Without seeing the circuit details it is impossible to give specific advice, however you should check and confirm the basics.
The middle leg of those 0.47 ohm resistors should be within 50mv or so of ground potential (both channels). Your voltage readings above show that should be the case for that channel, but check them both.
Look to see if there is a speaker relay, and if so is it being activated to connect the speakers to the amp.
The middle leg of those 0.47 ohm resistors should be within 50mv or so of ground potential (both channels). Your voltage readings above show that should be the case for that channel, but check them both.
Look to see if there is a speaker relay, and if so is it being activated to connect the speakers to the amp.
Hi Mooly, it doesn't appear to use relays judging from the previous photos.
Perhaps a bad solder joint, or the speaker switches have gone bad?
Does this amp have 'pre-out and power in' jumper links I wonder.You were right Mooly!! It did work.That's more like it
So where are you up to? Is it all working with no problems now?
I soldered back in the two resistors just now. I tested the speaker lines and saw the voltage jumping around on the left speaker line. Plugged in the speaker and the sound was good.
Wise might be correct about a bad solder joint : \
So glad it is finally working! Should I recap it? I have most the caps in stock already (dont have those big boys).
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I wouldn't see the need to recap the unit, it's not that old.
Recapping things has been some kind of internet "fix all" and it's gotten out of hand.
Part of the "recapping" craze was due to products that were manufactured between 2003 and 2006, because of a bunch of stuff was breaking down because of the infamous "bad capacitor" scandal - resulting from a stolen formula that was missing a key ingredient, whereby capacitors made with that formula soon failed.
That has since been taken care of, yet the "craze" goes on - promoting paranoia about anything with capacitors.
It's nonsense of course.
Older, 1940's to early 1970's products could use a,... ahem, "recapping" to bring back reliability and improve performance.
As for bad solder joints, it happens.
Best to concentrate with a magnifying glass the whole board(s), looking for "ring joints" around soldered components and wires, switches, jacks, etc.
A ring type crack may appear, and if the part is wiggled, it will show up.
good to hear it works. I'd agree with the comments on re-capping and also follow on and say that we see so many examples on the forum where things go wrong and an otherwise good amp ends up with multiple issues. Not saying that would be the case here... but it happens, a lot.
The comments on dries are spot on. Check out the last couple of images at the end of post #1 here for typical examples:
Sony CDP790 and KSS240 Restoration Project
I'd agree with the comments on re-capping and also follow on and say that we see so many examples on the forum where things go wrong and an otherwise good amp ends up with multiple issues. Not saying that would be the case here... but it happens, a lot.
The comments on dries are spot on. Check out the last couple of images at the end of post #1 here for typical examples:
Sony CDP790 and KSS240 Restoration Project
I'll add to those comments from that link - nice work Mooly.
A few months ago one of my Technics "Mash" CD players started annoying me.
Sometimes wouldn't read or spin up the disk.
Drove me nuts.
Diving into it, I eventually found a 1uf cap on the servo board was leaky, and read 0.4uf.
Changing it restored perfect operation again.
But man!.... was it a delicate procedure to do!
I'm retired after 45 years of professionally fixing other people's broken crap, and I want a darn rest now, but this stuff just happens anyway.
Apologies for reviving an 18 month old thread, but I'm having trouble with the same model receiver. It stopped working recently after a horrible squeal from one speaker; the fuse at the PS was blown. I disconnected everything, checked carefully for shorts in the speaker outputs (and any other likely areas I could reach), and tested the visible resistors. One, a 220K resistor connecting the power input to the case, is blown, but I assume that is either unrelated to, or another consequence of, the main problem.
I put in a new fuse so I could carry out other diagnoses in situ, and it blew again immediately. My assumption at this point is that it is a bad power transistor or rectifier -- does that seem plausible? If so, this probably puts repair costs beyond the value of the unit.
I put in a new fuse so I could carry out other diagnoses in situ, and it blew again immediately. My assumption at this point is that it is a bad power transistor or rectifier -- does that seem plausible? If so, this probably puts repair costs beyond the value of the unit.
ETA: Since the transistors are fairly accessible I decided to take another look, and found a short between the collector and the emitter on transistor T322. If it is just that one transistor then it is probably worth my while to replace it. Any other components that I should be looking at that are likely to be blown together with this transistor?
Thanks in advance. This kind of thing was in my wheelhouse 40+ years ago, but I've long since forgotten most of what I knew about this kind of diagnosis.
Thanks in advance. This kind of thing was in my wheelhouse 40+ years ago, but I've long since forgotten most of what I knew about this kind of diagnosis.
I don't see how that's possible, since the 2SC2577/2SA1102 are no longer made. At best I can probably source a Sanken 2SA1102 pull (the transistors in my unit are Sanken). Or, I can replace two or all 4 with more recent equivalents, if I can figure out which ones will slot in with no other changes and also sound good.
Although zapped output stages are common, fixing them for the unwary can be fraught with difficulties.
