Just bought this lovely amp and after pots and input/ output contacts cleaning, from quick listening it’s worthy to bring it back to its potential.
Maybe not at once, and my abilities are limited, so I would like to ask about what to do first.
I know one enthusiast living nearby, and he focuses more on valve amps and CD players, but he is a pedant, he could help.
The issue is channels imbalance, and perhaps slight tonal differences between them when balanced out.
Left channel is about 40% or more quieter. And also, when listening at low volume, it can just disappear. When louder, it seems stable.
I did not check anything, but DC presence on speakers terminals, there is no DC there.
In the amp there should be just 2 ICs, so mostly it’s solid state.
What would be the procedure to do first to make it balanced, then second, to make it tonally equal?
Thank you.
Maybe not at once, and my abilities are limited, so I would like to ask about what to do first.
I know one enthusiast living nearby, and he focuses more on valve amps and CD players, but he is a pedant, he could help.
The issue is channels imbalance, and perhaps slight tonal differences between them when balanced out.
Left channel is about 40% or more quieter. And also, when listening at low volume, it can just disappear. When louder, it seems stable.
I did not check anything, but DC presence on speakers terminals, there is no DC there.
In the amp there should be just 2 ICs, so mostly it’s solid state.
What would be the procedure to do first to make it balanced, then second, to make it tonally equal?
Thank you.
Attachments
This is definitely a nice piece to work on 😉
The first thing to do would be to visually check the general condition of the amplifier boards - lack of cracks and loose wires, need for resoldering etc.
Then you should download the service manual (e.g. from Hifiengine) to get the schematics.
After that- to test for the channel imbalance - you could feed the amplifier from computer sound card with e.g. 400Hz or 1kHz sine wave and start measuring the AC values along the signal path with a simple digital multimeter (DMM) until you come to the spot where substantial differences between channels occur.
Rgearding tonal imbalance - causes may be many, usual suspects are dried lytics in signal path, easiest way to check the device status is to use RMAA software and a sound card. Probably the problem will be located in the preamplifier area so you can start with measuring preamplifier output.
When preamplifier is fixed then the power amplifier should be addressed.
The first thing to do would be to visually check the general condition of the amplifier boards - lack of cracks and loose wires, need for resoldering etc.
Then you should download the service manual (e.g. from Hifiengine) to get the schematics.
After that- to test for the channel imbalance - you could feed the amplifier from computer sound card with e.g. 400Hz or 1kHz sine wave and start measuring the AC values along the signal path with a simple digital multimeter (DMM) until you come to the spot where substantial differences between channels occur.
Rgearding tonal imbalance - causes may be many, usual suspects are dried lytics in signal path, easiest way to check the device status is to use RMAA software and a sound card. Probably the problem will be located in the preamplifier area so you can start with measuring preamplifier output.
When preamplifier is fixed then the power amplifier should be addressed.
Assuming it has them this is a classic case of tarnished speaker relay contacts. Higher volume (so more current) temporarily clears a conductive path through the insulating tarnish layer.
I think it has. When switched on, it will click after one-two seconds. I just did it again and again, and what happened, is that the sound didn’t come out at all. Then moved the volume pot up, the sound appeared, but now with almost equal for both channels, now the right channel is slightly weaker. I feel stupid a bit, it possibly will just need better cleaning. I have emptied deoxit cleaner doing the all contacts cleaning, didn’t sort out speaker relay contacts, will need to find where they are first 🙈.. And wait for new contact cleaner. Thank you for the tip!
Wow, thank you for these suggestions! I am not sure I will be able to do that everything, but will consult with the guy I have mentioned before. 👍
Yes, it just sounds lovely this amp..
If the relays have clip on covers that can be prised off then use a thin piece of folded paper soaked in iso and pull it through the contacts to clean them.
These are microswitches but the principle is the same. A bit of paper is pulled through the contacts:
The easiest way to check for relay is to tap on it slightly with a pencil, srewdriver head or something similar. Volume should be low and I would prefer thist test with earphones (to preserver the speakers). If sound levels change then relay is the first culprit to look at. Otherwise it may also be the volume pot, connecting cables (solder joints) etc.
I have downloaded the service manual, the relay is Type MSLU 24VDC (the black box with DEC on it) fully covered with no access.. there is replacement available on eBay for at about £24..
This is the replacement MSLU 24VDC available from Germany on eBay, worthy into the future? Hm.. Are these relays prone to fail?
Unsoldering and removal of the relay has to be done before determining if it can be opened up to service the contacts.
Before purchasing any replacements (no objecton to that - a new relay is always better than decades old one) perhaps it would still make sense to measure test signal level before and after the relay contacts (in trheir closed state)?
If the levels are equal then the relay is not the culprit.
If the levels are equal then the relay is not the culprit.
Manufactured in 1980, I'd say it's ready for some sour capacitors to be replaced.
And going over, checking soldering is a good idea.
And going over, checking soldering is a good idea.
Absolutely you must do this first. Only by removing it can you see if it can be pried apart.
Now me being me 🙂 I would want the decimal point on that price moving over. The pin outs are the problem for sourcing replacements, if you are prepared to rehash the mounting of it then there are probably lots of other types that will be fine. Coil resistance is an important consideration so measure that when it is unsoldered.
You could even cut the top off the relay case off if it looks non openable. Wash it all out in iso to get any bits out, then clean the contacts... all doable.
Although the relay is favourite for your fault it is always nice to prove it so first.
By the way this is probably
That was another useful advice, thank you! Yes, I did connect the source, playing on headphones, tapped on the relay with screwdriver and there were changes, sometimes one channel disappeared, or they changed volume a bit, so great, there is issue with that relay 👍
I burnish relay/switch contacts with a strip of 500/800 grit emery paper, folded and massaged between closed contacts.
Yeah, absolutely agree 🤨, it is actually half price of the whole amp.. but it has potential..
so get it unsoldered 😉 and see if you can prise the cover off. Many just clip on, you might get lucky with a bit of force without removing it.
Thank you, or like Mooly suggested, but first I will try to remove the relay itself to see whether it’s possible to open it..
And thank you all guys, appreciate the help, even though I am not able to react to everything, I will re-read the advices again carefully.. 👍
On some similar occasions I have built riser boards i.e. vertical pieces of breadboard with relay soldered to it and wires soldered to initial printboard holes.
Another approach would be to attach the new relay to the back panel and resolder the wiring to it.