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Old 5th May 2022, 8:40 pm   #1
robertsozo
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Default Sharp GF-555 boombox preamp gain

Hello!
I am restoring a Sharp GF-555 that I got for a very reasonable price. So far I have fixed some usual suspects of this machine (bad belts, broken wiper in volume pot, e.t.c.)
Now however I have encountered a problem that I cant find the solution to. Speaker output of the machine is incredibly loud. It is excessively loud even when volume pot is at less than 1/4 of its travel. Also when radio is tuned to a station, the VU meters are in the red almost all the time. This leads me to believe that there is a problem in one of its amplifier IC`s and somewhere the signal level is too high.
I started to investigate this by inputting a 0,3V 1kHz sine wave signal into ''Line in'' of the unit. When probing ''Line out'' I got a 1,4V output signal. I feel this is strange, because I think that Line in and Line out output levels should be almost the same. Please correct me if I am wrong.

After that I moved my suspicion to ''IC 104'' which is a Rec & Line amp according to service manual. When probing its output pin I got an even higher 2.6 V signal level.
After this I tried to find a datasheet for this IC to maybe understand how the input gain is controlled. No luck here, there is no datasheet for this IC. All I could find was a block diagram of it. I am not an electronics expert at all, and this tells me very little about how the gain is controlled.

Can someone please advise if I am looking in the right direction, maybe these signals are as they should be. Also maybe someone has knowledge on how input gain is controlled for this preamp IC.

Attached pictures are as follows:
1) Probing ''Line out''
2) The results: CH1 is what I input to ''Line In'' and CH2 is output of ''Line Out''
3) Probing output pin of IC104
4) Results of probing IC104
5) The IC in question in the schematic diagram:

Any help is appreciated, Thanks!
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Old 5th May 2022, 8:48 pm   #2
robertsozo
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Default Re: Sharp GF-555 boombox preamp gain

I forgot to add block diagram of this preamp IC
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Old 5th May 2022, 8:51 pm   #3
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Default Re: Sharp GF-555 boombox preamp gain

My first thought is a poor common continuity somewhere, a pure guess, but based on old experiences, a fair one.
Reseat the connectors and clean them of course
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Old 5th May 2022, 9:57 pm   #4
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Default Re: Sharp GF-555 boombox preamp gain

Common cause of near full volume which can not be varied much is a missing / broken 0V to the volume control 'cold' end although in this case it would have to be missing / disconnected from the cold ends of both gangs of the volume control, assuming both channels have this problem. it sounds as though you have carried out some work on the volume control already so is it possible that something has not been reconnected or resoldered in that area?

Also as always with this type of thing, try 'exercising' the record-play switch while running some switch cleaner into it before looking for anything more complicated. I used to find that a dirty / tarnished record-playback switch could be responsible for all manner of oddball faults on radio-cassettes because there were sections of it all over the circuit.

This is obviously a stereo unit, I assume you are sure that the volume is equally excessive on both channels?

The only pin on that chip which is common to both channels is the 'AGC Bias' pin, 16. Pin 16 in turn is driven by a DC voltage derived from the 'Line Out' outputs of the chip (pins 7 and 10) through C175 / R213 and C176 / R214, rectified and combined by D107 / D108 and smoothed by (C178? C179? - 100uF / 16V) before being fed back into the AGC Bias pin (16). R240 lets the charge pushed onto the capacitor through D107 / D108 decay at a controlled rate so that it does not stay topped up indefinitely. There is also a 3K3 resistor R222 which should get connected across R240 (to 0V) by a single section of the record play switch (S101-J) when the unit is in playback mode. There is another line leading away down from the capacitor's +ve and off the edge to who knows where.

Is there an online link for the whole circuit?
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Old 5th May 2022, 10:30 pm   #5
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Default Re: Sharp GF-555 boombox preamp gain

I have checked the pot. I think it is not the problem. The volume is controllable, you can get it silent, it is just very sensitive. When reaching about 1/4 of the travel (it is a slider pot) the volume is already unbearably loud. I dont want to risk the remaining 3/4 because I feel the speakers would blow.

