Silvertone 1464 solid state 100 amp

[DrGonz78]

Well it is definitely cross-over distortion from the output transistors and not the preamp. I scoped the output at the 5981 driver (Q6) on the 1423 schematic, which is the same as the 1464. The output on that driver is a very nice looking sine wave form. I don't have an 11ohm resistor or a 1ohm resistor on hand so I will have to order those with my next parts order. Funny thing is that I ordered everything down from 10ohms. 9.2, 8, 7, 6 ohms. I guess I thought we might be lowering bias a bit and not raising it. Well if I put a 11ohm resistor in there I will expect to see 495mv and 12ohm should be 540mv. Anything higher than that and I don't think would be a good idea. Worst off worst as Juan said we can at least salvage the amp by putting in a LM3886. First off first I will try to increase the bias by a small amount and try to get the amp usable. Oh and here are some pics Juan... Oh and thanks you guys.

BTW- I did test to see if the amp biased correctly when it was hot and no it did not. Just X. O. Dist.

[J M Fahey]

Thanks.
You have 2 very good heatsinks there, by the way.
I bet just one would be enough for an LM3886.

[DrGonz78]

So I got some resistors and installed an 11 ohm instead of the 10 ohm for the bias resistor. The measurement across the resistor went from 450mv to about 500mv. It did clear up some cross over distortion and I could see that on the scope. I tried the amp with a speaker and there was still some distortion though it improved, but I went up to 12 ohms. Across the resistor now is around 544mv and it did really improve the sound. The only problem now is the distortion is still there even at high volumes. The distortion now is almost a more musical sounding breakup and kinda sounds cool. I guess I will talk to my client about how to proceed from this point as far as changing the output to a LM3886 chip. At this point the amp is basically a labor of love and a good learning tool. The person I am fixing the amp for is just trying to get it working good enough to sell it anyway. If I was not trying to fix it then it might just end up as parts etc...

I am resistant to go to a 13 ohm resistor and have 585mv or so across the bias resistor. Should I see though if that might just bring the bias where it needs to be or do you think that is pushing it to far? It might just be that little bit that gets the amp sounding clean.

[J M Fahey]

*I* would be happy with what you got so far, but if you wish, try 13 ohms *but* be certain to heat the amp up as I said earlier.
A customer will not complain *much* about some X-over distortion but certainly will about a burnt amp and a smoke filled room.
And it *is* obsolete technology after all.
There's a reason transformer driven amps are not used any more.
That said, they have their charm and they often overdrive nice, with more "bite" than a regular modern SS one.
Just try that

[The Dude]

Keep in mind, if your readings are correct, you're not substantially changing the current. You must do the math.
V/R=I. Therefore 450 mv/10 ohms=45 ma, 500 mv/11 ohms=45.45 ma, 544mv /12 ohms=45.33 ma, etc.
I think you've got plenty of "breathing room" to continue your resistor experiments.

[J M Fahey]

Keep in mind, if your readings are correct, you're not substantially changing the current. You must do the math.
V/R=I. Therefore 450 mv/10 ohms=45 ma, 500 mv/11 ohms=45.45 ma, 544mv /12 ohms=45.33 ma, etc.
I think you've got plenty of "breathing room" to continue your resistor experiments.

That's not the current we are taking about
In fact *that* current is constant

But voltage drop across that resistor is output transistor bias and there a few mV can cause *huge* idle current changes.

[DrGonz78]

So I did try the amp out for a while letting it heat up to see if the bias was just slow to go. Nope. The amp sounded the same as it did cold, just a touch of un-ignorable distortion breakup. So I went with a 13ohm resistor and you know what...? It sounds great! I think 13 was the magic number. I am measuring about 580-590mv across each resistor and about 525mv on the base of the Q6 output transistor at idle. I definitely will have play the amp some more and see what happens when it heats up. I am pretty happy so far with the improvements and the 13 ohm resistors put it right where it needs to be to have a nice clean sound. Anymore than 13 ohm resistors would be pushing my luck! Hopefully it remains stable as I test it for good long period of time... I will let you all know how it turns out, thanks again.

