GK 1001RB no output

[MarkusBass]

I was just going to suggest measuring DC on the output jack . If you look at the schematic, please read the remark on bias adjustment procedure (bottom-right of the schematic). It says that the voltage accross J5 should read 10mV. I think that if you measure it, it will be much higher than expected. If this is the case, the protection circuit is working correctly. But be careful when adjusting R28 - the amp may be very sensitive to this (Teemu please correct me if I'm wrong). The voltage accross J5 is measured by following two transistors (I cannot read the symbols - the first one may be marked Q31) and if it's to high, signaled as a fault. I my opinion there is still some faulty transistor in the amp. But since the amp is almost simetrical, you can measure voltages on the (+) side and compare them to (-) side. Appart from this, there are already some voltages on the schematic.

Marek

[lowell]

Hi yes, the J5 reads 50mv with the bias trim pot ALL the way down. I still cannot figure this out though. I feel I've checked everything, transistors and all. Obviously I'm missing a bad part or 2 somewhere. Will keep diggin through it. What might

[lowell]

more info on this:

the negative outputs outer 3 to the speaker jack measure only 56k while the positive outputs (all 6) are OL. There must be something on the negative side that's pulling things down. The inner 3 negative outputs are about double the resistance to the speaker jack. I checked the transient suppressor diode, is fine. Can anyone suggest a list of top 3 potential causes? What i mean is if you're experienced w/ solid state stuff and with all the data I've given what would be the first 3 things you might check?

[MarkusBass]

the negative outputs outer 3 to the speaker jack measure only 56k while the positive outputs (all 6) are OL.

I'm not sure whether I understand this statement. Such measurements may depend on the order of leads of the meter. I'm not experienced with GK1001, but just to make sure where we are, I would check the following (without input signat and without speaker):
1. voltages on collector of every output transistors (it's hard to read on the schematics but it should be about 42V - positive and negative).
2. voltages between emiter resistors of every output transistors pair (please note that with J5 you can measure the voltage only for the last pair of output transistors). The voltages should be close to 50mV (at the moment) and 10mV when the amp is fixed.
3. Voltages in the biasing circuit - on Zener diodes and between emiter and colector of the second transistor. The voltages should be symetrical. The voltages on each of the Zener diodes is provided on the schematic but I cannot read them (something like 4.5V). I would also check (very carefully) that if you try to change the bias voltage with the trimer, it changes as expected.
4. Voltages of the preceeding stages are provided on the schematic. If the amp is not working correctly, they cannot be the same as on the schematic. Check which of them are different than expected.

Marek

[lowell]

Marek,
I'll double check those voltages... however the voltage on the schematic for the collectors of the OUTER 3 output transistors should be 101V and the inner 3 are around 45V. Right now however I'm getting 30V and 11V on outer and inner sets. The reading of 56k is from the NEGATIVE collectors on outer 3 transistors and with the meter leads negative lead on the collector and positive on speaker jack... I put them this way because of the transient suppressor diodes, they would not allow an accurate reading w/ leads the other way. The inner 3 negative transistors are reading almost double resistance to speaker jack.

[MarkusBass]

Lowell,

I had some problems with understanding your post not because it was written incorrectly but rather due to my poor English . Are you saying that instead of 101V and 45V you get 30V and 11V? Then I would check the power supply. I would disconnect it from the power amp and test with some resistors.
It is important to understand how the amp works. With a small input signal the output transistors use the lower power supply (45V). When the amp gets higher input signal, it is connected (by outer output transistors) to 101V. The input stages use 101V rail (all the time). So if measured voltages of the power supply are completely different than expected, I would check the power supply itself - separately from the power amp - if this is possible.

Marek

[lowell]

Markus,
There is a protection circuit in this amp that is switch-controlled. I think that this partially shuts down the power supply in event of an error. Would this cause the voltages to read low as they are? Also, as stated before the outer 3 transistors of the negative output are measuring quite low while all other output transistors measure OL. I am being careful to put the meter leads with the positive to speaker and negative to collectors on the negative transistors and vice versa on positive ones. Also, when measuring resistance between D18 and 101V supply rail I only get about 5K ohms. Does this clue you into any possibilities?

