SMC1 amp rebuild

I've had my SMC1 running for awhile now (big thanks to Ron for advice and parts), but I'm not really happy with the sound. It seems the treble is not very 'crisp' (really not sure how to describe the sound - just not clear). Also one channel seems a little louder than the other.

When I first got it I replaced all of the electrolytic caps in the amp except for the non polarized ones in the crossover network. I also replaced the needles.

My question is are there any transistors I should be looking at? Maybe the bias is off on one channel? Just not sure what my next step should be.

I do have a copy of Tony Millers great SHP book.

Any help is appreciated.

Don

Don,
The bias setting is critical on these amps-follow Tony's ( actually my ) procedure, in his book !
Which amp do you have--what code ? Which "needles" did you use--??
Ron Rich

Ron,

Thanks for the quick reply.

The amp is the SHP3. It looks like code 3, but the info on the label is really hard to see. It looks like it's been worn off.

The needles I got from Needlesforjukeboxes. They are the yellow ones listed for the Pickering 340D cartridge.

Don

Hi Don,
That's probably a "Code B", amp. If so, it has the two output trannies mounted to the chassis and has the "older" bias circuit ( see above--"Errors known in printed materials--")
You can check to see if the problem is inside, or out side the amp fairly easily, by first switching inputs--if no different, switch out-puts ( which should be connected to the "50" screws, if no other speakers are connected). If, when switching either, the "bad side",changes--the problem is not in the amp--
While "better then some, aftermarket needles", I have tested those, and found them still to be "lacking" in tone, and volume, when compared to the "gen-u-whine" Seeburg/Pickering product. (Just a note--- Despite what my friend, and others say--COLOR, IMHO, does mater)
Ron Rich

I find this true on my SMC1 as well. The highs sound distorted, or over modulated.
When I connect a normal pick-up to the amp it all sounds very very good and lower in volume.
This made me think the signal I was getting from the cartridge / needle. I have replaced the needles with NOS yellow see burg needles.

My amp has been completely rebuilt and I'm following this thread for any usable advice regarding this matter.

With kind regards,
William

Don,
I re-read you first post-- I'm wondering, did you check the relay--the NC contacts ( when de-energized), if not making well , will cause it to sound "muddy"--- Ron Rich

Thanks for the info Ron.

I worked late last night and am on mid-shift today, so I won't get a chance to look at things until tonight.

As far as needles go, is there a source for 'the good ones'?

Thanks
Don

Don,
I have a few pair left--however you won't like my price ( I paid an arm and leg, for what I have)!
PM, if you wish--
Ron Rich

Finally got a chance to do some work on this.
First thing - it looks like I wasn't as through as I should have been when re-capping the amp. I didn't chance the filter cape (C65, C66) or the bi-polar output caps. Got those all on order now.

After examining under the heatsink, it looks like my amp is not code B like I thought. It looks like this has the later biasing circuit.







I did all of the checks on page 39 of Tony's SHP book. Everything checked out OK except for items g and h (after making the noted corrections in the book). Tony states that these measurements should all be under 1 ohm. It looks like these measurements are checking the connection on the edge fingers of the driver board. However, when I did the measurements for g and h I get a reading of 100 ohms. Looking at the schematic, it looks like I'm reading the 100 ohm resistors R5176 and R5199. Is this correct?

I think I might re-pin the edge connectors, as I had to slide it on and off a little to get correct readings on some of the measurements.

I will also use a burnisher to clean the contacts on the mute relay (looks like someone already had the relay top off).

Two questions:

In the bias adjustment instructions, Tony recommends replacing the 75 ohm wirewound pot with a cermet unit. Then when the adjustment is set, remove the pot, measure it, and then solder a 1% resistor in parallel with the thermistor. My question is - if you're going to remove the adjustment pot, why replace it with a new one first. I would think you could adjust the old one and measure that.

Number 2 - I see no mention of the R and L gain adjustment pots. Should these be adjusted? What is the proper setting for these?

Sorry for the long winded post.

Don

In the bias adjustment instructions, Tony recommends replacing the 75 ohm wirewound pot with a cermet unit. Then when the adjustment is set, remove the pot, measure it, and then solder a 1% resistor in parallel with the thermistor. My question is - if you're going to remove the adjustment pot, why replace it with a new one first. I would think you could adjust the old one and measure that.

Number 2 - I see no mention of the R and L gain adjustment pots. Should these be adjusted? What is the proper setting for these?

Personal opinion based on dealing with voltage-current amplifiers in-general. There is no need to replace the pot simply for this test. I just make -sure- there it is clean with no dead spots. Test the pot after cleaning using a fast acting meter.

The recommendation to replace the pot with a fixed resistor is good practice and what I've done on consumer equipment since the early 70's. The 1% thing is overkill. 5% with the difference going to the lower side is fine. With transistors a slight under-bias is OK, the quiescent current will naturally increase as the amp conducts and heats up.

The "gain pots" are unnecessary. If an amp has been properly rebuilt the channels should stay within 1 -1/2 db as presented to the volume control. On a home stereo a balance control might occasionally be useful when dealing with poorly balanced material.

On the few SHP series I've been stuck dealing with, among the most important mods I installed were 4 amp fast fuses on each B+ and B- to teh finals. With that done at least there won't be those messy flameouts if they fail. Not my favorite design.

Rob/NYC

FWIW: this talk of the SHP series caused me to dig one out that I had worked on back in June 1997.

Among other things I found on a note attached was power output is 36.6 rms -one channel driven and 32.7/ch for both channels driven. These are max clean readings -just before clipping occurs.

So essentially, this is a 65 watt amp, not 100 watts, but still more than adequate for most applications.

Rob/NYC

Just an update.

I replaced the two 3000uF filter caps.
I couldn't find replacements for the non polar output caps.
The schematic calls for 7uF non polar caps for the crossover. I was able to find 10uF. Will this change the cut-off freq. too much (or at all)?
I had to change out the 6 pin AMP connector at location J5103. One of the locking tabs broke off and the mating connector was very loose. I had to replace it with a molex, as I could not find a replacement AMP connector.
Currently waiting on some 39 ohm resistors to replace the bias pots.

Small rant - I hate the fact I can't get simple replacement parts anywhere. Radio Shack is pretty much useless. Projects the used to take 2 days now take 3 weeks because you have to order everything online. (end rant).

Don

MrGorf wrote:Small rant - I hate the fact I can't get simple replacement parts anywhere. Radio Shack is pretty much useless. Projects the used to take 2 days now take 3 weeks because you have to order everything online. (end rant).
Don

Consider yourself lucky.

I service consumer electronics for a living. - AKA 5 days a week.
How'd ya like to have MY job?

Don,
Both halves of that TEC/AMP (formerly A-MP) connector is available --I stock them---"pins too"--
Ron Rich

MattTech wrote:Consider yourself lucky.

I service consumer electronics for a living. - AKA 5 days a week.
How'd ya like to have MY job?

I can imagine how tough that is nowadays.