ADD THIS TOO: The schematic for the 1423 shows 200mv at the base of Q6 with the germaniums installed. With the 12 ohms resistors in I get a great 500mv at the base of Q6, but the sound still had slight XO dist. 525mv at base of Q6 was where I ended up. The amp measured 145-150mv on that base before I started modifications to the circuit. I think the drift of the circuit may have been why I need to go just touch above 500mv on that base. I don't know if that makes any sense, but it was a thought that came to me.

[J M Fahey]

So I did try the amp out for a while letting it heat up to see if the bias was just slow to go. Nope. The amp sounded the same as it did cold, just a touch of un-ignorable distortion breakup. So I went with a 13ohm resistor and you know what...? It sounds great! I think 13 was the magic number. I am measuring about 580-590mv across each resistor and about 525mv on the base of the Q6 output transistor at idle. I definitely will have play the amp some more and see what happens when it heats up. I am pretty happy so far with the improvements and the 13 ohm resistors put it right where it needs to be to have a nice clean sound. Anymore than 13 ohm resistors would be pushing my luck! Hopefully it remains stable as I test it for good long period of time... I will let you all know how it turns out, thanks again.

ADD THIS TOO: The schematic for the 1423 shows 200mv at the base of Q6 with the germaniums installed. With the 12 ohms resistors in I get a great 500mv at the base of Q6, but the sound still had slight XO dist. 525mv at base of Q6 was where I ended up. The amp measured 145-150mv on that base before I started modifications to the circuit. I think the drift of the circuit may have been why I need to go just touch above 500mv on that base. I don't know if that makes any sense, but it was a thought that came to me.

COOL !!!!
Yes, on biasing just a couple mV make all the difference.
Easy to get to the exact point when you have low current and a trimmer pot, tedious if you must solder resistors in and out, but, hey, looks like you found it
Congratulations
Burn it in and let it go.

[The Dude]

That's not the current we are taking about
In fact *that* current is constant

But voltage drop across that resistor is output transistor bias and there a few mV can cause *huge* idle current changes.

Oops. My bad. I was for some reason thinking emitter resistors. I should have realized that emitter resistors would not likely be that high in value. Sorry for the misdirection and excuse my idiocy.

Glad you got it goin' Dr.!

[52 Bill]

I have a couple of 100 ohm wirewound pots that I keep here to use either as a temporary replacement for these resistors or to use in parallel with existing resistors. Adjust until the circuit works and then read the value with a meter and replace with a standard resistor.

And just to tie up loose ends on this thread/amp, did you measure the offset on the output?

[DrGonz78]

Yeah DC offset is measuring around 15mv or it was 25mv on the output after my final adjustments. Gonna give another burn in tonight to make sure it does not burn up or something... Thanks again to all you who devote your time to give advice to novice amp techs like me. I promise to carry the torch of your knowledge as far as I can.

Edit: I forgot exactly what DC offset was so I just measure it again to make double sure. It actually went up higher to 56mv on the output under 8ohm dummy load. So not too bad and seems reasonable.

[minions]

I know this is old, but I'm just posting it for those who may come to this thread later.

Below is the link the the Sears/Silvertone 1464 Solid State 100 Schematic (Chassis No. 185.12030)

https://drive.google.com/open?id=0B_...DJDVnRnajROeVk

[minions]

Just in case the above link ever goes dead, here's another one:

Sears Silvertone 1464 Service Manual - Album on Imgur

PM me if that ever goes dead and I'll fix it.

[Butch808]

I know this is an old post but it was very helpful to me in troubleshooting my 1464. Looks like the same transistors mentioned above went out on mine. I'm getting a short to chassis ground from the heatsinks so I pulled the transistors and got very different readings than those sited above. I'm going to implement your solution and see how that goes.

[freegan72]

I have been troubleshooting my own 1464. It has this low-frequency hum that sucks most of the power. Otherwise, the input controls, reverb, tremolo work. I started lifting components to remove different parts of the circuit and have been using a clean external DC power supply. I replaced all the e-caps (many of which were bulging and/or a little burnt-looking), with the exception of the expensive power supply ones. That just made the hum brighter and more powerful-sounding.

If I disconnect everything before Q5, the hum goes away. I can plug a guitar into the base of Q5, and it sounds great, super loud. If I add Q3 and Q4 back, the hum comes back. I checked continuity on the leads of Q3 and Q4, nothing is shorted. They are marked 67467296. Any suggestions on replacements? Anything else I should consider?

schematic link from earlier in this thread:
https://imgur.com/a/15npj