All that's bypassed by pre-out/main-in

[killdozzer]

I have a rather good A class amp, 2 x 60Wpch into 6.

It’s an integrated and the amp section still works very well. Preamp section is another story altogether. Not a single switch still works as it should. It loses contact on input selector, balance, speaker selector, loudness… all of these went south on me. It is almost 40 years old so I bear no grudge.

I don’t want to dispose of a good a-class amp, I don’t know how to throw away stuff that still works at least for the most part.

I’m not being completely rational about it. It was my fathers, it has sentimental value. He recently passed and I’m reluctant to throw it away.

I want to slap a working pre-amp onto it and use it as a power-amp, balance being the only thing that’s giving me some headache.

I’m really in no position to bring a dozen of pre-amps home and test this, and this is why I’m asking here.

I believe pre-out / main-in bypasses most of the switches, but I’m not sure for balance.

Is there a general rule for these or is it every company for itself? What would be the most common scenario, does pre-out / main-in bypasses the entire pre-amp section of an integrated, is there a prevailing convention on how these are being put together? Will a pre-amp sort this out for me?

Thank you all for any info you can provide.

(the manual is long gone although my father bought it new, I am a member of the hifi engine and will try to get the manuals, but in my experience this is not an info you usually get in the manuals, perhaps it’s possible to read it from the schematics, but I wouldn’t be able to that)

 

[WaynePflughaupt]

If it has pre-out and main-in jacks, it functions no different than a separate pre-amp and amplifier connected with cables would. The pre-amp section, as with a stand-alone pre-amp, encompasses all the source input connections, the capability to switch between them, and tone controls. The amplifier section is – well, the amplifier.

With any integrated amp with these connections, the pre-out jacks are basically pre-amp outputs that bypass the amplifier. Thus, the integrated can be used as a stand-alone pre-amp.

The main-in jacks are a direct amplifier inputs that bypasses the pre-amp. Thus the integrated can also be used as a stand-alone amplifier.

Regards,
Wayne A. Pflughaupt

 

[TLS Guy]

Yes, connect to the main in. That connects to the power amps. However I don't think you can be certain that the power amps are OK, as the balance problem may be due to a defective power amp. I guess when you try it, you will find out.

I mention this as that is a great age for a class A amp, which are not known for longevity as the current through the output devices is independent of power output.

The only integrated class A amp, that I have known to have long legs, is the old AR Sugden A 21, 20 watt per channel amp. This amp was also made under license by Richard Allan. It dates from 1969 and rates as one of the greatest integrated amps of all time. They still fetch really high prices on eBay.

My sister has one of these driving some Labyrinth speaker I made in the late sixties. The amp has never been opened.

 

[lovinthehd]

Assume you've attempted cleaning of the switches with deoxit or similar?

 

[KenM10759]

Assume you've attempted cleaning of the switches with deoxit or similar?

Agreed, and there are good YouTube videos on you to "DeOxit" this type of unit.

 

[killdozzer]

Thank you all for the input. You're most helpful.

It loses contact on input selector, balance, speaker selector, loudness… all of these went south on me.

It would appear that switches are not the only thing that went south on me. I haven't seen this amp in a while and had a false memory of it being equipped with pre-out/main-in. It has two sets of rec-out/in for recording onto a tape. So, I think it may be beyond help in the end.

as the balance problem may be due to a defective power amp. I guess when you try it, you will find out.

I mention this as that is a great age for a class A amp, which are not known for longevity as the current through the output devices is independent of power output.

I don't think it's balance. And this answer goes to @lovinthehd and @KenM10759 as well; sure, I tried cleaning several times. I tried myself and took it to the shop. It seems to me that as long as there's some "contact-fluid" it works and as soon as it evaporates it stops working. This is why I think that the amp section works fine.

It is this Technics (pic is not that of my unit):

It is the SU-V5 model and it gives 60 into 8, I was wrong earlier. A lot of good ideas went into this one, like the subsonic filter for the TT rumble, high filter, both MM and MC, straight DC. Rather good for the 80'. I would like to keep using it, but it would seem I'm asking too much.

