HMV Stereomaster 2419 left channel output help

[savmot22]

Hi everyone,

I was wondering if I could get some advice on a HMV Stereomaster 2419. This was my grandad's, and I was trying to get it working again after moving in to their old house. I opened it up to try to figure out why the left channel wasn't working.

Using an audio probe, I traced the signal in the left channel from the input to the output, and concluded that it must be a faulty speaker jack - there was audio all the way to the speaker out and no issues, and the left speaker made a sound in the right channel socket. I could have saved myself some time by just plugging into the headphone socket or tracing from the speaker connection backwards, but live and learn I guess.

Unfortunately, as I went back to compare the output volume between the channels, I accidentally slipped with my probe and, I think, connected the collector of TR33 (labelled RCA BRC5296) to ground. There was a small arc, and now there is no sound in the output stage of the left channel...

As a disclaimer, I'm not very familiar with transistor amplifiers, but I have a background building valve guitar amps, so I could be way off with my guesses here.

I was thinking that I've damaged TR33 or TR29, and the reason there's no audio output from TR25 is that TR33 or TR29 was providing a bias to TR25?

Some part of the output circuit seems to drop the 40v B+ down to around 17v, but that isn't happening. The voltages seems fine (within a few volts) across the right channel.

I was wondering if anyone could give me an idea of where to go from here, and how to narrow my search down a little?

Also, is the BRC5296 equivalent to a 2N5296?

Thanks for any help, it's very much appreciated.

[paulsherwin]

I suspect you will find multiple transistor failures. DC coupled transistor amps like these are notorious for cascading component failure when something goes bang, and are difficult to fix. If you want to repair it I would identify cheap alternatives for the transistors and change *all* of them - it sounds drastic, but needn't cost a lot.

Alternatively, if originality isn't important then you could replace one or both power amps with cheap IC modules. These are available for a pound or two from Chinese eBay sellers.

[DMcMahon]

Originally Posted by savmot22

Also, is the BRC5296 equivalent to a 2N5296?

From the information in this link it sounds like they are.

https://www.google.com/url?client=in...Q6zUjGNpXPh8v2

David

[savmot22]

Originally Posted by paulsherwin

I suspect you will find multiple transistor failures. DC coupled transistor amps like these are notorious for cascading component failure when something goes bang, and are difficult to fix. If you want to repair it I would identify cheap alternatives for the transistors and change *all* of them - it sounds drastic, but needn't cost a lot.

Alternatively, if originality isn't important then you could replace one or both power amps with cheap IC modules. These are available for a pound or two from Chinese eBay sellers.

Thanks for your reply.

Multiple transistor failure was what I was worried about. I am happy playing around and trying to get it sorted, replacing the power amps is a good last resort though. I'll see how frustrated I get with it!

On replacing the transistors, do you have any tips on finding the modern equivalents? The first number below is the service manual part id, the second is the part number on the transistor. I have had a go at finding a replacement for each.

AF Amp: AF20, ME4102 - 2N5088 modern alternative?
AF Amp: AF22, ME0412 - 2N5087?
Driver: AF21, ME0404-2 - 2N2907A?
Driver: AF23, ME6002 - 2N2907A?
Output: OP8B, BRC 5296 - 2N5296?
Output: OP8A, BRC 6109 - 2N6109?


Two transistors (shown to the left of TR29 on the schematic) aren't listed in the service manual as transistors, but are labelled W11/W13 on the PCB and D11/13 in the services manual, with part number C1129 printed on them. I'm assuming that these are 2SC1129 transistors.

This website suggests that the BF959 are replacements for the C1129, but they seem to be fairly different in spec. I can always try to find 2SC1129 or 28 too.

Finally - do you happen to know what this type of speaker connector is called? I have attached a pic.

Thanks again.

[savmot22]

Originally Posted by DMcMahon

From the information in this link it sounds like they are.

https://www.google.com/url?client=in...Q6zUjGNpXPh8v2

David

Cheers!

[paulsherwin]

I've no personal experience of changing transistors in these amps, but they all appear to be standard general purpose silicon transistors. The small signal ones could all be replaced with BC547/BC557s or 2N2222A/2N2907s, and the output pair look like medium power output transistors like the TIP31C/TIP32C.

I'm not sure why 2SC1129s were used in this design, as there's nothing special about them.

[music-centre]

The plug looks like a standard 2 pin Din plug
Steve.

[savmot22]

Originally Posted by paulsherwin

I've no personal experience of changing transistors in these amps, but they all appear to be standard general purpose silicon transistors. The small signal ones could all be replaced with BC547/BC557s or 2N2222A/2N2907s, and the output pair look like medium power output transistors like the TIP31C/TIP32C.

I'm not sure why 2SC1129s were used in this design, as there's nothing special about them.

Great - I actually have a bag of BC547s, so I will order the rest. I'll update on here when I have them installed!

Thanks again.

[savmot22]

Originally Posted by music-centre

The plug looks like a standard 2 pin Din plug
Steve.

Great, thank you!

[DMcMahon]

Originally Posted by savmot22

Two transistors (shown to the left of TR29 on the schematic) aren't listed in the service manual as transistors, but are labelled W11/W13 on the PCB and D11/13 in the services manual, with part number C1129 printed on them. I'm assuming that these are 2SC1129 transistors.

