Musical Fidelity A1 problem

[Byter]

I have a Musical Fidelity A1 integrated with a problem. After an hour or so of play the sound begins to become distorted and one channel loses most of its volume. All the electrolytic caps have been replaced but this did not fix the problem. Does anyone have any suggestions? Perhaps even a suitable repairer in Melbourne, as cap replacement may be about the limit of my tecnical abilities. Thanks in advance.

[mbz]

Initial response is the device (diode, transistor,,,) that performs temperature compensation of the bias (though would expect bias to increase with temp if faulty). Either way, suggest you monitor the voltage, ie, with no audio, measure bias, then wait 10min or so for amp to warm up then remeasure, then play audio until problem occurs (after 1hr?) then stop audio, vol=0 and remeasure bias.

 

You could also use RCA cables to cross from Left pre-out RCA into right main-in likewise from right pre-out to left main-in. Does problem still occur on same channel. Assume amp has pre-out/main-in RCA's. Will try and find service manual.

[Byter]

Thank you very much for your help. I think I will need someone more technical than me to help implement your suggestions. Much appreciated, though. 

[Guest Old Man Rubber]

Dirty volume control?  Can sometimes be heat related - if you fiddle with the volume does it improve / is obviously affected by the volume?

[Decky]

Everything is heat related in that thing. 

[mbz]

Odd design, didn't see any temperature compensation of bias. Bias appears "hard coded" with no adjustment, set by voltages at output emitters. Maybe you've got a dying transistor, still like the idea of checking/monitoring the bias (voltage across emitter resistors). An oscilloscope would be handy.

[Jackybrown]

Have you checked this? maybe the same cause of the "Noisy Pots and Switches".

 

https://www.markhennessy.co.uk/mf_a1/technical.htm

 

 

"Noisy pots and switches

The bane of my life! It's so embarrassing to have a half-decent hi-fi with an unusable volume control! The failure mode is deposits on the track, particularly at the lower end of the scale. Unfortunately, the design of the preamp compounds the problem: as the wiper hits dirt and goes high-resistance, the volume is effectively max'd for a brief time. If the configuration was conventional, the sound would do the opposite, making the effect much less objectionable.

Also, the switches are affected for the same reason - the input op-amp stage puts dc currents through the switches. Replacing the pot and switches is a short-term solution. If you retain the original preamp, the new components will go the same way."

[Byter]

Thanks everyone for your replies. It doesn't seem to be the pot or switches, unfortunately. Mike, I don't suppose you would like to have a look at it? I think it's time to pay someone who knows what they are looking at, as I'm afraid in this case I don't.

[mbz]

On 09/12/2020 at 2:47 PM, Byter said:

Mike, I don't suppose you would like to have a look at it?

I'm out of town for a few days. I'm in W Brunswick, if you are within 15-20min then one off/no-fee can have a look when I get back. Please don't drive 40min.

 

EDIT:If it's an easy fix, will fix, otherwise give feedback what needs to be done.

Edited December 10, 2020 by mbz
edit

[Byter]

17 hours ago, mbz said:

I'm out of town for a few days. I'm in W Brunswick, if you are within 15-20min then one off/no-fee can have a look when I get back. Please don't drive 40min.

 

EDIT:If it's an easy fix, will fix, otherwise give feedback what needs to be done.

That is very kind of you. I work in Richmond and live in the outer east, so drive 40 mins + twice a day! Please let me know when you are available and what you drink

[Wimbo]

Replacing all the caps can be enlightening but doesnt mean you have fixed a problem. Good you did it yourself mate

These things work under extreme conditions and when you think of the age of the unit, I would most probably replace the output trani's and repaste them. Sounds to me like a thermal problem.

The good thing is that its a great amp and well worth repairing.

[mbz]

On 05/12/2020 at 8:49 PM, Jackybrown said:

Have you checked this?

 

https://www.markhennessy.co.uk/mf_a1/technical.htm

Very helpful, shows the amp is a real PITA to work on. Hopefully can access bias points from underneath?

