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This is the G25R guitar amplifier model made by Marshall. It's part of the "Park" series amplifiers and it's made in Korea. It's a ~20 year old solid state combo with a 10" inch speaker, a Belton reverb tank, two channels (clean and OD), 3-band EQ, guitar input and headphones out.The problems:
This combo amp came in with quite a few issues. Most potentiometers were scratchy, there were leaky capacitors and the reverb did not always work. The channel select switch worked some times, the power cable was a bit messy and the amplifier was really hard to operate.
The restoration:
We decided to replace all the electronic components that acted up for brand new ones. I cleaned up the amp and removed the pcb board. Then, I replaced most potentiometers with brand new Alpha ones. I also replaced the channel select switch with a heavy duty one.
The electrolytic capacitors of the amp were old and leaky so there was a bit of hum coming from the amp even with no guitar connected to it. Leaky caps is a common site on most old amplifiers. These caps have to be replaced with quality ones for a smoother operation. Be sure to replace an old cap with a new one that has the right voltage, polarity and capacity ratings.
The reverb tank was made by Belton and it was in a pretty good shape but the reverb effect did not work... The all plastic RCA jacks were to blame so I replaced them for a new set of gold plated high quality jacks. The reverb worked better than ever before! It sound so full and natural and it couldn't be compared to modern cheap IC-based reverb effects used on affordable marshall combos.
The 3-prong electrical plug of the power cable was messed up. So I cut down the old wire and rewired the EU-type grounded plug to make sure no one gets hurt while plugging in or using the amp...
Once the circuit was good to go, I faced a small problem as the full shaft Alpha pots were too large for the old knobs... Therefore I bought a new matched set of plastic hex knobs with side screws. I bolted the new knobs on the pots and turned the amp ON...
The result:
What a sound! Once all the problems were addressed, I was able to test both channels of the amp with a few guitars. The amp is loud and the clean channel is so punchy and clear. OD/Dist pedals such as the MI audio crunchbox, the Tech21 gt2 or the Marshall JH-1 sound great through its clean channel. The EQ works great and it's really versatile. The OD channel has also a nice classic Marshall-ish tone. At higher gain level settings, it can roar and produce decent hard rock tones. Tone wise, I totally prefer this old Park G25r to modern MG and AVT series marshall combos!
Here are a few pictures of the amp:
Cheers,
Thanos
Hello! My G25R also had pot issues so I had to replace them.
ΑπάντησηΔιαγραφήWhat is the problem with the pots on your G15R?
Here is a schematic of the G15R CD were the control pots and their resistance values are pictured: http://www.eserviceinfo.com/downloadsm/48727/Marshall_G15RCD.html
I would first clean the pots using an oil-free contact cleaner spray can.
http://youtu.be/0QvOepREO4k
If this doesn't fix anything, consider replacing a few pots with similar ones (in terms of size and taper resistance value).
Cheers,
Thanos
Spraying all the pots and jacks inside the amp with a contact cleaner could probably fix this issue. If not, some parts need to be replaced, usually some capacitors (electrolytics) and/or some transistors/chips. You need to take it to a qualified tech if you're not familiar with electronics.
ΑπάντησηΔιαγραφήCheers,
Thanos
The volume drop issue is probably caused by dust and rust on jacks and pots. That's why you should clean them using contact cleaner.
ΑπάντησηΔιαγραφήThe distortion on the clean channel is probably caused by voltage drops. Replacing the large power electrolytic capacitors will usually fix that. If not, then you may have to replace a few large power resistors (see the universal resistor color coding and g15r amp schematic for these). If nothing changes, I'm pretty sure the last step would be to replaced an Opamp chip. Opamps are usually labeled U1, U2 etc on the printed circuit board (PCB). Check the electronic parts of your amp for visual signs of wear, burn marks, color fading etc, capacitor leakage and so on. Here are a few helpful links:
http://en.wikipedia.org/wiki/Capacitor_plague
http://guitar-dreamer.blogspot.gr/2012/11/diy-repair-your-solidstate-combo.html
http://www.eserviceinfo.com/downloadsm/48727/Marshall_G15RCD.html
Check out my full article on DIY amp repairs above. Be really careful when working inside your amp. Unplug it from the AC power for 1-2 days before opening it up (so the Caps can fully discharge)!
hi mr Sottos maybe you can help,since we have same model park g25r possible you can send me the value of all 7 potentiometers, because somebody work on and they lost it.
