bama & ND.

This morning I removed the DBT and checked the bias trimpots to ensure that they were at a minimum. Since I still have the 3058 boards hanging loose I decided to check the voltage across both of the emitter resistors in both channels. This is a little confusing since Sansui used the same board for each channel but reversed the nomenclature of the TRs between channels, so I will use the TR type instead of the nomenclature.

Right Channel -
Emitter resistor tied to the NPN OP TR (2SC2581): 2.9mv
Emitter resistor tied to the PNP OP TR (2SA1106): 0.1mv

Left Channel -
Emitter resistor tied to the NPN OP TR (2SC2581): 0.1mv
Emitter resistor tied to the PNP OP TR (2SA1106): 2.9mv

The DC Offset at the speaker terminals is +2.28VDC Right and -1.932VDC Left, or approximately the same as I measured between the emitter resistor junctions and ground in yesterday's post.

This morning I read LbPete's thread about Bias Currents and DC Offsets & I think someone said that the bias readings would be the same across each of the emitter resistors in a channel, so I am not gonna touch this mutha again until some one can tell me what is wrong with these readings, or that they are AOK.

The attached schematic is something I started working on because the SM I DL'd from Vinyl Engine was difficult to read. I later bought a better 'PDF file on line and used that to finish this off. Please note that L01,R1,R3,C3&R81 have been added to this part of the drawing from the Main Amp side, for clarity. Same for the right channel. I have checked it numerous time but cannot say that it is 100% correct (but neither can Sansui!), so please let me know if you spot an error.

Please PM me if you want a TurboCadd copy of this.
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[Overundr1]

Offset--- adjust vr1 and vr2 for null (0) volts. Those two trimpots should have been centered for initial startup. They will also affect the bias readings a bit so leave the bias trimmers where they are for the moment. Getting close.

 

[ghazzer]

Lord Love a Duck! ! ! I think I just fried one of my new OP TRs! Time to go take a break and read a book.

I got the DC Offset back to ~0.0mv in both channels, but they were moving around a little. Then I pulled out another DMM and put the grabbers on to go check the bias in the Right channel. It read ~35mv so I decided to look at the left channel before I did any tweaking. I think I must have shorted the collector to the emitter when I was removing the grabber. Saw an arc. Heard a pop. Saw some smoke. All before I could shut anything down.

The 3058 boards are still hanging loose and I had better access to those jumpers that I do to the "test points". Bummer!

Put it back on the DBT with 135W bulb - Bright, then dim.
Safety Operate light does not come on
Main caps are ±48.3 instead of the ±51.3 I had yesterday
All the fuses are OK

I'm just gonna back off and take a fresh look tomorrow.
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[ghazzer]

It took a while to get back to this thread, partly thanks to a virus or malware, but here is my summary:

First I checked the NPN OP TR (2SC2581) on the right ch F-3058 board because I think I shorted the base to the collector. The readings I got indicated an open, so I pulled the TR. Tested out of the circuit it looks good so I checked the PNP OP TR (2SA1106) and ……

Replaced the 1106 and tried it again, but the safety operate light and relay do not come on. Main caps are still at ±48. I checked a lot of things but could not find anything specific that had failed. I have misplaced a page of notes, so cannot tell you everything I tried.

The waveform widget was showing TR18 as more of a capacitor than semiconductor junctions and I decided to pull it for a clean check. Surprise: What should have been a 2SA968 was really a C3229, so I pulled some more of them.
TR17 was a NEC-B356, not a 2SA968.
TR15 was a NEC-D381, not a 2SC2238.
TR16 was a NTE-292, not a 2SC2238.

All of these TRs seem to check OK, but the specs say they were not good choices for substitutes. Sounds like the repair tech cleared out his breadbox rather than order replacements.

Then I was getting a 1µƒ capacitance waveform across the empty C-B socket of TR18, and across C40 (33pƒ), in the right channel. C39 (left channel) showed a waveform like a semiconductor.

I moved on to TRs9&10 and found the same scenario with the 22pƒ caps across them (C27&28). Removing TR9&10 (they were 2SC2071s as the schematic said) made The readings across C27 & C39 were now an open circuit. I removed C27&C28 and C39&C40. C27&39 still read open, while C28&C40 read like a 1µƒ cap. There are some odd “back door” paths that could be causing these readings, but I cannot figure it out.

Ohm meter readings across all four empty sockets are open (0.L), but C28&C40 (right ch) read as a momentary short (00.00 with auto ranging DMM), then rapidly climbing in the MΩ range before settling as open (0.L).

The four 220µƒ,80V caps (C29-32) all read the same and have almost identical waveforms.

Zener diodes D604 & 605 are in parallel with C628 & C629 and read as a short, then ~1K, then climb slowly as the cap charges up. The waveform is a pure ellipse, vertical.

