shfa-2 amp problems

Rob

I am trying to understand this thread in case I have this problem someday. You say:"Touching pins 2 or 3 at the 3-pin pickup plug should induce a loud hum when the pickup is unplugged. If neither pin results in a hum, it indicates a common fault such as B+."

Are you referring to the three pin J101 Audio input plug? If so I tried this on my DS160 and did not get any hum with/without a record playing. My DS160 has great sound. Is there some other 3 pin plug I should be doing this test on?

Also what is meant by "common fault B+"?

Best
Darrell

Darrell,
Yes touching pins 2, or 3 ( each pin goes to one channel), on the socket ( or inside the rear door skin ), should produce some hum if the plug is removed, and the motor is running (record playing position), and the volume is turned up--
If the 5U4 or associated circuits ( the "B+") is not functioning, no "hummmmm" will be heard--
NOTE--can't re-call which ones., but some of the SHFA's had a "speaker test"--If you get hum, using this, it does NOT indicate that the amp is working, just that the speakers are connected and"working" !
HTH, Ron Rich

Hello again: I have checked the test socket #2 and do get the voltage needed as I mentioned before I have checked all of the voltages on the high voltage and the low except for T I will check that when I get back to the amp again. I will also check the preamp transistors voltage. is there anything I need to be aware of in this check? I do not want to break a transistor. Ron, I will look at the mech book again as per your suggestion, however would still like to know if there is any other circuits that merge with the squelch. This amp has the test on the speaker output selector. that test is good as it connects the heater voltage to the speaker output. so I know that the speaker connection is good. as the speaker selector, the volume control plug have tested ok. I will move onto the cartridge wiring. what should be the resistance of the wire between the cartridge and the plug going into the amp? thanks in advance. Mark

mb9513 wrote: I will move onto the cartridge wiring. what should be the resistance of the wire between the cartridge and the plug going into the amp? thanks in advance. Mark

That's an easy one.
Zero.

ds100h wrote:Rob

I am trying to understand this thread in case I have this problem someday. You say:"Touching pins 2 or 3 at the 3-pin pickup plug should induce a loud hum when the pickup is unplugged. If neither pin results in a hum, it indicates a common fault such as B+."
Are you referring to the three pin J101 Audio input plug? If so I tried this on my DS160 and did not get any hum with/without a record playing. My DS160 has great sound. Is there some other 3 pin plug I should be doing this test on?
Also what is meant by "common fault B+"?
Best Darrell

Darrell, my suggestion there was based on conditions I assumed to be true in Mark's machine. that was: Mech in play mode. I should always remember that someone else may read these post out of context and get a bit confused. Sorry about that....

You are correct that in only play mode will you hear a hum.

Darrell, the complete quote was :"If neither pin results in a hum, it indicates a common fault such as B+." This simply means to look for something common to both channels --in this case it might be B+. Based on Mark's recent post, that appears likely.

Rob

mb9513 wrote:Hello again: I have checked the test socket #2 and do get the voltage needed as I mentioned before I have checked all of the voltages on the high voltage and the low except for T I will check that when I get back to the amp again. I will also check the preamp transistors voltage. is there anything I need to be aware of in this check? I do not want to break a transistor.. I will move onto the cartridge wiring. what should be the resistance of the wire between the cartridge and the plug going into the amp? thanks in advance. Mark

Mark what-where is "T"-?

As Darrell & Ron pointed out, the mech must be in play mode for there to be a hum when you touch the pins on the input plug.

You are wise to be careful when testing around transistors. That is why I suggest looking first at the B+ source going into the preamp section. As a kid back in the 1960s I --hated-- working on transistor circuits because any slip-up that grounded a base or emitter (or other mistake) killed a transistor...and they were expensive back then.

I doubt you'll find the fault in the tonearm-cart wiring. But the easiest test is to measure from pin 1 on the male to pins 2 then 3. You should see approx 570 ohms for a black cart in a 222 and approx 600 ohms for a red one. Anything near these values is fine.

Rob

Hi All,
Going from my great, but not too long memory, I think Rob made a typo here--seems to me (and I'm too lazy to dig one out and check this !) that the meter reading for a "Redhead" is more likely to be 1600, rather than 600 ?? I seem to recall some reading around 2K, that sounded just fine ---Ron Rich

Thank you everyone for helping me understand the lingo and how to test my amps. Once it was clear I was to touch the "pin sockets" instead of the "pins on the plug" I did get the desired hum which I should get on my working amp. Have a great weekend.

Best
Darrell

Ron, 570-600 is correct range for a T-needle stereo here. The 1600+ is the mono redhead.

Rob

Thanks Rob--
My bad--I was in my normal state of confuzzion when I rit that--did not allow the coffee to hit the brain---
Of course, you are correct !! Ron Rich

Hello again: Rob, T is the voltage that comes off C128 and R201, and it is correct. I measured the cartridge an wire resistance and got right at 570 ohms(is blackhead). measured the collector voltage at both of the transistors. it is -15. I have just about exhausted everywhere to look on this one. I am going to signal inject at the screen pins on all of those 12AX7's to try to trace back where I am loosing it at. but if anyone else wants to weigh in I am listening. Thanks Mark

Hi all: FOUND IT! I was looking in the wrong part of the amp. I was searching near the preamp and AVC because they are common to both channels. in looking further down I noticed there is one more common part, the balance control. it was dirtier than I thought. I had cleaned it already just not enough. moving it around got us proper sound again. Thanks again everyone! till next time. Mark

Well, I am glad you found the problem, Mark.

But that is really a new one -to have both pots open up at once. Normally, that would indicate that one channel was already either dead or extremely weak.

FWIW: I've always gotten rid of balance pots in any commercial amp, including jukes. If the amp is properly serviced it should balance within 1½ db w/volume control @full. Similarly, with the TSA-SHP series I leave both gain pots @ max. In the later amps the gain controls throw a load across that channel's audio which causes reactance in the previous coupling capacitor when the control is set to attenuate level. That isn't the intended effect, but it happens and can affect bass response in addition to overall level.

In seeburg stereo tube amps the audio goes through the control and it becomes another failure point. For the SHFA amps I just removed the wire from the arm tab and put it on the high-side tab.

Again, Mark, that was an odd-bird. Glad you caught it.

Rob

Hi Rob,
On SHP type amps, setting the gain at "full", will result in blown horns, and sometimes speakers as well !
Ron Rich

Ron, well that 'splains it. I had always assumed it was due to the fact that loud spikes (scratches or other transients) would trigger the protection circuit which was a bit hyper on the SHP models.

The only applications I ever used those amps for were non-original settings with speakers that could easily handle the power.

Rob