collapse

Show Posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.


Topics - gbono

Pages: [1] 2 3
1
Amplifier Discussion / Gallien Kruger GT200
« on: September 07, 2019, 06:24:51 PM »
I'm working on a GT 200 where the preamp board is working great but no level from the output board. I found Q1/2 were damaged but also noted that R1 and C1 are swapped on the output board. I get a strong signal from the preamp board to the PA but almost no signal at base of Q1. Assuming C1 provides DC block for preamp out??? What am I missing?

2
Amplifier Discussion / Sunn Beta Lead Input Switching Issues
« on: November 06, 2018, 03:20:29 PM »
I have a Sunn Beta Lead that originally had an issue where the "B" channel would stop working (pulsing on and off) when the footswitch was connected - worked fine without FS.

I replaced IC103 (74C04 HEX Inverters) and now the amp powers up with "B" channel LED on (NO INPUT). The amp stays in this state for 20 sec and then turns the green LED (B channel) off. If you plug a jack into the B input the amp works BUT when unplugging from the B input the LED stays on for 20 sec. The "A" channel and "both" work as they should (LEDS go of immediately after removing input). Now when you hook up the footswitch to the amp,all inputs behave as normal (i.e. fixed original issue!!). This is a digital switch/control issue only since the amp passes an input (sig gen, guitar, etc) without issue. All voltage supplies look normal.

I'm assuming that IC106 (MC14506 expandable AND-OR-INVERT gate) is the issue: B channel LED is on without input because Q102 is getting turned on by PIN7 of IC106. Any other ideas?

3
Amplifier Discussion / Ibanez IBZ10A schematic
« on: May 18, 2017, 02:02:27 AM »
Anyone have a copy?

4
Amplifier Discussion / PT Laney HC50R
« on: April 10, 2017, 02:05:51 AM »
Laney HC50 that had a history of cutting out. The issue turned out to be the PT. With the PT on the bench (secondary disconnected from PCB) and the primary connected to a VARIAC there is no secondary voltage present at any primary voltage applied. Primary windings also measure open with DVM.

The transformer in the amp has markings that identify the secondary as 2 x 21VAC (120VAC primary) - the schematic for the amp is attached for reference.

Looking for a low cost PT for this repair - found this http://www.antekinc.com/content/AN-1222.pdf but I'm wondering if it is too low of a VA rating?

5
Amplifier Discussion / Hartke 5000 Mystery
« on: February 14, 2017, 05:57:35 PM »
I picked up this amp off of Craig's List and the only info from the owner was that the amp would power up but no signal at output. Took the cover off the amp and found that the voltage regulator IC401 was hanging off the board probably damaged when the amp was dropped (one of the rack ears were bent). New regulator in place and the amp powers up but I only get a good clean/strong signal out of the send jack. If I inject a signal into the return jack or apply it to the front panel input jacks I get a very small output level at all 4 speaker output jacks on the back panel. I have the amp in Non-biamp mode (Mono).

I started to chase a signal from the return jack and found some very weird layout issues. The amp is a 5000HA with the following main board marked as: Model 3500B/5000/7000B 4005198010A. Looking at the signal coming off the EFX jack board I get a nice clean signal at the RE terminal of AN401 through the cable but when I look at the terminals of AN401 on the main board they are marked SE/RE/G but do not connect to the same components shown in the attached schematic. For example terminal RE on AN401 main board connects to R202 to C201 to pin 6 of IC107-B. "G" terminal connects to pin 3 and 5 on the same opamp. The SE terminal does connect to pin 1 of IC107 so that is why the send circuit is working.

With .5 to 1 VRMS at the return jack the output pin 7 of IC107 is not anywhere near the level of what is input at the return jack and if I chase the signal down to pin 1 of IC202 there is no way I'm any where near 12dB of gain.

Does anyone have the schematic for the above main board - the ones I find on the net don't match the layout I have? What am I missing?

6
Amplifier Discussion / Peavey Blaze 158
« on: June 26, 2016, 11:20:51 PM »
I'm unable to fix this little beast - symptom is that the clean channel distorts at almost any volume setting and the maximum output level is about 3-4VRMS into 4 ohms. If I put in a 1kHz sinewave at J1- I get a nice clean signal until the node between R9 and R10  (positive half of cycle is clipped). I assume these resistors are part of a feedback loop for the darlington pair of Q3/4 - is this right?

  DC voltages on Q3/4 look good and all the supply voltages are correct. If I inject a signal into the power IC U1 (lift C23) I get a nice clean signal that meets the rated power into a resistive 4 ohm load. There is a nice clean signal (undistorted) at R14/C9 - reverb send. Checked and repaired any broken solder joints too.

I'm baffled.. ???

7
Amplifier Discussion / Frontman 212 reverb question
« on: June 07, 2016, 08:29:42 PM »
See the section of the schematic from a Fender Frontman 212. What is the purpose of D15? It's back biased by a -16V supply.

8
Amplifier Discussion / Altec 770
« on: February 04, 2016, 05:46:00 PM »
Question about grounding on this "electronic" crossover amplifier from Altec Lansing. See page 1/8 - where does this amp design return/use chassis ground. I have several of these amps and I notice on 1/3 units that if a measure the +52 supply referencing chassis ground I get get strange reading on a DMM - like I'm reading AC? The transformer, rectifier didoes and C13 test good. Same thing happens at C12. When I check chassis ground continuity (screw on transformer) to a point like negative terminal of C12/13 there isn't any. The other two working amps have continuity between the above points and the supply voltages measure fine when using chassis ground as reference.

9
Amplifier Discussion / Peavey Transtube 212 EFX
« on: May 27, 2015, 12:16:10 AM »
Found one these on CL with a "for parts" status. With the amp connected to a LB limiter I get a soft orange glow and the channel select button does nothing. FX controls appear to work- LEDs light. No signal from the preamp and when driving the power amp input I get a very weak distorted signal at the output.

