Shed find: is a GM 400 guitar amp worth rescuing?

lasseo · April 23, 2020

Hello,

WIth the new place along came a shed, and along with the shed came quite an inconspicuous-looking box that looks like a guitar amplifier. What might this be, approximately how old is it, and would it be worth doing anything about or should it rather go to on top of the hard rubbish pile?

It powers up, and all the usual hallmarks of electronics that has been stored out of house for a long time are there: AC hum, crackly pots (one of them seized actually), intermittent dropouts of the input. It does sounds all right when feeding it with line out off a little non-iPod music player, so I'm wondering whether this would be worth bringing back to life? If yes, any insider knowlede would be welcome, starting by identifying what it actually is. Quick google search was rather slim pickings.

Cheers

lasseo

pwstereo · April 23, 2020

Tandy Realistic brand from what I can see.

So not a big name in guitar amps or other stage equipment.

Monkeyboi · April 28, 2020

A good DIY project to learn how amplifiers work. If you don't want it, offer it up for collection by someone interested in tinkering around with it rather than chucking it at the tip. As for seized and scratchy potentiometers are concerned - nothing a spray of CRC or WD-40 won't fix.

Cheers,

Alan R.

Edited April 28, 2020 by Monkeyboi

VK3RX · April 28, 2020

DeoxIT D5, never WD40

aussievintage · April 28, 2020

Yes, an old Tandy amp. Not worth much, it's the sort of thing a kid would sit in his bedroom and practice with.

But if you have a use, and can fix it for the sake of a cleanup and spray - why not.

lasseo · April 28, 2020

4 hours ago, aussievintage said:

Yes, an old Tandy amp. Not worth much

Yeah that figures, otherwise it would hardly have been left behind.

4 hours ago, VK3RX said:

DeoxIT D5, never WD40

So what exactly does DeoxIT do that WD40 doesn't, other than doubling the price? WD40 did a fine job of making that stuck pot moveable again.

7 hours ago, Monkeyboi said:

A good DIY project to learn how amplifiers work.

I already started! I mean, other than a few hours what's there to lose? I checked the power supply and so far as I can tell the capacitors and rectifying diodes measured okay - this baby is old (or cheap?) enough to have discrete components rather than a bridge.

The AC hum and buzz remain and are rather persistent, and so far I haven't found anything wrong per se - no overheated components, no broken traces on PCB, nada. However, permutating cables on and off various inputs and outputs in much the same way our three-year-old would I did discover that if I stick a mono TRS connector (no cable) to the EFF in socket, the hum and buzz are effectively gone. Unfortunately, so too is anything that is panned stereo in the recording, such vocals of the music... the source is a little battery operated Sansa music player with a 3.5 mm headphone jack out. I tried a mono cable between Eff out and Eff which too removes the hum, along with any ability for volume control; it just goes really, really stupendously loud!

He Said What Now?

andyr · April 29, 2020

Post content deleted

Andy

Edited April 29, 2020 by andyr
Move to correct thread

Monkeyboi · April 29, 2020

19 hours ago, VK3RX said:

DeoxIT D5, never WD40

I agree with using DeoxIT or similar in something that you wanted to keep or restore with unobtainium parts, but it's a discarded guitar amplifier that most likely uses off the shelf readily available generic parts! If the OP wanted to do a proper restoration then replacing the worn out and seized potentiometers would be a better long term solution. Depending on how much previous use these controls had they are probably worn out and well past their use by date in any event. Considering replacement pots are just a few dollars each if it was me I'd just replace the old pots once I'd determined if the rest of the device was worth investing the time, effort and money on.

Whilst I would never use WD40 or CRC on a client's piece equipment in a potentiometer or electrical switch these products have their uses in electronics servicing and other applications. DeoxIT make some really good products for electronic servicing and I have some of these in my workshop for regular use but most others that don't do electronic servicing are unlikely to buy a 142g can of DeoxIT D5 for $45 to spray a couple of pots in the hope it might unseize them. On the other hand anyone who has a lawnmower or some mechanical equipment is possibly going to have an $8 can of WD-40 or CRC226 in the back shed.

I'm making the assumption that the OP is not an electronics service technician simply because I doubt he would have asked the questions in the first place. If he was a service tech he would most probably used DeoxIT or a similar product out of preference.