In my mind the hint that something might not be right with the preamp IC is the high VU meter readings I mentioned before. VU meter circuit is before volume pots in the circuit. VU meter readings are not affected by volume controls by design.

Full schematic can be viewed here:https://www.manualslib.com/manual/10...p-Gf-555h.html
For easier viewing I suggest downloading it as a pdf.
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Old 5th May 2022, 11:18 pm   #6
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Default Re: Sharp GF-555 boombox preamp gain

Possibly a lack of negative feedback around one of the components? If the same chip is used for both recording and playback then (as previously said) the record/playback switch could be faulty or a poor connection could also be the issue. I doubt that the IC itself is faulty if it is amplifying the signal.
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Old 6th May 2022, 12:08 am   #7
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Default Re: Sharp GF-555 boombox preamp gain

Quote:
I have checked the pot. I think it is not the problem
How about the voltage on pin 16 of the chip, what do you see there? (It should be a DC voltage which is proportional to the amount of audio output coming from the 'line out' pins of the chip, but it might be worth looking at it with a scope as well).
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Old 6th May 2022, 12:34 am   #8
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Default Re: Sharp GF-555 boombox preamp gain

It hasn't by any chance been fitted with linear rather than log type pots, has it?
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Old 6th May 2022, 10:03 am   #9
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Default Re: Sharp GF-555 boombox preamp gain

Methinks Herald could have a good point. Are there signs that the pot's been replaced before?
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Old 6th May 2022, 5:34 pm   #10
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Default Re: Sharp GF-555 boombox preamp gain

According to the service manual the volume control is a 20K dual ganged slider. These sort of sliders can be prone to the common silver loaded track becoming o/c near the rivets, it needs checking carefully out of circuit ideally using croc clips on the meter probes so that the slider can be slid for testing correct operation.

The pre-amp chip is type LB1416 (VHILB1416XX-I), one for each channel, the output amp is type HA1392 (VHIHA1392XX-I), a stereo chip.

The balance slider is 100K across both channels before the volume controls, just a thought but is it OK and not o/c.

The fault does appear to be typical of an o/c earthy end to the volume control/s.

Dave
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Old 8th May 2022, 9:36 pm   #11
robertsozo
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Default Re: Sharp GF-555 boombox preamp gain

Quote:
Originally Posted by SiriusHardware View Post
Quote:
I have checked the pot. I think it is not the problem
How about the voltage on pin 16 of the chip, what do you see there? (It should be a DC voltage which is proportional to the amount of audio output coming from the 'line out' pins of the chip, but it might be worth looking at it with a scope as well).
I probed pin 16 of the chip. There is no signal and no voltage present referenced to ground. Any ideas?

So far I have sprayed with contact cleaner and excersised all switches, no improvements.
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Old 9th May 2022, 10:56 am   #12
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Default Re: Sharp GF-555 boombox preamp gain

If post 11 is referring to IC104 pin 16 then 0v could well be correct. Pin 16 is used in record mode and if you follow the circuit, it brings you to a manual/auto switch which in turn goes to a contact on the rec/play switch to one of the record contacts.

Given that the overly loud volume is affecting all all output modes of the unit, the fault is likely something in the pre-amp (IC601 & IC601), the volume circuit control circuit or possibly the output IC701. Personally I would check the volume control for being open circuit in itself, probably on the earthy contact part, as other members have also commented on. Bear in mind that some of these sliders have 2 earthy contacts to the main pcb. It is not uncommon to find that one has gone o/c and it's that one that is actully being used in circuit.

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Old 9th May 2022, 1:43 pm   #13
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Default Re: Sharp GF-555 boombox preamp gain

If you consider the line out feeds pin 10 of P104 after a potential divider into the tone and volume control for that channel. The other channel does the same using pin 9 of P104.
The line output is fixed at the full level, before the volume and tone controls.
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Old 9th May 2022, 6:53 pm   #14
robertsozo
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Default Re: Sharp GF-555 boombox preamp gain

Quote:
Originally Posted by G6ONEDave View Post
If post 11 is referring to IC104 pin 16 then 0v could well be correct. Pin 16 is used in record mode and if you follow the circuit, it brings you to a manual/auto switch which in turn goes to a contact on the rec/play switch to one of the record contacts.

