I'm going to go ahead and try to compare my 3505 with the photo and schematic of the 5502 based on the component values in order to identify any differences that could result in the different tone of my 3505 as compared to the 5502.

I'm planning on treating the 5502 schematic as a starting point and checking for the indicated connections using a meter to check for near-zero resistance between components with the power disconnected?  Would I be risking damage to any components by checking for connections this way?

I found information regarding capacitor colour coding and worked out all the capacitor and resistor values on the board of my 3505.  I was planning on comparing these to the 5502 schematic and/or board photo, but it seems there are some other differences that I simply don't know enough about to explain.

It looks to me like there is an unlabeled capacitor soldered across R4 on the 5502 board (directly below C1 in the photo, the label is blocked by one of the heat sink legs).  I cannot make out the labeling on the 5502 schematic in this area, but there does appear to be something in addition to R4 across pins 1 & 2 in this area.  I find it strange that the component numbering on the 5502 board does not include this part, but the photo and schematic seem to show a component in place.  On my 3505 it seems that C1 performs this function since there is zero resistance between the left side of C1 and the left side of R3 and the same on the right sides.  Is this a reasonable way to check that two components are connected in parallel?  I don't know whether this indicates any significant difference between these two amps, but it seems like it's in the preamp so it has the potential to affect the treble content and/or the amount of overdrive.

Both boards also have two arrangements each consisting of a diode, capacitor, and resistor in the upper-right corner.  These diodes labelled ZD1 & ZD2 on the 5502 board are not installed and I could not find any reference to ZD1 & ZD2 in the 5502 schematic.  Based on where these capacitors and resistors are in the schematic, they appear to be at the transition from the preamp to the power amp.  If that is the case, is it reasonable to assume that the presence of these diodes in my 3505 (ZD2 & ZD3) would not significantly impact either the treble content or the amount of overdrive since they are not in the preamp?

Thanks to everyone who has chimed in so far.  I feel like I'm rambling, so please let me know whether any of this makes sense.

Well that makes more sense.  So do the coloured bands on C1, C4, C5, C7, C8, C9, & C11 indicate their capacitance values?

Just a thought from a complete novice ...

I've attached an image of the 5502 and 3505 boards side-by-side.  I noticed that some of the marked component locations on the 5502 board are vacant.  These are the components labelled "K/B".  I assume that means the same board was used in both the 5502 bass amp and the 5302 keyboard amp and they simply installed the components appropriate to the individual amp.

I'll go through the component numbers in detail later today, but is it possible that this accounts for the differences in component numbering between the 5502 and my 3505?  If so, would it be possible to make the 5502 preamp by substituting the appropriate components on my 3505 board?

I've tried playing this amp through a couple of different speakers and cabs with both bass and guitar, and it just has no overdrive at all and very little treble.

Although I can't find a schematic that is specific to the model 3505, a few sources have led me to believe that it "should" be the same as that of the 5502.  I've attached a PDF of the only schematic I can find for the 5502, but it is quite hard to read and it includes schematics of other amps too.  The schematic for the 5502 is the one at the bottom of the page that says "Bass Amplifier 5502 (Preamp)" above it.  It seems to match the component values and connections that I can make out on my board (fixed and variable resistors seem to be the same values, traces appear to make the connections shown in the schematic), but I can't be certain that it is identical.

I noticed is that in comparison to the 5502 Bass 20 amp in this video https://www.youtube.com/watch?v=h289WBGK7vU, my amp has none of this overdrive when I plug in a guitar.  Please take note however that this video is a comparison and switches between the 5502 Bass 20 and the 5501 Bass 12, so you have to look at the amp behind him to see which one you're actually hearing (the Bass 20 is the larger one and has a less overdriven tone).  The circuit in my amp may not be identical to the one shown in this video because some of the component numbering on the board is different than in my 3505.  That may also mean that my circuit is not identical to the schematic either.

Sorry for the long post and convoluted description.  Any thoughts on where I could begin?

I had been lurking on this site for a while in order to learn more before I bought my Marshall Lead 12 3005 Micro Stack a few months ago.  I've been playing and loving that little guitar amp ever since.  So much so that I just bought a 1987 Marshall 3505 Micro Bass stack to go along with the 3005.

The 3505 is similar in appearance and controls to the 3005, and it has the same controls and layout as the 5502 Bass 20 combo, but it is a 30W bass head running into a 4 ohm load.  It was part of Marshall's Integrated Bass System (IBS) line and came with two 10" cabs loaded with 8 ohm Celestion G10L-35 speakers.

I've been having fun playing it, but I've found that is has essentially no top end or any overdrive breakup at all.  Although I only use it for bass, I'd love to know whether anyone has any suggestions on how to increase the treble and add some gritty overdrive to the tone of this amp?

In addition to these two Marshalls, I also own Danelectro Nifty Fifty and Nifty Seventy combos.  I don't know how much input I'll be able to offer this forum since I'm certainly not an electronics expert, but I will continue to enjoy reading, learning, and contributing as much as I can.