[MarkusBass]

Lowell,

I cannot read the schematic but it seems to me that in case of a fault detected in the amp, the input of the power amp is muted (which can also be done manually) but this rather shouldn't influence the voltages. At least 45V should always be present.
The amp does not have "a protection circuit in this amp that is switch-controlled". It has only a "Tunning Mute" button, which should be switched ON when tuning up. It causes that no signal is sent to the speakers, or the P.A. system. But it has nothing to do with PROTECTION circuit. Protection state is only signaled with a LED - you cannot switch it ON nor OFF.

I suggest to test the power supply separately from the amp although this may not be easy - it may require desoldering several wires (and yu have to be quite sure which wires should be desoldered).

Marek

[lowell]

Markus,
Ok thanks. There is a "fault" node on the schematic that connects to a separate section of the amp which has an electromechanical switch. This section of the amp has an opamp of sorts and seems to be connected to the protection LED. Does this not have a play in protecting the power supply or the amp? Just want to be sure, sorry if being stubborn.

[MarkusBass]

Markus,
Ok thanks. There is a "fault" node on the schematic that connects to a separate section of the amp which has an electromechanical switch. This section of the amp has an opamp of sorts and seems to be connected to the protection LED. Does this not have a play in protecting the power supply or the amp? Just want to be sure, sorry if being stubborn.

Lowell,

I wrote you that I cannot read the schematic. Can you post a better one?
But even looking at the current one, you can see that the main power supply (45V) goes directly from the transformer to the bridge, later to filtering capacitors and through D22, D23, D24 diodes to output transistors (I cannot read the symbols - could be for example Q27). I cannot see any device that would disconnect the voltage from power transistors in case of a failure.
But since the schematic is incomplete (preamp is missing along with MUTE switch) I cannot rule out any special protection curcuits.
I suggest the following (very simple) tests:
1. measure AC voltage from the transformer (you can measure it on the bridge),
2. measure DC voltage on the filtering capacitors,
3. measure DC voltage after D22, D23 and D24 diodes.
4. post here the results, so that other people could help you.

Marek

[lowell]

Marek,
I appreciate your still helping me w/ this... I cannot figure out how to get a 4meg pdf file uploaded... it's too big and when I try to make it smaller you cannot read it... frustrating. if you give me your email i can send it to you.

[MarkusBass]

Lowell,

I sent you a PM - please read it. But still, I suggest that you perform those simple tests listed above just to make sure that the power supply is OK.

Marek

[lowell]

Ok,
the AC voltage on the power transformer connection on rectifiers BR1 and BR3 are 20VAC. BR2 which is in between those on the schematic is 28VAC.

the DC voltages on the power supply filter caps are +-40VDC respectively on lower rails and +-95VDC on outer rails.

the DC voltage after D22, D23 and D24 diodes is +-40VDC on each side.

[MarkusBass]

Lowell,

The voltages seem to be OK apart from the AC voltage on BR1 and BR2. Instead of 20VAC they should be rather 40VAC. Are you sure that you measured them correctly? You can check DC voltage on C38 - it should be close to 56V (which would prove that the AC voltage is rather 40VAC).
Since the power supply voltages look OK, I would check:
1. +-4.9V between D9 and D5 (as marked on the schematic),
2. voltages directly on D9 and D5 - they should be exactly 3.6V,
3. voltages on Q13 and Q12, it should be 2.6V on Q12 and it should change slightly when you try to adjust R28 trimmer.
4. voltages in the input section (listed on the schematic).

Have you noticed that several resistors in this amp are marked as "FUSE" (e.g. R39, R33, R31 ...)? Have you checked them?

Marek

[lowell]

Markus,
The test points on D1 (-4.9VDC) and D12 (+4.9VDC) are measuring off! They are BOTH measuring -13VDC. Something is wrong in the servo isn't there..? What do you think?