Proline/Solid State series amplifier transformers

[peterb_2795]

Hi All,

Just throwing this question out there. I seem to have collected a couple of 925's and a 330 which all have amplifier transformers (023788, 023770) in various states of "decay" (for want of a better word). All are still working but I am sure that there will come a time when one or more of these transformers will "pop it's socks".

Does anyone know of an equivalent transformer that would be a suitable replacement (and is readily accessible) in today's market?

Thanks in advance for any suggestions.

Cheers,
Peter

[bnelson218]

Are you talking about the interstage driver transformers on the amp boards? Have you tried Hammond Suzuki service department? (630)543-0277. I bought a few of these several years ago. I think the new part # is 140560.

[peterb_2795]

HI,

Thank you most kindly for that information. It does indeed appear to be a correct match for the treble and auxiliary driver transformer.
When I checked against the 925/760/330 manual board layouts, I am left a tad confused by the bass driver transformer pin outs.

Using your notation, the new part: 140560 matches 023788 as per
(bottom view)
6 2 1 (red/black dot)
4 5 3

However, in the case of the bass driver transformer 023770, the pin outs noted,
(bottom view)
5 2 no pin (red/black dot)
4 5 3


I have not removed either board or transformer to investigate, but would this not be a different transformer? Or is there a possibility that
it is just another tap off (5)?

Either way, I appreciate the information that you have given and certainly, if the need should ever arise, then HS would be a first port of call.

Cheers,
Peter

[bnelson218]

Well, at some point they stopped using separate xfmrs for the bass and treble amps and settled on just one of them, but I forget it it was the treble or bass xfmr. But if you look at a service manual for the HL722 you'll see that all 4 amps use the same xfmr, part number 140560.

[David Anderson]

Well, at some point they stopped using separate xfmrs for the bass and treble amps and settled on just one of them, but I forget it it was the treble or bass xfmr.

It was the bass transformer. Also, since the two transformers are different sizes, the layout of the PCB traces and mounting hardware is different for the board with the smaller treble transformer -- including the standoffs on the black metal panel. (I ran into this in a repair where one of the transformers was damaged; you can't easily swap one type of transformer or driver board for the other.)

Later 760s had two identical bass driver boards, probably because, at that point, production was winding down and they were building Leslies from the stock of parts they had -- less complexity.

I would say that the best way to avoid losing an interstage transformer is to do as much work as possible to prevent the runaway conditions that could destroy one of them.

[peterb_2795]

Thank you David,

A font of knowledge as always. It would appear to me then, that in the case of the early Bass transformers (023770), prevention is going to be easier than cure! Equally, I will now always keep an eye open for parted out Solid State Leslies as I might be able to pick a spare up.

Again, thank you to your goodself and also to bnelson218 for the sharing of knowledge.

Cheers,
Peter

[David Anderson]

By the late 1970s, interstage-coupled solid-state power amps were pretty much obsolete, replaced by direct-coupled circuits.

The other company that used them, Rhodes, was also owned by CBS at the time. I have seen one early-mid 70s Japanese Ace Tone bass amp with this circuit. Rhodes went to the 100W direct-coupled power amp around 1976-77.

As such, I wouldn't expect that interstage driver transformers like those used by Rhodes or Leslie would be easy to find. Leslie was the last company I know of to build amps using that circuit topology.