SA-7800 restore
- Thread starter loweran
- Start date
loweran
Super Member
Parts are in, but doubt I will have time to work on it with Christmas on-goings at play. Just want to verify that on the trimmers, I need to turn the fully counter clockwise to start. On the 50K trimmers, that reads 50K ohms on both the middle and outside leads. This model ties the middle with on of the outsides.
that is correct - MAXIMUM resistance.
Are the GOOD channel ones behaving the same on your meter? Sometimes it's not the resistor, it's the meter. Measuring 0.2 ohms is HARD, unless you are using an ESR meter - Then with wirewound resistors, can you TRUST the reading since the ESR meter uses AC, the "blue" meter for example uses 150khz and I DO see inductance effects!!
But yes, 5w wire wound will do if it fits. Not intended to blow up to limit damage.
loweran
Super Member
No its not an ESR meter. All the emitters measure the same. I guess I can put them back in, install the new components to fix the issues and fire it up on a DBT to see how it behaves. I assume I should go ahead and replace all the outputs to make them equal since I already have the good channel unsoldered. I will need to make up some leads for them and install them. Not going to be able to look at this until next weekend. I went ahead and ordered the replacement emitters just in case. I am tired of having this one sitting around and ready to get it back up and running.
Mark, once I get it repaired, I will be replacing the caps and some of the transistors that are listed as suspect. I have all the parts to replace all the transistors except the drivers and the doubles. Would you recommend a full replacement, or just the ones on the top 10 list?
Mark, once I get it repaired, I will be replacing the caps and some of the transistors that are listed as suspect. I have all the parts to replace all the transistors except the drivers and the doubles. Would you recommend a full replacement, or just the ones on the top 10 list?
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NOT straightaway. There is no feedback connection at the driver level, it is formed by the presence of the outputs.
SO, to do it:
It would need 2 jumpers, one for each channel. And they MUST be removed BEFORE installing the outputs!!!!
The left channel has one end of the first jumper connected to the trace between the resistors R57 (150 ohms 1/2 w) & R59 (150 ohms 1/2 w)
and the other end of the first jumper connected to the trace between the resistors R61 (0.22 ohms 2 w) & R63 (0.22 ohms 2 w)
The right channel has one end of the second jumper connected to the trace between the resistors R58 (150 ohms 1/2 w) & R60 (150 ohms 1/2 w)
and the other end of the second jumper connected to the trace between the resistors R62 (0.22 ohms 2 w) & R64 (0.22 ohms 2 w)
Possible mica, mouser.com 534-4661
The mt-200 mica insulators I've been getting from eBay.
Shown ON the transistors, along with the 534-4661 as appropriate.
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Sigh.... I was only using the pictures as an illustration of the transistors involved and their attendant mica's and the mica's sources.
like the 4661 - since I am unsure what you are using as transistors and what was originally in there.
Nits cannot be picked without pictures. Period.
like the 4661 - since I am unsure what you are using as transistors and what was originally in there.
Nits cannot be picked without pictures. Period.
loweran
Super Member
Shew, that was terrifying! Powered on on the DBT. Got a dim bulb and I got relay click, tested both sets of trimmers and the meter responds! I did not put in the new outputs on the good channel yet. Next steps unless otherwise advised will be to check the power board voltages, and move to recap and replace known suspect transistors.
loweran
Super Member
Power voltages look good. I powered it down took a look at the output install again. I used fairly large 16 ga stranded wire. I think I am going to get some 22 ga solid wire and redo them. They dont look so pretty right now. I also prodded them again with the MM and got continuity on the collector of one. I unmounted it from the heat synch and pulled it away only to still get a beep on the MM. Not sure what that is about, but I think I will clean them up and check again.
Once that is settled, I am going to rebuild the power supply.
Edit: Upon inspection, it looks like the foil for Q4 goes to pin 13 that goes across a .01 cap to ground. Would that be the cause of the continuity with the collector and the heat sink?
Once that is settled, I am going to rebuild the power supply.
Edit: Upon inspection, it looks like the foil for Q4 goes to pin 13 that goes across a .01 cap to ground. Would that be the cause of the continuity with the collector and the heat sink?
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loweran
Super Member
Dang it, I didnt order all the caps I needed. Placed another order. Got all new outputs in, recapped the power supply, cleaned the relay, cleaned and tightened the fuses, replaced Q11 - Q16 and Q19 - Q22 on the amp. With power off, had a spark and the display went blank. Had to look at some old threads about that and traced it down to R104. Tested bad and replaced. Now have display back. I am dialing in the idles again for a test run.