[Cecil B]

Hiya, have one of these that was refurbed by an SNA member........they are "hot" amps so heat is always an issue.....mine has just started doing exactly what you are experiencing, have eliminated obvious things like faulty/suspect interconnects, speaker connections and drive devices.........in mine it appears to be the Aux input RCA's as all other inputs work ok. This only happens when amp has been working for 30+ min sometimes 1hr .......think it is a heat problem but because of the way the amp is built if you remove the covers (heatsink)  you will require a particular heat paste compound to replace. Anyway, try some other inputs and see if the drop outs still happen..........as Wimbo says ....worth fixing........love the sound of this very underrated amp from the 80's.

 

Cheers, John

[mbz]

8 hours ago, Cecil B said:

in mine it appears to be the Aux input RCA's as all other inputs work ok.

In vintage, it's not uncommon for "excessive" heat, over time, to "unsolder" components such as voltage regulators. Worthwile to check solder connections, google "dry solder joints" will give you pixs on what to look for.

 

8 hours ago, Cecil B said:

if you remove the covers (heatsink)  you will require a particular heat paste compound to replace

Available "everywhere" even jaycar. Need to throughly clean away old past then apply even thin film, use a credit card or similiar to evenly spread THIN film. Less is better in this case.

 

@Byter did you try different inputs (TAPE, TUNER...) maybe a problem with the input selector?

 

Appears the amp is a PITA to work on, you need to open it up to take measurements etc... however you can't run it with the top lid removed since it forms part of the heat sink.  Doubtful there is a removable base plate.

[Byter]

Thanks again to everyone for your help. I only ever use the CD input so trying some others now. Would completely fix my problem if even one other input worked! 

[Byter]

Going strong on AUX! I'll keep it running until bedtime but thanks very much, especially to John for the suggestion and Mike for your generosity. Cheers, Geoff

[Byter]

I have tuner and aux working so am out of the woods. I'll have a look next time I have it open and see if I can identify the problem but, as has been noted here, it is a bit of a pain to work on so that may be a while. Thanks to everybody for your help.

[tubularbells]

Im working on one right now that appears to have  a short somewhere in the transformer (blowing fuses) so am looking at a replacement transformer. As others commented heat is the enemy inside these amps as nest to no internal ventilation and the top heatsink in the high 50 degree range is a torture test for the internal components.

 

Last one i worked on had caps ready to pop!

 

Not a pot to be adjusted anywhere so have to rely on the components to be in spec. in order for it to perform correctly but a lovely sounding piece when working correctly.

[Byter]

On 12/12/2020 at 10:23 PM, Cecil B said:

Hiya, have one of these that was refurbed by an SNA member........they are "hot" amps so heat is always an issue.....mine has just started doing exactly what you are experiencing, have eliminated obvious things like faulty/suspect interconnects, speaker connections and drive devices.........in mine it appears to be the Aux input RCA's as all other inputs work ok. This only happens when amp has been working for 30+ min sometimes 1hr .......think it is a heat problem but because of the way the amp is built if you remove the covers (heatsink)  you will require a particular heat paste compound to replace. Anyway, try some other inputs and see if the drop outs still happen..........as Wimbo says ....worth fixing........love the sound of this very underrated amp from the 80's.

 

Cheers, John

Hi again everyone. It's been nearly four years now since I "sidestepped" my problem by using a different input, thanks to suggestions given here. It seems, though, that my luck has run out and the same fault has returned to what were the remaining operable inputs on my amp. John, did you find a solution? If it was to replace the input selector, may I ask where you sourced it? My technician is expressing some frustration over parts availability. Thanks to everyone for their time.

[Cecil B]

Hi Byter, 

My only solution at the time was to change inputs. If they have all failed on you I can only suggest that the heat these amps produce has compromised a lot of solder joints or other stuff. 
Suggest you contact Tubularbells who is on this forum, he did all the refurb work on the amp I had. 
Cheers, John. 

[Byter]

Thanks very much, John. The amp is with a tech at the moment but if he gives up (I can feel it sliding towards the too-hard basket) I will do just that. I appreciate your help. Geoff

[Cecil B]

[mbz]

Reads like either a bad connection on input RCA's or a dirty function selector. Using a scope would quickly identify problem area. In both cases more likely that one channel has failed or is distorted than both. Visual inspection of RCA solder connections also reveal problem. In vintage it's quite common to completely disassemble switches for deep cleaning since replacements are not available.

eg, lifted