ΑπάντησηΔιαγραφήGain
Volume
Clean volume
Treable
Middle
Bass
Reverb
Thanks in Advance
Hello!
ΑπάντησηΔιαγραφήIf I remember correctly, these are the pot values:
Gain B250K
Volume A10K
Clean volume B10K
Treble B200K
Contour/mids B20K
Bass A1M
Reverb B20K
"A" stands for "Audio type" or Log pot and "B" stands for Linear pot.
If you need to tweak the amp for more gain and volume, you could replace the gain and Vol pots for a bit higher value ones.
Cheers,
Thanos
hi every one!
ΑπάντησηΔιαγραφήI have a g25R. Two components are burned. The two small blue resistances between the cooler and the tw0 black components 35V/2200uF (capacitors?). resistances R40 and R41. If anybody can give me the correct vallues?? PLease you would help me a lot
Hello! The right question is: Why were these components burnt out? I think there was some short circuit inside the amp or a mains voltage surge incident.
ΑπάντησηΔιαγραφήThe Power amp chip could also be dead inside. I would replace that as well.
The capacitors are indeed rated at 35 Volts and have a capacitance of 2200uF. Any good quality electrolytic capacitor rated above 35V and 2200uF will be a good replacement. Also, try to find capacitors suitable for 105oC Celcius temperature tolerance rather than 85oC. The 105oCs last longer!
The two resistors are hard to remember because I don't have the amp anymore. However, you could identify their value by looking at their color stripes/bands and using this online tool:
http://www.digikey.com/us/en/mkt/calculators/4-band-resistors.html
Use replacement resistors rated at 1Watt.
Cheers,
Thanos
Hi, would it be possible for you to confirm the value of the treble pot? The values of the other pots on my G25r are inconsistent with the values posted in reply to a previous question even though the code no on my PCB is identical the the one shown in your images. The values shown on my other pots are:
ΑπάντησηΔιαγραφήGain B200K
Volume A100K
Clean Vol A10K
Treble ?
Middle B2.2KA
Bass B20KA
Reverb B20KA
Thanks
Hello!
ΔιαγραφήIt seems that the circuit of your amp is quite different. That's common for amps that were produced for years and changes/upgrades were made along the way. That's why we see these "rev._" and "mk_' sings on some PCBs.
I could only guess what the correct Pot value would be based on other schematics.
( Unfortunately I also sold this g25r but it wasn't even the same amp)
A good starting value for the treble pot would be around 20K based on other "Park series"
You could experiment with different pot values using alligator clips and a bunch of pots.
But because pots are not too cheap, you could start by using some software emulation. The Treble pot is part of the "Tone stack" circuit inside your amp.
Here's a tone stack simulator software:
http://www.duncanamps.com/tsc/
As said, a good starting value for the treble pot would be around 20K. Emulating the "Marshall Tonestack" circuit using these pot values would give you the frequency response of this EQ.
Cheers,
Thanos
Morten Rasmussen
ΔιαγραφήI'm in the processs of restoring my own G25R. It has been stowed away in the attic for several years because of the really REALLY cheap potentiometers. I'm now replacing these with decent ones.
My pot readings are in full agreement with Simon's.
The missing value for the treble pot reads B20K, just like bass and reverb.
I'm not sure if anyone else has stumbled across this page, the values on all my pots are as follows and I am looking at doing the refurbish on all my pots as well like the rest of you. overdrive=B200K 3CW
ΑπάντησηΔιαγραφήoverdrive vol=JH-700KA
clean vol=JH-70KA
treble=B20k 3CW
mid=B20k 3CW
bass=B2.2k 3CW
reverb=B20k 3CW
Hi,
ΑπάντησηΔιαγραφήI've just cleaned up ones of these combos and replaced the speaker with a Jensen C10R.
It sounds lovely!!