Yes, seems quite common for 2SC transistors to have the 2S missing.

[Radio Wrangler]

Do NOT try fixing it by replacing one transistor at a time then seeing if it works.

As Paul said above, these DC coupled circuits are notorious for cascade failures. Find one bad transistor and fit a new one... but the one of the other bad ones pops your new device. You look around and find one of the other bad devices and change it and your new (now bad) transistor pops that new one. You can go around in circles indefinitely.

There are two efficient ways to fix transistor amps like these:

1) Remove all the semiconductors, test each one quite thoroughly at the voltages they will be run at (meaning pocket transistor testers aren't trustworthy here) Replace the ones found bad, and refit the lot.

2) Remove all the semiconductors, and just replace the lot. Check the resistors while the transistors are out.

These may sound like the long way round, but all the short cuts wind up taking longer or in the repair getting abandoned.

It's a known difficult task, but with self-discipline, it's do-able.

David

[savmot22]

Thanks David. I’m still getting audio from the preamp/eq section, it’s the power amp (from TR25) where the audio fails - I was planning on replacing all the transistors in the power amp section in one go, but are you suggesting I should go further back into the preamp too?

[savmot22]

Originally Posted by DMcMahon

Yes, seems quite common for 2SC transistors to have the 2S missing.

Thanks, that’s good to know.

[paulsherwin]

No, you don't need to go back before the power amp stage. There's no DC path to these earlier transistors so they should be fine. You should be able to treat the power amp as a discrete subsystem.

The 2SC1129s (if that's what they are) seem to be being used to set the bias for the output pair. It's not clear why the designers decided to do things that way.

Be careful about leadout differences when subbing transistors.

[AC/HL]

This seems to be the Thorn series 74 chassis. The ERT sheet shows them as Thorn D26, which is C1129 (with b-c strapped) in their equivalents book. Also C1574.
Incidentally, now that I've found it, the book gives OP8A and B as TIP32A and TIP31A as above. These Thorn numbers are a PITA.

[savmot22]

Originally Posted by paulsherwin

No, you don't need to go back before the power amp stage. There's no DC path to these earlier transistors so they should be fine. You should be able to treat the power amp as a discrete subsystem.

The 2SC1129s (if that's what they are) seem to be being used to set the bias for the output pair. It's not clear why the designers decided to do things that way.

Be careful about leadout differences when subbing transistors.

Originally Posted by AC/HL

This seems to be the Thorn series 74 chassis. The ERT sheet shows them as Thorn D26, which is C1129 (with b-c strapped) in their equivalents book. Also C1574.
Incidentally, now that I've found it, the book gives OP8A and B as TIP32A and TIP31A as above. These Thorn numbers are a PITA.

Thanks both for the information.

This morning I pulled out the transistors and tested them (just using a pocket tester). There's something a bit weird with the C1129s though - one has a green bit of paint, the other has a white bit of paint.

Both tested as two diodes with a common point rather than a transistor (I don't know the significance of this or how my cheap transistor tester makes that decision), but the one with white paint is testing as PNP, while the one with green paint is testing as NPN. This is at odds with the schematic, which has them listed both as NPNs. Should I follow the schematic, or try installing one NPN and one PNP as it seems to be? The right channel transistors also has the green/white dots.

Incidentally, both of the audio drivers and the BRC 5296 tested as damaged.

[paulsherwin]

You should follow the circuit diagram. If the transistors are damaged then this will confuse the tester.

The two C1129s are essentially being used as two silicon diodes in series to set the bias conditions, which would be a more normal way of doing things.

As described earlier, it is normal practice for most or all of the transistors to go bang in these designs when a fault develops or is introduced.

[savmot22]

Originally Posted by paulsherwin

You should follow the circuit diagram. If the transistors are damaged then this will confuse the tester.

The two C1129s are essentially being used as two silicon diodes in series to set the bias conditions, which would be a more normal way of doing things.

As described earlier, it is normal practice for most or all of the transistors to go bang in these designs when a fault develops or is introduced.

Thanks again! I installed all the transistors today and used some servisol on the left speaker plug and everything works perfectly.

The left channel ended up sound nicer than the right - it was clearer, fuller, and louder when compared to the right. So I replaced all the same transistors in the right hand side too.

It sounds really nice! I'm very happy with it. I installed a bluetooth module too while I was in there (only cost 8.99) which is powered using a raspberry pi plug I pulled apart and wired to the transformer. The bluetooth module feeds the tape input.

I recorded a little video here: https://www.youtube.com/watch?v=zmQ_...ature=youtu.be

[paulsherwin]

Good result. Wholesale 'shotgun' component changing is generally frowned on in the vintage radio community, but with these sorts of amps it's usually the best course of action.

[Techman]

I agree!

Very well done!

It's not often that there's an outcome as good as this when it comes to this type of amplifier.

As a side note - I wouldn't have used a transistor tester for testing, but rather an Avometer to test the junctions and would have 'probably' got away with replacing just the ones that definitely tested faulty...note that I said 'probably'!