 

[KenM10759]

If you've tried cleaning and it exhibits that characteristic of working for a short time, it might have contacts that are too worn to maintain good connections. Can you see inside the switches and pots to confirm? Maybe there's some foreign matter that isn't getting completely removed and the cleaning fluid provides a temporary conduit.

That it works for even a short time is a good sign, but I question the thoroughness of a shop if they got the same result you get. Find another, better one?

 

[PENG]

It is this Technics (pic is not that of my unit):
View attachment 28085
It is the SU-V5 model and it gives 60 into 8, I was wrong earlier. A lot of good ideas went into this one, like the subsonic filter for the TT rumble, high filter, both MM and MC, straight DC. Rather good for the 80'. I would like to keep using it, but it would seem I'm asking too much.

I check it out on hifiengine and found the schematic and brochure etc. No service manual, but from the schematic there are a 3 pots that you have to replace if you have already tried cleaning. Did you clean it with the the cover opened so you can access the contact points though? If not, you should try that first because cleaning from the outside typically either don't work or work to a point only.

By the way, that amp is not class A, they don't call it "New Class A" for no good reason. It is actually a class AB amplifier.

 

[TLS Guy]

Thank you all for the input. You're most helpful.


It would appear that switches are not the only thing that went south on me. I haven't seen this amp in a while and had a false memory of it being equipped with pre-out/main-in. It has two sets of rec-out/in for recording onto a tape. So, I think it may be beyond help in the end.


I don't think it's balance. And this answer goes to @lovinthehd and @KenM10759 as well; sure, I tried cleaning several times. I tried myself and took it to the shop. It seems to me that as long as there's some "contact-fluid" it works and as soon as it evaporates it stops working. This is why I think that the amp section works fine.

It is this Technics (pic is not that of my unit):
View attachment 28085
It is the SU-V5 model and it gives 60 into 8, I was wrong earlier. A lot of good ideas went into this one, like the subsonic filter for the TT rumble, high filter, both MM and MC, straight DC. Rather good for the 80'. I would like to keep using it, but it would seem I'm asking too much.

I think that unit is restorable. You can download a service manual here. It is nice to know what unit we are talking about.

I suspect the switches will clean up. The controls have likely dried out and I would replace them The balance is a single 100 Kohm linear taper pot. The volume control is 100Kohm ganged pot log taper.

These became very hard to get for a long while. Now I see there is a plentiful supply again which is good news for restorers.

One note of caution though, there are quite a few trim pots the make critical adjustments. These may be a problem. If these are to be cleaned then the unit will need to be on a service desk with appropriate equipment. Of particular concern to me is that the DC offset has to be critically set and must be mo more than 30 mv. This is an early DC coupled amp and does have offset protection, if somewhat crude. The protection circuit operates a relay that can not be reset without shutting down the unit. All this is critical to avoid speaker damage.

It should not be a lot of work for a competent tech to restore this unit. I would however submit it to the restorers touch before connecting it to any speakers you care about.

One other word of caution. Do not attempt any adjustments using a passive standard multimeter. A high impedance amplified meter is essential, or you could do serious damage.

 

[killdozzer]

I think that unit is restorable. You can download a service manual here. It is nice to know what unit we are talking about.

I suspect the switches will clean up. The controls have likely dried out and I would replace them The balance is a single 100 Kohm linear taper pot. The volume control is 100Kohm ganged pot log taper.

These became very hard to get for a long while. Now I see there is a plentiful supply again which is good news for restorers.

One note of caution though, there are quite a few trim pots the make critical adjustments. These may be a problem. If these are to be cleaned then the unit will need to be on a service desk with appropriate equipment. Of particular concern to me is that the DC offset has to be critically set and must be mo more than 30 mv. This is an early DC coupled amp and does have offset protection, if somewhat crude. The protection circuit operates a relay that can not be reset without shutting down the unit. All this is critical to avoid speaker damage.

