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Topics - gbono

I'm stumped why the output of this amp is limiting or at least I think that's the case.
DC tests:
The bases of Q301,302 are both at 12v and Q304, the bias transistor, has VC=48.3 and VE=46.1. The limiter circuit has the collector of Q316 at 8.55V and the voltage at node R333/4 is 8.48V? The base of Q316 is .259V
I replaced Q315,Q316 and Q303 and the tantalums.
CR301-304 are neither open or short.
Attached waveforms are voltage at R301 (sinewave) and the voltage waveform at the base of Q310

What am I missing?
Amplifier Discussion / Acoustic G120-112
December 02, 2020, 08:35:54 PM
Reassembling this amplifier and looking at the schematic I'm questioning the lack of large aluminum electrolytic in the output (quasi-complementary?)? I've seen pictures of amps and some have a 1000uF/50V in series with the output terminals. Also this amp has an extra large hole in the back panel for an isolating fiber washer(s) for the output jack but NOT for the extension speaker jack?? Doesn't this defeat the isolation?
Amplifier Discussion / Altec 770 Biamplifier
July 07, 2020, 07:39:10 PM
I have several Altec 770/771 biamplifiers that have been rebuilt but have an issue with excessive heating at idle. The schematics are for the 770 version but the design is the same for the 771.

The problem is that the heat ink will get up to 125F/50C when the amps are idling with no input and 8 ohm loads attached. I have tested many of these amps and they appear to operate in class AB and usually don't have this issue. The amps that I have now all meet their output specifications and play program very well but get hot. I replaced the zeners on the EB of the output transistors, cleaned and reapplied new heatsink compound to the mica insulators and checked the "stabistors" (CR1/2).

Can the amp run in class A?
Amplifier Discussion / Sunn Dymos
June 06, 2020, 07:02:30 PM
I have a Sunn Dymos with all the output/driver/reverb transistors removed. Looking at the schematic, it looks like these transistor's collectors (they are all TO-3 2N3055) should be isolated from the chassis - is that correct? Point A and B on the schematic would be grounded without isolation but i don't have any mounting hardware with this chassis to verify how the collectors should be connected.

The jpeg is off the net for reference only.
Amplifier Discussion / Backline 600 no output
May 25, 2020, 05:52:49 PM
G&K backline 600 which initially required new output transistors (drivers also changed). Amp powers up, supply voltages look good +-50V, +-14V, etc., and has strong signal into power amp board (base of Q7a) but no signal at output or at D6 anode.

I believe the issue is the mute/fault circuit. Voltage across R4 is 0V. Checked all transistors in fault/mute circuits but nothing is damaged. Amp powers up and preamp stage works so power on and protect is working but no output voltage on mute node?? How does the mute/fault work on this design?
Amplifier Discussion / Peavey CS400
March 09, 2020, 09:27:41 PM,%207-79)%20Schematic.pdf

CS400 with damaged (but functional) R16/17 - part of a zobel network? The amp is causing the light bulb limiter to glow brightly but the amp did not blow fuse. Is this an issue with high frequency oscillations on the output?
Amplifier Discussion / Gallien Kruger GT200
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?
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?
Amplifier Discussion / Ibanez IBZ10A schematic
May 18, 2017, 02:02:27 AM
Anyone have a copy?
Amplifier Discussion / PT Laney HC50R
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 but I'm wondering if it is too low of a VA rating?
Amplifier Discussion / Hartke 5000 Mystery
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?
Amplifier Discussion / Peavey Blaze 158
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.. ???
Amplifier Discussion / Frontman 212 reverb question
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.
Amplifier Discussion / Altec 770
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.
Amplifier Discussion / Peavey Transtube 212 EFX
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?
Amplifier Discussion / Peavey Stereo Chorus
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.
Amplifier Discussion / Ampeg BT15
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?
Amplifier Discussion / Polytone tone control issue
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).
Amplifier Discussion / G&K Backline 600
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???
Amplifier Discussion / Marshall MG100HDFX no FX
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.