GK 400 rb

[catstrat]

This came to me blowing fuses. I replaced 4 output trans. along with the drivers. My question, the schematic says to set the bias for 10MV across r26/r27. When i check that with my meter i get nothing. Could it be my meter isn't sensitive enough to measure voltage that low? I tried it with the cross over notch method and it seemed to me the o/p transistors were getting un usually hot in a short time with a signal. Does anyone had any knowledge with these?

[g1]

If memory serves, that 10mV(DC) is across the 2 resistors, so you should have 5mV each. Depending on your meter, accuracy at such a low level may be a problem. Does your meter have a mV range?
Please post the schematic you are working from.

[catstrat]

GK 400 rb

Here are further details. This had mj15001 and 15002 for outputs. Audikarma gave a list of suitible replacements. I installed MJ 15022 and 15023. Should i have changed the emitter resistors to and to what value. The originals are .33 ohms. The sine wave is good and clean, no problems i can see. I'm thinking it's a bias problem but again, the sine doesn't show it. Any help is apprecaiated. Thanks all.

[catstrat]

GK 400 rb.

Hi G-one. It has a 200 mv range. I'm thinking my meter isn't sensitive enough to measure voltage that low. If that's the case, is there any alternate to set the bias? Thanks.

[MarkusBass]

You don't have to guess. You can easily test the meter, for example with 9V battery and two resistors in series (to form a voltage divider). Take two resistors which such values that voltage on one of them will be in a range 5-20 mV. This will allow you for checking the meter.
I would suspect that the meter is OK but there is not sufficient voltage on bases of the driver transistors. Have you checked that when you change the bias trimmer setting, the voltage between bases of the drivers change as well? If yes, in what range?

Mark

[catstrat]

gk400 rb

Let me test the meter per your suggestion and see whare i'm at. Let me check the voltage change again to be sure but i think the base voltage does change. Let me see if i can figure out how to post a schematic on here. I've never done that here before. Thanks.

[catstrat]

400rb

After i tested the meter i powered up the amp and it blew a fuse. It blew 2 fuses. I tested it with my trusty variac method and sure enough, one of the mj15022 is heating up,again. So what ever caused the problem in the first place is back or it was never fixed. I'll replace the 15022 and try to post a shematic on here. Thanks everyone.

[catstrat]

stratcat50@hotmail.com

[MarkusBass]

You post schematic with the Manage Attachments button (when replying to the thread).
The variac method is not always good with transistor amps. The problem is that some parts of the amp may not work at all unless the power supply voltage is greater than x Volts. For example 7815/7915 stabilisators will not work correctly unless the input voltage is greater than 18V. This may case that other parts of the amp don't work (eg. preamp, input IC in the power amp, protection circuit). So one has to be careful when checking amp with variac. Sometimes it's better not to use it .

Mark

[catstrat]

rb400

Hi Markus. I found another post on here by Lowell who posted about an rb400 not having bias voltage. Sounds like my problem. He posted a schematic.I'll try to post it again but i didn't have much luck the first time. On the variac use i have a question, If the amp is blowing fuses at a full 115 volts then how do you check for shorted o/p transistors? That only thing i use a variac for is trying to find shorted O/P transistors. The shorted one will heat up, usually. Once i stop it from blowing fuses then i can proceed with voltage checks at a full 115 volts. Many on here know i'm a tube amp guy and i just don't get enough solid state to sharpen my troubleshooting skill. That's where you guys come in handy. I can look at the schematic on this 400 rb and tell what the componants are and i can identify most of the circuit except the bias circuit. In the meantime i'll replace the shorted o/p transistor. Thanks.

[MarkusBass]

On the variac use i have a question, If the amp is blowing fuses at a full 115 volts then how do you check for shorted o/p transistors?

This is a difficult question. I usually find shorted transistors with a meter set to Ohm range - without turning on the amp. If you use variac just to find shorted transistors, that's OK. I only wanted to say that measuring voltages (when variac is used) does not always makes sense because part of the amp may not work and the rest of the amp may work in an unpredictable way.
Quite often (if this is possible) I put small value resistors (22 Ohms, low power) between the power supply rails and the power amp. If the power amp has shorted transistors, the resistors will burn and nothing else. I'm sure that everyone here uses different methods of fixing amps. I prefer to find all failed components without turning on the amp. Of course in some cases it is not possible.

Mark

[catstrat]

400rb

Do you remove the transistors from the circuit to test with a meter? I used to do that way until i tried using the variac and that seems to find the shorted ones faster but, you have to have a full 115 volts on the amp to get the proper voltage readings. Anyway, i removed the two 15022s that were overheating and one of them was shorted with a meter check. And as i increased the voltage Q4 blew which i've already replaced once. I'm right back to where i was when it first came here. Puzzles me, earlier today it was working but with the o/p transistors getting hot. Clean sine wave. Normal amperage draw. Now this. To me it's a bias problem. I've already replaced q10/q3/q6/q4/ and q15. Some of those i replaced because they didn't test right with my meter and i had them in stock. Let me try to post the schematic again. Thanks.

[nsubulysses]

Or the diode function on a DMM can be useful to find shorted transistors as well. Because then it just beeps and you don't even have to look up.

Testing from base to collector then base to emitter in one direction, then switching the meter leads to check for the opposite polarity. You should have beeps in one direction and no beeps when the leads are switched.

[catstrat]

400rb

i just uploaded the schematic and board layout, problem is, i don't know where it went.Unless they haven't made it public yet.

[MarkusBass]

Do you remove the transistors from the circuit to test with a meter?

No, I don't. But... I usually get schematic of the amp, read it and try to understand the role of every component. Sometimes transistor leads are shorted with a low value resistor and in such cases I need to distinguish whether low resistance on the meter is caused by a shorted transistor, or by this resistor. Only in very rare cases I desolder transistor to measure it.

EDIT: when attaching a file to a post you need to perform 2 steps (described on the screen):
1. Click Add Files to upload new files or select from existing files below
2. Drag previously uploaded files into this area to attach them.
You most probably performed step #1 but forgot to perform the step #2. Step #1 uploads the file to the server (to a general storage) and the step #2 inserts previously uploaded file into the currently edited post.

Mark