It should not be a lot of work for a competent tech to restore this unit. I would however submit it to the restorers touch before connecting it to any speakers you care about.

One other word of caution. Do not attempt any adjustments using a passive standard multimeter. A high impedance amplified meter is essential, or you could do serious damage.

I read this a couple of times and gave it time to settle. It surpasses what is available to me. It would also raise the price way too high. I was aiming at a 60-100$ preamp. Because, here, you can find people able to fix it, but none who stock the components. They would have to buy every part at retail price just for my amp, charge shipping, hands and the parts with a slight chance of not fixing the problem which is the least of my problems, but still there.

By the way, that amp is not class A, they don't call it "New Class A" for no good reason. It is actually a class AB amplifier.

I really didn't know this. They loose a couple of points in my book for calling AB class A class. I did wonder what's new about it, but I was hoping it had something to do with the heat or some improvements... It also felt light to be 60Wpch into 8ohm.

Thanks anyway.

 

[PENG]

It also felt light to be 60Wpch into 8ohm.

Thanks anyway.

It still looks like a good quality amp, if in good condition it is worth replacing the two volume pots. The balance pot should most likely be fine. Again, did you try to do a better cleaning job by removing the cover for full access?

 

[killdozzer]

It still looks like a good quality amp, if in good condition it is worth replacing the two volume pots. The balance pot should most likely be fine. Again, did you try to do a better cleaning job by removing the cover for full access?

Yes, yes, thank you for your concern. It went into a shop a couple of times in the hands of some very appreciative people. They enjoyed working on the thing, but still... It works perfectly for some time and than goes back to its usual MO of crackling sound and loosing a channel at a time.

I do believe it can be fixed, don't get me wrong, but I think it would cost a lot in the end. I'll ask just for the peace of mind.

 

[PENG]

Yes, yes, thank you for your concern. It went into a shop a couple of times in the hands of some very appreciative people. They enjoyed working on the thing, but still... It works perfectly for some time and than goes back to its usual MO of crackling sound and loosing a channel at a time.

I do believe it can be fixed, don't get me wrong, but I think it would cost a lot in the end. I'll ask just for the peace of mind.

Okay so cleaning properly and thoroughly didn't work, so you need to replace the volume pot. According to the schematic diagram it is a 100KB, probably looks similar to this:

100KB means 100 kohm, linear, I know TLSGuy said it was log taper, but it is actually linear according the the service manual/schematic. You may actually prefer log taper though and it is probably easier to find.

I doubt you can find one that is exactly the same as the original, but should be able to find one that is physically compatible. If you can find one, it shouldn't cost much, below is one that cost under $5, just to give you some idea. A high quality one should still cost less than $15.

https://www.parts-express.com/100k-audio-taper-stereo-potentiometer-1-4-shaft--023-652

 

[killdozzer]

When you say volume pot, do you mean the acctual mechanism of the main volume knob? Or perhaps all these are called volume pots?

I ask because I loose signal at most of the switches; input selector, speaker selector... all of them, and even with the loudness button and balance knob.

And selectors should have a required number of positions in order to be compatible. It should match the number of audio inputs on the back of the amp.

 

[PENG]

When you say volume pot, do you mean the acctual mechanism of the main volume knob? Or perhaps all these are called volume pots?

I ask because I loose signal at most of the switches; input selector, speaker selector... all of them, and even with the loudness button and balance knob.

And selectors should have a required number of positions in order to be compatible. It should match the number of audio inputs on the back of the amp.

Yes it is the rotary potentiometers that you turn via the volume knob.

I thought you were losing signal only when you were moving the volume knob. Now it sounds like you have to clean or replace all the other switch contacts too. Still fixable, just more work, time and money. My Marantz sm-7 and sc-7 are older than your su-v5, and have more switches, yet they only needed cleaning. I think storage conditions have a lot to do with this.