Given that the overly loud volume is affecting all all output modes of the unit, the fault is likely something in the pre-amp (IC601 & IC601), the volume circuit control circuit or possibly the output IC701. Personally I would check the volume control for being open circuit in itself, probably on the earthy contact part, as other members have also commented on. Bear in mind that some of these sliders have 2 earthy contacts to the main pcb. It is not uncommon to find that one has gone o/c and it's that one that is actully being used in circuit.

Dave
I double checked the volume pot. Resistance of both tracks is within spec. Both ground connections are also OK. Resistance is smoothly changing when measured between wiper and one track end. Once again I am certain the issue is not in the pot.

Returning to the preamp IC. What would be considered a normal ''Line out'' level? If I feed 0,5V signal in, I get 2,4V in ''Line out''. It is almost 5 times amplification. Shouldnt ''Line in'' and ''Line out'' signals be almost the same?

This brings me back to my original question: How is gain (amplification) of this or similar preamp or controlled/defined? My guess it should be some sort of voltage divider circuit that controls one of opamps inputs referenced to its output, but I lack electronics knowledge to distinguish it in this particular circuit.
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Old 10th May 2022, 10:56 am   #15
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Default Re: Sharp GF-555 boombox preamp gain

Thanks for re-checking the volume control, that can now be totally ruled out.

Looking at the circuit around IC104 the feedback pins for each op-amp section are pins 4 and 13. If there is minimal feedback signal the amp will ramp up it's amplication until it sees a signal level that is pre-set by the values in the feedback circuit.

Since this fault is affecting both channels then it must be due to something that is common to both. Although possible it would seem unlikly that IC104 has failed.

I notice that some of the signal earthing is via the paint type tracks on the component side of the pcb. My next suggestion is that you do continuity tests on all of them in that part of the circuit. Measure them from the non component side of the pcb (copper track) via the plated through links. Also check that the printed carbon resistors on the component side of the pcb measure the correct resistance and have not gone o/c.

To try to prove the point re the feedback, you could try putting the balance fully left or right and then temporarily hold a 10k resistor between either pins 4/7 or 11/13, depending on which way you have set the balance. This hopefully will prove that IC104 is working correctly internally, the output signal should noticeably drop if the chip is behaving.

Look for a common earth link for R199 and R200, as they are both going from each feedback pin to earth and at 180 ohms would drop the feedback signal somewhat. You could do a continuity test from the earthy end of each to the common earth 0v.

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Old 10th May 2022, 12:02 pm   #16
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Default Re: Sharp GF-555 boombox preamp gain

As a further thought you probably should use a 0.1uf capacitor in series with the 10K resistor if you intend to try my feedback check. The capacitor will ensure that you get ac feedback and not any dc component to it. Be very careful not to short out any of the chip pins as that could cause damage to the chip.

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Old 12th May 2022, 10:33 pm   #17
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Default Re: Sharp GF-555 boombox preamp gain

I've looked at one these units some years back, and yes, as Dave mentions those painted silver tracks often go open circuit. They cause some weird faults. A simple 'point to point' check with an ohmeter will often reveal the fault. They look perfect, but read otherwise. You can't solder to the tracks, but replace with wire links instead. I'd be surprised if you have a genuine component breakdown fault, rather than a pcb related problem. You could save a lot of time by simply checking as many tracks as you can get to. SJM.
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Old 14th May 2022, 11:28 am   #18
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Default Re: Sharp GF-555 boombox preamp gain

I suggest that when checking the tracks that you connect the black meter probe to the battery 0v terminal and use that as your reference to 0v/earth/chassis (whatever you want to call it). There should be a near zero ohms reading to all of the earthy returns, anything that goes above say 0.5 ohm should be checked carefully.

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