While it usually rates as 'an easy fix' it is quite possible to find that slight differences in production processes (historic processes vs current manufacture) mean that occasionally things like bias current adjustments can be out of range. Although usually easy to correct it is something to be aware of.
Of more concern is the fact that faster modern parts can sometimes throw up stability issues when just slotted into older equipment.
99% of failed output stages are fixed by replacement of the output transistors, the drivers and checking all low value resistors for damage around that area. Also check for dries generally around parts that run hot.
Use of a DBT (dim bulb tester) is an absolute must when working on anything like this.
While it usually rates as 'an easy fix' it is quite possible to find that slight differences in production processes (historic processes vs current manufacture) mean that occasionally things like bias current adjustments can be out of range. Although usually easy to correct it is something to be aware of.
Of more concern is the fact that faster modern parts can sometimes throw up stability issues when just slotted into older equipment.
99% of failed output stages are fixed by replacement of the output transistors, the drivers and checking all low value resistors for damage around that area. Also check for dries generally around parts that run hot.
Use of a DBT (dim bulb tester) is an absolute must when working on anything like this.
Thanks for the input.
DBT: I have a variac on my tools wish list, maybe I should uprank it.
NTE36/37: I see these are available at digikey as a matched pair, that is certainly an advantage, but won't the ft mismatch from the original transistors be a problem? And would the mismatch mean I'd have to replace all 4 transistors? Sanken gives the 2SA1693/2SC4466 pair as the modern replacement, this has the same ft as the 2SA1102.
Is there that big a risk of problems if I just slot a working pull A1102 in for the old one? Was the manufacturing variation so great as to cause real issues? I have a practical reason for asking this: the A1102 is accessible enough from the top of the board that I can replace it by cutting the old leads and soldering the new one onto the stubs. The 2577 is hiding behind a resistor and to replace it I might have to remove the board(s), which will elevate it from a quick repair job to a project and will require commandeering our dining table for a day or three.
DBT: I have a variac on my tools wish list, maybe I should uprank it.
NTE36/37: I see these are available at digikey as a matched pair, that is certainly an advantage, but won't the ft mismatch from the original transistors be a problem? And would the mismatch mean I'd have to replace all 4 transistors? Sanken gives the 2SA1693/2SC4466 pair as the modern replacement, this has the same ft as the 2SA1102.
Is there that big a risk of problems if I just slot a working pull A1102 in for the old one? Was the manufacturing variation so great as to cause real issues? I have a practical reason for asking this: the A1102 is accessible enough from the top of the board that I can replace it by cutting the old leads and soldering the new one onto the stubs. The 2577 is hiding behind a resistor and to replace it I might have to remove the board(s), which will elevate it from a quick repair job to a project and will require commandeering our dining table for a day or three.
As long as that transistor is the only failed part then you might be OK doing as you suggest and fitting a pulled part, but tbh its not a good way to approach a repair.
If it works it works, if it doesn't then what you do do next?
Whatever you try, I would suggest turning the bias setting down before powering up.
If it works it works, if it doesn't then what you do do next?
Whatever you try, I would suggest turning the bias setting down before powering up.
I don't disagree, but it is more likely to get it working than the alternative, which is to bring it to the recycling center.
Put it back in the box (which we still have), and add it to the stack of repair projects for after I retire, a stack that including our old betamax VCR and a shocking number of clocks.
However, if I can figure out how to replace the second transistor without a complete disassembly I'll make replacing the pair my first step, either with the suggested NTE36/37 or with a 2SA1693/2SC4466 pair, especially if I can source a matched set . And I can certainly turn the bias down, though access to either a service manual or to the bottom of the PCB to determine the trimpot's range would give me more confidence.
Thanks for the helpful advice.
Just an update. The replacement parts finally arrived; I replaced the two transistors on the right channel, turned the gain trimpots down to 0, did a quick check of the other nearby components, and powered up through a bulb tester. No light on the bulb, so then I tried again without the dim bulb and the fuse blew immediately. Turns out someone in my household put a dead light bulb back in the cabinet 
With the new bulb and a new fuse, powering the receiver on lit the bulb to full bright. So, the bad transistor (the A1102 was genuinely dead, it reads as a pair of resistors on my tester) was obviously not the (only) cause of the problem.
Before I put the bottom back on the receiver and stick it into storage I'll take a look for obvious shorts in the power supply stage of the unit, but I think diagnosis at this point is beyond the limit of my expertise (and the intrinsic value of the receiver, which is mainly emotional attachment on the part of my wife, who bought it new while a student).
Thanks again for all your help.
With the new bulb and a new fuse, powering the receiver on lit the bulb to full bright. So, the bad transistor (the A1102 was genuinely dead, it reads as a pair of resistors on my tester) was obviously not the (only) cause of the problem.Before I put the bottom back on the receiver and stick it into storage I'll take a look for obvious shorts in the power supply stage of the unit, but I think diagnosis at this point is beyond the limit of my expertise (and the intrinsic value of the receiver, which is mainly emotional attachment on the part of my wife, who bought it new while a student).
Thanks again for all your help.
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