Hohner/Marlboro 1550 Bass Amp Schematic

[Steve Fundy]

Hello All. I have been brought a Hohner 1550 Bass Amp made by Marlboro which needs repair. I have looked all over the net, but, I haven't been able to find a schematic. Any chance there's one out there? The owner would be so happy if I can fix it!
Thanks,
Steve

[Enzo]

What is wrong with it?

[Steve Fundy]

Hello Enzo. It's got almost no output. I have found the power amp section definitely works. I have discovered parts of the circuit, put test tone directly into the PA and it comes out loud! I have been tracing the tone through the preamp. I have amplification in the 1st stage of the preamp as i see on the scope, but it disappears shortly after. I have tested components and replaced a few dodgy caps, but still no output via the input volume. On odd anomaly: If I crank the "Mid" and "Boost" controls it will start to amplify as the volume knob is turned up, starting at "7" and all the way up getting louder to "10". I have tested transistors in circuit, and so far they seem "OK". I could pull them out one by one I guess. I was given a similar guitar amp some time ago, it has same symptoms & I never repaired it either (no schematic!) Thanks, Steve

[Enzo]

Can we get a photo of the board? Is it s straight transistor preamp or is it op amps plus transistors?

[vintagekiki]

https://i.imgur.com/Ync6vos.jpg

[Steve Fundy]

Hi Vintagekiki. I have this same schematic. It's similar, but not the same. The amp is actually a 1550 not a 1500B. The 1st stage of the preamp is the same, but it deviates from there. I had been using it, but it has gotten me nowhere. Also the 1500B has no mid control. Many thanks.

[Steve Fundy]

Enzo. It is a straight transistor preamp. I am going to take that pic of the circuit board now. I didn't have a chance last night. It's still AM here in Hawaii

[Steve Fundy]

OK, here are 2 pics. I took one of the entire board and another backlit to make the traces easier to see. The orange .47uF cap was changed yesterday as I tested the one that was there and it was somewhat higher in value than I expected. No change.
Thanks

[vintagekiki]

Hi Vintagekiki. I have this same schematic. It's similar, but not the same. The amp is actually a 1550 not a 1500B. The 1st stage of the preamp is the same, but it deviates from there. I had been using it, but it has gotten me nowhere. Also the 1500B has no mid control. Many thanks.

Schematics 1500B use as a base, and on the basis of 1500B draw difference and here are the schematics for 1550.
1500B has a fixed Mid control with R9 (3.3k)
Question:
Are the DC voltages on the transistors within the limits as on the schematics?
Some transistors have been replaced. Are they correctly replaced as schematics?
Does the 470n have a severed wire and "hang out" in the air?

[Steve Fundy]

Thanks, that's actually a good idea, to draw the differences between the schematics. As for the cap, no, it's formed in a curve and then 90 degree bend down soldered into the board. I removed it from another circuit board & it was formed that way. The other cap (also 470n) is the original & it's soldered securely. I haven't checked the DC voltages on the transistors yet, another good idea. Any replaced transistors were in there already and though it's an assumption, I would imagine they are correctly installed, or this amp wouldn't have worked before it's recent failure. I will be working on it again tomorrow. Hopefully your suggestions will help me find the culprit.
Thanks,
Steve

[vintagekiki]

To determine more closely whether the fault is in the preamp or in the power amp, disconnect the connections from the hot end of the volume potentiometer and connect a guitar to it (power amp test) .
If you get clear sound and decent power, the fault is in the preamp.

[Steve Fundy]

Thanks vintagekiki. I have already done this and I know it's in the preamp, somewhere between Q1 & Q2. I put my tone into the base of Q2 & it's loud at the output. I also have amplification out of Q1. It's disappearing somewhere in the tone stack/volume control. I will be charting the differences between the schematic and the actual circuit today & continue hunting for a bad component. I have replaced 2 caps already, but no change. Mahalo

[Steve Fundy]

Did some work on the amp today. After lots of tracing, I finally decided to test the DC voltages on the transistors. As it turns out Q2 is actually bad. It isn't original. The original was supposed to be a BC549C. Not sure because, It was replaced previously with TCG199. I looked them up. Apparently it is crossed with a NTE/ECG199. I don't have a proper substitute, but I decided to try a few similar NPN's 2N3904, a N2222 and one I pulled from the other marlboro I have a NS138 just as a test. So: Now I have preamp signal, but the amp now has a serious motorboat going on. The test tone is audible & rises & falls with turning the vol up/down, but it mixed with a loud motorboating sound. I began to check the power supply etc. I am a bit baffled why the motorboating goes away when either the bad transistor or no transistor is installed. I figure I would hear that noise no matter what. Hmmmm

[Jazz P Bass]

One cause of 'motorboating' is faulty power supply capacitors.
It didn't rear it's ugly head until you replaced Q2 because now the amp is drawing current.

[vintagekiki]

... ...

Any NPN silicon low noise general purpose transistor can be used for the preamp transistor with VCEO> 30V; hFE 420 to 800. When replacing transistor, pay attention to the correct pinout.

Motorboating goes due to poor DC voltage filtration. If the cap looks nice on the eye, it does not necessarily mean that it is good in terms of parameters (ESR, capacity ...)
Check or replace C3 (100u / 25V); C12 (220u / 35V); C13 (100u / 35V); C14, C16 (2200u / 40V)

Check and remove any cold soldering on the pcb.
Check the condition of the jacks, and the hardware connections to the chassis. Clean any of rusted or corroded traces.