Would I use WD-40 or CRC226 on a customer's bit of hifi gear to unseize a potentiometer or switch? - NO, on a piece of test equipment? - NO, on medical equipment? - ABSOLUTELY NOT. On a piece of verge collection kit just to unseize a part if I didn't have any DeoxIT D5 on hand? - HELL YEAH.

Would I go out in the middle of a pandemic just to buy a $45 can of DeoxIT D5 for a one off job in a lifetime? - NAH.

So what are the differences between the said products?

DeoxIT D5 - Pros: good for improving or restoring oxidised contacts, has some lubricating properties.

Cons: not ideal for unseizing moving parts. Uneconomical unless used often for its intended purpose (i.e. electronic servicing).

WD-40 / CRC226 - Pros: good water displacing properties, good for lubricating some parts, good for unseizing moving parts, multiple other uses and is inexpensive.

Cons: not ideal for electrical conductivity because over time the lubricant can attract dust which could eventually create a conductivity problem.

Cheers,

Alan R.

Monkeyboi · April 29, 2020

16 hours ago, lasseo said:

The AC hum and buzz remain and are rather persistent, and so far I haven't found anything wrong per se - no overheated components, no broken traces on PCB, nada. However, permutating cables on and off various inputs and outputs in much the same way our three-year-old would I did discover that if I stick a mono TRS connector (no cable) to the EFF in socket, the hum and buzz are effectively gone. Unfortunately, so too is anything that is panned stereo in the recording, such vocals of the music... the source is a little battery operated Sansa music player with a 3.5 mm headphone jack out. I tried a mono cable between Eff out and Eff which too removes the hum, along with any ability for volume control; it just goes really, really stupendously loud!

The persistent hum might be caused by a ground loop or just the high sensitivity of the amplifier input. At the effects sockets, the levels are generally higher so you aren't picking up as much mains 50Hz hum as you would from the more sensitive guitar input.

The very loud volume is because the normal input from an electric guitar is just a few millivolts, whereas the output from your music player is more likely to be hundreds of millivolts.

Cheers,

Alan R.

pwstereo · April 29, 2020

1 hour ago, Monkeyboi said:

WD-40 / CRC226

CRC 5-56 is the equivalent to WD-40. 2-26 is more akin to an electrical contact cleaner/lube.

VK3RX · April 29, 2020

22 hours ago, lasseo said:

So what exactly does DeoxIT do that WD40 doesn't, other than doubling the price? WD40 did a fine job of making that stuck pot moveable again.

What monkeyboi said

lasseo · April 30, 2020

21 hours ago, VK3RX said:

What monkeyboi said

All right then, doesn't look like I did any major harm.

On 29/04/2020 at 12:36 PM, Monkeyboi said:

The persistent hum might be caused by a ground loop or just the high sensitivity of the amplifier input. At the effects sockets, the levels are generally higher so you aren't picking up as much mains 50Hz hum as you would from the more sensitive guitar input.

That's the thing, it's the other way round. If I plug the music player into the eff in, it's deafeningly loud, and volume controls have seemingly no or little effect. If I plug the player into Hi-Z or Lo-Z in, both input level and volume seem to work, and there is hum. If I plug a TRS to RCA converter plug into eff in, the hum is gone. I presume the converter plug effectively shorts one of the inputs to the shield. If I keep the converter plug in the eff in, and connect the player through the Hi-Z or Lo-Z input, then there is no hum, but the vocals (stereo centre) are all but gone, too. I tried unplugging the effect board from the main, but that didn't do any favours either.

A cable had been added that had been soldered to negative leg of one of the supply capacitors, and connected to the chassis in the other. It didn't help with the hum, and it didn't look like it would belong. I might try and get a couple of photos up off the boards so you get some idea what I'm talking about.

Wrong thread or not, Monkeyboi's lockdown project looks way more interesting than mine...

lasseo · May 2, 2020

Well here are a couple of photos of the amplifier section. As you can see, not much to it...

Main board, 1

Main board, 2

Hi-Z and mic input

Effects loop (Effects out and in). Phone ans speaker out underneath

Overview photo. Power caps measured okay but I tried replacing them anyway. Added a wannabe Colorbond transformer shielding... these steps didn't hurt, and they didn't help either. What helps (the hum) is a TS to RCA adapter plugged into EFF in. But vocals are gone...