Still getting some anomalous readings, so I removed R37&38 so I could get a clean reading on D701&702. The waveform looks like a pure semiconductor junction in both directions. D701 reads open (0.L) in both directions, but D702 (right ch) reads open anode to cathode and 22.7 MΩ cathode to anode.

I guess I’d better start looking for a replacement STV-3H.
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[Overundr1]

Ahhh, those pesky stv3h's. Takes a good dmm to test them. Anyways, if you are unsure there is a workaround that will let you test the circuit before spending $$$ for replacements. Use two 1n3593 diodes in series being carefull of anode/cathode orientation. Will track reasonably close to the stv-3h albeit @1/10 volt drop low which will not hurt anything. Obviously don't let the amp run for hours with them floating about since they are not thermally tied to the heatsinks.
dbt time again.

 

[ghazzer]

After further testing I have concluded that the STV-3Hs are both OK.

I did find that TR08 E-C only reads 40Ω both ways and C-B reads ~5KΩ, effectly a short when the other 2SA969s read open and ~ 25MΩ.
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[ghazzer]

I also found that R36 is reading 12Ω instead of 150Ω and that R44 is open. I believe that these went out when I shorted some points with a probe when trying to adjust the bias. My notes say that they checked OK during initial testing.

Here are the TRs that I will be ordering, formatted as a table in a WORD document. The data for the original TRs is hilighted in yellow and the name of the selected substitute is hilighted in yellow. I have tried to match the original specs as best I can, and would appreciate hearing from anyone who has had a bad experience with any of my selections.

The KTA968 looks like a better sub for the 2SA968, but I could not find a good source in the US. Plenty of places in China offered them at a range of $0.26 to $3.00 each. I will avoid that, thank you!
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[Overundr1]

Use the mje 15032 and 15033 subs.

 

[DCinDC]

Yeah
I put those things in everything....

 

[ghazzer]

Sounds like the best analogy is to use a 426 hemi when a 240 Ford would still get you to the church on time. More reliable, and plenty of oomph left in reserve. Am I right to assume that the driver circuitry will keep the MJEs operating in the optimum portions of the curve to give the desired performance? That is, just because they are rated at twice the power (50W) and 5+ times the current does not mean that they are going to burn the veneer off the case.

 

[ghazzer]

I have ordered replacements for all the TRs in the driver section, some of the resistors, and most of the caps on the 2980 board. While I was at it, I did a little research on the G-4700, identified suitable subs for the driver TRs, and order them as well.

The suggested sub for the 2SD357 in the G-4700 is KSC2690. It suprised me to learn that the 2690 is a TO126 (ECB) instead of the TO220 (BCE) of the 357, but I guess that it can just be rotated 180° in the socket. Pc of the 357 is 10W and the 2690 is 1.2W, so maybe this is not a good sub for TR601 (G-4700) which is mounted to a massive heatsink.

It will be like Christmas when all these parts come in - - -
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[petehall347]

 

[ghazzer]

Post #1,000!

This is my 1,000 post here on AudioKarma, and I am really enjoying the ride. THANKS - - -

I never said I was fast!

It has taken a long time, but I carefully listed all the components in the driver section of the G-6700, then added all the electrolytic caps. Then I spent weeks looking at the options for replacements for the 1970s transistors. Before ordering the parts I decided to include the two G-4700s I have waiting in the wings, thus consuming more time.

Parts were ordered from Mouser, MCM Electronics, and Tayda Electronics (Bangkok) (no affiliation with any of them) I carefully avoided the temptation to get some good deals on fleabay after reading some reports of people getting scammed with counterfeit parts.

One of my dilemmas has been the F-3058 boards that the output transistors are mounted on. Mine must have been worked on by the ‘technician from hell’, or a well meaning enthusiast like me, but with even less skills. The insulation of the jumpers were burnt in several places, tracks were lifted, some tracks were missing, and the solder work was really sloppy. Recently I lucked out and found a pair of boards on fleabay where someone was parting out a G-6700, so I quickly pounced on them – they are in very good shape.

Bonus Hour: Testing reveals that the original Sanken TRs are OK on the new boards.

These pictures show the stash of parts that I have now. Many of the ¼W, 1% resistors and ceramic caps were so cheap that there was a minimum order of 10 each, plus I stocked up on some common values “just in case”.

Remember that this is for 3 receivers, plus spares, plus some for the stockpile:

Capacitors
{EDIT: 3 photos of parts lost on PhotoBucket}

Transistors

Resistors

Replacement F-3058 board {P1030252}



Now I need to knuckle down and start getting some of these installed.
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[Overundr1]

Glad to see you back at it, keep up the progress reports.
-Lee