I pulled the secondary connectors and get 70VRMS across J6-8 and 35VRMS across J18-22. Reconnect secondary and get +-35VDC instead of +-47V rails. Quick DMM check of filter caps, Q3 and Q6 show no issue. What's loading down the 47V rails?

10
Amplifier Discussion / Peavey Stereo Chorus
« on: May 06, 2015, 01:20:00 AM »
Early 90's stereo chorus which needed U8 replaced to get the chorus working again. Now I find that I only get chorus effect at one output amp - I believe it is a "stereo" effect   ???

I get the correct outputs at the preamp 1/2 (dry/mix). Where or how is the stereo chorus generated or routed?

BTW - why do people dis these amps? I have nothing but praise for Peavey designs and support.

11
Amplifier Discussion / Ampeg BT15
« on: April 13, 2015, 03:53:42 PM »
I received this amp in a box - it has the original "fliptop" speaker enclosure with the Altec 418 option - even has the detachable dolly. I replaced the filter caps, output cap and many of the bypass capacitors and replaced the output transistors. The amp worked okay but I noticed that it was not putting out the rated 65W more like 50W (tested with 1KHz sine wave - just before clipping - 4 ohms resistive load) and I had a few bad solder joints that need to be reflowed.

After reflowing the solder joints I have noticed a pretty strong hum that wasn't presnt before  :grr (need to check if its 60 or 120Hz) and measuring the voltage at the filter caps I'm getting 56VDC.

I'll scope the supply to see what kind of ripple is present but I'm a bit baffled why I don't see at least 78VDC at the caps - the secondary measures 58V RMS. I will pull one leg of the 6.8K resistor to see if something is loading down the supply - no bright light when using a light bulb limiter though. Any other ideas - suggestions?

Also what is the function of the diode (D1 connected to Q10 emitter and 300 ohm power resistors - crossover distortion compensation?

12
Amplifier Discussion / Polytone tone control issue
« on: March 23, 2015, 06:02:13 PM »
I don't usually post the same issue to multiple forums (MEF, etc) but ............

Polytone tiny mini brute with proper output levels from power amp and preamp but no tone controls working? Checked all connections and components and I have an output signal from pin 13 and 1 on U1A/B. head scratch icon here.

The preamp circuit is the lower one in the attachement - very simple volume, bass and treble.

Last effort is to swap U4 and U1 (RC4739).

13
Amplifier Discussion / G&K Backline 600
« on: February 28, 2015, 03:39:55 PM »
The initial fault was traced to Q24-26 which were shorted. Replaced three output transistors and found Q20 (+15 rail series regulator) getting hot. It appears there is an issue in the mute/fault circuit since I can disconnect J2 and still have Q20 getting hot.

Can S1 (thermal switch?) fail? I assume it should be normally closed.
I haven't measured voltages on Q10,12,13 but Q10 EB measures short with DMM check though I'm assuming that S1 closed?

How does this mute/fault circuit work?  I assume that pins 2 and 6 in the 555 must be held in some "state" to get output at pin 3???


https://docs.google.com/folderview?id=0BwnWRG81HCWEYmJjMDFkYzUtOGZiYy00OTEwLThmM2QtYWMxMjllZTk5ODcx&usp=drive_web

14
Amplifier Discussion / Marshall MG100HDFX no FX
« on: January 21, 2015, 12:27:16 AM »
I have 2 of these amp heads which both don't have working FXs. The 5 volt regulator is working fine and the FX board has 5V at the connector (no schematic for the FX board available?). I connected a footswitch but I can only change channels but not enable FX....

I was going to trouble shoot the footswitch (FS_c) connection components and the function of IC2 (DG212 MOS switch) but the reverb is not working as well and this signal originates within the FX board and is not switched by IC2. So it looks like the issue is the FX board and not the control circuitry.

Do I have 2 dead FX boards - really? Is there way to trouble shoot the FX board - reload the config bit stream for the DSP??

edit - now that I look at the schematic again - IC3 may also be the issue. I will look at the signal or lack of it at + terminal of C8/7.

15
Amplifier Discussion / VOX VBM1
« on: September 02, 2014, 07:12:16 PM »



I need some extra eyes (brain cells would help too) looking at this problem.....

What "we" know - the amp will power on and not blow fuse(s) - you can chase a signal from Q1 (NPN) to Q2 (PNP) but there is no signal at the collector of Q2. When checking the supply voltages I get a +15/+9 and -15/-3.5 - checking the 9V zener diode (ZD5) and it is okay. When I lift R63 on the -9V supply I get -9V at ZD5 - someting is loading down the supply - but what? I checked Q2 through Q5 with a DMM in circuit and found no issues. D1 and D2 (are these bias diodes helping the BJT turn on??) also test good and the ESR on all the bypass caps (C8,12,20) all test out okay. Check the voltages on Q2 - Q5 but they have to be wrong since the -9V supply line is pulled down. I hate to start removing parts without a strategy.......

Pages: [1] 2 3

* User Controls

 
 
Welcome, Guest. Please login or register.
Did you miss your activation email?

* Recent Posts

signal tracing by g1
[December 10, 2019, 02:26:30 PM]


Eight Audio Kit TDA7294 and TDA2050 from India by laikairawk
[December 10, 2019, 01:57:45 AM]


The Dangers of Speaker Impedance Mismatching by Enzo
[December 10, 2019, 01:15:39 AM]


Fender Mustang II v.2 by Enzo
[December 08, 2019, 04:46:37 PM]


Gallien Kruger GT200 by gbono
[December 04, 2019, 02:54:12 PM]

* Sponsors